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Brad C. Motter; Crowding and object integration within the receptive field of V4 neurons. Journal of Vision 2002;2(7):274. doi: https://doi.org/10.1167/2.7.274.
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© ARVO (1962-2015); The Authors (2016-present)
Target detection is constrained during active visual search and during letter crowding studies by the proximity of neighboring stimuli to the target. Eccentricity (E) dependent spatial interactions between stimuli occur over a range of ∼0.5 E. Cortical receptive fields (RF) capable of integrating across this range are first encountered in area V4. We investigated the impact of flanking stimuli on the responsiveness of V4 neurons to a stimulus centered in the RF. Fifty-five V4 neurons with RFs centered within 10 degs of the fovea and differentially responsive to a letter-like set of stimuli, 0.75 to 2.5 deg in length, were studied in monkeys. Attention was directed to the fixation point and not at the stimuli. Flanker spacing varied over a 0.0 to 6.4 deg range. Response interactions occurred only when flanking stimuli were within the classic RF. Interactions were not simple summations, and can be classified as either crowding suppression or integrative fusion. Crowding suppression occurred as soon as flanking stimuli entered the RF. The flanker(s) typically suppressed the response below that elicited by the centered stimulus when presented alone. The degree of suppression observed did not correlate with the difference in responsiveness observed for the center and flanking stimuli when presented alone. An integrative fusion, that reduced or reversed the suppression, could often be observed when physically identical flanking and center stimuli were separated by less than 0.5 deg. Sensitivity to stimulus size and shape could limit the degree of fusion observed. Stimuli that differed in color did not fuse. A series of flanking stimuli were chosen such that when they fused with the center stimulus a different member of the letter-like set was formed. Integrative fusions of this subset (spacing < 0.5 deg) could produce dramatic changes in responsiveness. Interactions were translationally invariant within the RF. Supported by the Veterans Affairs Medical Research Program.
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