Electrophysiological assessment of the human depth-perception threshold.

The cortical response to stereoscopic stimuli was measured as a function of disparity using visual evoked potentials (VEP). The stereoscopic stimulus consisted of a dynamic random-dot pattern that portrayed a three-dimensional horizontal grating. Disparity of the grating was variable between 0 and 18 arc min, step size being 4.22 arc s. Evoked responses were recorded using a "random-sequence sweep technique." The VEP amplitude increased approximately linearly with the logarithm of disparity and allowed an estimation of the stereo threshold by linear extrapolation. The evoked potential estimates of the stereo threshold of 16 subjects were compared to psychophysically obtained thresholds. Several threshold-estimation techniques are discussed. Evoked potential and psychophysical threshold estimates had the lowest discrepancy when the VEP amplitude was linearly extrapolated to the logarithmic disparity axis. The difference between the evoked potential estimate and the psychophysical threshold was less than 20% in 56% of all cases; 16 of 18 cases (89%) agreed within a factor of 2.