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.

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.

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.

We measured visual-adaptation strength under variations in visual awareness by manipulating phenomenal invisibility of adapting stimuli using binocular rivalry and visual crowding. Results showed that the threshold-elevation aftereffect and the translational motion aftereffect were reduced substantially during binocular rivalry and crowding. Importantly, aftereffect reduction was correlated with the proportion of time that the adapting stimulus was removed from visual awareness. These findings indicate that the neural events that underlie both rivalry and crowding are inaugurated at an early stage of visual processing, because both the threshold-elevation aftereffect and translational motion aftereffect arise, at least in part, from adaptation at the earliest stages of cortical processing. Also, our findings make it necessary to reinterpret previous studies whose results were construed as psychophysical evidence against the direct role of neurons in the primary visual cortex in visual awareness. binocular rivalry ͉ crowding ͉ vision V isual adaptation has been dubbed the psychologist's microelectrode (1) because the resulting visual aftereffects presumably reveal response properties of neural mechanisms that are activated by adapting stimuli. Also, measuring visual adaptation under visual conditions that render the adapting stimulus invisible allows one to draw inferences about the neural concomitants of the conditions that produce stimulus invisibility. Specifically, a result showing a full-strength aftereffect that is generated by an invisible stimulus implies normal, unperturbed neural activation at the site of adaptation. This outcome implies that the neural correlates of the visual phenomenon that are used to render the adapting stimulus invisible lie beyond the neural mechanisms that are responsible for the aftereffect. This line of reasoning has been applied to the study of binocular rivalry and visual crowding, which are two extensively studied visual phenomena that are used to ''erase'' visual stimuli from awareness (2). The results have shown that full-strength pattern and motion aftereffects (MAEs) can be induced even when the high-contrast inducing stimuli were absent from awareness for a substantial portion of the adaptation period during binocular rivalry (3-7) and crowding (8, 9). Because adaptation producing these aftereffects includes neural events that presumably occur within cortical areas ranging from the primary visual cortex (V1) (10-12) to the middle-temporal visual area (12, 13), these psychophysical findings have reasonably been interpreted as evidence for the high-level origin of both rivalry (14, 15) and crowding (8,14). Also, these same results were regarded by some workers as key psychophysical evidence against the direct involvement of V1 neurons in conscious visual awareness (16)(17)(18)(19). Measurement of full-strength aftereffects under conditions of rivalry and crowding shows a clear dissociation between the abolished perceptual awareness of the adapting stimulus and unperturbe...

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 observer's 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.