Hochstein S, Ahissar M, 2003, "Cortical sites and mechanisms of two types of visual experience" Perception 32 ECVP Abstract Supplement
Cortical sites and mechanisms of two types of visual experience
S Hochstein, M Ahissar
Everyone is aware of a strange dichotomy of visual experience: rapid 'vision at a glance' and a slower 'vision with scrutiny'. On the one hand, we are able to perceive complex images, objects, categories and scenes even when these are presented briefly or at very rapid rates (RSVP). On the other hand, we are surprisingly unable to report details outside the focus of our attention, large changes in flashed pictures (change blindness), and even the viewpoint of recalled objects. We suggest that bottom - up processing is implicit, and that perceptual consciousness only begins when visual information reaches post-processing high cortical levels. Thus, rapid conscious vision with spread attention is attributed to the large receptive fields of high-level cortical areas, where neurons respond to complex images but generalise over--ie are insensitive to--precise object location, orientation, size, lighting, or the particular details that originally formed the represented category. In contrast, slower perception with focused attention is seen as a conscious return to lower cortical levels following reverse-hierarchy feedback connections to the sources of higher-level activation, recovering detailed information available in the specialised receptive fields found there. The dual correspondence is especially apparent in two types of visual search: spread-attention feature-search (perceiving 'what' without 'where'), deriving from high-level position-insensitive receptive fields, versus scanning-attention or conjunction search deriving from the return to low-level position-encoding receptive fields. Physiological data support this reverse hierarchy theory and the characteristics of the two perceptual modes indeed match those of higher and lower areas, respectively.
[Supported by a 'Center of Excellence' grant from the Israeli Science Foundation of the Israeli Academy of Sciences and Humanities, and by a grant from the US - Israel Binational Science Foundation.]
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