Mutual contrast effects of perceived average size between groups of objects: Neural and behavioural evidence

The human visual system simultaneously and accurately computes summary statistics of object features, such as average size or average orientation, across groups of objects. Evidence suggests that summary statistics across groups are not independent but are influenced by one another. In this study, we examined whether the average size of one set of objects affects the perceived size of another set, and whether such changes are reflected in altered functional activation patterns in retinotopic regions of the visual cortex. The average size of a task-irrelevant group of objects altered the perceived size of a task-relevant group in a contrast-like manner: When the task-irrelevant set was larger, participants perceived the average size of the task-relevant set as smaller; conversely, a smaller task-irrelevant set led to a larger perceived average size in the task-relevant set. A greater number of activated voxels in retinotopic brain regions representing task-relevant sets perceived as larger reflected these perceptual contrast effects. Coding of task-irrelevant sets was similarly modulated suggesting that their perceived average size was also altered. Overall, our data indicate that summary statistics of different object groups are contrasted and that these interactions alter neural representations in early visual cortex.