Contextual influences on the directional responses of tectal cells in pigeons

Contrary to the traditional view that receptive fields are limited in spatial extent, recent studies have indicated that the response of neurons to a local stimulus within the receptive field can be modulated by stimulation of the surrounding region. Here we quantified the nature of these contextual effects on visual motion responses of neurons in the pigeon's optic tectum using standard extracellular recording techniques. All of the cells tested responded well to a test spot moving across their receptive fields. When a background pattern was moved in the same or in a similar direction as that of the test spot, the responses of most deep tectal neurons to the test spot were maximally inhibited. Movement of the background in the opposite or near opposite direction produced minimal inhibition or even facilitation. For some deep tectal neurons, this directionally selective modulation by the moving background was maintained when the background motion was paired with different movement directions of the test spot (including both the preferred and least preferred directions). Thus, this selectivity for opposing motion was independent of the absolute direction of either the test spot or the background, a finding which is consistent with the results reported by Frost and Nakayama (1983), although they did not include all test spot directions. For some other neurons, identified here for the first time, the background movement selectively modulated the response only when the test spot moved in the neuron's preferred directions. These neurons lost selectivity for opposing motion when the test spot moved in nonpreferred directions. The significance of these contextual effects on the motion response of tectal neurons may be related to how the brain distinguishes self-induced motion from object motion and segregates figure from ground.