NEURONAL RESPONSES IN EXTRASTRIATE CORTEX TO OBJECTS IN OPTIC FLOW-FIELDS

During locomotion, observers respond to objects in the environment tha t may represent obstacles to avoid or landmarks for navigation. Althou gh much is known about how visual cortical neurons respond to stimulus objects moving against a blank background, nothing is known about the ir responses when objects are embedded in optic flow fields (the patte rns of motion seen during locomotion). We recorded from cells in the l ateral suprasylvian visual area (LS) of the cat, an area probably anal ogous to area MT. In our first experiments, optic flow simulations mim icked the view of a cat trotting across a plain covered with small bal ls; a black bar lying on the balls served as a target object. In subse quent experiments, optic flow simulations were composed of natural ele ments, with target objects representing bushes, rocks, and variants of these. Cells did not respond to the target bar in the presence of opt ic flow backgrounds, although they did respond to it in the absence of a background. However, 273/423 cells responded to at least one of the taller, naturalistic objects embedded in optic flow simulations. Thes e responses might represent a form of image segmentation, in that cell s detected objects against a complex background. Surprisingly, the res ponsiveness of cells to objects in optic flow fields was not correlate d with preferred direction as measured with a moving bar or whole-fiel d texture. Because the direction of object motion was determined solel y by receptive-field location, it often differed considerably from a c ell's preferred direction. About a quarter of the cells responded well to objects in optic flow movies but more weakly or not at all to bars moving in the same direction as the object, suggesting that the optic flow background modified or suppressed direction selectivity.