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Unilateral hemispheric activation does not affect free-viewing perceptual asymmetries
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Wallesch, C. W., & Fehrenbach, R. (1988). On the neurolinguistic nature of language abnormalities in Huntington's disease. Journal of Neurology, Neurosurgery, and Psychiatry, 51, 367-373. Whitworth, A., Lesser, R., & McKeith, I. (1999). Profiling conversation in Parkinson's disease with cognitive impairment. Aphasiolog3, 13, 407-425. Yorkston, K. M., & Beukelman, D. R. (1984). Assessment of intelligibility of dysarthric speech. Austin, TX: Pro-Ed.
Unilateral Hemispheric Activation Does Not Affect Free-Viewing Perceptual Asymmetries Michael E. R. Nicholls Department of Ps3'chology, University of Melbourne, Australia
and John L. Bradshaw and Jason B. Mattingley Department of Psychology, Monash University, Australia Strong leftward perceptual biases have been reported for the selection of the darker of two left/right mirror-reversed luminance gradients under free-viewing conditions. This study investigated the effect of unilateral hemispheric activation on this leftward bias in two groups of dextrals (N = 52 and N = 24). In Experiment 1, activation was manipulated by asking participants to tap with their left or right fingers along their midline. In Experiment 2, participants clenched their left or right hands in their respective hemispaces. Participants selected the stimulus that was darker on the left-hand side 73% of the time. Despite manipulations of activation strength and hemispace, activation had no effect on the asymmetry. If activation was important, the leftward bias should have been enhanced when the left hand/right hemisphere was active and reduced (or reversed) when the right hand/left hemisphere was active. The contribution of left-to-right scanning biases to free-viewing perceptual asymmetries is discussed as an alternative. © 2ool AcademicPress
Introduction
Perceptual asymmetries in unilateral neglect patients have been researched extensively. In general, patients with lesions to the fight parietal cortex neglect the leftward features of their environment or the leftward portions of individual objects (Walker, 1995). Perceptual asymmetries under free-viewing conditions, also referred to as 'pseudo-neglect,' have been observed in nonclinical populations as well (Bowers & Heilman, 1980). In general, such participants attend more to the leftward features of stimuli than the right, a situation that is the reverse of clinical patients. Thus, for line bisection tasks, participants bisect the line slightly to the left of its true center: the opposite of what is observed for neglect patients (McCourt & Olafson, 1997). Particularly strong perceptual asymmetries have been reported for the 'gray scales' task (Nicholls, Bradshaw, & Mattingley, 1999). This task requires participants to judge the relative brightness of two simultaneously presented horizontal bars. Each bar is a luminance gradient which changes incrementally from white on one side to black on the other. The bars are arranged so that they are left/right mirror reversals of one another (see Fig. 1). When asked to determine which stimulus was overall darker, Nicholls et al. (1999) found that nonclinical participants chose the gray scale that was darker on the lefthand side 67% of the time. This bias occurred despite the fact that the gray scales within each pair were always equally dark. Similar leftward
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biases have been reported for judgments of relative size (Nicholls et al., 1999) and numerosity (Luh, 1995; Nicholls et al., 1999). Luh (1995) and Nicholls et al. (1999) suggested that the perceptual asymmetries observed for their tasks could be the result of an attentional bias directed to the left hemispace. According to Kinsbourne (1970) spatial processing would activate the right hemisphere more than the left hemisphere, leading to a bias of attention to the left hemispace. In support of this proposition, Schiff and Truchon (1993) found that right-hand activity reduced the leftward bias for a chimeric faces task. However, Fischer (1994) has reported negative results for a line bisection task. Attention was manipulated by requiring participants to report cues in a left-right or right-left order. Order of report was found to have no effect on perceptual asymmetry. The present study sought to clarify the role of unilateral activation on perceptual asymmetries for the gray scale task. In Experiment 1, activation was manipulated by requiring participants to tap with the left or right hands while carrying out the gray scale task. A midline hand placement was used to the effects of hemispace. In Experiment 2, strength of activation was increased by requiring participants to clench their left or right hand. In light of the suggestion that hemispace can facilitate the effect of hemispheric activation (Robertson & North, 1992), the left and right hands were placed in their respective hemispaces. If unilateral hemispheric activation plays an important role in the perceptual asymmetry, the leftward bias should be accentuated in the left-hand activity condition relative to the baseline condition. In contrast, the right-hand activity condition should produce activation in both the left and the right hemispheres, resulting in a reduction, or reversal, of the leftward bias.
Method Participants. Fifty-two (12 male, 40 female) and 24 (8 male, 16 female) righthanded (Oldfield, 1971) students participated in Experiments 1 and 2, respectively. Apparatus. The experiments were controlled with an IBM clone interfaced with a VGA monitor. Responses were recorded using a seven-button response panel. One button was centered in the panel and was used as a target for tapping responses. Buttons were placed 50 mm to the near and far sides of the central button. The remaining four buttons were placed symmetrically to either the left or the right side of the center of the panel. These buttons allowed simultaneous bimanual responses from the index and middle fingers of both hands. The near and far buttons (for unimanual and bimanual responses) were used to select the lower and upper stimuli, respectively. Stimuli. Stimuli were viewed at a distance of 500 ram. The horizontal midline of stimulus pairs was aligned with the center of the display screen. The upper and lower stimuli were placed with their centers 29 mm above and below the middle of the screen. The length of the stimuli varied between 88, 110, 132, 154, 176, and 198 mm. The upper and lower stimuli within the pairs were arranged so that they were left/right reversals of one another. It should be noted, however, that each stimulus within a pair had a slightly different configuration. Thus, while the images were identical at a global level, they differed at a local level. An example of the gray scale stimulus is shown in Fig. 1. The stimuli were 22 mm high. Each stimulus changed in 50 increments from black on one side to white on the other. Pixels were added progressively to each increment to create the impression of a change from black on one side to white on the other. A smooth change in brightness was achieved by randomizing the vertical location of the pixels (see Nicholls et al. (1999) for more details on the construction of the stimuli).
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221
FIG. 1. Example of the gray scale stimuli. Fifty percent of trials were arranged so that the upper stimulus was dark on the left while the lower stimulus was dark on the right. The reverse was true for the remaining trials.
Procedure. The baseline, right-tap/clench and left-tap/clench conditions were run in separate blocks. Eight practice trials prior to each block of testing. The order in which the blocks were administered was balanced between subjects. Each block contained 96 trials. For half of the trials, the upper stimulus was darker on the left and the lower stimulus was darker on the right. The reverse was true for the remaining trials. The six different line lengths were represented an equal number of times within each block. The order in which these factors occurred was randomized for each subject. For baseline testing, each trial began with the presentation of the gray scale stimuli. After a period lasting between 2500 and 3500 ms, the computer emitted a tone which served as a signal for participants to make their response. Participants selected the stimulus that appeared to be darker. Bimanual responses were made with the middle and index fingers using the buttons placed to either side of the center of the response panel. Trials with responses longer than 2000 ms were rejected and replaced by an identical trial. The screen was cleared after a response was made and a new trial was begun in 1500 ms. In the right-tap/clench condition, participants were instructed to tap their right index finger on the central button (Experiment l) or clench their right hand (Experiment 2) as quickly as possible upon presentation of the gray scale stimuli. After 2500 and 3500 ms, a tone sounded and this served as the signal for participants to make their response. The right index finger selected one of the buttons located above or below the central button. If the participant did not respond within 2000 ms, the trial was rejected and replaced by an identical trial. Following the response, the display was cleared and a new trial was begun. Participants resumed movements of their right hand as soon as they had made a response. The procedure used for the left-tap/ clench condition was the same as the right-tap condition except that the left hand was used to make all responses. In Experiment 1, tapping movements and stimulus selection responses were made along the participant's midline. In Experiment 2, clenching movements of the left and right hands were made 50 ° to the left or right of the midline, respectively. To avoid the influence of asymmetrical visual distractions, movements were made underneath a cloak.
222
TENNET XI
--L~--Left-hand --~Baseline --D--Right-hand Tapping
Clenching
-2 t~ .O
-4
oQ .
-6-8-
-10 -12
I 88
I
I
I
I
[
110 132 154 176 198
I 88
110 132 154 176 198
Length(mm)
FIG. 2. Mean response bias values for the gray scale task across the left-hand,baseline, and righthand conditionsfor the tapping and clenching tasks. Negative scores indicate a leftward bias.
Results Experiment 1. Responses were categorized according to whether participants selected the stimulus that was dark on the left or right side. Response bias was measured by subtracting leftward responses from rightward responses. The mean response biases for the left-tap, baseline, and right-tap conditions were - 6 . 3 , - 7 . 4 , and -8.4, respectively, all of which were significantly different from zero [t(51) = 6.8, p < .001], [t(51) = 7.4, p < .001], and It(51) = 8.4, p < .001]. An ANOVA with condition (left tap, baseline, and right tap) and length (six levels) as within subjects factors revealed a significant increase in bias with increases in length [F(5, 255) = 4.3, p < .005]. There was no main effect of condition [F(2, 102) = 0.6, ns] and no interaction between length and condition [F(10, 510) = 0.8, ns] (see Fig. 2). Experiment 2. The mean response bias for the left-clench, baseline, and rightclench conditions was -8.4, -7.6, and -7.4, respectively. A t test revealed that the levels of leftward bias for the left-tap [t(23) = 6.7, p < .001], baseline [t(23) = 6.1, p < .001], and fight-tap [t(23) = 4.8, p < .001] conditions was significantly different from zero. An ANOVA revealed a significant increase in bias as a function of increases in length [F(5, 115) = 5.9, p < .001]. There was no main effect of condition [F(2, 46) = 0.5, ns] and no interaction between length and condition [F(10, 230) = 0.8, ns] (see Fig. 2). Discussion Both experiments show no effect of unilateral hemispheric activation on fleeviewing perceptual asymmetries. This lack of effect occurred despite manipulations in the level of activation and hemispace. It could be argued that the present studies failed to disprove the null hypothesis because of insufficient statistical power. However, the power of the present study should have been equal to, or better than, the other studies that have found an activation effect. For example, Schiff and Truchon (1993) assigned 60 participants to one of three between-participants testing conditions, each of which contained 30 trials. The present study used a within-participants design where 52 (Experiment 1) or 24 (Experiment 2) participants were required to
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complete 288 trials. This within-participants design should have provided a test that was at least as sensitive as that used by Schiff and Truchon (1993). The finding that unilateral activation does not affect perceptual asymmetries for the gray scales task m a y not be so surprising when considered within the cerebral lateralization literature. Right hemisphere lateralization o f spatial functions such as line bisection and the discrimination o f brightness are notoriously weak (Bradshaw & Nettleton, 1983). Bearing in mind the weak lateralization that probably exists for brightness discrimination, it is somewhat surprising that such a strong leftward bias was observed in the present study. Indeed, the strong leftward bias for the gray scales suggests that processes other than hemispheric activation m a y be at work. Manning, Halligan, and Marshall (1990) proposed that scanning habits play an important role in pseudoneglect. Left-to-right scanning habits, which predominate a m o n g English readers, m a y lead to an overrepresentation o f the leftward extent o f a stimulus compared to the right. This proposition could be tested in the future using the gray scale stimuli with groups o f readers with different scanning habits (e.g., Chokron, Bernard, & Imbert, 1997) or through directions to scan one way or the other (e.g., Brodie & Pettigrew, 1996).
REFERENCES Bowers, D., & Heilman, K. M. (1980). Pseudoneglect: Effects of hemispace on a tactile line bisection task. Neuropsychologia, 18, 491-498. Bradshaw, J. L., & Nettleton, N. C. (1983). Human cerebral as3'mmetr).'. Englewood Cliffs, NJ: Prentice Hall International. Brodie, E. E., & Pettigrew, L. E. L. (1996). ls left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction. Neuropsychologia, 34, 467-470. Chokron, S., Bernard, J. M., & Imbert, M. (1997). Length representation in normal and neglect subjects with opposite reading habits studied through a line extension task. Cortex, 33, 47-64. Fischer, M. H. (1994). Less attention and more perception in cued line bisection. Brain and Cognition, 25, 24-33. Kinsbourne, M. (1970). The cerebral basis of lateral asymmetries in attention. Acta Psychologia, 33, 193-201. Luh, K. E. (1995). Line bisection and perceptual asymmetries in normal individuals: What you see is not what you get. Neuropsycholog 3. 9, 435-448. Manning, L., Halligan, P. W., & Marshall, J. C. (1990). Individual variation in line bisection: A study of normal subjects with application to the interpretation of visual neglect. Neuropsychologia, 28, 647-655. McCourt, M. E., & Olafson, C. (1997). Cognitive and perceptual influences in visual line bisection: Psychophysical and chronometric analyses of pseudoneglect. Neuropsychologia, 35, 369-380. Nicholls, M. E. R., Bradshaw, J. L., & Mattingley, J. B. (1999). Free-viewing perceptual asymmetries for the judgement of brightness, numerosity and size. Neuropsychologia, 37, 307-314. Oldfield, R. C. (1971). The assessment of handedness: The Edinburgh Inventory. Neuropsychologia, 9, 97-133. Robertson, I. H., & North, N. (1992). Spatio-motor cueing in unilateral neglect: The role of hemispace, hand and motor activation. Neuropsychologia, 30, 553-563. Schiff, B. B., and Truchon, C. (1993). Effect of unilateral contraction of hand muscles on perceiver biases in the perception of chimeric and neutral faces. Neuropsychologia, 31, 1351-1365. Walker, R. (1995). Spatial and object-based neglect. Neurocase, 1: 371-383.
219
Wallesch, C. W., & Fehrenbach, R. (1988). On the neurolinguistic nature of language abnormalities in Huntington's disease. Journal of Neurology, Neurosurgery, and Psychiatry, 51, 367-373. Whitworth, A., Lesser, R., & McKeith, I. (1999). Profiling conversation in Parkinson's disease with cognitive impairment. Aphasiolog3, 13, 407-425. Yorkston, K. M., & Beukelman, D. R. (1984). Assessment of intelligibility of dysarthric speech. Austin, TX: Pro-Ed.
Unilateral Hemispheric Activation Does Not Affect Free-Viewing Perceptual Asymmetries Michael E. R. Nicholls Department of Ps3'chology, University of Melbourne, Australia
and John L. Bradshaw and Jason B. Mattingley Department of Psychology, Monash University, Australia Strong leftward perceptual biases have been reported for the selection of the darker of two left/right mirror-reversed luminance gradients under free-viewing conditions. This study investigated the effect of unilateral hemispheric activation on this leftward bias in two groups of dextrals (N = 52 and N = 24). In Experiment 1, activation was manipulated by asking participants to tap with their left or right fingers along their midline. In Experiment 2, participants clenched their left or right hands in their respective hemispaces. Participants selected the stimulus that was darker on the left-hand side 73% of the time. Despite manipulations of activation strength and hemispace, activation had no effect on the asymmetry. If activation was important, the leftward bias should have been enhanced when the left hand/right hemisphere was active and reduced (or reversed) when the right hand/left hemisphere was active. The contribution of left-to-right scanning biases to free-viewing perceptual asymmetries is discussed as an alternative. © 2ool AcademicPress
Introduction
Perceptual asymmetries in unilateral neglect patients have been researched extensively. In general, patients with lesions to the fight parietal cortex neglect the leftward features of their environment or the leftward portions of individual objects (Walker, 1995). Perceptual asymmetries under free-viewing conditions, also referred to as 'pseudo-neglect,' have been observed in nonclinical populations as well (Bowers & Heilman, 1980). In general, such participants attend more to the leftward features of stimuli than the right, a situation that is the reverse of clinical patients. Thus, for line bisection tasks, participants bisect the line slightly to the left of its true center: the opposite of what is observed for neglect patients (McCourt & Olafson, 1997). Particularly strong perceptual asymmetries have been reported for the 'gray scales' task (Nicholls, Bradshaw, & Mattingley, 1999). This task requires participants to judge the relative brightness of two simultaneously presented horizontal bars. Each bar is a luminance gradient which changes incrementally from white on one side to black on the other. The bars are arranged so that they are left/right mirror reversals of one another (see Fig. 1). When asked to determine which stimulus was overall darker, Nicholls et al. (1999) found that nonclinical participants chose the gray scale that was darker on the lefthand side 67% of the time. This bias occurred despite the fact that the gray scales within each pair were always equally dark. Similar leftward
220
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biases have been reported for judgments of relative size (Nicholls et al., 1999) and numerosity (Luh, 1995; Nicholls et al., 1999). Luh (1995) and Nicholls et al. (1999) suggested that the perceptual asymmetries observed for their tasks could be the result of an attentional bias directed to the left hemispace. According to Kinsbourne (1970) spatial processing would activate the right hemisphere more than the left hemisphere, leading to a bias of attention to the left hemispace. In support of this proposition, Schiff and Truchon (1993) found that right-hand activity reduced the leftward bias for a chimeric faces task. However, Fischer (1994) has reported negative results for a line bisection task. Attention was manipulated by requiring participants to report cues in a left-right or right-left order. Order of report was found to have no effect on perceptual asymmetry. The present study sought to clarify the role of unilateral activation on perceptual asymmetries for the gray scale task. In Experiment 1, activation was manipulated by requiring participants to tap with the left or right hands while carrying out the gray scale task. A midline hand placement was used to the effects of hemispace. In Experiment 2, strength of activation was increased by requiring participants to clench their left or right hand. In light of the suggestion that hemispace can facilitate the effect of hemispheric activation (Robertson & North, 1992), the left and right hands were placed in their respective hemispaces. If unilateral hemispheric activation plays an important role in the perceptual asymmetry, the leftward bias should be accentuated in the left-hand activity condition relative to the baseline condition. In contrast, the right-hand activity condition should produce activation in both the left and the right hemispheres, resulting in a reduction, or reversal, of the leftward bias.
Method Participants. Fifty-two (12 male, 40 female) and 24 (8 male, 16 female) righthanded (Oldfield, 1971) students participated in Experiments 1 and 2, respectively. Apparatus. The experiments were controlled with an IBM clone interfaced with a VGA monitor. Responses were recorded using a seven-button response panel. One button was centered in the panel and was used as a target for tapping responses. Buttons were placed 50 mm to the near and far sides of the central button. The remaining four buttons were placed symmetrically to either the left or the right side of the center of the panel. These buttons allowed simultaneous bimanual responses from the index and middle fingers of both hands. The near and far buttons (for unimanual and bimanual responses) were used to select the lower and upper stimuli, respectively. Stimuli. Stimuli were viewed at a distance of 500 ram. The horizontal midline of stimulus pairs was aligned with the center of the display screen. The upper and lower stimuli were placed with their centers 29 mm above and below the middle of the screen. The length of the stimuli varied between 88, 110, 132, 154, 176, and 198 mm. The upper and lower stimuli within the pairs were arranged so that they were left/right reversals of one another. It should be noted, however, that each stimulus within a pair had a slightly different configuration. Thus, while the images were identical at a global level, they differed at a local level. An example of the gray scale stimulus is shown in Fig. 1. The stimuli were 22 mm high. Each stimulus changed in 50 increments from black on one side to white on the other. Pixels were added progressively to each increment to create the impression of a change from black on one side to white on the other. A smooth change in brightness was achieved by randomizing the vertical location of the pixels (see Nicholls et al. (1999) for more details on the construction of the stimuli).
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221
FIG. 1. Example of the gray scale stimuli. Fifty percent of trials were arranged so that the upper stimulus was dark on the left while the lower stimulus was dark on the right. The reverse was true for the remaining trials.
Procedure. The baseline, right-tap/clench and left-tap/clench conditions were run in separate blocks. Eight practice trials prior to each block of testing. The order in which the blocks were administered was balanced between subjects. Each block contained 96 trials. For half of the trials, the upper stimulus was darker on the left and the lower stimulus was darker on the right. The reverse was true for the remaining trials. The six different line lengths were represented an equal number of times within each block. The order in which these factors occurred was randomized for each subject. For baseline testing, each trial began with the presentation of the gray scale stimuli. After a period lasting between 2500 and 3500 ms, the computer emitted a tone which served as a signal for participants to make their response. Participants selected the stimulus that appeared to be darker. Bimanual responses were made with the middle and index fingers using the buttons placed to either side of the center of the response panel. Trials with responses longer than 2000 ms were rejected and replaced by an identical trial. The screen was cleared after a response was made and a new trial was begun in 1500 ms. In the right-tap/clench condition, participants were instructed to tap their right index finger on the central button (Experiment l) or clench their right hand (Experiment 2) as quickly as possible upon presentation of the gray scale stimuli. After 2500 and 3500 ms, a tone sounded and this served as the signal for participants to make their response. The right index finger selected one of the buttons located above or below the central button. If the participant did not respond within 2000 ms, the trial was rejected and replaced by an identical trial. Following the response, the display was cleared and a new trial was begun. Participants resumed movements of their right hand as soon as they had made a response. The procedure used for the left-tap/ clench condition was the same as the right-tap condition except that the left hand was used to make all responses. In Experiment 1, tapping movements and stimulus selection responses were made along the participant's midline. In Experiment 2, clenching movements of the left and right hands were made 50 ° to the left or right of the midline, respectively. To avoid the influence of asymmetrical visual distractions, movements were made underneath a cloak.
222
TENNET XI
--L~--Left-hand --~Baseline --D--Right-hand Tapping
Clenching
-2 t~ .O
-4
oQ .
-6-8-
-10 -12
I 88
I
I
I
I
[
110 132 154 176 198
I 88
110 132 154 176 198
Length(mm)
FIG. 2. Mean response bias values for the gray scale task across the left-hand,baseline, and righthand conditionsfor the tapping and clenching tasks. Negative scores indicate a leftward bias.
Results Experiment 1. Responses were categorized according to whether participants selected the stimulus that was dark on the left or right side. Response bias was measured by subtracting leftward responses from rightward responses. The mean response biases for the left-tap, baseline, and right-tap conditions were - 6 . 3 , - 7 . 4 , and -8.4, respectively, all of which were significantly different from zero [t(51) = 6.8, p < .001], [t(51) = 7.4, p < .001], and It(51) = 8.4, p < .001]. An ANOVA with condition (left tap, baseline, and right tap) and length (six levels) as within subjects factors revealed a significant increase in bias with increases in length [F(5, 255) = 4.3, p < .005]. There was no main effect of condition [F(2, 102) = 0.6, ns] and no interaction between length and condition [F(10, 510) = 0.8, ns] (see Fig. 2). Experiment 2. The mean response bias for the left-clench, baseline, and rightclench conditions was -8.4, -7.6, and -7.4, respectively. A t test revealed that the levels of leftward bias for the left-tap [t(23) = 6.7, p < .001], baseline [t(23) = 6.1, p < .001], and fight-tap [t(23) = 4.8, p < .001] conditions was significantly different from zero. An ANOVA revealed a significant increase in bias as a function of increases in length [F(5, 115) = 5.9, p < .001]. There was no main effect of condition [F(2, 46) = 0.5, ns] and no interaction between length and condition [F(10, 230) = 0.8, ns] (see Fig. 2). Discussion Both experiments show no effect of unilateral hemispheric activation on fleeviewing perceptual asymmetries. This lack of effect occurred despite manipulations in the level of activation and hemispace. It could be argued that the present studies failed to disprove the null hypothesis because of insufficient statistical power. However, the power of the present study should have been equal to, or better than, the other studies that have found an activation effect. For example, Schiff and Truchon (1993) assigned 60 participants to one of three between-participants testing conditions, each of which contained 30 trials. The present study used a within-participants design where 52 (Experiment 1) or 24 (Experiment 2) participants were required to
TENNET XI
223
complete 288 trials. This within-participants design should have provided a test that was at least as sensitive as that used by Schiff and Truchon (1993). The finding that unilateral activation does not affect perceptual asymmetries for the gray scales task m a y not be so surprising when considered within the cerebral lateralization literature. Right hemisphere lateralization o f spatial functions such as line bisection and the discrimination o f brightness are notoriously weak (Bradshaw & Nettleton, 1983). Bearing in mind the weak lateralization that probably exists for brightness discrimination, it is somewhat surprising that such a strong leftward bias was observed in the present study. Indeed, the strong leftward bias for the gray scales suggests that processes other than hemispheric activation m a y be at work. Manning, Halligan, and Marshall (1990) proposed that scanning habits play an important role in pseudoneglect. Left-to-right scanning habits, which predominate a m o n g English readers, m a y lead to an overrepresentation o f the leftward extent o f a stimulus compared to the right. This proposition could be tested in the future using the gray scale stimuli with groups o f readers with different scanning habits (e.g., Chokron, Bernard, & Imbert, 1997) or through directions to scan one way or the other (e.g., Brodie & Pettigrew, 1996).
REFERENCES Bowers, D., & Heilman, K. M. (1980). Pseudoneglect: Effects of hemispace on a tactile line bisection task. Neuropsychologia, 18, 491-498. Bradshaw, J. L., & Nettleton, N. C. (1983). Human cerebral as3'mmetr).'. Englewood Cliffs, NJ: Prentice Hall International. Brodie, E. E., & Pettigrew, L. E. L. (1996). ls left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction. Neuropsychologia, 34, 467-470. Chokron, S., Bernard, J. M., & Imbert, M. (1997). Length representation in normal and neglect subjects with opposite reading habits studied through a line extension task. Cortex, 33, 47-64. Fischer, M. H. (1994). Less attention and more perception in cued line bisection. Brain and Cognition, 25, 24-33. Kinsbourne, M. (1970). The cerebral basis of lateral asymmetries in attention. Acta Psychologia, 33, 193-201. Luh, K. E. (1995). Line bisection and perceptual asymmetries in normal individuals: What you see is not what you get. Neuropsycholog 3. 9, 435-448. Manning, L., Halligan, P. W., & Marshall, J. C. (1990). Individual variation in line bisection: A study of normal subjects with application to the interpretation of visual neglect. Neuropsychologia, 28, 647-655. McCourt, M. E., & Olafson, C. (1997). Cognitive and perceptual influences in visual line bisection: Psychophysical and chronometric analyses of pseudoneglect. Neuropsychologia, 35, 369-380. Nicholls, M. E. R., Bradshaw, J. L., & Mattingley, J. B. (1999). Free-viewing perceptual asymmetries for the judgement of brightness, numerosity and size. Neuropsychologia, 37, 307-314. Oldfield, R. C. (1971). The assessment of handedness: The Edinburgh Inventory. Neuropsychologia, 9, 97-133. Robertson, I. H., & North, N. (1992). Spatio-motor cueing in unilateral neglect: The role of hemispace, hand and motor activation. Neuropsychologia, 30, 553-563. Schiff, B. B., and Truchon, C. (1993). Effect of unilateral contraction of hand muscles on perceiver biases in the perception of chimeric and neutral faces. Neuropsychologia, 31, 1351-1365. Walker, R. (1995). Spatial and object-based neglect. Neurocase, 1: 371-383.