Contextual modulation in primary visual cortex of macaques

Recent studies have suggested that V1 neurons extract figures from their ba ckgrounds, in that they respond better to interior features of figures than to equivalent features of background stimuli. This is reportedly true even when the figure boundaries are distant from the borders of the classical r eceptive field (RF). To test the role of V1 neurons in figure-ground segreg ation, we recorded their responses to texture figures on texture background s, centered on the RF. The texture elements of the figures remained identic al across trials, and figure boundaries were defined by orientation differe nces between the elements in the background texture relative to elements in the figure. For nearly all neurons (98/102), responses to a large texture figure did not differ from the responses to a uniform-texture background. A lthough many neurons gave enhanced responses to texture boundaries, this oc curred only when the boundaries were within or close to the RF borders. Sim ilar effects were found in V2. For neurons in V1, the limited spatial exten t of the contextual modulation was not increased either at low stimulus con trast or when the animal was rewarded for detecting an orientation-defined figure. Thus, V1 neurons appear to signal texture boundaries rather than fi gures per se. Unexpectedly, many V1 neurons gave significant long-latency r esponses to texture stimuli located entirely outside the classical RF, up t o 5 degrees from the RF border in some cases. However, these responses did not depend on the stimulus forming a figure that contained the RF. Although V1 neurons are influenced by stimuli outside the classical RF, they do not appear to segregate figures from ground.