Motion detection and directional tuning.

A random dot pattern was presented which made two jumps in various directions with a variable delay between them. The jumps occurred at the frame transitions of a 3-frame apparent motion sequence. The variation in detectability with the directional difference and the temporal separation of the jumps allows us to make inferences about directional tuning and the temporal response of the motion detection mechanism. The detectability of a pair of jumps was highest when the delay between the jumps was short and the difference in the jump directions was small. In all cases the data were well fitted with a vector version of the speed energy model earlier proposed by Simpson. The model supposes that the two input vectors are temporally filtered, squared and integrated. Using the model, the autocorrelation function of the motion system's temporal impulse response can be found. This function shows the filter to be lowpass. According to the model, the shape of the threshold or d' locus as a function of the difference in the directions of the two jumps does not show the tuning of a motion mechanism. A tuned mechanism will respond well to a jump in its preferred direction, but less well to any other jump. Instead we show that the apparent tuning evident in the threshold and d' loci is due to the way in which the two jump vectors, each fully recovered, are combined in a vector sum.