Neuronal correlates of optic flow stimulation.

Neurons in a region of monkey extrastriate cortex, MSTd, respond to the components of optic flow stimulation. Some of these neurons (single-component neurons) are selective for a single type of motion such as inward- or outward-radial motion. Other neurons respond to multiple types of rotation, for example, rightward planar, clockwise circular, and inward radial. Rather than forming discrete groups, we think these neurons represent a continuum covering the range from single-component sensitivity to multiple-component sensitivity. By combining the optic flow stimuli, we have also been able to recognize that such combinations alter the response of cells in the continuum to varying degrees. At this point, while our evidence is consistent with the hypothesis that cells in area MSTd contribute to the processing of optic flow stimuli, we do not know whether these neurons do in fact serve this function. As in all single-cell recording experiments, even those in awake animals performing tasks closer to real-world tasks than we have succeeded in emulating here, the activity of the cell in relationship to the visual stimulation is simply a correlate of the optic flow stimulation and may or may not contribute to the processing of optic flow stimulation upon which behavior depends. Further information on a number of characteristics of these cells might clarify their role. Information on such factors as whether heading in the environment is conveyed by individual neurons, or whether this property is more likely to be conveyed over a population of neurons, and the role of changes in the point of fixation of the eyes are critical points. Generation of behavior on the basis of the optic flow stimulation and determination that this behavior is modified by selective lesion of MSTd would also strengthen the argument that visual motion processing in this area is related to analyzing optic flow information.