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Suzanne P. McKee, Preeti Verghese, Bart Farell; Edges and gratings: Interactions between 1st and 2nd order stereo systems. Journal of Vision 2002;2(10):74. doi: 10.1167/2.10.74.
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© ARVO (1962-2015); The Authors (2016-present)
A sinusoidal grating is perceived in depth at or near the disparity specified by its edges (Cumming & Parker, l997). Interocular phase differences also affect grating depth. To explore the interaction between envelope-defined depth (‘edge disparity’) and carrier-defined depth (phase disparity), we measured disparity increment thresholds as a function of the standing disparity separating a test target from a reference. The test was a 3 cpd sinusoidal grating, 6 deg wide, presented for 200 ms in a rectangular envelope. The reference was a thin black line that was always presented in the fixation plane. From trial-to-trial, the phase disparity of the grating was varied in small increments around the standing disparity; the subject judged whether the grating appeared closer or farther from the reference than the mean depth. The standing disparity was created in one of two ways: 1) the phase disparity of the carrier was shifted by 0 – 180 degrees while the edges remained matched at retinal correspondence, or 2) the half-images were shifted laterally in equal and opposite directions to produce edge disparities equal to the phase disparities. Although these two operations produce identical effects on the interocular phase of the carrier, they have different effects on disparity increment thresholds. When the edges remain matched in the fixation plane, increment thresholds rise steeply once the standing phase disparity exceeds 72 deg. When the edges are shifted to the standing disparity plane, increment thresholds for large standing disparities are significantly reduced (∼0.3 log unit). We speculate that the edge disparities produce responses in a 2nd order system that coarsely codes surface depth location, thereby overcoming the depth ambiguities associated with the periodic grating. The carrier produces responses in 1st order phase mechanisms that finely modulate depth around this surface location.
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