Processing of second-order motion stimuli in primate middle temporal area and medial superior temporal area

Two rhesus, monkeys were subjects in a direction-discrimination task involv ing moving stimuli defined by either first- or second-order motion. Two dif ferent second-order motion stimuli were used: drift-balanced motion consist ing of a rectangular field of stationary dots and theta motion consisting o f the same rectangular field with dots moving in the direction opposite to that of the object. The two types of stimuli involved different segmentatio n cues between the moving object and the background: temporal structure of the luminance (flicker) in the case of drift-balanced motion and opposed mo tion in the case of the theta-motion stimulus. Our monkeys were able to cor rectly report the direction of each stimulus. Single-unit recordings from t he middle temporal (MT) and medial superior temporal (MST) areas revealed t hat 16 out of 38 neurons (41%) from area MT and 34 out of 68 neurons (50%) from area MST responded in a directionally selective manner to the drift-ba lanced stimulus. The movement of an object defined by theta motion is not e xplicitly encoded in the neuronal activity in areas MT or MST. Our results do not support the hypothesis that the neuronal activity in these areas cod es for the direction of stimulus movement independent of specific stimulus parameters. Furthermore, our results emphasize the relevance of different s egmentation cues between figure and background. Therefore the notion that t here are multiple sites responsible for the processing of second-order moti on is strongly supported. (C) 2001 Optical Society of America.