The influence of adapting velocity and luminance on the movement after-effect.

The response of movement-sensitive mechanisms to both velocity and luminance may be investigated by means of the movement after-effect (MAE). A high contrast (98%) drifting grating having a spatial frequency of 1c deg-1 was used with four adapting velocities of 0.90, 2.87, 4.70 and 7.55 deg sec-1 to confirm their significant effect on initial MAE velocity (F = 29.83, P less than 0.01) and duration (F = 12.36, P less than 0.01). Four mean luminance levels of 4.5, 12.6, 42.9 and 126 cd m-2, spanning 1.5 log units of illumination, demonstrate its subtle effect on both initial MAE velocity (F = 4.11, P less than 0.01) and duration (F = 3.00, P less than 0.05). It is argued that adaptation of movement-sensitive mechanisms is more accurately reflected by the initial MAE velocity than its duration. The tracking technique which was employed, whilst accurate at low velocities, progressively underestimates the MAE velocity with increasing velocity. With a velocity of 1.2 deg sec-1 the under-estimate is 12%, rising to 23% at 2 deg sec-1. The tracking procedure, however, remains a reliable technique for assessing low velocity MAEs.