Abstract
Purpose:
Object perception depends not only on physical stimulus properties but also on endogenous, top-down factors that affect the observer’s perceptual state. In this study we equated physical properties to investigate the neural mechanisms of top-down contributions in object perception.
Methods:
We recorded EEG and compared neural activity elicited by varying perceptions of the same physical image - a bistable moving image in which perception spontaneously alternates between dissociated fragments and a single, unified object.
Results:
A time-frequency analysis of EEG changes associated with the perceptual switch from object to fragment and vice versa revealed a greater decrease in alpha band (8-12Hz) power accompanying the switch to object-percept than to fragment-percept. Recordings of event-related potentials elicited by irrelevant probe flashes superimposed on the image revealed an enhanced positivity in the latency range of the P2 component (~184-212ms) when the probes were contained within the perceived unitary object. The topography of this positivity elicited by probes during object- relative to fragment-perception was distinct from the topography of the P2 elicited by probes during fragment perception, suggesting that neural processing of probes differed as a function of perceptual state. Two source localization algorithms estimated the neural generator of the difference positivity to lie in the lateral occipital cortex (LOC), a region associated with object perception.
Conclusions:
These data suggest that objects attract attention and modulate the processing of individual elements occurring within their boundaries, perhaps reflecting the perceptual binding of the elements into a unified object. Importantly, these effects were observed when the perceived "object" in this case emerged as a function of the fluctuating perceptual state of the viewer.