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The true receptive field of more than 90% of neurons in the middle temporal visual area (MT) extends well beyond the classical receptive field (crf), as mapped with conventional bar or spot stimuli, and includes a surrounding region that is 50 to 100 times the area of the crf. These extensive surrounds are demonstrated by simultaneously stimulating the crf and the surround with moving stimuli. The surrounds commonly have directional and velocity-selective influences that are antagonistic to the response from the crf. The crfs of MT neurons are organized in a topographic representation of the visual field. Thus MT neurons are embedded in an orderly visuotopic array, but are capable of integrating local stimulus conditions within a global context. The extensive surrounds of MT neurons may be involved in figure-ground discrimination, preattentive vision, perceptual constancies, and depth perception through motion cues.

We stimulated with microelectrodes the face representation in precentral motor cortex in macaque monkeys. Responses were very discrete; at threshold current levels the usual response was a small focus of movement in part of a muscle. Facial muscles cluster together in the posterior and anterior portions of the precentral gyrus with tongue movements represented in the intervening region and along the lateral extent. Within each cluster there are multiple representations of individual muscle movements. In long penetrations down the anterior wall of the central sulcus we were able to advance the electrode tangentially through cortex. In these penetrations we encountered a series of discrete zones each of which was related to the movement of a particular muscle or part of a muscle in the face. The lowest threshold points were found in the center of each zone, and as the microelectrode progressed toward the edge, thresholds rose until there was a shift to a new muscle movement. Successive stimulation points separated by as little as 50 micrometer could yield different responses. These zones could be either roughly cylindrical or take the form of narrow curving bands running mediolaterally across cortex. There is a tendency for adjacent muscles to occur together, and the representation may be roughly topographical within the limits set by the morphological structure of the muscles themselves. The most commonly evoked muscle response was in zygomaticus, which retracts the corners of the mouth in expressions of fear and anger.

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