Movement direction analysers: independence and bandwidth.

Opponent, or ratio, models of movement direction perception propose that pairs of analysers, sensitive to opposite directions, are linked in an opponent fashion. Alternatively, distribution models posit independence of movement direction analysers. To investigate analyser independence, a direction-selective adaptation experiment was conducted in which motion coherence thresholds for random-dot apparent motion stimuli were measured for upward, downward, leftward and rightward motion with and without prior adaptation to rightward motion. Coherence thresholds for rightward motion were elevated on average by 0.64 log units after adaptation. Thresholds for motion in the other directions remained unchanged. Since opponent models predict enhanced sensitivity after adaptation for the direction opposite that of adaptation, these results suggest that movement direction analysers operate independently. In a second experiment subjects adapted to a bidirectional stimulus containing an equal number of leftward and rightward moving dots. Leftward and rightward coherence thresholds were elevated even though no motion aftereffects were produced. A third experiment in which coherence thresholds for test directions within a narrow range were measured with and without prior adaptation to motion in the direction of the range's mean showed that threshold elevation was maximal when test and adaptation direction were matched and fell off as the difference in adaptation and test direction increased. The resulting functions indicate that the bandwidth of movement direction analysers is between +/- 35 and +/- 40 deg, a value consistent with reported mean directional tuning functions of movement sensitive units in the middle temporal area but smaller than previously reported psychophysical values.