Resolving the projecjion of an occluded stimulus on the human cortical surface

The human visual system is capable of tracking multiple visual targets under a variety of task constraints and configurations. For nearly two decades, the psychophysical literature has shown that moving, occluded visual targets -- targets that are momentarily invisible as they pass behind an occluding bar -- are differentially represented by the visual system compared to their moving, non-occluded counterparts. Here, I sought to examine the neurophysiological basis of this behavioral difference in response to occluded versus non-occluded visual targets. I used brain imaging to conduct modern retinotopic mapping experiments in human participants. Once· their early visual cortices were mapped, I was able characterize the neural representations for both targets and distractors as well as during moments of occlusion and non-occlusion. The results show that, using our method, we can distinguish visual targets from distractors; furthermore, there appears to be a representation in retinotopically organized early visual cortex for visual targets that have momentarily disappeared from the visual field due to occlusion

Electrophysiological Advances on Multiple Object Processing in Aging

EEG research conducted in the past five years on multiple object processing has begun to define how the aging brain tracks the numerosity of the objects presented in the visual field for different goals. We review the recent EEG findings in healthy older individuals (age range: 65-75 years approximately) on perceptual, attentional and memory mechanisms- reflected in the N1, N2pc and Contralateral Delayed Activity components of the EEG, respectively- during the execution of a variety of cognitive tasks requiring simultaneous processing of multiple elements. The findings point to multiple loci of neural changes in multi-object analysis, and suggest the involvement of early perceptual mechanisms, attentive individuation and WM operations in the neural and cognitive modification due to aging. However, the findings do not simply reflect early impairments with a cascade effect over subsequent stages of stimulus processing, but in fact highlight interesting dissociations between the effects occurring at the various stages of stimulus processing. Finally, the results on older adults indicate the occurrence of neural overactivation in association to good levels of performance in easy perceptual contexts, thus providing some hints on the existence of compensatory phenomena that are associated with the functioning of early perceptual mechanisms

The cognitive neuroscience of visual working memory

Visual working memory allows us to temporarily maintain and manipulate visual information in order to solve a task. The study of the brain mechanisms underlying this function began more than half a century ago, with Scoville and Milner’s (1957) seminal discoveries with amnesic patients. This timely collection of papers brings together diverse perspectives on the cognitive neuroscience of visual working memory from multiple fields that have traditionally been fairly disjointed: human neuroimaging, electrophysiological, behavioural and animal lesion studies, investigating both the developing and the adult brain