PubMed · 13987130
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H SCHUCKMAN. 1963. Attention and visual threshold.. https://doi.org/10.1097/00006324-196305000-00005
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Can the brain attend to more than a single location at one time? In this issue of Neuron, McMains and Somers report psychophysical and fMRI evidence showing that subjects can attend to two separate locations concurrently and that divided spatial attention leads to separate zones of attentional enhancement in early visual cortex.
Spatially directed attention strongly enhances visual perceptual processing. The metaphor of the "spotlight" has long been used to describe spatial attention; however, there has been considerable debate as to whether spatial attention must be unitary or may be split between discrete regions of space. This question was addressed here through functional MR imaging of human subjects as they performed a task that required simultaneous attention to two briefly displayed and masked targets at locations separated by distractor stimuli. These data reveal retinotopically specific enhanced activation in striate and extrastriate visual cortical representations of the two attended stimuli and no enhancement at the intervening representation of distractor stimuli. This finding of two spotlights was obtained within a single cortical hemisphere and across the two hemispheres. This provides direct evidence that spatial attention can select, in parallel, multiple low-level perceptual representations.
A question of long-standing interest to philosophers, psychologists and neuroscientists is how the brain selects which signals enter consciousness. Binocular rivalry and attention both involve selection of visual stimuli, but affect perception quite differently. During binocular rivalry, awareness alternates between two different stimuli presented to the two eyes. In contrast, attending to one of two different stimuli impairs discrimination of the ignored stimulus, but without causing it to disappear from consciousness. Here we show that despite this difference, attention and rivalry rely on shared object-based selection mechanisms. We cued attention to one of two superimposed transparent surfaces and then deleted the image of one surface from each eye, resulting in rivalry. Observers usually reported seeing only the cued surface. They were also less accurate in judging unpredictable changes in the features of the uncued surface. Our design ensured that selection of the cued surface could not have resulted from spatial, ocular or feature-based mechanisms. Rather, attention was drawn to one surface, and this caused the other surface to be perceptually suppressed during rivalry. These results raise the question of how object representations compete during these two forms of perceptual selection, even as the features of those objects change unpredictably over time.