Feature-specific effects of selective visual attention

Vision Res. Vol. 35, No. 5, pp. 621 634, 1995

Pergamon

0042-6989(94)00156-1

Copyright ,~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-6989/95 $9.50 + 0.00

Feature-specific Effects of Selective Visual Attention A N D R E W F. ROSSI,* M I C H A E L A. PARADISO*~" Received 4 August 1993: in revised form 18 March 1994; in final form 27 June 1994

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observer's primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties. Selective attention Spatial frequency Orientation Detection

INTRODUCTION

uli? Moreover, if one can attend to features, can this be dissociated from location? A number of previous studies are relevant to the first of these questions, particularly concerning attention to spatial scale. There have been several demonstrations that observers can selectively attend to one spatial scale over others contained in an image (Julesz & Papathomas, 1984; Miller, 1981; Navon, 1977, 1981). It has been hypothesized that observers use spatial frequency labeled lines in identification and detection tasks, attending to a subset of channels to reduce uncertainty and noise (Davis & Graham, 1981; Watson & Robson, 1981; Graham, Kramer & Haber, 1985). In an experiment specifically looking at the issue of spatial scale and attention, Shulman and Wilson (1987) transiently presented stimuli consisting of large letters of the alphabet made from many smaller letters, followed by a grating stimulus. They found that when observers were told to categorize the large global letters they were relatively more sensitive to subsequent low frequency gratings compared to trials in which they were told to categorize the smaller component letters. One explanation of this finding is that performance on the global categorization task is enhanced by selectively monitoring low frequency channels and this is reflected in the heightened sensitivity to subsequent low frequency gratings. Concerning the second question posed above, whether attention to features can be dissociated from location, the results are more mixed. While it has been proposed that attentive selection by location is only quantitatively not qualitatively different from selection by other attributes (Bundensen, 1990; Duncan, 1980; Keren, 1976; von Wright, 1970), some studies find that

Our daily experience reveals a powerful ability to selectively attend to a portion of our total sensory input. We can isolate for conscious processing a conversation in a noisy environment, one object in the midst of others in a visual scene, or input from one sensory modality over that from other modalities. While the brain mechanisms responsible for this ability are still unclear, psychophysical experiments have begun to quantify the characteristics of attention, particularly in the visual system. Most studies of visual attention concern the finding that when attention is drawn to one location in the visual field, visual performance at that location is enhanced. In one commonly used paradigm, subjects must detect a target that might appear at any of a number of peripheral locations. It is found that reaction times are shorter (Eriksen & Hoffman, 1972; Posner, 1980) and detection thresholds lower (Bashinski & Bachrach, 1980; Downing, 1988) if the peripheral target is preceded by a cue that indicates where the target will appear. Presumably the cue selectively increases attention at one peripheral location and this increased attention makes one more sensitive to the subsequent stimulus. While there is unquestionably a strong locationspecificity to visual attention, we are interested in the extent to which there is more to attention than just location. In particular, can the preferential processing associated with attention be specific to features of stim*Department of Neuroscience, Box 1953, Brown Providence, RI 02912, U.S.A. *To whom all correspondence should be addressed.

University.

621

622

ANDREW F. ROSSI and MICHAEL A. PARADISO

attention is fundamentally tied to location (Kwak & Egeth, 1992; Nissen, 1985). In our research, we have sought to develop a paradigm which can further our understanding of attention to particular visual features, by the use of carefully controlled stimuli. We wanted a technique which could be applied to a range of visual attributes such as orientation, spatial frequency and color. In addition we wanted the technique to be quantitative in the sense that the attributes could be precisely specified. To satisfy these requirements, we have used discrimination and detection tasks with Gabor patches and sine-wave gratings as stimuli. We report the results of four experiments which use a new paradigm to address two questions. First, do the spatial frequency and orientation of stimuli used in a foveal discrimination task selectively affect the detectability of near-threshold surround gratings which have similar features? Second, if the central stimuli do affect the detectability of surround gratings, is this effect influenced by the nature of the central task (i.e. orientation or spatial frequency discrimination).

METHOD

Overview o[ procedure The authors and two naive observers participated in this study. Four experiments were conducted in the order in which they are presented below. Observers' attention was engaged by having them perform a temporal twoalternative forced-choice discrimination in a window approx. 1 deg in radius about the point of fixation. In two of the experiments this task was orientation discrimination and in the other two it was spatial frequency discrimination. On one-third of the trials a nearthreshold grating was presented after the second discrimination target. Sensitivity for detection of the grating was measured at a variety of spatial frequencies and orientations when the central task was orientation or spatial frequency discrimination. In this way, we established whether the nature of the discrimination task being attended to, or the specific characteristics of the attended central targets, affected the detectability of the surround grating. Stimuli and visual display Stimuli were generated by a Number Nine Graphics Board installed in a PC clone and displayed on a NEC GS2A grayscale monitor with 640 × 480 pixel resolution (28 pixels/cm) and a refresh rate of 60 Hz. At the viewing distance of 82 cm, the screen subtended 16 deg and there were 40 pixels per deg. Fine control of stimulus contrast was achieved by driving the monitor with the output of a video attenuator connected to the computers RGB outputs (Pelli & Zhang, 1991). The mean luminance of the display was 11 cd/m 2. The target stimuli consisted of Gabor patches presented in the center of an otherwise blank meanluminance screen. The Gabor patches were defined by a

cosine grating with a frequency of 0.5, 1, or 4c/deg damped by a Gaussian envelope (Fig. 1). The peak contrast of the cosine grating was 50%. The Gaussian envelope had a standard deviation of 39 min arc. At times when a Gabor stimulus was not on the screen, there was a fixation point at the center of the display. The luminance of the surround gratings was sinusoidally modulated about the average screen luminance. In order that the grating and the central Gabor targets were never spatially coincident, the central region of the surround grating was deleted by cutting out a hole 1.5 deg in radius. Within this hole the screen was at the mean luminance level. To eliminate the sharp edge at the border of this hole, the contrast near the edge was weighted by a Gaussian having a standard deviation of 25min arc. With the cutout in the surround grating there was a gap approx. 0.5 deg in width between the outer edge of the Gabor target and the inner edge of the surround grating (though they were never simultaneously on the screen).

Central discrimination task In two of the four experiments the central task was orientation discrimination. Observers made discriminations with stimuli near the baseline orientations of vertical and - 6 0 deg from vertical (our convention is that negative orientations are counterclockwise from vertical). In a given experimental session a single baseline orientation was used, they were not intermixed. For each session a set of Gabor targets was constructed with one oriented at exactly the baseline orientation, three others at clockwise orientations and three more at counterclockwise orientations from the baseline. The seven different targets differed in orientation from each other by an offset which yielded approx. 75% correct in a preliminary series of discrimination experiments. Across observers, for discriminations made with 0.5c/deg Gabors, the offsets were in the range 1.2-1.6 deg, with 1.0c/deg Gabors offsets were 0.8 1.2deg, and with 4.0 c/deg Gabors the offsets were 0.65 1.0 deg. For each trial of the experiment, a Gabor patch was chosen at random from the five targets nearest the baseline orientation and it was presented for 96 msec. After an interstimulus interval of 496 msec, a second Gabor patch was presented for 96 msec with an offset either clockwise or counterclockwise from the first Gabor target (Fig. 2). This procedure requires the subject to attend to both Gabor patches to make the correct discrimination, unlike paradigms with a fixed reference orientation (see Paradiso, Carney & Freeman, 1989). As an explicit example, if a subject required an offset of I deg in order to get 75% correct in preliminary experiments, the first Gabor target used for discriminations near vertical would have an orientation of - 2 , - 1, 0, 1, or 2 deg chosen at random. If the orientation of the first Gabor was - 2 deg then the second Gabor target would have an orientation of either - 3 or - l d e g . Importantly, in these experiments employing orientation discrimination and the subsequent experiments using frequency discrimination, the contrast of

FEATURE-SPECIFIC

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patches was always the same as described contrast was not adjusted in order to get 75% the central task). second two experiments, observers made

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spatial frequency discriminations between two consecutively presented Gabor patches. A procedure analogous to that used for orientation discrimination was employed. For each experimental session, seven stimuli

(a) and surround grating (b). For the purpose of illustration, the contrast increased from the near-threshold value used in the experiments.

of the grating

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ANDREW F. ROSSI and MICHAEL A. PARADISO presented. The surround grating, when presented, would occur 16 msec after the presentation of the second Gabor for a duration of 48msec. (A grating was never presented on trials without the "gratings?" prompt.) Within an experimental session the background grating was presented five times at each of a number of randomly interleaved orientations ( - 9 0 , - 6 0 , - 3 0 , 0, 30, and 60 deg from vertical) or spatial frequencies (0.5, 1, 2, and 4 c/deg) covering the full range displayed in the figures. Each observer performed each experiment 7-10 times.

FIGURE 2. Presentation sequence of stimuli. Presentation of a surround grating, as illustrated here, occurred on only one-sixthof the trials. The actual luminance modulation of the grating was a sinusoidal wave, not the square wave shown here. were constructed which differed by a frequency offset that yielded approx. 75% correct in a preliminary series of discrimination experiments. All observers used the same offset values for the experiments involving frequency discrimination. For discriminations made with 0.5c/deg Gabors, the offset was 0.03c/deg, with 1.0 c/deg Gabors the offset was 0.06c/deg, and with 4.0 c/deg Gabors the offset was 0.1 c/deg. The spatial frequency of the first Gabor patch was randomly chosen from the set of five stimuli nearest the baseline frequency (i.e. the Gabor at the baseline frequency, two at lower frequencies and two at higher frequencies). Trials with different baseline spatial frequencies were not intermixed. The second Gabor patch differed from the first by the threshold offset value and it had either a higher or lower spatial frequency than the first. The presentation sequence was identical to that which was used for orientation discrimination. In both discrimination tasks, observers responded by pressing one of two buttons to indicate the direction in which the second Gabor patch differed from the first. Auditory feedback was provided for incorrect discriminations.

Peripheral detection task In each experimental session there were 120-180 discrimination trials. On a random one-third of the trials, the observer was prompted with the message "grating?" in the center of the monitor after the response to the central discrimination task had been made. The observer pressed one of two buttons to indicate whether they had detected the presence of a surround grating. This secondary detection task was only performed on a third of the trials, and no feedback on the secondary task was provided, to minimize the possibility that the observer would adopt a strategy in which they tried to allocate attention to the peripheral as well as the central task. On half of the detection trials (i.e. those with the "gratings?" prompt) a grating had been presented in the surround immediately after the second foveal target and on the other half no surround grating had been

Control experiments Two control experiments were conducted after the initial four experiments. In the first of these controls, observers made detection judgments with the surround stimuli, as described above, but no central targets were presented. This allowed us to assess the effect of the central stimuli and task on the detectability of the surround gratings. In the second control, Expts 1-4 were repeated with the single change that the observers were instructed not to discriminate the central targets. They simply fixated the central targets and made detection judgments with the peripheral gratings when prompted. As in the main experiments, detection judgments were prompted on one third of the trials, meaning that on two thirds of the trials in the control experiment no decision was made. This control was done to determine what effect attending to the central stimuli had in Expts 1-4. Data collection and presentation Observers' responses were collected under computer control. The percent correct for the central discrimination task, and the hit and false alarm rates for the peripheral task were computed for each experimental condition. The average and standard error, over sessions, of the detection and discrimination rates were computed for each observer. The average hit and false alarm rates for the surround grating are plotted as a function of either the grating's spatial frequency or orientation, for each of the two discrimination tasks. In the graphs of the data, hit rates are shown for each spatial frequency or orientation at which the peripheral gratings were presented. However, because we interleaved a full range of spatial frequencies or orientations in each session, and because the subject performed a yes/no detection task, there is a single false alarm rate for each data curve rather than a false alarm rate associated with the individual spatial frequency or orientation.

RESULTS

Four variations of the experimental paradigm were conducted: the detection rate for the surround grating was measured as a function of either its orientation or spatial frequency when the central task was either orientation o r spatial frequency discrimination.

FEATURE-SPECIFIC

Experiment 1: Central orientation discrimination with detection of surround gratings that vary in orientation An observer's ability to detect the presence of a surround grating at different orientations was measured while the observer was engaged in the orientation discrimination of foveal Gabor targets. In a given experimental session the foveal discriminations were made with targets near either vertical or 60deg counterclockwise from vertical (i.e. - 60 deg). In either case the surround gratings appeared at each of six orientations ( - 9 0 , - 6 0 , - 3 0 , 0, 30, and 60 deg from vertical). The spatial frequency of the Gabor targets and the surround gratings was 1.0 c/deg. Results in Fig. 3 show the detection rate of the surround grating as a function of its orientation for three observers. Plots with solid lines represent data obtained when the central discrimination task is performed with Gabors in the vicinity of vertical. For all three observers, when the central discriminations are made near vertical, the detection rate is highest for surround gratings also near vertical. In contrast, when the central task is performed with Gabors roughly - 6 0 deg from vertical (dotted lines in Fig. 3) the detection rate is maximal when the surround grating is also - 6 0 d e g from vertical. The bandwidths of the peaks in the detection plots are approx. 15 45 deg (half width at half height), roughly centered on the orientation of the Gabor targets discriminated in the central task. A concise summary of the data for three observers is provided in Fig. 7(a) which plots the difference in hit rate, expressed as Az(Hit) when the central discriminations are made near vertical vs - 60 deg from vertical. A value of zero indicates that the detectability of the surround grating is the same with the two different orientation ranges of the central Gabor targets. This is the case for surround gratings with orientations intermediate between vertical and - 6 0 deg from vertical. The largest differences in the detection rates are seen at the two surround grating orientations that match the orientations about which the Gabors were discriminated (0 and - 60 deg). The data in Fig. 3 are presented in terms of z-scores in addition to the percentage of hits and false alarms. This allows one to assess the question of whether changes in criterion might underlie the changes in the hit rate. Clearly, a simple criterion change cannot account for the differences seen between the pairs of curves since the curves cross rather than being vertically displaced. The possibility that the differences result from orientation-tuned shifts in criterion is considered in the general discussion.

Experiment 2: Central orientation discrimination with detection o[" surround gratings that vary in spatial frequency The results of Expt 1 show that the detectability of surround gratings is highest for orientations similar to the orientation of the central Gabor patches. This second experiment was conducted to determine whether

VISUAL ATTENTION

625

detectability is also highest when the surround grating and central Gabor have the same spatial frequency. The central task was orientation discrimination with Gabor patches near vertical and having a spatial frequency of either 0.5 or 4.0c/deg. Within a session the spatial frequency of the central targets was fixed and vertical surround gratings were presented at 0.5, 1, 2, and 4 c/deg. Figure 4 shows the detection rate for the surround grating as a function of its spatial frequency. It should be noted that the shapes of these curves are not

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Orientation of Surround Grating (deg) F I G U R E 3. M e a n hit rates for s u r r o u n d gratings as a function of their orientation. The vertical axis on the left side shows the p e r c e n t a g e of hits and the axis on the right shows c o r r e s p o n d i n g z-scores. Orient a t i o n on the abscissa refers to degrees from vertical where negative values represent angles c o u n t e r c l o c k w i s e from vertical. The central task was o r i e n t a t i o n d i s c r i m i n a t i o n near vertical (solid squares) or - 6 0 deg from vertical (open circles). E r r o r bars indicate _+ 1 SE from the mean. The spatial frequency of b o t h the central targets and the s u r r o u n d gratings was 1.0c/deg.

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Experiment 3: Central spatial frequency discrimination with detection of surround gratings that vary in spatial frequency Comparing the results of Expts 1 and 2, one sees that when the central task is orientation discrimination the detectability of the surround gratings depends on the orientation but not the spatial frequency of the central target. Experiment 3 examines whether there is an effect of spatial frequency when the central task is spatial frequency discrimination rather than orientation discrimination. In the central task the observer made spatial frequency discriminations in the vicinity of either 0.5 or 4.0c/deg. Both the central G a b o r targets and the

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contrast of the surround grating was set separately for each frequency to the value yielding roughly 75% correct in control experiments without the central discrimination task. Each plot shows the hit rate for the detection of the surround grating when the observer was engaged in discriminating small orientation changes of a 0.5 c/deg (solid lines) and 4.0 c/deg (dotted lines) G a b o r target. While there are small differences in the curves, there is no consistent or systematic difference as a function of the central spatial frequency. The data are summarized in Fig. 7(b) which shows Az(Hit) for the surround gratings between conditions in which the central target was 0.5 or 4 c/deg. The data points are all near zero and the curves have relatively flat slopes indicating that there is not a significant effect of the central G a b o r

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FEATURE-SPECIFIC VISUAL ATTENTION

surround gratings were always vertical. Figure 5 shows the hit rate for the surround grating as a function of its spatial frequency when the central frequency discriminations are made near either 0.5 c/deg (solid lines) or 4.0 c/deg (dotted lines). When the central discriminations are made near 0.5c/deg the low frequency surround gratings are relatively more detectable compared to conditions in which the central discriminations are made near 4c/deg. Conversely, when the central discriminations are made near 4c/deg, the higher frequency surround gratings are more detectable. As in Expt 1, the changes in detection rates cannot be due to overall shifts in criterion because of the crossing of the curves in Fig. 5. As above in Expt 2, the shapes of the curves are not significant since the contrasts were set independently for each spatial frequency and they do not assure equivalent detectability across spatial frequency. Figure 7(d) shows the difference in hit rate, expressed as Az(Hit), for the surround gratings between conditions in which the central targets were near 0.5 and 4 c/deg. The negative slopes of the difference curves corroborate the observation made above that low frequency surround gratings are more detectable when the central task is frequency discrimination at low frequencies and higher frequencies become more detectable with frequency discrimination at high frequencies. There is an effect of spatial frequency on detectability in this frequency discrimination experiment unlike the results of Expt 2 which used nearly identical stimuli but involved orientation discrimination.

Experiment 4: Central spatial .frequency discrimination with detection (?f surround gratings that vary in orientation The preceding experiments demonstrate that foveal discrimination of orientation has an orientation-specific effect on detectability in the surround (Expt 1) and discrimination of spatial frequency has a frequencyspecific effect (Expt 3). There is no systematic effect of spatial frequency when the central task is orientation discrimination (Expt 2). This fourth experiment examines whether there is symmetry in the effects of spatial frequency and orientation (i,e. no effect of Gabor orientation when the central task is frequency discrimination). This will establish whether the changes in the detectability of the surround gratings observed in Expt 1 were due to the fact that an observer had to concentrate on orientation to perform the discrimination or merely due to the presence of that orientation in the target. The observer's primary task was to make spatial frequency discriminations with Gabor patches having spatial frequencies near 1 c/deg. The Gabor targets were oriented at either vertical or - 60 deg from vertical (fixed at one orientation within a session). The surround grating had a spatial frequency of 1.0 c/deg. Figure 6 shows the average hit rate for detection of the surround grating when the central targets are vertical (solid lines) or - 6 0 d e g from vertical (dotted lines). When the central targets are near vertical, the hit rate for vertical surround gratings is significantly higher than when the targets are - 60 deg from vertical. Conversely, when the

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targets are - 6 0 deg from vertical there is a significant increase in the detectability of surround gratings at this orientation relative to the condition with vertical targets. In addition to the differences in hit rate associated with the change in orientation of the central targets, there is a detection bias for vertical which is small for two of the observers and relatively large for the third. The difference in z(Hit) between the conditions in which the central targets are near vertical or - 60 deg from vertical is shown in Fig. 7(c). As in the results for Expt 1 [Fig. 7(a)], the largest differences in the hit rates are seen at the surround grating orientations nearest the orientations of the Gabor targets used in the discrimination task. Comparison of the data in Fig. 6 with those in Fig. 3

628

A N D R E W F. ROSSI and MICHAEL A. PARADISO

reveals that overall the detection rates are lower, for the identical surround gratings, when the central task is spatial frequency discrimination compared to orientation discrimination. For observer AT this difference might be due to a change in criterion since the lower hit rates in Fig. 6 are accompanied by lower false alarm rates (in comparison to Fig. 3). However, observers AR and BM actually have higher false alarm rates in Fig. 6, indicating that criterion changes are not the sole basis for the overall higher detection rates in Expt 3.

Control experiments: Effects of presence and attention to central stimuli The experiments described above demonstrate that under some conditions there are feature-specific changes in the detectability of peripheral gratings. For example, in Expt 1 vertical surround gratings are most detectable when foveal orientation discriminations are made near vertical and oblique surround gratings are most detectable when the foveal discriminations are made near a similarly oblique angle. We were interested in whether the relatively greater detection rate at one orientation results from an increase in sensitivity at that orientation, a decrease in sensitivity at the other orientations, or a combination of these effects. To address this point we conducted control experiments in which the same sur-

round gratings were used as in Expts 1-4, but there were no central targets. The hit rates obtained in these control experiments are shown in Fig. 8 ( × symbols) where they are compared to the hit rates obtained in Expts 1-4. Interestingly, in most cases the detection rates in the absence of the central targets are lower than the hit rates when the targets are present (solid symbols). In the experiments in which the surround gratings varied in orientation [Fig. 8(a,c)] the hit rates are higher when there are central targets despite the fact that the false alarm rates are lower. This implies that the better performance obtained when there are central stimuli is not a consequence of a simple criterion shift. The changes in the shapes of the curves, with and without the central stimuli, suggest that the basis for the orientation tuned effects is an overall increase in sensitivity when the central targets are introduced, plus particular enhancement near the orientation being discriminated. Somewhat different results are seen in the experiments in which grating frequency was varied [Fig. 8(b,d)]--both the hit rates and the false alarm rates tend to be higher when the central task is present. This opens the possibility that shifts in criterion might account for some part of the improved performance when the central stimuli are introduced. In a second control experiment we examined the

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F l G U R E 8. Summary of results from two control experiments compared with results from Expts 1 4. Solid symbols show the detection rates for the surround gratings in Expts 1-4 [(a) Expt 1, (b) Expt 2, (c) Expt 4, (d) Expt 3]. x s show detection rates from control experiments in which there were no central targets. Open symbols show the detection rates from control experiments in which observers saw the same central and peripheral stimuli as in Expts 1 4 but they did not make discriminations with the central targets [(a) squares, central targets near 0 deg; triangles, targets near - 6 0 deg; (b) squares, targets at 0.5 c/deg; triangles, targets at 4.0 c/deg; (c) squares, targets at 0 deg; triangles, targets at - 6 0 deg: (d) squares, targets near 0.5 c/deg; triangles, targets near 4.0 c/deg].

question of whether the effects seen in Expts 1 4 require that the observer attend to and discriminate the central targets. To answer this question, we repeated the four experiments with exactly the same center and surround stimuli originally used. However, in the control experiments the subjects were instructed to perform the surround detection task when prompted, but to simply fixate and not make discriminations with the central targets. We find that when central Gabor patches are presented near 0 or - 60 deg one does not see the hit rate for the surround gratings significantly biased toward these orientations as they are when central discriminations are made [Fig. 8(a), open symbols]. Similarly, when the central targets are near 0.5 or 4.0 c/deg, detectability for the gratings is not biased toward these frequencies [Fig. 8(d)]. These findings indicate that the feature-specific effects observed in Expts 1--4 depend on the performance of a discrimination task with the central targets, not simply their presence. Comparing the curves for data collected with and without the central discriminations, one sees that in most cases the effect of adding the central task is to increase

sensitivity (i.e. the open-symbol curves in Fig. 8 lie below the solid-symbol curves). The exception to this observation concerns the experiment in which the surround grating varied in orientation and central spatial frequency discriminations were made. In that case, the hit rates generally decrease when the central discriminations are made. Importantly, in the three experiments in which hit rates increased when discriminations were made with the central targets, the false alarm rates decreased. This suggests that the changes are not due simply to a difference in criterion. In the fourth experiment [Fig. 8(c)] the results are the opposite of the other three in that the hit rates are lower and the false alarm rates higher when discriminations are made with the central targets. This also is inconsistent with a change in criterion, but it implies that the central task had a different effect on detection in the surround than in the other experiments. Comparing the data obtained in this second set of control experiments with those obtained in the previous control, in which there were no central targets, it is consistently found that hit rates are higher with, than without, the central targets. Usually hit rates are higher

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still when discriminations are made with the central targets. DISCUSSION The results of the four main experiments show that the detectability of peripheral gratings is clearly influenced by the orientation and spatial frequency of foveal targets, as well as the nature of the foveal discrimination task being performed. When the central task is orientation discrimination with targets near vertical, surround gratings near vertical are relatively more detectable than gratings at other orientations. Likewise, when the discriminations are performed near an oblique angle, sensitivity is highest for oblique gratings. Thus it appears that sensitivity is related to the similarity of features in the central targets and the surround gratings. However, there is no enhanced sensitivity for surround gratings with the same spatial frequency as the central targets when the central task is orientation discrimination. Only when the central task is spatial frequency discrimination, is there a specific effect of frequency such that surround gratings are most detectable when they have a spatial frequency near that of the central targets. Based on these findings, one might expect that there would be no orientation specificity when the central task is frequency discrimination just as there is no frequency specificity when the central task is orientation discrimination. This is not the case. Even when the central task does not explicitly involve orientation, we find that surround gratings are more detectable when they have an orientation similar to that of the central stimuli. This may imply a special status for orientation, a possibility that is discussed below.

Accounts of the findings not involving attention The original goal of our experiments was to determine whether one can selectively attend to particular features of visual stimuli and, if so, whether this affects contrast sensitivity. However, there are other possible accounts of the data which must be considered before reaching the conclusion that attention is involved. Those that seem to be particularly important are spatial interactions not involving attention, subthreshold summation, and effects of shifts in criterion. The feature-specific effects we observed might be due to interactions between visual filters activated by the central and surround stimuli. Perhaps relevant to this point, there have been demonstrations with spots of light (Westheimer, 1967), letters (Bouma, 1970), and most recently G a b o r patches (Adini & Sagi, 1992; Polat & Sagi, 1993), showing that sensitivity at one location on the retina is influenced by the presence of stimuli at other locations. Through subthreshold summation it might be possible that the central Gabor stimuli we used affected the detection thresholds for the surround gratings in a feature-specific manner. Polat and Sagi (1993) used stimuli most similar to those in our study and they showed clear spatial interactions affecting threshold. They demonstrated that a Gabor patch at one location

can decrease or increase the detection threshold for a Gabor patch at another location and that this interaction is generally strongest when the patches have the same spatial frequency and orientation. Although they used higher spatial frequencies than those in our experiments, it is conceivable that the sort of interaction they observed could underlie the results of Expts 1 and 3 in which orientation-specific and spatial frequency-specific effects were seen. However, for several reasons, Polar and Sagi's "non-attentional" spatial interactions combined with subthreshold summation do not seem to be a viable explanation for our results. Most importantly, if the spatial interactions are based solely on the attributes of the central and surround stimuli, the results should not depend on whether discriminations are being made with the central stimuli. Yet, we find that the orientation- and frequency-specific effects seen in Expts 1-4 do depend on whether discriminations are made. Figure 8 shows that when identical stimuli in both the center and surround are used, the increases and decreases in sensitivity seen in Expts 1-4 are lost if the observer fixates the central stimuli without making discriminations. Evidently, attending to, and making discriminations with, the central targets is critical. In a similar vein, spatial interactions between center and surround which do not depend on attention should not be affected by which feature of the central stimuli is being discriminated. However, a comparison of the results of Expts 2 and 3 shows that changing the central task from orientation to spatial frequency discrimination produces significant differences in sensitivity even though the stimuli were nearly identical in the two experiments (see Methods for details of the stimulus differences). In summary, since we observe sensitivity changes not based simply on the spatial frequency and orientation of the stimuli, the changes are probably not the result of spatial interactions and subthreshold summation. Another possible account of the data is that there were differences in hit rate for the surround gratings in different situations because of shifts in the observers' criterion. For instance, in Fig. 4 observer AT has a higher hit rate when the Gabor targets are at 0.5 c/deg than 4.0 c/deg, and this is accompanied by an increase in the false alarm rate. It is quite likely that these changes resulted from a shift in criterion. The critical question is whether the hit rate changes in Expt 1 (Fig. 3) and Expt 3 (Fig. 5), which depend on the orientation and spatial frequency of the central targets, are based on shifts in criterion. Additionally, one wants to know whether the differences seen in comparing the results of Expt I with Expt 4 and particularly Expt 2 with Expt 3 result from criterion changes. To be clear, one must distinguish two different types of criterion change which are possible. The first possibility is that there are orientation non-specific and frequency non-specific changes. This is what we see in the data for observer AT in Fig. 4: across all spatial frequencies there is an increase in hit rate associated with a higher false alarm rate. However, such non-specific

FEATURE-SPECIFIC VISUAL ATTENTION

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T A B L E 1. C o m p a r i s o n of the hit and false alarm rates (percent and z-scores) for detection of a 0.5 c/deg s u r r o u n d grating with different central tasks and G a b o r target frequencies

Central task Spatial frequency discrimination Orientation discrimination

P z P z

0.5 c/deg G a b o r targets

4.0 c/deg G a b o r targets

Hit

False alarm

Hit

False alarm

P = 7 (2.1) z = - 1 . 5 (0.2) P = 7 (2.1) z = - 1 . 5 (0.2)

P = 55 (3.4) _- = 0 . 1 3 (0.1) P = 62 (3.9) z =0.31 (0.1)

P = 6 (2.2) z = 1.6 (0.2) P = 9 (1.7) z = - 1 . 3 (0.1)

= 76 (3.1) =0.71 (0.1) = 65 (6.2) = 0 . 3 9 (0.2)

Standard errors are s h o w n in parentheses.

effects clearly cannot account for the significant aspects of the data in Expts 1-4. In almost all cases in Figs 3-6 it is not possible for an overall shift in criterion to account for the differences between the dotted and solid lines because the curves associated with the featurespecific changes in hit rate cross each other. The only way that the key results could be based on criterion shifts is if these shifts are a function of spatial frequency or orientation. It is not clear that such featuretuned shifts could occur in experiments in which the different conditions are randomly interleaved. Nonetheless, to prove that feature-tuned criterion shifts do or do not underlie the sensitivity changes observed in the experiments, one must make a point-by-point comparison of hit and false alarm rates. Because of the procedure we used, in which a full range of orientations or spatial frequencies was randomly interleaved, we have false alarm rates for each curve in Figs 3-6 rather than for each point. Since these data are insufficient for testing for feature-tuned criterion shifts, we re-collected data for several key conditions, obtaining hit and false alarm rates for each point. The most interesting aspect of the data in the main experiments is the difference in the detection rates as a function of spatial frequency when the central task is orientation (Expt 2) or spatial frequency (Expt 3) discrimination. Therefore, we separately measured the hit and false alarm rates for the detection of a 0.5 c/deg surround grating for two central target frequencies (0.5 and 4.0c/deg) and the two discrimination tasks (orientation and spatial frequency). The results are shown in Table I. These data make two important points. First, they confirm the finding from Expt 3 that when the central task is frequency discrimination the hit rate for a 0.5 c/deg surround grating is significantly higher when the central targets are at 0.5 c/deg than 4.0 c/deg (76% vs 55%). The difference in z(Hit) is more than 2 SEs greater than the difference in z(False Alarm) suggesting that the difference in de-

tectability cannot be accounted for by a change in criterion. Second, the difference in the hit rates seen when the central task is frequency discrimination is lost when the central task is orientation discrimination [Az(Hit) is less than Az(False Alarm) in this case]. Importantly, the hit rate with central targets at 0.5 c/deg changes significantly when the central task changes even though the false alarm rate stays the same. These findings indicate that the important aspects of the feature-specific changes in sensitivity seen in Expts 2 and 3 are not due to criterion shifts, even if it were possible for such shifts to occur independently at different spatial frequencies. The role of visual attention Since it appears likely that the changes in detection rates observed in Expts 1-4 were not due to criterion shifts or "passive" effects such as subthreshold summation, we hypothesize that they result from changes in visual attention. Presumably, an observer attends to the central targets, specifically the feature being discriminated, and this attention influences sensitivity for the surround gratings. The experimental procedure incorporated a number of measures designed to maintain the observers' attention strictly on the central targets rather than the surround gratings. First, the offset for the central discriminations was chosen to make the task difficult enough to require the subject to vigilantly attend to the central stimuli. The offset was set at the value which yielded 75% correct discriminations in preliminary trials without the surround detection task (see Methods for details). If the subject were to shift attention to the periphery, this would degrade performance on the central task. However, we find that the addition of the secondary detection task did not significantly lower the percent correct in the primary discrimination task. For example, in Table 2 we show the percentages of correct orientation

T A B L E 2. C o m p a r i s o n of the percentages of correct orientation discriminations using the central G a b o r patches with and without the addition of the peripheral detection task

Percent correct orientation discriminations with 0.5 c/deg Gabor targets Observer AR AT BM

Percent correct orientation discriminationswith 4.0 c/deg Gabor targets

Withoutsecondary task

With secondary task

Withoutsecondary task

With secondary task

72 75 78

77 75 74

80 70 75

83 84 72

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ANDREW F. ROSSI and MICHAEL A. PARADISO

discriminations of central Gabor targets with and without performance of the peripheral task. The percents correct in the two situations tend to be very similar. The same result was found in the experiments in which the spatial frequencies of the central Gabor patches were discriminated. This suggests that subjects kept their attention on the central task, even when the peripheral task was added. A second feature of the paradigm which held the observers' attention on the central targets was that they had to attend to both central targets (and not shift attention to the periphery after the first target). To accomplish this a procedure was used in which the orientation or spatial frequency of the first target was chosen at random from within a small range. The second target was offset from this random setting (i.e. there was no fixed reference angle or spatial frequency). This procedure requires the subject to attend to both Gabor patches to make the correct discrimination, unlike paradigms with a fixed reference orientation (see Paradiso et al., 1989). Finally, the subject had to make central discriminations on every trial whereas he/she was prompted to indicate whether a surround grating was present on only one-third of the trials. On only half of these detection trials was a grating actually present. The relative infrequency of the peripheral task along with the fact that feedback was only provided for incorrect answers on the central task, should have discouraged subjects from trying to allocate attention to both central and peripheral parts of the display monitor. If the hypothesis is correct that the sensitivity changes seen in the experiments are due to attention, then the attentional influences are based on two different but interacting factors--the attributes of the stimuli and the nature of the task being performed. Our hypothesis is that the discrimination instructions affect the results because it is only when the central task is discrimination of spatial frequency that the observer must attend specifically to this feature. We can offer two possible reasons why Expt 4 showed increased sensitivity for surround gratings having similar orientations to the central target even when the central task was frequency discrimination. One possibility is that orientation has a special status in the sense that it is attended to even when the primary discrimination task does not involve orientation (i.e. orientation is obligatory). A second possibility is that orientation information is utilized, and thus attended to, in the spatial frequency discrimination task. Feature-specific attention If attention is responsible for the sensitivity differences in the experiments, then our results suggest that attention can be feature-specific. This is in addition to the preferential processing of information based on location which has been reported in numerous previous studies of attention (e.g. Eriksen & Hoffman, 1972; Bashinski & Bacharach, 1980; Downing, 1988; Posner, 1980). Our data suggest that it is possible to attend to one feature of a stimulus more than other features and, further, it is possible for there to be preferential processing in a relatively narrow range of the attended feature. Previous

studies derived from several different paths of research have arrived at similar conclusions. For example, a number of studies have demonstrated that observers can selectively attend to one spatial scale over others contained in an image (Davis, 1981; Julesz & Papathomas, 1984; Miller, 1981; Navon, 1977, 1981; Shulman & Wilson, 1987), though this point has been disputed (Gellatly, 1983). The data most closely related to our experiments is contained in the work of Shulman and Wilson (1987). In their primary task, attention was drawn to either the local or global information in stimuli which consisted of large letters of the alphabet composed of many copies of a second letter at a smaller scale. The subject categorized either the large or small letters and there was a secondary task to detect full-field low or high spatial frequency gratings. They found that detection of low frequency gratings was facilitated when the subjects categorized the large central letters and detection of high frequency gratings was facilitated when categorizations were made with the small central letters. Our findings are consistent with those of Shulman and Wilson in that both studies show a frequency-specific effect on detection of sinusoidal gratings. Our findings extend the previous research in a number of important directions. First, the use of Gabor targets in our central task allows us relatively good control over the spatial frequency content of the stimuli being attended to. Consequently, we were able to directly compare the spectrum of the central stimuli with the sensitivity changes in the surround task and demonstrate that they are at the same frequencies. Thus, the effects of attention can be concentrated on narrow spatial frequency bands rather than simply local or global scales (see also Davis, 1981). For the sake of interpreting the results we also feel that it is advantageous that our central and surround stimuli did not overlap as in other studies, making it less likely that aftereffects were involved. Our results also suggest that the specific effects of attention may extend to orientation. Since orientation-biased results were obtained in both Expt I and 4, we cannot conclude that they were definitely the result of attention to the central stimuli. However, this interpretation is worth considering in light of the spatial frequency results. We are presently conducting experiments to determine whether the same experimental paradigm can be used to test for attention to other stimulus attributes such as color. Finally, our experiments suggest that one can attend to one feature over another (e.g. orientation over spatial frequency in Expts 1 and 2) even when they are attributes of the same stimulus. Can attention to features be dissociated)Crom attention to location? A spotlight metaphor is frequently used to describe attention because of the many demonstrations that attention can be allocated on the basis of location. Moreover, partial-report experiments which examined the question of whether attention can be allocated on the basis of stimulus attributes such as color and shape

FEATURE-SPECIFIC VISUAL ATTENTION

concluded that attention is fundamentally tied to location (Nissen, 1985; von Wright, 1970). Nonetheless, there are alternative theories of attention in which selection by color or shape is not qualitatively different from selection on the basis of location (Bundensen, 1991; Duncan, 1980). An intriguing aspect of our results is that they hint at the idea that attention to features might be dissociated from attention to location. This suggestion derives from the structure of our stimuli in which the central targets and surround gratings were nonoverlapping and had a mean-luminance annulus separating them. The results show that the features of the central stimuli and the nature of the central task affect the detectability of the surround grating despite the fact that they are non-overlapping. This is also a difference between our study and that of Shulman and Wilson (1987) because their gratings filled their display, including the area where the targets were presented. The question is whether in our experiment there is a featurespecific attentional effect outside the central spotlight of attention. Even though there was a gap between the central target and the surround gratings, one cannot be sure that the peripheral effect is outside the spotlight since it is possible that the spotlight extends from the center outward including a portion of the surround. Relevant to this point, Eriksen and St James (1986) have shown not only that the size of the attentional spotlight can be changed, but also that there is a gradient of attention falling off at the edges of the attended area. The principal reason for suspecting that attention was focused only on the central task in our experiments is that the discrimination performance for the central task for trials that included the surround detection task was no worse than in trials without a surround task (Table 2). Thus there is no indication that attention was being diverted from the center to the surround, and subjects may well have been focusing their attention entirely on the central targets. Given the uncertainty which presently exists about the extent to which attention can be diffusely allocated or split, it is not possible to reach a definite conclusion. However, the suggestion that attention to features can be separate from attention to location deserves further study.

Physiological evidence for feature-specific modulation of neuronal responses There is considerable evidence that the responses of neurons in some visual cortical areas are affected by the behavioral task an animal is performing (Artim & Bridgeman, 1989; Fuster, 1990; Goldberg & Bruce, 1985; Haenny & Schiller, 1988; Haenny, Maunsell & Schiller, 1988; Maunsell, Nealey, Sclar & DePriest, 1991; Mountcastle, Andersen & Motter, 1981). Results from a number of these studies demonstrate feature-specific modulation of neuronal responses. For example, in the experiment conducted by Haenny and Schiller (1988), rhesus monkeys were trained to perform an orientation match-to-sample task. After presentation of a sample stimulus, the animal viewed a succession of test stimuli and made a behavioral response when a match to the

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sample occurred. For many neurons in area V4 they found that the response to a particular test stimulus depended on which sample preceded it. In other words, the cell's response to a stimulus with a given orientation depended on whether that was the orientation being "looked for". In other studies (e.g. Haenny & Schiller, 1988; Spitzer, Desimone & Moran, 1988) it has been reported that orientation tuning curves for individual neurons are narrower and have a larger peak response when a stimulus has behavioral relevance. Haenny and Schiller (1988) suggested that the changes in tuning curves which they observed might be the result of a selective feedback mechanism from higher cortical areas capable of modulating the activity of cells that respond to features useful in the performance of a particular task. Obviously, these physiological results cannot establish a causal relationship between our psychophysical results and the activity of neurons in visual cortex. However, the finding of state-dependent changes in neuronal responses can be taken as supportive evidence for the idea that the changes in perceptual sensitivity, which we observed, may result from attention to a specific task and a specific feature. Furthermore, if stimulus detection is based on activity "early" in the visual system, then our results suggest that attention has an effect at this early stage. In this regard it may be relevant that Motter (1993) has recently found evidence of attentional modulation of neural responses in area V1.

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Acknowledgements--This research was supported by a grant from the National Eye Institute (EY09050). The authors wish to thank Dr Denis Pelli for helpful comments on the manuscript.