((LL Kontsevich & CW Tyler))
Smith-Kettlewell Eye Research Institute, San Francisco
Methods. We measured local disparity tuning at a location 5º to the left of the fovea by a masking threshold paradigm, which gives a direct measure of the underlying channel tunings. Test bars were presented in a 2 sec cosine bell and mask bars were static. With the test bar of a particular width at 2X detection threshold, the contrast of the masking bar at a range of set disparities was varied to measure the mask contrast required to return the test to its detectability criterion. Masking thresholds were determined by a new minimum-entropy variant of the QUEST staircase that we have developed for optimal measurement of both the threshold and slope parameters of the psychometric function.
Results. The disparity tuning functions for zero-disparity test targets showed a remarkable ‘batwing’ form, with masking threshold increasing up to peaks at from 10 to 40 arc min (according to Gaussian bar width over a range from 4-60′), then plunging precipitously within about 10′ for larger disparities. Monocular masking by the sum of the dichoptic components showed a narrower spread matching the Gaussian bar width.