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The relationship of visual extinction to luminance-contrast imbalances between left and right hemifield stimuli
Neuropsychologia 43 (2005) 542–553
The relationship of visual extinction to luminance-contrast imbalances between left and right hemifield stimuli Sarah Geeraertsa , Karla Michielsb , Christophe Lafossec , Erik Vandenbusschea , Karl Verfaillied,∗ a Laboratory of Neuropsychology, University of Leuven, Belgium Physical Medicine and Revalidation, University Hospital Leuven, Belgium c Scientific Unit Rehabilitation Centre ‘Hof ter Schelde’, Antwerp, Belgium Laboratory of Experimental Psychology, Department of Psychology, University of Leuven, Tiensestraat 102, B-3000 Leuven, Belgium b
d
Received 3 February 2004; received in revised form 16 July 2004; accepted 21 July 2004
Abstract Visual extinction was investigated in six right brain-damaged patients with left visual neglect, using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. When the visibility of target and distractor gratings was subjectively equalized in this way, neglect patients still showed a significant extinction effect, i.e. a significant interference of the right hemifield distractor with left hemifield orientation sensitivity. By manipulating the luminances of left and right hemifield gratings during bilateral simultaneous stimulus presentation, we demonstrated the role of luminance-contrast imbalances in eliciting visual extinction. Both decreasing the right distractor luminance and increasing the left target stimulus luminance resulted in an elimination of the observed extinction effects. These results show that not the absolute salience of one of two simultaneously presented stimuli, but the relative salience of both stimuli, is the crucial factor for inducing extinction. © 2004 Elsevier Ltd. All rights reserved. Keywords: Attention; Orientation thresholds; Salience imbalance; Stimulus competition; Visual hemineglect
1. Introduction Visual extinction is a frequently observed consequence of unilateral, mainly right hemispheric, brain damage. Patients with visual extinction can reliably see a single stimulus in the hemifield contralateral to the brain lesion, but fail to see the same stimulus (or at least process the stimulus less efficiently) when another one is simultaneously presented on the ipsilesional side. Visual extinction is often observed as part of the hemineglect syndrome (Heilman, Watson, & Valenstein, 1993). Current theories of neglect and extinction propose either attentional or spatial representation deficits. According ∗
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to attentional theories, mechanisms for orienting attention towards the contralesional side of space and/or for disengaging attention from ipsilesional stimuli are affected (Corbetta, Kincade, Ollinger, McAvoy, & Shulman, 2000; Kinsbourne, 1993; Posner, Walker, Friedrich, & Rafal, 1984). According to representational theories, the spatial representation of the contralesional side of space is damaged (Bisiach & Luzzatti, 1978; Rizzolatti & Berti, 1990). However, Pouget and colleagues (Pouget & Driver, 2000; Pouget & Sejnowski, 2001) suggested that the distinction between attentional and representational accounts of neglect may be artificial. On the one hand, neglect and extinction may arise because the lesion produces a gradient in the number of (parietal) neurons for particular regions of space. On the other hand, it has been shown that, also in the intact brain, the parietal
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representation is a highly selective representation of the scene being viewed, with several neurons only responding to attended stimuli (Gottlieb, Kusunoki, & Goldberg, 1998). In this scheme, parietal damage would not only affect the representation of different spatial positions, but also the attentional salience of those positions. Imbalances in the attentional salience then becomes the key concept for a better understanding of extinction and neglect: When two stimuli are presented simultaneously, an imbalance in the salience of the stimuli results in extinction of the more contralesional stimulus. If an imbalance in the attentional “weight” of stimuli (also see Duncan et al., 1999) causes extinction, it should be possible to reduce extinction by reducing the salience imbalance in a bottom-up way. However, whereas visual extinction has been shown to be strongly influenced by top-down factors such as task-defined perceptual set and stimulus relevance (Vuilleumier & Rafal, 2000), there is much less evidence for modulation of extinction by bottom-up factors, such as the brightness or size of contralesional stimuli. Di Pellegrino and De Renzi (1995) observed that extinction persisted unmodified when stimulus presentation was lengthened from 200 to 500 ms, or when the size of the contralesional stimulus was fourfold that of the ipsilesional stimulus. The only condition in which extinction was reduced was when the right brain-damaged patient was instructed to ignore the right stimulus and only respond to the left stimulus. However, in these experiments, stimulus durations of left and right hemifield stimuli were lengthened together. Since this manipulation changes the salience of both stimuli but not the salience imbalance, no effect on extinction is expected on the hypothesis of Duncan et al. (1999). The size increment of the left stimulus, on the other hand, marginally altered the extinction rate with bilateral presentation from 100 to 90%. Smania, Martini, Prior, and Marzi (1996) manipulated the intensity of the stimuli in an extinction task. Extinction rate did not benefit when the contralesional stimulus was almost eight times more intense than the ipsilesional stimulus, compared to an equal luminance condition. In a similar extinction study, Zihl and von Cramon (1979) observed that increasing the contralesional stimulus luminance and decreasing the ipsilesional stimulus luminance resulted in a small reduction of the extinction effect. However, these effects were observed in a patient with a left hemispheric lesion while the patients in the study of Di Pellegrino and De Renzi and of Smania et al. had lesions in the right hemisphere. Smania et al. (1996) suggested that the nature of extinction following right and left hemisphere lesions might be different. The absence of an influence of stimulus brightness and size on extinction is often used as a strong argument for an attentional rather than a sensory deficit causing extinction in right brain-damaged patients. As attention is supposed to be mediated mainly by the right hemisphere (Parasuraman, 1998), sensory rather than attentional imbalances might induce extinction in left brain-damaged patients.
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Most visual neuropsychological studies use suprathreshold stimuli to assess extinction. A recent tactile extinction study by Meador, Ray, Day, and Loring (2001) determined tactile perception thresholds and studied the relationship of extinction to the thresholds for single stimuli. In contrast to the previous results, Meador et al. did observe effects of asymmetries in stimulus intensity on extinction (relative to singlestimulus perceptual thresholds). Moreover, using close to threshold stimuli, Gorea and Sagi (2002) have documented extinction-like effects in normal observers who had to discriminate between simultaneously presented visual stimuli with different intensities. Their results suggest that extinction is, at least in part, triggered by a sensitivity imbalance between simultaneous events. The relationship of extinction to visual perceptual thresholds for single stimuli remains uncertain. The purpose of the present experiments, was to study visual extinction via a psychophysical paradigm and to assess the influence of changes in the relative salience of ipsilesional and contralesional stimuli on extinction. An adapted version of the psychophysical paradigm developed by Geeraerts, Lafosse, Vandenbussche, and Verfaillie (in press) was used. Two features characterize the paradigm. First, the extinction paradigm was more sensitive than usually employed in standard testing. More specifically, thresholds for orientation discrimination were measured psychophysically. Orientation sensitivity was determined for left and right hemifield gratings presented with or without a contralateral distractor grating. Attention was always cued towards the target stimulus position and away from the distractor position, making the distractor stimulus task irrelevant. Extinction was assessed by measuring the influence of the contralateral distractor grating. Second, to control for possible lower-level sensory deficits, we attempted to equalize the visibility of left and right hemifield gratings. More specifically, before administering the orientation discrimination task, luminance thresholds for single stimuli presented in either the left or right hemifield were measured in a horizontal–vertical grating discrimination task. Gratings with luminances of 20 times these thresholds were then used in the orientation discrimination task, in which possible extinction effects were examined. We observed that, even after eliminating contra-ipsilesional differences in stimulus visibility, neglect patients still displayed an increased interference effect from an ipsilesional distractor. Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients (without neglect) who demonstrated extinction on standard extinction testing. This suggests that extinction in neglect patients, but not necessarily in patients without neglect, is based on an attentional imbalance between ipsilesional and contralesional stimuli. This paradigm was adapted to examine whether the relative salience of target and distractor grating can influence extinction. Right brain-damaged patients with left neglect were tested in three separate experiments. In Experiment 1, the luminance of the distractor grating was manipulated, in
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order to investigate whether left extinction can be reduced by decreasing the right hemifield distractor luminance and whether right extinction can be induced by increasing the left hemifield distractor luminance. In Experiment 2, the luminance of the target grating was manipulated, in order to examine whether left extinction can be reduced by increasing the left hemifield target stimulus luminance and whether right extinction can be induced by decreasing the right hemifield target stimulus luminance. In Experiment 3, a random dot distractor was used instead of a grating distractor to assess whether luminance onset is enough to induce extinction, or whether the contrast pattern of the grating distractor is a necessary condition.
2. Experiment 1 Extinction was examined in two right brain-damaged neglect patients. Subjects performed an orientation discrimination task, in which they had to decide whether a square-wave grating, presented either in the left or in the right hemifield, deviated clockwise or counterclockwise from vertical. Extinction was assessed by comparing conditions in which left or right targets were presented in isolation with conditions in which a task-irrelevant distractor grating was presented in the contralateral hemifield. The main purpose of Experiment 1 was to investigate the effect of changing the luminance of the distractor grating. There were two control conditions: a condition without distractor grating and a condition in which target and distractor were equally visible (based on luminance thresholds for single hemifield stimuli in a horizontal–vertical grating discrimination task). First, we examined whether decreasing the luminance of the ipsilesional distractor grating (in comparison to the condition with equally visible gratings) would reduce the extinction effect. Second, we determined whether increasing the luminance of the contralesional distractor grating would induce an (otherwise absent or even reversed, cf. Geeraerts et al., in press) extinction effect. In addition, one normal control subject performed the same task in order to determine whether an extinction effect could be induced in normal subjects, comparable to that of extinction patients, only by changing the relative luminance of target and distractor gratings. For both left and right hemifield distractor conditions, a condition in which the distractor luminance was much higher than the target luminance was compared with a condition in which target and distractor luminances were equalized. 2.1. Methods 2.1.1. Subjects Two right brain-damaged patients with left visual neglect (patients 1 and 2 in Table 1) and one control subject without a neurological history (patient NC in Table 1) participated in the study. Patients were recruited from a rehabilitation unit
for stroke patients. They all had a single, unilateral vascular lesion as revealed by CT/MRI and clinical symptoms and did not show hemianopia or upper visual field defects on perimetric testing (see Geeraerts et al. (in press) for more details on the procedure). Visual neglect was assessed with line bisection (Schenkenberg, Bradford, & Ajax, 1980) and star cancellation (Behavioural Inattention Test; Wilson, Cockburn, & Halligan, 1987) tasks. In order to be included in the study, the patients had to show neglect on both tasks and had to obtain reliable thresholds in our luminance and orientation task conditions without distractors. Table 1 summarizes the clinical and demographic characteristics of the patients. 2.1.2. Luminance thresholds Luminance thresholds were first determined for individual subjects. These thresholds were used to determine the stimulus luminances in the orientation tasks. Subjects were seated in front of the computer screen at a distance of 114 cm with the head restrained by a forehead and chin rest. A central red fixation point (diameter 0.2◦ of visual angle) was presented throughout the experiment, together with a peripheral cue, indicating the position of the next stimulus. The cue was a white circle (diameter 4.5◦ ), the centre of which was presented at 5◦ eccentricity from the fixation point on the 45◦ diagonal in either the left or right upper visual field quadrant. The stimuli only appeared at the cued position (left or right upper quadrants). Stimuli were circular patches of square-wave gratings (diameter 4.5◦ , spatial frequency 1 c/◦ ) with either horizontally or vertically oriented bars. The gratings were luminance-modulated with dark bars of a constant background luminance (0.001 cd/m2 ) and bright bars of luminances that varied according to the subjects’ performance. Bright bars were constructed from random noise patterns. The computer program could produce only two luminance levels (black and white) on the screen. The black bars were always uniform black. The luminance of the white bars was reduced by changing some of the pixels to black. The viewing distance was large enough for the ‘random noise’ to be spatially integrated by the observer’s visual systems, so that the bright bars appeared lighter or darker gray. Luminance thresholds were determined with an adaptive staircase procedure (MUEST; Snoeren & Puts, 1997) converging around a performance level of 70% correct. The starting luminance was 0.04 cd/m2 . The threshold was the luminance for which the subject reached a 70% correct discrimination between horizontal and vertical gratings. Each threshold estimation was based on 40 trials. Only subjects that reached a performance level between 65 and 75% correct (indicating a reliable threshold measurement) were included in the study. The trial sequence was the following. After central fixation was maintained for 500 ms, the cue circle was replaced by the grating stimulus for a period of 300 ms. The subject had to respond to the orientation of the grating by pressing a left response key for horizontal and a right response key for vertical orientations. When motor difficulties in the
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Table 1 Clinical and demographic characteristics of patients Subject
Age
Sex
Etiology
Lesion location
Months s/lesion
Visual field
Line bisection
Star cancellation
P1 P2 P3 P4 P5 P6 NC
53 64 53 56 46 60 61
F F M M M M F
Infarct ICB ICB Infarct ICB Infarct
R p–o R f–t–p R f–t–p R t–p–o Rp R f–p
35 11 14 6 8 26
Normal Normal Normal LLQ LLQ LLQ
13.1 21.2 12.6 8.7 7.0 12.8
4/2 3/0 3/0 4/2 8/4 3/2
P, patient; NC, control subject; M, male; F, female; ICB, intracerebral bleeding; R, right lesion; p, parietal; o, occipital; f, frontal; t, temporal; LLQ: quadrantanopia in left lower visual field; line bisection: % rightward deviation; star cancellation: number of omissions on left/right half of the page.
contralesional hand were suspected, patients responded with two fingers of the ipsilesional hand. Subjects were urged to guess when they were unsure about the grating’s orientation. Response time was unlimited. Feedback was given in the form of an auditory signal for incorrect responses. Only after a response was made and after a 2000 ms intertrial interval the next trial was initiated. Eye movements were monitored with the ASL 210 eye tracker (Applied Science Laboratories, Bedford, MA). Trials with eye movements out of a 2◦ fixation window were discarded and repeated. Stimuli were presented in four blocks of 20 trials (not counting trials with eye movements). Within a block, the stimulus was always presented in the same position. Blocks with left and right hemifield stimulus positions were alternated. Thus, threshold estimation for each hemifield consisted of 40 trials. 2.1.3. Orientation thresholds Unless stated otherwise, the stimuli and procedures for the orientation threshold measurements were the same as for the luminance threshold measurements. Stimuli could be either target gratings or distractor gratings, both of them having the same size and spatial frequency as the luminance gratings. Target gratings were always presented at the cued (left or right hemifield) position; distractor gratings were always presented in the opposite hemifield in the homologue position (i.e., in the mirror-imaged position about the vertical mid-line; Fig. 1A). The distractor grating was presented simultaneously with, and for the same duration as the target grating. Orientation thresholds were determined for target gratings in left and right visual fields, with and without the simultaneous presentation of a distractor grating. Patients indicated whether the grating orientation deviated clockwise or anticlockwise from vertical. We had to use a constant stimuli paradigm to measure orientation sensitivity independently of the point of subjective visual verticality, because a contraversive tilt of the subjective visual vertical has been observed in neglect patients (Geeraerts et al., in press; Kerkhoff, 1999). Via a short adaptive staircase procedure, five equally spaced and informative orientations spanning the estimated subjective visual vertical were obtained for each patient and separately for left and right hemifield positions. These orientations were then used as target stimulus orientations in the actual
task. Distractor orientations were chosen randomly in each trial and could be any orientation between 0◦ and 360◦ . A total of six conditions or six threshold measurements was carried out, three left and three right target stimulus conditions. The luminance of left and right target gratings was set at 20 times the luminance threshold that was obtained for the respective positions. The luminance of the distractor gratings was manipulated. The three left target stimulus conditions were: a condition without distractor; a condition with a right hemifield distractor of 20 times the right luminance threshold, such as the target stimulus; and a condition with a right hemifield distractor of only twice the luminance threshold (factor of 10 decrease, but still suprathreshold). The three right target stimulus conditions were: a condition without distractor; a condition with a left distractor of 20 times the left luminance threshold; and a condition with a left distractor of 200 times the threshold (factor of 10 increase). Blocks of 30 trials with a left hemifield target stimulus were alternated with 30-trial blocks with a right hemifield target. Within a block, the three distractor conditions (10 trials each) were randomly presented. The five target stimulus orientations were randomly presented within these 10 trials. Subjects were instructed to ignore the distractor as much as possible. The cue always indicated the position of the target to be presented. If the target orientation deviated clockwise from vertical, subjects had to press the right response key; for an anticlockwise deviation, the left response key had to be pressed. All experiments were carried out in a completely darkened room (the edge of the screen was also invisible), so that there was no reference frame for comparison with the grating orientations. Trials with eye movements out of a 2◦ window were discarded and repeated. A total of at least 100 trials per condition (20 trials per orientation level, not counting trials with eye movements) had to be completed. 2.2. Results and discussion The two patients obtained elevated left hemifield luminance thresholds (0.26 and 0.15 cd/m2 for patients 1 and 2, respectively) as compared to their right hemifield thresholds (0.15 and 0.11 cd/m2 ). In the normal control subject, the right hemifield threshold (0.17 cd/m2 ) was slightly higher than the left hemifield threshold (0.21 cd/m2 ). These results show that
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Fig. 1. Schematic depiction of stimuli and trial sequences for the orientation discrimination tasks.
the luminance adaptations in the orientation tasks are necessary to obtain the same subjective visibility for left and right hemifield gratings. For the analysis of the orientation thresholds, the normalized percentages of “clockwise deviation from vertical” responses were plotted against the presented orientations (ordered from the largest anticlockwise to the largest clockwise deviation), for each subject in each condition. Fig. 2 shows plots of patients 1 and 2. Factorial multiple regression analyses were performed for each subject, to determine whether there was a significant difference between the conditions with and without distractor and between the different distractor conditions. The dependent variable was the normalized proportion clockwise responses; the predictor variables were stimulus orientation and distractor condition. A significant interaction between the predictor variables indicates a significant difference between the slopes of the regression lines. When left target stimulus conditions without and with the standard right hemifield distractor (with equally visible target and distractor) were compared, both left neglect patients showed a significant interaction between the predictor variables (F(1,6) = 15.1, P < .008, for patient 1, and F(1,6) = 16.2, P < .007, for patient 2). For the right target stimulus conditions with and without standard distractor, this interaction was not significant (F(1,6) = 0.12, P < .74, for patient1, and F(1,6) = 0.03, P < .87, for patient 2). Thus, significant right distractor interference with left hemifield orientation sensitivity was observed, while no significant interference from the left distractor with right hemifield orientation sensitivity was observed. This replicates the findings of Geeraerts et al. (in press). When the left target condition without distractor was compared with the condition with a right distractor with reduced luminance, no significant interactions were observed (F(1,6)
= 4.0, P < .09, for patient 1 and F(1,6) = 0.002, P < .97, for patient 2). This indicates that decreasing the luminance of the ipsilesional distractor eliminated extinction in the contralesional hemifield. The difference between the right target condition without distractor and with a distractor with increased left distractor luminance was in the expected direction, but was not significant in neither patient (F(1,6) = 3.6, P < .11, for patient 1 and F(1,6) = 2.4, P < .17, for patient 2). Apparently, increasing distractor luminance in the contralesional hemifield was not a sufficient condition to induce extinction in the ipsilesional hemifield. A direct comparison of the two distractor conditions supported the same conclusions. In both patients, there was a significant difference for left hemifield target stimuli (F(1,6) = 10.6, P < .02, for patient 1 and F(1,6) = 17.7, P < .006, for patient 2), but not for right hemifield stimuli (F(1,6) = 1.9, P < .21, for patient 1 and F(1,6) = 1.5, P < .26, for patient 2). In summary, decreasing the right distractor luminance resulted in a significantly smaller interference with left hemifield orientation sensitivity, while increasing the left distractor luminance resulted in a slightly but non-significantly larger interference with right hemifield orientation sensitivity. Via least squares linear regression analyses, just noticeable differences (JNDs) in orientation were estimated. The JND is an inverse measure of the slope of the regression line. JNDs could only be determined for conditions in which a significant correlation between grating orientation and normalized percentage clockwise responses was observed. A significant correlation was observed for both patients in the conditions without distractor and for patient 2 in the conditions with distractor (Pearson product moment correlations higher than 0.90, P < .05). However, patient 1 did not show a significant correlation (r = 0.29, P > .05) between orientation and
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Fig. 2. Results of Experiment 1. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
percentage clockwise responses in the condition with a contralesional target and an ispilesional distractor with comparable luminance. Apparently, distractor interference in this condition was so strong that the patient was not able to perform the orientation discrimination task. Consequently, a reliable JND could not be estimated in this condition. The magnitude of the distractor effects was determined for each subject by dividing the JNDs from the conditions with a distractor by the JNDs from the conditions without distractor. Table 2(A) shows the distractor effects for the conditions with reliable JNDs in patients 1 and 2. Standard distractors of 20 times
the luminance thresholds resulted in an asymmetric distractor interference, with a larger (in patient 1 even not estimable) interference for the contralesional target than for the ipsilesional target. Decreased right and increased left distractor luminances eliminate this asymmetry. The normal control subject (Fig. 3) showed no distractor effects in left or right target stimulus conditions with distractors of 20 times the luminance thresholds. When distractor luminances were increased up to 200 times the thresholds, a slightly larger, but non-significant interference effect was observed. This indicates that, in normal subjects, extinction
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Fig. 3. Results of Experiment 1 (control subject). Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line. Table 2 Distractor effects (ratio of just noticeable differences in orientation in distracter vs. no distractor conditions) Left hemifield target stimulus
Right hemifield target stimulus
Standard
Experimental
Standard
Experimental
(A) Experiment 1 P1 n.r. P2 3.01 NC 1.07
1.68 1.01 1.14
1.06 1.04 1.01
1.41 1.49 1.07
(B) Experiment 2 P3 n.r. P4 n.r.
1.19 1.26
1.03 1.04
1.38 1.36
(C) Experiment 3 P5 3.18 P6 3.45
1.34 1.01
1.03 0.88
0.97 0.97
Standard: target stimuli and grating distractors of 20 times the luminance thresholds; Experimental: increased/decreased distractor luminance (Experiment 1) or target stimulus luminance (Experiment 2), or random dot distractor (Experiment 3); n.r.: threshold not reliable.
cannot be induced by increasing the luminance of the contralateral distractor.
3. Experiment 2 In Experiment 1, the distractor luminance was manipulated. In Experiment 2, the luminance of the target grating was manipulated, in order to examine whether left extinction can be reduced by increasing the left hemifield target stim-
ulus luminance and whether right extinction can be induced by decreasing the right hemifield target stimulus luminance. First, we examined whether extinction in the contralesional hemifield could be reduced by increasing the luminance of the contralesional target grating (in comparison to the condition with equally visible gratings). Second, we determined whether decreasing the luminance of the ipsilesional target grating could induce extinction in the ipsilesional hemifield. The luminance of left and right distractor gratings was set at 20 times the respective luminance thresholds. 3.1. Methods Two right brain-damaged patients with hemineglect (patients 3 and 4 in Table 1) participated in this experiment. Stimuli and procedures were the same as in Experiment 1, with two exceptions. First, the luminance of left and right distractor gratings was set at 20 times the respective luminance thresholds. Second, the luminance of the target grating was manipulated. The three left target stimulus conditions were: a condition with a left target of 20 times the luminance threshold but without a distractor, a condition with a left target of 20 times the threshold and a right distractor, and a condition with a left target of 200 times the threshold (factor of 10 increase) and a right distractor. The three right target conditions were: a right target of 20 times the luminance threshold without a distractor, a right target of 20 times the threshold with a left distractor, and a right target of only twice the threshold (factor of 10 decrease) with a left distractor.
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3.2. Results and discussion Luminance thresholds measured during single stimulus presentation in a horizontal–vertical discrimination task were 0.95 and 0.42 cd/m2 in the left hemifield and 0.13 and 0.31 cd/m2 in the right hemifield for patient 3 and 4, respectively. Normalized “clockwise” responses in the orientation discrimination task are plotted in Fig. 4, separately for patients 3 and 4. Statistical methods were the same as in Experiment 1. Comparison of the conditions without distractor and the
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conditions with equally visible distractors again resulted in significant interactions between stimulus orientation and distractor condition for left hemifield target stimuli (F(1,6) = 16.4, P < .007, for patient 3 and F(1,6) = 17.8, P < .006, for patient 4), but not for right hemifield target stimuli (F(1,6) = 0.004, P < .95, for patient 3 and F(1,6) = 0.2, P < .70, for patient 4). When the left target stimulus luminance was increased, the interaction was no longer significant (F(1,6) = 0.4, P < .56, for patient 3 and F(1,6) = 2.0, P < .20, for patient 4), indicating that enhancing the salience of the contralesional target
Fig. 4. Results of Experiment 2. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences in orientation (JNDs) are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
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grating relative to the ipsilesional distractor grating was sufficient to eliminate the right distractor interference effect. When the right target stimulus luminance was decreased, the left distractor interference slightly increased, but this effect was not significant (F(1,6) = 0.5, P < .50, for patient 3 and F(1,6) = 0.7, P < .42, for patient 4). This implies that reducing the relative salience of the ipsilesional target was not sufficient to induce interference from a contralesional distractor stimulus. Direct comparison of the two distractor conditions yielded a significant interaction effect for left hemifield target stimuli (F(1,6) = 9.4, P < .02, for patient 3 and F(1,6) = 7.9, P < .03, for patient 4), but not for right hemifield target stimuli (F(1,6) = 0.6, P < .47, for patient 3 and F(1,6) = 0.5, P < .51. for patient 4). JNDs were determined as in Experiment 1. The distractor effects (ratio of the JND in the conditions with a distractor and the JND in the conditions without distractor) in Table 2(B) show an asymmetric distractor interference in the standard (20 times the threshold) luminance conditions, with a large right distractor interference with left hemifield orientation sensitivity (in fact, interference was so strong that thresholds could not be determined), and no interference from a left distractor with right hemifield orientation sensitivity. However, increasing the left and decreasing the right target stimulus luminances resulted in more symmetric distractor interference.
4. Experiment 3 In Experiment 1, the luminance of the distractor grating was manipulated. Because we only changed luminances of the bright bars, leaving background luminances constant, the luminance variations at the same time resulted in a change in the contrast. In Experiment 3, the interference from grating distractors with contralateral orientation sensitivity was compared with the interference from random dot distractors (Fig. 1B). Random dot distractors had no luminance modulation (or contrast between dark and bright bars). The random dot pattern was the same as the one that was used for the bright bars of the grating distractors. Also the luminance of the random dot pattern was the same as that of the bright bars of the grating distractors. The rationale of the experiment was to examine whether the presence of a grating contrast pattern is necessary to induce extinction, or whether the luminance onset of the distractor alone is sufficient. For both hemifields, orientation discrimination thresholds were measured without a distractor stimulus, with a contralateral distractor grating, and with a contralateral distractor random dot pattern. Both target and distractor were presented at a luminance of 20 times the luminance threshold for the respective positions.
Stimuli and procedures were the same as in Experiment 1, with the following exceptions. Six conditions were carried out. The luminance of left and right target gratings was set at 20 times the luminance thresholds that were obtained for their respective positions. The three left target stimulus conditions were: a condition without distractor, a condition with a right hemifield grating distractor, and a condition with a right hemifield random dot distractor. The three right target stimulus conditions were: a condition without distractor, a condition with a left grating distractor, and a condition with a left random dot distractor. 4.2. Results and discussion For patient 5, left and right hemifield luminance thresholds were 0.52 and 0.20 cd/m2 , respectively. For patient 6, the left hemifield luminance threshold was 0.56 cd/m2 and the right threshold was 0.15 cd/m2 . For the analysis of the orientation thresholds, the same methods were used as in the previous two experiments. Data plots of patients 5 and 6 are shown in Fig. 5. Comparison of left target stimulus conditions without and with the standard grating distractor, resulted in significant interaction effects between the predictor variables (F(1,6) = 12.8, P < .01, for patient 5 and F(1,6) = 19.7, P < .004, for patient 6), replicating the interference effects observed in Experiments 1 and 2 and in Geeraerts et al. (in press). Significant right distractor interference with left hemifield orientation sensitivity was, however, eliminated when a random dot distractor was used instead of a grating distractor (F(1,6) = 1.1, P < .33, for patient 5 and F(1,6) = 0.004, P < .95, for patient 6). Direct comparison of right hemifield grating and random dot distractor conditions resulted in a close to significant interaction effect in patient 5 (F(1,6) = 5.1, P < .06) and a significant interaction effect in patient 6 (F(1,6) = 16.3, P < .007), again suggesting that processing of the contralesional target grating is hampered by the presence of an ipsilesional grating, but not by the presence of an ipsilesional random dot pattern with the same luminance. There were no significant differences in the right hemifield target stimulus conditions. Distractor effects are shown in Table 2(C). Like in Experiments 1 and 2, distractor effects were determined for each subject by dividing the JNDs from conditions with a distractor by the JNDs from the conditions without distractor. With grating distractors, there is a strong asymmetry, with larger interference for the contralesional distractor than for the ipsilesional distractor. However, with random dot distractors, this asymmetry is drastically reduced. In fact, in patient 6, the asymmetry is completely absent.
5. Discussion 4.1. Methods Two right brain-damaged patients with unilateral neglect (patients 5 and 6 in Table 1) participated in this experiment.
A psychophysical paradigm was used to study extinction in right brain-damaged patients with left neglect. In a previous study using this paradigm (Geeraerts et al., in press) we
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Fig. 5. Results of Experiment 3. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
already showed that, in extinction patients without neglect, extinction disappeared when left and right stimulus luminances were subjectively equalized based on single stimulus luminance thresholds. However, this was not the case for neglect patients, who still suffered an increased interference from an ipsilesional distractor when they had to attend to and evaluate the orientation of the contralesional grating. This effect was replicated in the present study. The luminances necessary for a 70% correct discrimination between horizontal and vertical gratings were first determined for both hemifields. To minimize differences in visibility between ip-
silesional and contralesional stimuli, gratings of 20 times these thresholds were subsequently used in the orientation task. Even with equalized luminances, significant extinction effects were observed in the six patients. This suggests that there is an additional (attentional/representational) imbalance in neglect. In three of the six patients, thresholds in the ipsilesional distractor condition were unreliable. The three other patients, however, obtained reliable thresholds in the ipsilesional distractor condition. Thus, even in the presence of an ipsilesional stimulus, they were able to perceive and identify the contralesional stimulus. Extinction, in these patients,
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appears to be not merely the failure to detect a contralesional stimulus in the presence of an ipsilesional stimulus, but rather, reflects the failure to discriminate the correct stimulus features (i.e. orientation). The main purpose of the present study was to assess whether extinction in neglect patients can be modulated by changing the luminance-contrast of simultaneously presented target and distractor stimuli. Extinction effects obtained with subjectively equalized luminances were compared with extinction effects obtained in conditions with either decreased or increased distractor or target stimulus luminances. Experiment 1 demonstrated that a 10-fold reduction of the ipsilesional distractor luminance resulted in a significant reduction of the interference with contralesional hemifield orientation sensitivity in neglect patients. A 10-fold increase of the contralesional distractor luminance, on the other hand, did not lead to more interference with orientation sensitivity in the ipsilesional hemifield. In Experiment 2, we observed that an increase in the luminance of the contralesional target grating eliminated extinction: right distractor interference with left hemifield orientation sensitivity was no longer significant after a 10-fold left target stimulus luminance increase. Reducing the right target luminance, on the other hand, did not result in a significant increase of the left distractor interference. In sum, when using subjectively equalized left and right stimulus luminances, asymmetric distractor interference (extinction) was observed. When left or right stimulus luminances were changed to the benefit of the left side or against the right side, asymmetric distractor interference disappeared. Consequently, a symmetric but non-significant distractor interference was observed. These results clearly show that not the absolute salience of the ipsilesional or contralesional stimulus is the important factor for inducing extinction, but the balance between both stimuli is, since extinction can be eliminated by changes on either side. Extinction depends on the relation between stimuli, and thus seems to be caused by a biased competition between the stimuli (Duncan, Humphreys, & Ward, 1997). Although a 10-fold reduction of the right distractor luminance resulted in a significant reduction of the interference with left hemifield orientation sensitivity in the patients, a comparably large increase of the distractor luminance in the normal control subject, did not result in a significantly larger extinction effect. This suggests that salience imbalance alone cannot explain extinction. An intact attentional (or spatial representation) system seems to protect against extinction and is able to suppress very salient distractor stimuli. If, however, this system is damaged, relatively small salience imbalances can have devastating effects on the perception of contralesional stimuli. In Experiment 3, the interference from a grating distractor was compared with the interference from a random dot distractor. Significant right distractor interference with left hemifield orientation sensitivity was observed when using a grating distractor but not when using a random dot distractor. Apparently, the luminance onset of the distractor alone
is not sufficient to induce extinction. The presence of a (grating) pattern seems to be necessary. There are several possible, not necessarily mutually exclusive explanations; First, although the mean luminance of the random dot distractor is higher compared to that of the grating distractor, the contrast is lower. Stimulus salience or representation strength is not just determined by the luminance onset but also by the complexity or contrast of the stimulus. Second, stimulus similarity might also be a factor of importance. It has frequently been shown that more similar stimuli induce stronger extinction effects (Baylis, Driver, & Rafal, 1993). Third, although the location of the contralateral target grating was pre-cued making the distractor grating task irrelevant, it is possible that the ‘magnetic’, involuntary, and obligatory attraction of attention towards the ipsilesional stimulus had a stronger interfering effect on the orientation discrimination task when that distractor also had an orientation (a grating) than when the distractor lacked an inherent orientation (a random dot pattern). Our results do not converge with the findings of Di Pellegrino and De Renzi (1995) and Smania et al. (1996). The latter two studies failed to show influences of stimulus brightness or size on extinction, whereas we observed a significant effect of stimulus luminance on the severity of extinction. One major difference is that we adapted stimulus luminance relative to the luminance thresholds for single stimuli, which was not the case in the previous visual extinction studies. In a tactile extinction study of Meador et al. (2001), effects of stimulus intensity on extinction were observed if intensity was adapted relative to single-stimuli perceptual thresholds. These results clearly show the advantages of using threshold measurements in extinction research. Zihl and von Cramon (1979) observed a small influence of stimulus intensity on extinction in a left brain-damaged patient, which according to Smania et al. (1996) might be suggestive of a difference in the nature of extinction after left and right hemispheric lesions. However, we demonstrated significant intensity effects on extinction in six right braindamaged patients. These effects suggest that the higher-order imbalances in extinction patients are strongly influenced by both top-down and bottom-up factors. In patients with a damaged attentional/representational system, salience imbalances seem to have a larger impact than in subjects with an intact attentional/representational system. As patients with focal brain damage frequently have additional sensory deficits, asymmetric sensory as well as attentional/representational deficits are likely to contribute to the production of extinction. References Baylis, G. C., Driver, J., & Rafal, R. D. (1993). Visual extinction and stimulus repetition. Journal of Cognitive Neuroscience, 5, 453– 466. Bisiach, E., & Luzzatti, C. (1978). Unilateral neglect of representational space. Cortex, 14, 129–133.
S. Geeraerts et al. / Neuropsychologia 43 (2005) 542–553 Corbetta, M., Kincade, J. M., Ollinger, J. M., McAvoy, M. P., & Shulman, G. L. (2000). Voluntary orienting is dissociated from target detection in human posterior parietal cortex. Nature Neuroscience, 3, 292– 297. Di Pellegrino, G., & De Renzi, E. (1995). An experimental investigation on the nature of extinction. Neuropsychologia, 33, 153–170. Duncan, J., Bundesen, C., Olson, A., Humphreys, G., Chavda, S., & Shibuya, H. (1999). Systematic analysis of deficits in visual attention. Journal of Experimental Psychology: General, 128, 450– 478. Duncan, J., Humphreys, G., & Ward, R. (1997). Competitive brain activity in visual attention. Current Opinion in Neurobiology, 7, 255– 261. Geeraerts, S., Lafosse, C., Vandenbussche, E., & Verfaillie, K. (in press). A psychophysical study of visual extinction: Ipsilesional distractor interference with contralesional orientation thresholds in visual hemineglect patients. Neuropsychologia. Gorea, A., & Sagi, D. (2002). Natural extinction: A criterion shift phenomenon. Visual Cognition, 9, 913–936. Gottlieb, J. P., Kusunoki, M., & Goldberg, M. E. (1998). The representation of visual salience in monkey parietal cortex. Nature Neuroscience, 391, 481–484. Heilman, K. M., Watson, R. T., & Valenstein, E. (1993). Neglect and related disorders. In K. M. Heilman & E. Valenstein (Eds.), Clinical Neuropsychology (pp. 279–336). New York: Oxford University Press. Kerkhoff, G. (1999). Multimodal spatial orientation deficits in left-sided visual neglect. Neuropsychologia, 37, 1387–1405. Kinsbourne, M. (1993). Orientational bias model of unilateral neglect: Evidence from attentional gradients within hemispace. In I. H. Robertson & J. C. Marshall (Eds.), Unilateral Neglect: Clinical and Experimental Studies (pp. 63–81). Hove, UK: Lawrence Erlbaum Associates Ltd.
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Meador, K. J., Ray, P. G., Day, L. J., & Loring, D. W. (2001). Relationship of extinction to perceptual thresholds for single stimuli. Neurology, 56, 1044–1047. Parasuraman, R. (Ed.). (1998). The attentive brain. Cambridge, MA: MIT Press. Posner, M. I., Walker, J. A., Friedrich, F. J., & Rafal, R. D. (1984). Effects of parietal injury on covert orienting of attention. Journal of Neuroscience, 4, 1863–1874. Pouget, A., & Driver, J. (2000). Relating unilateral neglect to the neural coding of space. Current Opinion in Neurobiology, 10, 242–249. Pouget, A., & Sejnowski, T. J. (2001). Simulating a lesion in a basis function model of spatial representations: Comparison with hemineglect. Psychological Review, 108, 653–673. Rizzolatti, G., & Berti, A. (1990). Neglect as a neural representation deficit. Review in Neurology (Paris), 146, 626–634. Schenkenberg, T., Bradford, D. C., & Ajax, E. T. (1980). Line bisection and unilateral visual neglect in patients with neurological impairment. Neurology, 30, 509–517. Smania, N., Martini, M. C., Prior, M., & Marzi, C. A. (1996). Input and response determinants of visual extinction: A case study. Cortex, 32, 567–591. Snoeren, P. R., & Puts, M. J. H. (1997). Multiple parameter estimation in an adaptive psychometric method: MUEST, an extension of the QUEST method. Journal of Mathematical Psychology, 41, 431–439. Vuilleumier, P. O., & Rafal, R. D. (2000). A systematic study of visual extinction: Between- and within-field deficits of attention in hemispatial neglect. Brain, 123, 1263–1279. Wilson, B., Cockburn, J., & Halligan, P. W. (1987). In B. St. Edmunds (Ed.), The Behavioural Inattention Test. UK: Thames Valley Test Company. Zihl, J., & von Cramon, D. (1979). The contribution of the ‘second’ visual system to directed visual attention in man. Brain, 102, 835–856.
The relationship of visual extinction to luminance-contrast imbalances between left and right hemifield stimuli Sarah Geeraertsa , Karla Michielsb , Christophe Lafossec , Erik Vandenbusschea , Karl Verfaillied,∗ a Laboratory of Neuropsychology, University of Leuven, Belgium Physical Medicine and Revalidation, University Hospital Leuven, Belgium c Scientific Unit Rehabilitation Centre ‘Hof ter Schelde’, Antwerp, Belgium Laboratory of Experimental Psychology, Department of Psychology, University of Leuven, Tiensestraat 102, B-3000 Leuven, Belgium b
d
Received 3 February 2004; received in revised form 16 July 2004; accepted 21 July 2004
Abstract Visual extinction was investigated in six right brain-damaged patients with left visual neglect, using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. When the visibility of target and distractor gratings was subjectively equalized in this way, neglect patients still showed a significant extinction effect, i.e. a significant interference of the right hemifield distractor with left hemifield orientation sensitivity. By manipulating the luminances of left and right hemifield gratings during bilateral simultaneous stimulus presentation, we demonstrated the role of luminance-contrast imbalances in eliciting visual extinction. Both decreasing the right distractor luminance and increasing the left target stimulus luminance resulted in an elimination of the observed extinction effects. These results show that not the absolute salience of one of two simultaneously presented stimuli, but the relative salience of both stimuli, is the crucial factor for inducing extinction. © 2004 Elsevier Ltd. All rights reserved. Keywords: Attention; Orientation thresholds; Salience imbalance; Stimulus competition; Visual hemineglect
1. Introduction Visual extinction is a frequently observed consequence of unilateral, mainly right hemispheric, brain damage. Patients with visual extinction can reliably see a single stimulus in the hemifield contralateral to the brain lesion, but fail to see the same stimulus (or at least process the stimulus less efficiently) when another one is simultaneously presented on the ipsilesional side. Visual extinction is often observed as part of the hemineglect syndrome (Heilman, Watson, & Valenstein, 1993). Current theories of neglect and extinction propose either attentional or spatial representation deficits. According ∗
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to attentional theories, mechanisms for orienting attention towards the contralesional side of space and/or for disengaging attention from ipsilesional stimuli are affected (Corbetta, Kincade, Ollinger, McAvoy, & Shulman, 2000; Kinsbourne, 1993; Posner, Walker, Friedrich, & Rafal, 1984). According to representational theories, the spatial representation of the contralesional side of space is damaged (Bisiach & Luzzatti, 1978; Rizzolatti & Berti, 1990). However, Pouget and colleagues (Pouget & Driver, 2000; Pouget & Sejnowski, 2001) suggested that the distinction between attentional and representational accounts of neglect may be artificial. On the one hand, neglect and extinction may arise because the lesion produces a gradient in the number of (parietal) neurons for particular regions of space. On the other hand, it has been shown that, also in the intact brain, the parietal
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representation is a highly selective representation of the scene being viewed, with several neurons only responding to attended stimuli (Gottlieb, Kusunoki, & Goldberg, 1998). In this scheme, parietal damage would not only affect the representation of different spatial positions, but also the attentional salience of those positions. Imbalances in the attentional salience then becomes the key concept for a better understanding of extinction and neglect: When two stimuli are presented simultaneously, an imbalance in the salience of the stimuli results in extinction of the more contralesional stimulus. If an imbalance in the attentional “weight” of stimuli (also see Duncan et al., 1999) causes extinction, it should be possible to reduce extinction by reducing the salience imbalance in a bottom-up way. However, whereas visual extinction has been shown to be strongly influenced by top-down factors such as task-defined perceptual set and stimulus relevance (Vuilleumier & Rafal, 2000), there is much less evidence for modulation of extinction by bottom-up factors, such as the brightness or size of contralesional stimuli. Di Pellegrino and De Renzi (1995) observed that extinction persisted unmodified when stimulus presentation was lengthened from 200 to 500 ms, or when the size of the contralesional stimulus was fourfold that of the ipsilesional stimulus. The only condition in which extinction was reduced was when the right brain-damaged patient was instructed to ignore the right stimulus and only respond to the left stimulus. However, in these experiments, stimulus durations of left and right hemifield stimuli were lengthened together. Since this manipulation changes the salience of both stimuli but not the salience imbalance, no effect on extinction is expected on the hypothesis of Duncan et al. (1999). The size increment of the left stimulus, on the other hand, marginally altered the extinction rate with bilateral presentation from 100 to 90%. Smania, Martini, Prior, and Marzi (1996) manipulated the intensity of the stimuli in an extinction task. Extinction rate did not benefit when the contralesional stimulus was almost eight times more intense than the ipsilesional stimulus, compared to an equal luminance condition. In a similar extinction study, Zihl and von Cramon (1979) observed that increasing the contralesional stimulus luminance and decreasing the ipsilesional stimulus luminance resulted in a small reduction of the extinction effect. However, these effects were observed in a patient with a left hemispheric lesion while the patients in the study of Di Pellegrino and De Renzi and of Smania et al. had lesions in the right hemisphere. Smania et al. (1996) suggested that the nature of extinction following right and left hemisphere lesions might be different. The absence of an influence of stimulus brightness and size on extinction is often used as a strong argument for an attentional rather than a sensory deficit causing extinction in right brain-damaged patients. As attention is supposed to be mediated mainly by the right hemisphere (Parasuraman, 1998), sensory rather than attentional imbalances might induce extinction in left brain-damaged patients.
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Most visual neuropsychological studies use suprathreshold stimuli to assess extinction. A recent tactile extinction study by Meador, Ray, Day, and Loring (2001) determined tactile perception thresholds and studied the relationship of extinction to the thresholds for single stimuli. In contrast to the previous results, Meador et al. did observe effects of asymmetries in stimulus intensity on extinction (relative to singlestimulus perceptual thresholds). Moreover, using close to threshold stimuli, Gorea and Sagi (2002) have documented extinction-like effects in normal observers who had to discriminate between simultaneously presented visual stimuli with different intensities. Their results suggest that extinction is, at least in part, triggered by a sensitivity imbalance between simultaneous events. The relationship of extinction to visual perceptual thresholds for single stimuli remains uncertain. The purpose of the present experiments, was to study visual extinction via a psychophysical paradigm and to assess the influence of changes in the relative salience of ipsilesional and contralesional stimuli on extinction. An adapted version of the psychophysical paradigm developed by Geeraerts, Lafosse, Vandenbussche, and Verfaillie (in press) was used. Two features characterize the paradigm. First, the extinction paradigm was more sensitive than usually employed in standard testing. More specifically, thresholds for orientation discrimination were measured psychophysically. Orientation sensitivity was determined for left and right hemifield gratings presented with or without a contralateral distractor grating. Attention was always cued towards the target stimulus position and away from the distractor position, making the distractor stimulus task irrelevant. Extinction was assessed by measuring the influence of the contralateral distractor grating. Second, to control for possible lower-level sensory deficits, we attempted to equalize the visibility of left and right hemifield gratings. More specifically, before administering the orientation discrimination task, luminance thresholds for single stimuli presented in either the left or right hemifield were measured in a horizontal–vertical grating discrimination task. Gratings with luminances of 20 times these thresholds were then used in the orientation discrimination task, in which possible extinction effects were examined. We observed that, even after eliminating contra-ipsilesional differences in stimulus visibility, neglect patients still displayed an increased interference effect from an ipsilesional distractor. Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients (without neglect) who demonstrated extinction on standard extinction testing. This suggests that extinction in neglect patients, but not necessarily in patients without neglect, is based on an attentional imbalance between ipsilesional and contralesional stimuli. This paradigm was adapted to examine whether the relative salience of target and distractor grating can influence extinction. Right brain-damaged patients with left neglect were tested in three separate experiments. In Experiment 1, the luminance of the distractor grating was manipulated, in
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order to investigate whether left extinction can be reduced by decreasing the right hemifield distractor luminance and whether right extinction can be induced by increasing the left hemifield distractor luminance. In Experiment 2, the luminance of the target grating was manipulated, in order to examine whether left extinction can be reduced by increasing the left hemifield target stimulus luminance and whether right extinction can be induced by decreasing the right hemifield target stimulus luminance. In Experiment 3, a random dot distractor was used instead of a grating distractor to assess whether luminance onset is enough to induce extinction, or whether the contrast pattern of the grating distractor is a necessary condition.
2. Experiment 1 Extinction was examined in two right brain-damaged neglect patients. Subjects performed an orientation discrimination task, in which they had to decide whether a square-wave grating, presented either in the left or in the right hemifield, deviated clockwise or counterclockwise from vertical. Extinction was assessed by comparing conditions in which left or right targets were presented in isolation with conditions in which a task-irrelevant distractor grating was presented in the contralateral hemifield. The main purpose of Experiment 1 was to investigate the effect of changing the luminance of the distractor grating. There were two control conditions: a condition without distractor grating and a condition in which target and distractor were equally visible (based on luminance thresholds for single hemifield stimuli in a horizontal–vertical grating discrimination task). First, we examined whether decreasing the luminance of the ipsilesional distractor grating (in comparison to the condition with equally visible gratings) would reduce the extinction effect. Second, we determined whether increasing the luminance of the contralesional distractor grating would induce an (otherwise absent or even reversed, cf. Geeraerts et al., in press) extinction effect. In addition, one normal control subject performed the same task in order to determine whether an extinction effect could be induced in normal subjects, comparable to that of extinction patients, only by changing the relative luminance of target and distractor gratings. For both left and right hemifield distractor conditions, a condition in which the distractor luminance was much higher than the target luminance was compared with a condition in which target and distractor luminances were equalized. 2.1. Methods 2.1.1. Subjects Two right brain-damaged patients with left visual neglect (patients 1 and 2 in Table 1) and one control subject without a neurological history (patient NC in Table 1) participated in the study. Patients were recruited from a rehabilitation unit
for stroke patients. They all had a single, unilateral vascular lesion as revealed by CT/MRI and clinical symptoms and did not show hemianopia or upper visual field defects on perimetric testing (see Geeraerts et al. (in press) for more details on the procedure). Visual neglect was assessed with line bisection (Schenkenberg, Bradford, & Ajax, 1980) and star cancellation (Behavioural Inattention Test; Wilson, Cockburn, & Halligan, 1987) tasks. In order to be included in the study, the patients had to show neglect on both tasks and had to obtain reliable thresholds in our luminance and orientation task conditions without distractors. Table 1 summarizes the clinical and demographic characteristics of the patients. 2.1.2. Luminance thresholds Luminance thresholds were first determined for individual subjects. These thresholds were used to determine the stimulus luminances in the orientation tasks. Subjects were seated in front of the computer screen at a distance of 114 cm with the head restrained by a forehead and chin rest. A central red fixation point (diameter 0.2◦ of visual angle) was presented throughout the experiment, together with a peripheral cue, indicating the position of the next stimulus. The cue was a white circle (diameter 4.5◦ ), the centre of which was presented at 5◦ eccentricity from the fixation point on the 45◦ diagonal in either the left or right upper visual field quadrant. The stimuli only appeared at the cued position (left or right upper quadrants). Stimuli were circular patches of square-wave gratings (diameter 4.5◦ , spatial frequency 1 c/◦ ) with either horizontally or vertically oriented bars. The gratings were luminance-modulated with dark bars of a constant background luminance (0.001 cd/m2 ) and bright bars of luminances that varied according to the subjects’ performance. Bright bars were constructed from random noise patterns. The computer program could produce only two luminance levels (black and white) on the screen. The black bars were always uniform black. The luminance of the white bars was reduced by changing some of the pixels to black. The viewing distance was large enough for the ‘random noise’ to be spatially integrated by the observer’s visual systems, so that the bright bars appeared lighter or darker gray. Luminance thresholds were determined with an adaptive staircase procedure (MUEST; Snoeren & Puts, 1997) converging around a performance level of 70% correct. The starting luminance was 0.04 cd/m2 . The threshold was the luminance for which the subject reached a 70% correct discrimination between horizontal and vertical gratings. Each threshold estimation was based on 40 trials. Only subjects that reached a performance level between 65 and 75% correct (indicating a reliable threshold measurement) were included in the study. The trial sequence was the following. After central fixation was maintained for 500 ms, the cue circle was replaced by the grating stimulus for a period of 300 ms. The subject had to respond to the orientation of the grating by pressing a left response key for horizontal and a right response key for vertical orientations. When motor difficulties in the
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Table 1 Clinical and demographic characteristics of patients Subject
Age
Sex
Etiology
Lesion location
Months s/lesion
Visual field
Line bisection
Star cancellation
P1 P2 P3 P4 P5 P6 NC
53 64 53 56 46 60 61
F F M M M M F
Infarct ICB ICB Infarct ICB Infarct
R p–o R f–t–p R f–t–p R t–p–o Rp R f–p
35 11 14 6 8 26
Normal Normal Normal LLQ LLQ LLQ
13.1 21.2 12.6 8.7 7.0 12.8
4/2 3/0 3/0 4/2 8/4 3/2
P, patient; NC, control subject; M, male; F, female; ICB, intracerebral bleeding; R, right lesion; p, parietal; o, occipital; f, frontal; t, temporal; LLQ: quadrantanopia in left lower visual field; line bisection: % rightward deviation; star cancellation: number of omissions on left/right half of the page.
contralesional hand were suspected, patients responded with two fingers of the ipsilesional hand. Subjects were urged to guess when they were unsure about the grating’s orientation. Response time was unlimited. Feedback was given in the form of an auditory signal for incorrect responses. Only after a response was made and after a 2000 ms intertrial interval the next trial was initiated. Eye movements were monitored with the ASL 210 eye tracker (Applied Science Laboratories, Bedford, MA). Trials with eye movements out of a 2◦ fixation window were discarded and repeated. Stimuli were presented in four blocks of 20 trials (not counting trials with eye movements). Within a block, the stimulus was always presented in the same position. Blocks with left and right hemifield stimulus positions were alternated. Thus, threshold estimation for each hemifield consisted of 40 trials. 2.1.3. Orientation thresholds Unless stated otherwise, the stimuli and procedures for the orientation threshold measurements were the same as for the luminance threshold measurements. Stimuli could be either target gratings or distractor gratings, both of them having the same size and spatial frequency as the luminance gratings. Target gratings were always presented at the cued (left or right hemifield) position; distractor gratings were always presented in the opposite hemifield in the homologue position (i.e., in the mirror-imaged position about the vertical mid-line; Fig. 1A). The distractor grating was presented simultaneously with, and for the same duration as the target grating. Orientation thresholds were determined for target gratings in left and right visual fields, with and without the simultaneous presentation of a distractor grating. Patients indicated whether the grating orientation deviated clockwise or anticlockwise from vertical. We had to use a constant stimuli paradigm to measure orientation sensitivity independently of the point of subjective visual verticality, because a contraversive tilt of the subjective visual vertical has been observed in neglect patients (Geeraerts et al., in press; Kerkhoff, 1999). Via a short adaptive staircase procedure, five equally spaced and informative orientations spanning the estimated subjective visual vertical were obtained for each patient and separately for left and right hemifield positions. These orientations were then used as target stimulus orientations in the actual
task. Distractor orientations were chosen randomly in each trial and could be any orientation between 0◦ and 360◦ . A total of six conditions or six threshold measurements was carried out, three left and three right target stimulus conditions. The luminance of left and right target gratings was set at 20 times the luminance threshold that was obtained for the respective positions. The luminance of the distractor gratings was manipulated. The three left target stimulus conditions were: a condition without distractor; a condition with a right hemifield distractor of 20 times the right luminance threshold, such as the target stimulus; and a condition with a right hemifield distractor of only twice the luminance threshold (factor of 10 decrease, but still suprathreshold). The three right target stimulus conditions were: a condition without distractor; a condition with a left distractor of 20 times the left luminance threshold; and a condition with a left distractor of 200 times the threshold (factor of 10 increase). Blocks of 30 trials with a left hemifield target stimulus were alternated with 30-trial blocks with a right hemifield target. Within a block, the three distractor conditions (10 trials each) were randomly presented. The five target stimulus orientations were randomly presented within these 10 trials. Subjects were instructed to ignore the distractor as much as possible. The cue always indicated the position of the target to be presented. If the target orientation deviated clockwise from vertical, subjects had to press the right response key; for an anticlockwise deviation, the left response key had to be pressed. All experiments were carried out in a completely darkened room (the edge of the screen was also invisible), so that there was no reference frame for comparison with the grating orientations. Trials with eye movements out of a 2◦ window were discarded and repeated. A total of at least 100 trials per condition (20 trials per orientation level, not counting trials with eye movements) had to be completed. 2.2. Results and discussion The two patients obtained elevated left hemifield luminance thresholds (0.26 and 0.15 cd/m2 for patients 1 and 2, respectively) as compared to their right hemifield thresholds (0.15 and 0.11 cd/m2 ). In the normal control subject, the right hemifield threshold (0.17 cd/m2 ) was slightly higher than the left hemifield threshold (0.21 cd/m2 ). These results show that
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Fig. 1. Schematic depiction of stimuli and trial sequences for the orientation discrimination tasks.
the luminance adaptations in the orientation tasks are necessary to obtain the same subjective visibility for left and right hemifield gratings. For the analysis of the orientation thresholds, the normalized percentages of “clockwise deviation from vertical” responses were plotted against the presented orientations (ordered from the largest anticlockwise to the largest clockwise deviation), for each subject in each condition. Fig. 2 shows plots of patients 1 and 2. Factorial multiple regression analyses were performed for each subject, to determine whether there was a significant difference between the conditions with and without distractor and between the different distractor conditions. The dependent variable was the normalized proportion clockwise responses; the predictor variables were stimulus orientation and distractor condition. A significant interaction between the predictor variables indicates a significant difference between the slopes of the regression lines. When left target stimulus conditions without and with the standard right hemifield distractor (with equally visible target and distractor) were compared, both left neglect patients showed a significant interaction between the predictor variables (F(1,6) = 15.1, P < .008, for patient 1, and F(1,6) = 16.2, P < .007, for patient 2). For the right target stimulus conditions with and without standard distractor, this interaction was not significant (F(1,6) = 0.12, P < .74, for patient1, and F(1,6) = 0.03, P < .87, for patient 2). Thus, significant right distractor interference with left hemifield orientation sensitivity was observed, while no significant interference from the left distractor with right hemifield orientation sensitivity was observed. This replicates the findings of Geeraerts et al. (in press). When the left target condition without distractor was compared with the condition with a right distractor with reduced luminance, no significant interactions were observed (F(1,6)
= 4.0, P < .09, for patient 1 and F(1,6) = 0.002, P < .97, for patient 2). This indicates that decreasing the luminance of the ipsilesional distractor eliminated extinction in the contralesional hemifield. The difference between the right target condition without distractor and with a distractor with increased left distractor luminance was in the expected direction, but was not significant in neither patient (F(1,6) = 3.6, P < .11, for patient 1 and F(1,6) = 2.4, P < .17, for patient 2). Apparently, increasing distractor luminance in the contralesional hemifield was not a sufficient condition to induce extinction in the ipsilesional hemifield. A direct comparison of the two distractor conditions supported the same conclusions. In both patients, there was a significant difference for left hemifield target stimuli (F(1,6) = 10.6, P < .02, for patient 1 and F(1,6) = 17.7, P < .006, for patient 2), but not for right hemifield stimuli (F(1,6) = 1.9, P < .21, for patient 1 and F(1,6) = 1.5, P < .26, for patient 2). In summary, decreasing the right distractor luminance resulted in a significantly smaller interference with left hemifield orientation sensitivity, while increasing the left distractor luminance resulted in a slightly but non-significantly larger interference with right hemifield orientation sensitivity. Via least squares linear regression analyses, just noticeable differences (JNDs) in orientation were estimated. The JND is an inverse measure of the slope of the regression line. JNDs could only be determined for conditions in which a significant correlation between grating orientation and normalized percentage clockwise responses was observed. A significant correlation was observed for both patients in the conditions without distractor and for patient 2 in the conditions with distractor (Pearson product moment correlations higher than 0.90, P < .05). However, patient 1 did not show a significant correlation (r = 0.29, P > .05) between orientation and
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Fig. 2. Results of Experiment 1. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
percentage clockwise responses in the condition with a contralesional target and an ispilesional distractor with comparable luminance. Apparently, distractor interference in this condition was so strong that the patient was not able to perform the orientation discrimination task. Consequently, a reliable JND could not be estimated in this condition. The magnitude of the distractor effects was determined for each subject by dividing the JNDs from the conditions with a distractor by the JNDs from the conditions without distractor. Table 2(A) shows the distractor effects for the conditions with reliable JNDs in patients 1 and 2. Standard distractors of 20 times
the luminance thresholds resulted in an asymmetric distractor interference, with a larger (in patient 1 even not estimable) interference for the contralesional target than for the ipsilesional target. Decreased right and increased left distractor luminances eliminate this asymmetry. The normal control subject (Fig. 3) showed no distractor effects in left or right target stimulus conditions with distractors of 20 times the luminance thresholds. When distractor luminances were increased up to 200 times the thresholds, a slightly larger, but non-significant interference effect was observed. This indicates that, in normal subjects, extinction
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Fig. 3. Results of Experiment 1 (control subject). Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line. Table 2 Distractor effects (ratio of just noticeable differences in orientation in distracter vs. no distractor conditions) Left hemifield target stimulus
Right hemifield target stimulus
Standard
Experimental
Standard
Experimental
(A) Experiment 1 P1 n.r. P2 3.01 NC 1.07
1.68 1.01 1.14
1.06 1.04 1.01
1.41 1.49 1.07
(B) Experiment 2 P3 n.r. P4 n.r.
1.19 1.26
1.03 1.04
1.38 1.36
(C) Experiment 3 P5 3.18 P6 3.45
1.34 1.01
1.03 0.88
0.97 0.97
Standard: target stimuli and grating distractors of 20 times the luminance thresholds; Experimental: increased/decreased distractor luminance (Experiment 1) or target stimulus luminance (Experiment 2), or random dot distractor (Experiment 3); n.r.: threshold not reliable.
cannot be induced by increasing the luminance of the contralateral distractor.
3. Experiment 2 In Experiment 1, the distractor luminance was manipulated. In Experiment 2, the luminance of the target grating was manipulated, in order to examine whether left extinction can be reduced by increasing the left hemifield target stim-
ulus luminance and whether right extinction can be induced by decreasing the right hemifield target stimulus luminance. First, we examined whether extinction in the contralesional hemifield could be reduced by increasing the luminance of the contralesional target grating (in comparison to the condition with equally visible gratings). Second, we determined whether decreasing the luminance of the ipsilesional target grating could induce extinction in the ipsilesional hemifield. The luminance of left and right distractor gratings was set at 20 times the respective luminance thresholds. 3.1. Methods Two right brain-damaged patients with hemineglect (patients 3 and 4 in Table 1) participated in this experiment. Stimuli and procedures were the same as in Experiment 1, with two exceptions. First, the luminance of left and right distractor gratings was set at 20 times the respective luminance thresholds. Second, the luminance of the target grating was manipulated. The three left target stimulus conditions were: a condition with a left target of 20 times the luminance threshold but without a distractor, a condition with a left target of 20 times the threshold and a right distractor, and a condition with a left target of 200 times the threshold (factor of 10 increase) and a right distractor. The three right target conditions were: a right target of 20 times the luminance threshold without a distractor, a right target of 20 times the threshold with a left distractor, and a right target of only twice the threshold (factor of 10 decrease) with a left distractor.
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3.2. Results and discussion Luminance thresholds measured during single stimulus presentation in a horizontal–vertical discrimination task were 0.95 and 0.42 cd/m2 in the left hemifield and 0.13 and 0.31 cd/m2 in the right hemifield for patient 3 and 4, respectively. Normalized “clockwise” responses in the orientation discrimination task are plotted in Fig. 4, separately for patients 3 and 4. Statistical methods were the same as in Experiment 1. Comparison of the conditions without distractor and the
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conditions with equally visible distractors again resulted in significant interactions between stimulus orientation and distractor condition for left hemifield target stimuli (F(1,6) = 16.4, P < .007, for patient 3 and F(1,6) = 17.8, P < .006, for patient 4), but not for right hemifield target stimuli (F(1,6) = 0.004, P < .95, for patient 3 and F(1,6) = 0.2, P < .70, for patient 4). When the left target stimulus luminance was increased, the interaction was no longer significant (F(1,6) = 0.4, P < .56, for patient 3 and F(1,6) = 2.0, P < .20, for patient 4), indicating that enhancing the salience of the contralesional target
Fig. 4. Results of Experiment 2. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences in orientation (JNDs) are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
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grating relative to the ipsilesional distractor grating was sufficient to eliminate the right distractor interference effect. When the right target stimulus luminance was decreased, the left distractor interference slightly increased, but this effect was not significant (F(1,6) = 0.5, P < .50, for patient 3 and F(1,6) = 0.7, P < .42, for patient 4). This implies that reducing the relative salience of the ipsilesional target was not sufficient to induce interference from a contralesional distractor stimulus. Direct comparison of the two distractor conditions yielded a significant interaction effect for left hemifield target stimuli (F(1,6) = 9.4, P < .02, for patient 3 and F(1,6) = 7.9, P < .03, for patient 4), but not for right hemifield target stimuli (F(1,6) = 0.6, P < .47, for patient 3 and F(1,6) = 0.5, P < .51. for patient 4). JNDs were determined as in Experiment 1. The distractor effects (ratio of the JND in the conditions with a distractor and the JND in the conditions without distractor) in Table 2(B) show an asymmetric distractor interference in the standard (20 times the threshold) luminance conditions, with a large right distractor interference with left hemifield orientation sensitivity (in fact, interference was so strong that thresholds could not be determined), and no interference from a left distractor with right hemifield orientation sensitivity. However, increasing the left and decreasing the right target stimulus luminances resulted in more symmetric distractor interference.
4. Experiment 3 In Experiment 1, the luminance of the distractor grating was manipulated. Because we only changed luminances of the bright bars, leaving background luminances constant, the luminance variations at the same time resulted in a change in the contrast. In Experiment 3, the interference from grating distractors with contralateral orientation sensitivity was compared with the interference from random dot distractors (Fig. 1B). Random dot distractors had no luminance modulation (or contrast between dark and bright bars). The random dot pattern was the same as the one that was used for the bright bars of the grating distractors. Also the luminance of the random dot pattern was the same as that of the bright bars of the grating distractors. The rationale of the experiment was to examine whether the presence of a grating contrast pattern is necessary to induce extinction, or whether the luminance onset of the distractor alone is sufficient. For both hemifields, orientation discrimination thresholds were measured without a distractor stimulus, with a contralateral distractor grating, and with a contralateral distractor random dot pattern. Both target and distractor were presented at a luminance of 20 times the luminance threshold for the respective positions.
Stimuli and procedures were the same as in Experiment 1, with the following exceptions. Six conditions were carried out. The luminance of left and right target gratings was set at 20 times the luminance thresholds that were obtained for their respective positions. The three left target stimulus conditions were: a condition without distractor, a condition with a right hemifield grating distractor, and a condition with a right hemifield random dot distractor. The three right target stimulus conditions were: a condition without distractor, a condition with a left grating distractor, and a condition with a left random dot distractor. 4.2. Results and discussion For patient 5, left and right hemifield luminance thresholds were 0.52 and 0.20 cd/m2 , respectively. For patient 6, the left hemifield luminance threshold was 0.56 cd/m2 and the right threshold was 0.15 cd/m2 . For the analysis of the orientation thresholds, the same methods were used as in the previous two experiments. Data plots of patients 5 and 6 are shown in Fig. 5. Comparison of left target stimulus conditions without and with the standard grating distractor, resulted in significant interaction effects between the predictor variables (F(1,6) = 12.8, P < .01, for patient 5 and F(1,6) = 19.7, P < .004, for patient 6), replicating the interference effects observed in Experiments 1 and 2 and in Geeraerts et al. (in press). Significant right distractor interference with left hemifield orientation sensitivity was, however, eliminated when a random dot distractor was used instead of a grating distractor (F(1,6) = 1.1, P < .33, for patient 5 and F(1,6) = 0.004, P < .95, for patient 6). Direct comparison of right hemifield grating and random dot distractor conditions resulted in a close to significant interaction effect in patient 5 (F(1,6) = 5.1, P < .06) and a significant interaction effect in patient 6 (F(1,6) = 16.3, P < .007), again suggesting that processing of the contralesional target grating is hampered by the presence of an ipsilesional grating, but not by the presence of an ipsilesional random dot pattern with the same luminance. There were no significant differences in the right hemifield target stimulus conditions. Distractor effects are shown in Table 2(C). Like in Experiments 1 and 2, distractor effects were determined for each subject by dividing the JNDs from conditions with a distractor by the JNDs from the conditions without distractor. With grating distractors, there is a strong asymmetry, with larger interference for the contralesional distractor than for the ipsilesional distractor. However, with random dot distractors, this asymmetry is drastically reduced. In fact, in patient 6, the asymmetry is completely absent.
5. Discussion 4.1. Methods Two right brain-damaged patients with unilateral neglect (patients 5 and 6 in Table 1) participated in this experiment.
A psychophysical paradigm was used to study extinction in right brain-damaged patients with left neglect. In a previous study using this paradigm (Geeraerts et al., in press) we
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Fig. 5. Results of Experiment 3. Normalized percentages of ‘clockwise deviation from vertical’ responses are plotted against the presented orientations. Just noticeable differences (JNDs) in orientation are estimated via linear regression analysis. The JND is an inverse measure of the slope of the regression line.
already showed that, in extinction patients without neglect, extinction disappeared when left and right stimulus luminances were subjectively equalized based on single stimulus luminance thresholds. However, this was not the case for neglect patients, who still suffered an increased interference from an ipsilesional distractor when they had to attend to and evaluate the orientation of the contralesional grating. This effect was replicated in the present study. The luminances necessary for a 70% correct discrimination between horizontal and vertical gratings were first determined for both hemifields. To minimize differences in visibility between ip-
silesional and contralesional stimuli, gratings of 20 times these thresholds were subsequently used in the orientation task. Even with equalized luminances, significant extinction effects were observed in the six patients. This suggests that there is an additional (attentional/representational) imbalance in neglect. In three of the six patients, thresholds in the ipsilesional distractor condition were unreliable. The three other patients, however, obtained reliable thresholds in the ipsilesional distractor condition. Thus, even in the presence of an ipsilesional stimulus, they were able to perceive and identify the contralesional stimulus. Extinction, in these patients,
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appears to be not merely the failure to detect a contralesional stimulus in the presence of an ipsilesional stimulus, but rather, reflects the failure to discriminate the correct stimulus features (i.e. orientation). The main purpose of the present study was to assess whether extinction in neglect patients can be modulated by changing the luminance-contrast of simultaneously presented target and distractor stimuli. Extinction effects obtained with subjectively equalized luminances were compared with extinction effects obtained in conditions with either decreased or increased distractor or target stimulus luminances. Experiment 1 demonstrated that a 10-fold reduction of the ipsilesional distractor luminance resulted in a significant reduction of the interference with contralesional hemifield orientation sensitivity in neglect patients. A 10-fold increase of the contralesional distractor luminance, on the other hand, did not lead to more interference with orientation sensitivity in the ipsilesional hemifield. In Experiment 2, we observed that an increase in the luminance of the contralesional target grating eliminated extinction: right distractor interference with left hemifield orientation sensitivity was no longer significant after a 10-fold left target stimulus luminance increase. Reducing the right target luminance, on the other hand, did not result in a significant increase of the left distractor interference. In sum, when using subjectively equalized left and right stimulus luminances, asymmetric distractor interference (extinction) was observed. When left or right stimulus luminances were changed to the benefit of the left side or against the right side, asymmetric distractor interference disappeared. Consequently, a symmetric but non-significant distractor interference was observed. These results clearly show that not the absolute salience of the ipsilesional or contralesional stimulus is the important factor for inducing extinction, but the balance between both stimuli is, since extinction can be eliminated by changes on either side. Extinction depends on the relation between stimuli, and thus seems to be caused by a biased competition between the stimuli (Duncan, Humphreys, & Ward, 1997). Although a 10-fold reduction of the right distractor luminance resulted in a significant reduction of the interference with left hemifield orientation sensitivity in the patients, a comparably large increase of the distractor luminance in the normal control subject, did not result in a significantly larger extinction effect. This suggests that salience imbalance alone cannot explain extinction. An intact attentional (or spatial representation) system seems to protect against extinction and is able to suppress very salient distractor stimuli. If, however, this system is damaged, relatively small salience imbalances can have devastating effects on the perception of contralesional stimuli. In Experiment 3, the interference from a grating distractor was compared with the interference from a random dot distractor. Significant right distractor interference with left hemifield orientation sensitivity was observed when using a grating distractor but not when using a random dot distractor. Apparently, the luminance onset of the distractor alone
is not sufficient to induce extinction. The presence of a (grating) pattern seems to be necessary. There are several possible, not necessarily mutually exclusive explanations; First, although the mean luminance of the random dot distractor is higher compared to that of the grating distractor, the contrast is lower. Stimulus salience or representation strength is not just determined by the luminance onset but also by the complexity or contrast of the stimulus. Second, stimulus similarity might also be a factor of importance. It has frequently been shown that more similar stimuli induce stronger extinction effects (Baylis, Driver, & Rafal, 1993). Third, although the location of the contralateral target grating was pre-cued making the distractor grating task irrelevant, it is possible that the ‘magnetic’, involuntary, and obligatory attraction of attention towards the ipsilesional stimulus had a stronger interfering effect on the orientation discrimination task when that distractor also had an orientation (a grating) than when the distractor lacked an inherent orientation (a random dot pattern). Our results do not converge with the findings of Di Pellegrino and De Renzi (1995) and Smania et al. (1996). The latter two studies failed to show influences of stimulus brightness or size on extinction, whereas we observed a significant effect of stimulus luminance on the severity of extinction. One major difference is that we adapted stimulus luminance relative to the luminance thresholds for single stimuli, which was not the case in the previous visual extinction studies. In a tactile extinction study of Meador et al. (2001), effects of stimulus intensity on extinction were observed if intensity was adapted relative to single-stimuli perceptual thresholds. These results clearly show the advantages of using threshold measurements in extinction research. Zihl and von Cramon (1979) observed a small influence of stimulus intensity on extinction in a left brain-damaged patient, which according to Smania et al. (1996) might be suggestive of a difference in the nature of extinction after left and right hemispheric lesions. However, we demonstrated significant intensity effects on extinction in six right braindamaged patients. These effects suggest that the higher-order imbalances in extinction patients are strongly influenced by both top-down and bottom-up factors. In patients with a damaged attentional/representational system, salience imbalances seem to have a larger impact than in subjects with an intact attentional/representational system. As patients with focal brain damage frequently have additional sensory deficits, asymmetric sensory as well as attentional/representational deficits are likely to contribute to the production of extinction. References Baylis, G. C., Driver, J., & Rafal, R. D. (1993). Visual extinction and stimulus repetition. Journal of Cognitive Neuroscience, 5, 453– 466. Bisiach, E., & Luzzatti, C. (1978). Unilateral neglect of representational space. Cortex, 14, 129–133.
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