Two mechanisms for detecting spatial contours defined by motion

We measured the discriminability of the linear motion trajectory of a line embedded in a random-line kinematogram. The signal line orientation was either parallel (iso-) or orthogonal (ortho-) to its motion direction and it was identical in all respects to the noise (orientation, length, baseline step size, frame rate) except for motion direction, rendering the signal line indistinguishable from the noise lines on a frame-to-frame basis. Our results show better discrimination of ortho-trajectories in most conditions, which (i) improved with duration up to 150 - 450 ms, (ii) was relatively independent of the speed, and (iii) whether the trajectory was straight or jittered each frame. However, discrimination of ortho-trajectory dropped when step size was high and lines were longer than 30 min of arc. Discrimination of iso-trajectory improved with increasing signal line speed and length, but was not affected by duration and was severely impaired by jitter. These results strongly suggest that discrimination of the signal line's trajectory is modulated by its orientation, and that iso- and ortho-trajectory discrimination relies upon at least two distinct mechanisms: an orientation-selective static detector that responds to iso-trajectories and a network of primary motion units that integrate local motion signals orthogonal to line orientation.