What masks utrocular discrimination

Utrocular discrimination, the ability to judge which eye has received monocular stimulation, was studied under conditions designed to reduce the salience of the cue mediating this discrimination. In one series of experiments, these conditions involved reducing the likelihood of motor command signals, overwhelming the motor command centers, and triggering motor command signals for both eyes. Those results indicate that utrocular discrimination is not mediated by ocular movements or command signals. In a second series of experiments, a visual masking paradigrn was used. A monocular test grating appeared superimposed upon a background of binocular masking noise, which was presented either simultaneously with the target or at some interval preceding or following the target (asynchronous masking). When presented simultaneously, the binocular masking noise interfered with utrocular performance, but only when the noise contained spatial frequencies near the test frequency. Masking functions obtained under conditions of asynchronous masking indicated that the time course of the forward and backward masking changed with spatial frequency. Finally, a sustained masking pattern that was ineffective in reducing utrocular performance could be made effective by pulsing the mask simultaneously with the target. Taken together, these results implicate a transient response to the target onset as a critical factor in generating the cue for successful utrocular discrimination. This conclusion may account for the fact that utrocular discrimination falls off for normal observers as the spatial frequency of the target grating increases above 4 cycles/deg.