Visual discrimination of direction changes based upon two types of angular motion

We address the question of how the visual system analyses changes in direction. Using plaid stimuli, we define type O direction changes which entail a change in the orientations of the plaid components, and type V direction changes in which the orientations of the components remain constant, relative to the observer but their relative speeds change. Lower thresholds for discriminating type O and type V direction changes were compared. Type O thresholds for clockwise/anticlockwise direction change were very low (0.2-0.5 degree), were resistant to directional noise, and showed a low-pass relationship with drift velocity. Type V thresholds on the other hand were higher (1-5 degrees), and exhibited a bandpass relationship with drift velocity. Type O direction changes gave low thresholds at short inter-stimulus intervals (ISI) (< 160 ms) and higher thresholds (successive orientation discrimination) at long ISI (240 ms-12.8 s). Type V thresholds, on the other hand, exhibited no short-range process and performance at short ISI, was no better than for successive direction discrimination at long ISI. A two-stage rotary motion model is sufficient to explain the discrimination of type O direction changes and results rule out a model based on velocity discrimination. For type V direction changes, a two-stage mechanism is insufficient and results are consistent with a minimum of three computational stages.