Attention alters certainty but not appearance

Does paying attention to a stimulus change its appearance or merely influence the decision mechanisms involved in reporting it? Recently we proposed an uncertainty stealing hypothesis in which subjects, when uncertain about a perceptual comparison between a cued and uncued stimulus, tend to disproportionately choose the cued stimulus. The result is a psychometric function that mimics the results that would be measured if attention actually changed the appearance of the cued stimulus. In the present study, we measure uncertainty explicitly. In three separate experiments, subjects judged the relative appearance of two Gabor patches that differed in contrast. In the first two experiments, subjects performed a comparative judgment, reporting which stimulus had the higher contrast. In the third experiment, subjects performed an equality judgment, reporting whether the two stimuli had the same or different contrast. In the first comparative judgment experiment and in the equality judgment experiment, one of the two stimuli was pre-cued by an exogenous cue. In the second comparative judgment experiment, a decision bias was explicitly introduced: one stimulus was followed by a post-cue and the subjects were instructed, when uncertain, to choose the cued target. In all three experiments, subjects also indicated whether or not they were certain about each response. The results reveal that in the pre-cue comparative judgment, attention shifted the subjects’ uncertainty and made subjects more likely to report that the cued stimulus had higher contrast. In the post-cue biased comparative judgment, subjects also were more likely to report that the cued stimulus had higher contrast, but without a shift in uncertainty. In the equality judgment, attention did not affect the contrast judgment, and the subjects’ uncertainty remained aligned with their decision. We conclude that attention does not alter appearance but rather manipulates subjects’ uncertainty and decision mechanisms.

A note on the equality judgment experiment in Itthipuripat, et al. (2019)

Itthipuripat, Chang, Bong, & Serences (2019) investigated the effects of attention on appearance and decision criteria. Using a series of comparative and equality judgment tasks, they determined that most of the effects of attention could be explained as changes in decision criteria; however, their equality judgment experiment did reveal an effect of attention on appearance at low contrasts. Because their analysis reported Cmax, the contrast at which the equality judgement is maximum, and because Cmax is not independent of potential low-contrast response biases, I decided to reanalyze their equality judgment data. Based on this reanalysis, I was not able to rule out a potential effect of attention on appearance at low contrast. But, due to the small number and asymmetric range of contrasts probed, their experiment was not well designed to test this contrast regime, and I am not confident about their result. For the higher contrasts they tested, the measured effects of attention trended in the opposite direction.

Topology-defined units in numerosity perception

What is a number? The number sense hypothesis suggests that numerosity is “a primary visual property” like color, contrast, or orientation. However, exactly what attribute of a stimulus is the primary visual property and determines numbers in the number sense? To verify the invariant nature of numerosity perception, we manipulated the numbers of items connected/enclosed in arbitrary and irregular forms while controlling for low-level features (e.g., orientation, color, and size). Subjects performed discrimination, estimation, and equality judgment tasks in a wide range of presentation durations and across small and large numbers. Results consistently show that connecting/enclosing items led to robust numerosity underestimation, with the extent of underestimation increasing monotonically with the number of connected/enclosed items. In contrast, grouping based on color similarity had no effect on numerosity judgment. We propose that numbers or the primitive units counted in numerosity perception are influenced by topological invariants, such as connectivity and the inside/outside relationship. Beyond the behavioral measures, neural tuning curves to numerosity in the intraparietal sulcus were obtained using functional MRI adaptation, and the tuning curves showed that numbers represented in the intraparietal sulcus were strongly influenced by topology.