Functional MRI reveals spatially specific attentional modulation in human primary visual cortex

Abstract

Selective visual attention can strongly inf luence perceptual processing, even for apparently low-level visual stimuli. Although it is largely accepted that attention modulates neural activity in extrastriate visual cortex, the extent to which attention operates in the first cortical stage, striate visual cortex (area V1), remains controversial. Here, functional MRI was used at high field strength (3 T) to study humans during attentionally demanding visual discriminations. Similar, robust attentional modulations were observed in both striate and extrastriate cortical areas. Functional mapping of cortical retinotopy demonstrates that attentional modulations were spatially specific, enhancing responses to attended stimuli and suppressing responses when attention was directed elsewhere. The spatial pattern of modulation reveals a complex attentional window that is consistent with object-based attention but is inconsistent with a simple attentional spotlight. These data suggest that neural processing in V1 is not governed simply by sensory stimulation, but, like extrastriate regions, V1 can be strongly and specifically inf luenced by attention.It has long been appreciated that selective attention can dramatically affect high-level visual perception (1). More recently, attention has been shown to influence low-level visual phenomena such as luminance detection (2, 3), motion perception (4, 5), orientation discrimination (6), contour detection (7), hyperacuity (6), and even "preattentive" visual search (8). These modulations of perception appear to result from selective spatial attention, because they depend on the location of directed attention. These studies exploited the fact that attention and eye position need not be directed to the same location; that is, attention may be covert (2). Under the same fixation conditions attention may be either directed toward a test stimulus, directed elsewhere, or not directed. Striking differences have been revealed when performance under directed attention is compared with performance when attention is engaged in a highly distracting task, such as identifying letters in a rapid serial visual presentation (RSVP) stream (5, 8, 9).These phenomena indicate that attention operates at low levels of visual processing, but they do not identify the specific cortical areas in which processing is influenced by attention. This question of the locus of selection is fundamental to the cognitive neuroscience of attention. Theories suggest that processing of the attended representation is enhanced and͞or that processing of unattended representations is suppressed. Enhancement and suppression may act directly on cells in the lower tiers of visual cortex that code retinotopic location or may act at higher cortical areas (10). One long-held theory, the spotlight model of spatial selection (2, 11), suggests that attention is directed to a connected visual field region that contains no topological holes. A competing theory, objectbased selection (12), suggests that attention is directed to ...

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