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Brad Motter; Visual Crowding in Area V4 Neurons is a Stimulus Integration Effect. Journal of Vision 2012;12(9):335. doi: 10.1167/12.9.335.
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The effect of visual crowding within a cortical area V4 neuron’s receptive field (RF) was studied in monkeys trained to discriminate letter-like symbols. The primary task required a fixation of a small dot until it was replaced by a 0.25 deg letter. A discriminative push of a R/L button was required. During fixation stimuli were presented within peripheral (1-8 deg ecc) RFs. The response to a stimulus placed in the center of the RF was examined as a function of the distance to one or two additional stimuli simultaneously presented at flanking positions. Single neurons were recorded with standard electrophysiological techniques. A change in response occurred only when the additional stimuli were within the RF. As the separation between stimuli narrowed within the RF one of three scenarios occurred: the response 1) increased or 2) decreased as the separation decreased, or 3) a decrease in response was rapidly reversed as the stimuli began to merge. The first scenario correlated well with the neuron’s size tuning. Neurons that responded better to larger stimuli, generally summated additional stimuli added to the RF. Neurons that decreased response in presence of flanking stimuli, were usually tuned to stimulus sizes smaller than half the RF size. Rapid reversals of response as stimulus positions merged are consistent with the emergence of a new preferred stimulus. For some neurons the monkeys were required to make a discrimination of targets appearing at the RF center along with the flanking stimuli. Typical visual crowding psychophysical performance was obtained. Attending to the RF stimuli resulted in a general increase in neural activation (relative to an attend-away condition) but did not markedly affect the crowding effect. The change in responsiveness for V4 neurons under visual crowding conditions is best explained as a stimulus integration that produces a less (or more) optimal stimulus configuration.
Meeting abstract presented at VSS 2012
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