Rapid serial visual presentation of motion: short-term facilitation and long-term suppression

The visual system can detect coherent motion in the midst of motion noise. This is accomplished with motion-sensitive channels, each of which is tuned to a limited range of motion directions. Our aim was to show how a single channel is affected by motions both within and outside its tuning range. We used a psychophysical reverse-correlation procedure. An array of dots moved coherently with a new, randomly chosen, direction every 14 or 28 ms. Human subjects pressed a key whenever they saw upwards movement. The results were analyzed by finding two motion directions before each key-press: the first preceded the key-press by the reaction time, and the second preceded the first by a variable interval. There were two main findings. First, the subject was significantly more likely to press the key when the vector average of the two motions was in the target direction. This effect was short-lived: it was only seen for inter-stimulus intervals of several tens of milliseconds. Second, motion detection was reduced when the target direction was preceded by a motion of similar direction 100-200 ms earlier. The results support the idea that a motion-sensitive channel sums sub-optimal inputs, and is suppressed by similar motion in the long term.