Michele Fornaciai (UMass Amherst) will be presenting “Temporal evolution of visual representation: From physical to perceived numerosity” in the Cognitive Brown Bag Wednesday Dec. 14 – all are welcome.
Time: 12:00pm to 1:15pm  Location:  Tobin 521B.
Abstract. Humans share with many animals a number sense, the ability to estimate rapidly the approximate number of items in a scene. Recent work has shown that like many other perceptual attributes, numerosity is susceptible to adaptation. It is not clear, however, whether adaptation works directly on mechanisms selective to numerosity, or via related mechanisms, such as those tuned to texture density. To disentangle this issue, we measured numerosity adaptation of 10 pairs of connected dots, as connecting dots makes them appear to be less numerous than unconnected dots. Adaptation to a 20-dot pattern (same number of dots as the test) caused robust reduction in apparent numerosity of the connected-dot pattern, but not of an unconnected dot-pattern. This suggests that adaptation to numerosity, at least for relatively sparse dot-pattern, occurs at neural levels encoding perceived numerosity, rather than at lower levels responding to the number of elements in the scene. However, little is known about the processes that make a given percept available in the content of subjective visual awareness, and when such percepts arise in the visual stream. We then exploited the connectedness illusion to investigate when the perceptual representation of numerosity emerges in the visual processing stream. We recorded brain activity by means of electroencephalogram while participants passively viewed a stream of arrays containing 16 or 32 dots, either isolated or pair-wise connected by straight lines. The results showed that the early latency visual evoked potentials (~ 100 ms) reflect physical, rather than perceived, numerosity, while the later latency potentials (~ 150 ms) reflect perceived, rather than physical, numerosity. A multivariate pattern analysis in the time domain confirmed such a pattern and further demonstrated that both the effects of physical and perceived numerosity persist until later latency (~ 400 ms). These results demonstrate that the physical information of a visual scene undergoes a series of manipulations along the visual stream that could radically change its content before being available in the subjective visual experience.