High -density electrical mapping of perceptual closure processes
Object recognition is achieved even in circumstances when only partial information is available to the observer. Perceptual closure processes are essential in enabling such recognitions to occur. In the present studies, sequences of fragmented pictures of common objects were presented, such that information content was incrementally increased until just enough information was present to permit closure and object recognition. High-density event-related potentials (ERPs) were recorded during these sequences, which permitted us to monitor brain activity during the perceptual closure processes leading up to object recognition. In the first study, we discovered a bilateral ERP component (Ncl) that tracks these processes (onsets ∼230 msec, maximal at ∼290 msec). Scalp-current density mapping of the Ncl revealed bilateral occipito-temporal scalp foci, consistent with generators in human ventral visual stream, and specifically the lateral occipital (LO) complex as defined by hemodynamic studies of object recognition. Perceptual closure presumably occurs because repeated and varied exposure to different classes of objects has caused the brain to undergo “perceptual learning,” which promotes a robust mnemonic representation, accessible under partial information circumstances.
The second study examined the impact of perceptual learning on closure-related brain processes. Periodic repetition of a subset of picture sequences was used to induce repetition priming due to perceptual learning. This priming had an electrophysiological signature—enhancement of the occipito-temporal N1 component—that is putatively generated in LO complex, but significantly precedes the electrophysiological correlate of closure. That the earliest priming-related activity occurs over LO complex suggests that the sensory trace itself may reside in these object recognition areas. Schizophrenia patients require significantly more visual information than normal individuals to achieve object recognition from partial information. In the third study, we assessed the neural integrity of perceptual closure processes in schizophrenia patients by the same methodology as the first study. Schizophrenia patients showed profound Ncl-impairment in addition to dramatic reduction in the early parieto-occipital P1 component. In contrast, the intervening occipito-temporal N1 component remained intact. These findings suggest a highly specific pattern of deficits in schizophrenia, in which aberrant dorsal stream gating impacts upon ventral stream processing at a relatively “late” stage in the processing hierarchy.
0989: Physiological psychology