Does image movement have a special nature for neurons in the cat's striate cortex?
The question of whether a moving image is especially effective for stimulating visual neurons was studied in the striate cortex of the cat. Receptive fields (RFs) of simple and complex neurons were stimulated with optimally oriented bright and dark bars that either moved smoothly or were presented statically at an array of positions across the RF. A linear prediction of responses to the smooth movement was calculated by superposition of the responses to stationary presentation of these bar stimuli. A comparison between responses to actual movement and their prediction showed that the relative effectiveness of a moving stimulus decreases with speed. Effects of "conditioning" stimuli and nitrous oxide anesthesia were also studied. Both simple and complex units exhibited on average slightly lower than predicted responses for both bright and dark bars, even when they moved in the preferred direction of the unit. Movement in the opposite direction usually elicited even lower response levels, suggesting nonlinear suppression. These results imply that a moving image has no special efficacy for visual neurons but rather that it has a special propensity to elicit suppression when it moves in the nonpreferred (null) direction of a neuron.