INTEGRATION ACROSS DIRECTIONS IN DYNAMIC RANDOM-DOT DISPLAYS - VECTORSUMMATION OR WINNER-TAKE-ALL

Recent studies have clearly demonstrated that the activity of directio nally selective neuronal populations in the middle temporal (MT) and m edial superior temporal (MST) cortical areas plays a direct role in th e judgment of the direction of visual motion, However, the way in whic h the information is derived from a population of neurons remains unkn own. Two principal models have been suggested in the past: the vector summation model suggests that the responses of neurons encoding all di rections of motion are weighted and pooled to obtain an accurate estim ate of the mean direction of motion; the winner-take-all model is base d on a competition between different direction-specific channels, so t hat decisions are cast in favor of the channel generating the stronges t directional signal. To discriminate between these two models we gene rated random dot stimuli that contained an asymmetric distribution of directions of motion. Human subjects were asked to adjust the global d irection of motion to the upward vertical direction. When the directio nal signals were of similar strength, subjects tended to perceive glob al motion in the mean direction of motion (corresponding to vector sum mation), but as one directional signal became more prominent, most sub jects' settings diverged from the mean towards the modal direction of motion. Some subjects could either match the mean or the modal directi on of motion in the display, depending on the task instructions. These results suggest that the perceptual judgment of direction of motion i s not based on any rigid algorithm generating a single valued output. Rather, human observers are able to judge different aspects of the dis tribution of activity in a cortical area depending on the task require ments. Copyright (C) 1996 Elsevier Science Ltd.