Center-surround interactions of Group I and Group II cells (Winters, Hickey and Pollack, 1973) were studied by simultaneously presenting a small spot in the center of a unit's receptive field and an annulus in the receptive field periphery. The responses to this target configuration were compared to those elicited by the center spot alone and those elicited by presenting an annulus alone. Group I cells gave a transient response to a maintained central spot while Group II cells gave a sustained response for the same stimulus. Both Group I cells and Group II cells gave on-off responses to a center spot plus peripheral annulus but the on portion of the response was much weaker in Group II cells than in Group I cells, indicating greater peripheral suppression in these units. Linear summation of central and peripheral responses could be demonstrated for Group II cells but not Group I cells. © 1973.
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