DIRECTIONAL AND NONDIRECTIONAL CODING OF A SPATIO-TEMPORAL MODULATED STIMULUS

Human contrast thresholds for apparent flicker and direction of movement were measured separately by using a subthreshold summation technique. The test stimulus consisted of a drifting grating superimposed on an identical grating moving in the opposite direction and set at different subthreshold contrasts. With increasing contrast of the subliminar grating movement, the thresholds increased, apparent flicker thresholds slightly decreased, and percentage of apparent flicker reports was increased from 0 to 100. The distinction between flicker and movement thresholds was justified by 2 subsidiary experiments which showed that flicker is not to movement what movement is to flicker. A moving subliminar grating facilitates the detection of an objectively flickering grating whereas the reciprocal is not true. This might indicate separate detection of drifting and flickering gratings.