Neural representation of two -dimensional contours in area V4 of the macaque monkey

Early in the visual shape processing pathway, in areas V1 and V2, cells encode shape in terms of oriented edges and bars. Beyond this, however, little is known about how shape is processed in the primate brain. In this thesis we demonstrate that in area V4, an intermediate stage along the shape processing pathway, shape is encoded in terms of curvature and other contour characteristics, with each neuron representing the configuration of a specific section of the contour.;Theoretical considerations and psychological evidence suggest that contour features, i.e. angles and curves along the object contour, may serve as intermediate level primitives in the processing of object shape. In the first set of experiments we tested for explicit neural representation of contour features in area V4. We studied the responses of 152 V4 neurons to contour features presented in isolation within the center of the receptive field. We found that many V4 neurons showed strong systematic tuning for contour features, responding preferentially to an angle and/or curve pointed in a specific direction. Cells showed tuning for the orientation of contour features and their acuteness, i.e., the angle subtended by the angle or curve.;We also studied the responses of V4 neurons to complex shapes constructed by a systematic combination of convex and concave contour features. V4 neurons responded consistently to shapes with a specific contour configuration at a specific angular position. For example, a neuron may respond to all shapes with a sharp convexity pointing to the upper left. The neuronal responses to complex shapes were well-described as a gaussian function of the curvature of the influential contour segment. In our population of 109 cells, we found a stronger representation of cells encoding shape in terms of sharp convexities as compared to broad convexities or concavities. A subsequent test showed that the object-centered position and tangential orientation of the influential contour segment also dictated the responses to complex shape.;In accordance with hierarchical shape processing theories, piecewise contour information encoded by V4 neurons may be integrated at subsequent stages as a step toward complex shape recognition.