Abstract
Neural hysteresis plays a fundamental role in stereopsis and reveals the existence of positive feedback at the cortical level (Wilson & Cowan, 1972). Using a model of cortical dynamics, we predicted that it should be possible to measure hysteresis as a function of orientation disparity in tilted gratings in which a transition is perceived between stereopsis and binocular rivalry.
Methods. The patterns were 2 cpd or 4 cpd sinewave gratings with orientation disparities (0–40 deg) resulting in various degrees of tilt. A sequence of 40 pattern pairs was used, with the orientation disparity increased by a fixed increment in successive pairs. During each experimental trial a movie of the 40 pairs was shown at a rate of 0.5, 1 or 2 pairs per second. In separate trials the same movie was shown in reverse order. Two transition points were measured: the point at which the single tilted grating broke into two rivalrous gratings (T1), and the point at which the two rivalrous gratings merged into a single tilted grating (T2).
Results. Transitions occurred at different orientation disparities (T1=24.7 deg, T2=17.8 deg at 2 cpd; T1=27.1 deg, T2=18.0 deg at 4 cpd) corresponding to a timing difference of 3.5–4.6 sec. This was consistent with hysteresis and far exceeded the effects which could be attributed to reaction time or adaptation.
The results are consistent with a cortical model with positive feedback arising from recurrent inhibition between binocular units coding different disparities and orientations. Stereopsis and rivalry are two possible states of the network.
Supported by an NSERC grant to HRW (#OP227224), a CIHR training grant in vision health research and an NSERC postdoctoral fellowship.