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Mazyar Fallah, Gene R Stoner, John H Reynolds; Competitive selection of superimposed stimuli moving through space. Journal of Vision 2003;3(9):37. doi: 10.1167/3.9.37.
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© ARVO (1962-2015); The Authors (2016-present)
Single unit studies in monkey extrastriate cortex have identified competitive circuits that could conceivably mediate either spatial or object-based attention. These studies have used stimuli at separate locations, confounding these two possibilities. We recorded responses of V4 neurons to stimuli that were spatially superimposed and could not, therefore, be selected by a purely spatial mechanism. We presented in the neuronal receptive field (RF) patterns of dots that rotated rigidly in opposite directions around a common center, yielding the percept of superimposed transparent surfaces. One set of dots was of the neuron's preferred color and the other was of an equiluminant non-preferred color. Psychophysical studies using these stimuli have found that when one surface is cued by delaying its onset relative to the other surface, human observers can more accurately judge brief changes in the motion of the new than of the old surface. This effect of cueing lasts for approximately 600 milliseconds following the onset of the new surface. This result would be expected if competition were biased in favor of neurons selective for the new surface. V4 neurons were driven preferentially by the new surface for approximately the same period of time as the perceptual effect. Object-based selection should survive moving the superimposed surfaces through space. We found that the cueing effect survived when cueing occurred outside the RF and the two superimposed surfaces then moved into the RF. These results show that competitive circuits in V4 are not limited to mediating competition between spatial locations, and that selection occurring outside the RF persists when the selected stimulus then moves into the RF.
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