Sang Chul Chong

Representation of statistical properties.

Everyday scenes often contain sets of similar objects. Perceptual representations may summarize these with statistical descriptors. After determining the psychological mean of two sizes, we measured thresholds for judging the mean with arrays of 12 circles of heterogeneous sizes. They were close to those for the size of elements in homogeneous arrays and single elements, and were little affected by either exposure duration (50-1000 ms) or memory delays (up to 2s). They were only slightly more accurate within the same distribution than across different distributions (normal, uniform, two-peaks, and homogeneous), confirming that subjects were indeed averaging sizes.

Statistical processing: computing the average size in perceptual groups.

This paper explores some structural constraints on computing the mean sizes of sets of elements. Neither number nor density had much effect on judgments of mean size. Intermingled sets of circles segregated only by color gave mean discrimination thresholds for size that were as accurate as sets segregated by location. They were about the same when the relevant color was cued, when it was not cued, and when no distractor set was present. The results suggest that means are computed automatically and in parallel after an initial preattentive segregation by color.

Attentional spread in the statistical processing of visual displays

We tested the hypothesis that distributing attention over an array of similar items makes its statistical properties automatically available. We found that extracting the mean size of sets of circles was easier to combine with tasks requiring distributed or global attention than with tasks requiring focused attention. One explanation may be that extracting the statistical descriptors requires parallel access to all the information in the array. Consistent with this claim, we found an advantage for simultaneous over successive presentation when the total time available was matched. However, the advantage was small; parallel access facilitates statistical processing without being essential. Evidence that statistical processing is automatic when attention is distributed over a display came from the finding that there was no decrement in accuracy relative to single-task performance when mean judgments were made concurrently with another task that required distributed or global attention.

Endogenous attention prolongs dominance durations in binocular rivalry

We investigated the effects of attention on dominance durations during binocular rivalry. In a series of three experiments, observers performed several tasks while viewing rival stimuli to ensure and control deployment of attention. We found that endogenous attention can prolong dominance durations of attended stimulus. We developed a novel single-task procedure where observers responses in an attentional task were used to objectively estimate dominance durations of the attended stimulus. Using this procedure, we showed that paying attention to the stimulus features involved in rivalry is necessary for prolonging dominance durations--mere engagement of attention during rivalry was insufficient. Finally, we were able to simulate the effects of endogenous attention by doubling the contrast of the attended stimulus while it was dominant. Attention may increase the apparent contrast of the attended stimulus, thereby prolonging its dominance duration. Overall, our results indicate that dominance durations in rivalry can be prolonged when observers are performing an attentionally demanding task on the rival stimulus.

Exogenous attention and endogenous attention influence initial dominance in binocular rivalry

We investigated the influence of exogenous and endogenous attention on initial selection in binocular rivalry. Experiment 1 used superimposed +/-45 degrees gratings viewed dioptically for 3s, followed by a brief contrast increment in one of the gratings to direct exogenous attention to that grating. After a brief blank period, dichoptic stimuli were presented for various durations (100-700 ms). Exogenous attention strongly influenced which stimulus was initially dominant in binocular rivalry, replicating an earlier report (Mitchell, Stoner, & Reynolds. (2004). Object-based attention determines dominance in binocular rivalry. Nature, 429, 410-413). In Experiment 2, endogenous attention was manipulated by having participants track one of two oblique gratings both of which independently and continuously changed their orientations and spatial frequencies during a 5s period. The initially dominant grating was most often the one whose orientation matched the grating correctly tracked using endogenous attention. In Experiment 3, we measured the strength of both exogenous and endogenous attention by varying the contrast of one of two rival gratings when attention was previously directed to that grating. The contrast of the attended grating had to be reduced by an amount in the neighborhood of 0.3 log-units, to counteract attentions boost to initial dominance. Evidently both exogenous and endogenous attention can influence initial dominance of binocular rivalry, effectively boosting the stimulus strength of the attended rival stimulus.

Statistical Processing: Not So Implausible after All

Myczek and Simons (2008) have shown that findings attributed to a statistical mode of perceptual processing can, instead, be explained by focused attention to samples of just a few items. Some new findings raise questions about this claim. (1) Participants, given conditions that would require different focused attention strategies, did no worse when the conditions were randomly mixed than when they were blocked. (2) Participants were significantly worse at estimating the mean size when given small samples than when given the whole display. (3) One plausible suggested strategy—comparing the largest item in each display, rather than the mean size—was not, in fact, used. Distributed attention to sets of similar stimuli, enabling a statistical-processing mode, provides a coherent account of these and other phenomena.

The effect of spatial attention on invisible stimuli

The influence of selective attention on visual processing is widespread. Recent studies have demonstrated that spatial attention can affect processing of invisible stimuli. However, it has been suggested that this effect is limited to low-level features, such as line orientations. The present experiments investigated whether spatial attention can influence both low-level (contrast threshold) and high-level (gender discrimination) adaptation, using the same method of attentional modulation for both types of stimuli. We found that spatial attention was able to increase the amount of adaptation to low- as well as to high-level invisible stimuli. These results suggest that attention can influence perceptual processes independent of visual awareness.

On the nature of the stimulus information necessary for estimating mean size of visual arrays

This paper explores the nature of the representations used for computing mean visual size of an array of visual objects of different sizes. In Experiment 1 we found that mean size judgments are accurately made even when the individual objects (circles) upon which those judgments were based were distributed between the two eyes. Mean size judgments were impaired, however, when a subset of the constituent objects involved in the estimation of mean size were rendered invisible by interocular suppression. These findings suggest that mean size is computed from relatively refined stimulus information represented at stages of visual processing beyond those involved in binocular combination and interocular suppression. In Experiment 2 we used an attentional blink paradigm to learn whether this refined information was susceptible to the constraints of attention. Accuracy of mean size judgments was unchanged when one of the two arrays of circles was presented within a rapid serial visual presentation sequence, regardless of task requirement (single vs. dual task) and the arrays time of presentation relative to the brief appearance of a target that was the focus of attention. Evidently the refined stimulus information used for computing mean size remains available even in the absence of focused attention.

Computation of mean size is based on perceived size

The present study investigated whether computation of mean object size was based on perceived or physical size. The Ebbinghaus illusion was used to make the perceived size of a circle different from its physical size. Four Ebbinghaus configurations were presented either simultaneously (Experiment 1) or sequentially (Experiment 2) to each visual field, and participants were instructed to attend only to the central circles of each configuration. Participants’ judgments of mean central circle size were influenced by the Ebbinghaus illusion. In addition, the Ebbinghaus illusion influenced the coding of individual size rather than the averaging. These results suggest that perceived rather than physical size was used in computing the mean size.

Accurate but Pathological: Social Anxiety and Ensemble Coding of Emotion

The current study examined whether social anxiety is associated with a biased perception of the overall emotional impression of facial crowds. Participants were presented with facial crowds that consisted of 6, 12, or 24 faces expressing either happiness or anger, after which they were asked to judge whether the crowds were positive or negative. We estimated each participant’s point of subjective equality (PSE) and precision when judging the overall emotion of facial crowds to be negative. The participants’ social anxiety levels were negatively associated with their PSEs to perceive the overall emotion of the crowds as negative. In contrast, there was no significant relation between social anxiety and the degree of precision in the participants’ judgments of facial crowds. These findings indicate that socially anxious individuals lack the positive biases that are present in non-anxious individuals and, thereby, perceive the overall emotion of crowds more negatively compared with their less anxious counterparts.