The rivalry stimulus was presented for 120 seconds during which time participants indicated every instance of a complete switch in perception (exclusive visibility) from one grating orientation to the orthogonal one using right (for 45 degrees grating) and left (for 135 degrees grating) arrow keys in the keyboard (
Fig. 2A). Participants were explicitly instructed to ignore periods of piece-meal rivalry that appear in between periods of complete switch in grating orientation.
23 This task was performed with 100% contrast presented to each eye (baseline; see
Fig. 1A) and for 6 levels of interocular contrast difference in the 2 eyes (see
Fig. 1B). This difference was induced by maintaining the contrast at 100% in the weaker eye of cases and in the nondominant eye of controls, whereas the contrast to the fellow eye was attenuated to 80%, 40%, 20%, 10%, 5%, and 2.5% contrasts in each session, in randomized order. These produced interocular contrast differences of 20%, 60%, 80%, 90%, 95%, and 97.5%, respectively. The dwell time on each grating orientation was calculated for each interocular contrast combination from the sum total of the elapsed time between the key presses over the entire 120 second duration (see
Fig. 2B). Accidental key presses indicating the same grating orientation sequentially were excluded from the calculations. A dwell time of 50% in one eye indicated that monocular percepts occupied equal durations of time over the entire task duration. A dwell time of 100% or 0% in one eye indicated that only one monocular percept dominated throughout the task duration, with no perceptual switches in grating orientation and, therefore, no binocular contrast rivalry. The percent dwell time on the weaker eye's percept for cases and on the nondominant eye's percept for controls was then plotted as a function of contrast and a spline interpolation function was fit to this data to determine the contrast at which 50% of dwell time was achieved (see
Fig. 2B). This value was considered as the contrast “balance point” and represented the extent of contrast attenuation in the stronger eye required for both eyes to contribute equally to the rivalry percept (see
Fig. 2B). Contrast balance points tending toward 0% indicated that the stronger eye's contrast needed to be attenuated extensively to achieve the balance point and, therefore, a deeper suppression of the weaker eye (see
Fig. 2B). Contrast balance points closer to 100% indicated relatively shallow suppression of the weaker eye (see
Fig. 2B). If the weaker eye's dwell time varied non-monotonically with a reduction in stronger eye's contrast such that the 50% mark was reached at multiple contrast values (see
Fig. 2B; mild keratoconus), the highest of these contrasts were considered as the balance point. This effectively translated into a liberal criterion for suppression depth of the weaker eyes of cases and in the nondominant eyes of controls.