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Abstract:
Perceptual closure refers to the process whereby fragmented
and unidentifiable images are completed and identified in the
mind's eye. High-density ERP recordings (64-channels) were used to
investigate the neural structures responsible for perceptual
closure. We tracked the process of perceptual closure by presenting
pictures (350 unique pictures) of familiar objects, initially in a
highly fragmented form and thereafter becoming progressively less
fragmented through eight distinct levels. Images were presented for
750 ms, followed 800 ms later by a prompt to make a forced choice
button push response (ID or no-ID). A no-ID response resulted in
the next most complete level being presented. After an ID response,
subjects were required to name the picture. We compared the ERP to
pictures when they were identified ("closed") versus the ERP to the
level preceding ID. Perceptual closure was accompanied by a marked
enhancement of the N2 component over occipital cortex. This
enhanced negativity had a unilateral right hemispheric onset at
~200 ms (during the peak of the P2), then became bilateral by ~230
ms, peaking at ~275 ms. Initial inspection of this negativity over
successive levels of fragmentation suggests that it builds in
amplitude progressively as subjects approach the level of ID. Scalp
current density (SCD) mapping of this negativity suggests that
bilateral extrastriate visual areas are involved in the perceptual
closure process. Subsequent to this occipital negativity, a
sustained fronto-central positivity was evident in the ID trials
versus the trials immediately preceding ID, perhaps indicative of
frontal executive access to identification information.
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