Two-pulse monocular and binocular interactions at the differential luminance threshold.

Interaction between two pulses at the differential luminance threshold was studied for stimuli pairs presented to the same eye or to opposite eyes with an interocular delay. With monocular stimuli, the results replicated the earlier observations by Ikeda (1965) and Rashbass (1970) indicating linear interaction followed by rectification occurring at about 50-60 msec into the integration epoch. Binocular results were different, in accord with observations made in the contrast domain by Green and Blake (1981). Binocular stimuli of opposite polarity showed no cancellation. Binocular facilitation at threshold was found when either the stimuli of the same sign (+ + or - -) occurred with little interocular delay (stimulus onset asynchrony, SOA less than 15 msec), or the stimuli of the opposite sign (+ - or - +) were presented with an interocular delay between 15 and 100 msec SOA; the latter effect was at maximum with flashes 50 msec in duration presented with 50 msec interocular SOA. These results imply that binocular interaction takes place between rectified internal effects of luminance pulses. From the two-channel binocular model of Cogan (1987), binocular facilitation is attributed to the "fused" response derived from multiplicative excitation between same-sign (half-wave rectified), internal pulse responses. The absence of cancellation between simultaneous opposite-sign dichoptic stimuli is attributed to the "either-eye" binocular process dealing with full-wave rectified internal pulse responses to transient stimuli.