On Sensory Eye Dominance Revealed by Binocular Integrative and Binocular Competitive Stimuli

Abstract

Purpose: Two core processes underlie 3-D binocular vision. The first, a binocular combination/summation process, integrates similar feature signals from the two eye channels to form a binocular representation. The second, a binocular inhibitory process, suppresses interocular conflicting signals or falsely matched binocular representations to establish single vision. Having an intrinsic interocular imbalance within one or both processes can cause sensory eye dominance (SED), related to imbalances of combination (SEDcombo) and/or inhibition (SEDinhibition). While much has recently been revealed about SEDcombo and SEDinhibition, the relationship between them is still unknown.

Methods: We measured observers' foveal SEDcombo and SEDinhibition, respectively, with a pair of dichoptic horizontal sine wave gratings with different phases and binocular rivalry stimulus with vertical and horizontal gratings. We then measured horizontal and vertical monocular contrast thresholds using sinusoidal grating stimuli, and stereo thresholds using random-dot stereograms.

Results: There exists a strong correlation between SEDcombo and SEDinhibition. An observer's interocular difference in contrast threshold was not always consistent with his/her SEDcombo and SEDinhihition, suggesting a partial binocular origin for the underlying imbalances. We also found stereo thresholds significantly increased with the magnitudes of SEDcombo, as well as with the magnitude of SEDinhibition.

Conclusions: Our findings suggest a common origin for interocular imbalance in the two different binocular processes and that both types of sensory eye dominance are significant factors in impeding stereopsis.

Figure 1

(a, b) Binocular rivalry stimulus used for measuring SED_inhibition. (a) Test 1: The LE balance contrast is obtained by varying the vertical grating contrast while keeping the contrast of the horizontal grating seen by the RE constant. The balance contrast is reached when the two eyes obtain an equal percentage of perceiving the two gratings (point of equality). (b) Test 2: Switching the grating orientation between the two eyes permits measurement of RE balance contrast. (c) Binocular combination stimulus for measuring SED_combo. The dichoptic horizontal gratings have a 90° interocular phase difference.

Figure 2

Random-dot stereogram for measuring stereopsis threshold. The disc target can be rendered in crossed (top) or uncrossed (bottom) binocular disparity.

Figure 3

Relating the two types of sensory eye dominance. The graph plots SED_combo as a function of SED_inhibition. Each symbol represents the data of an observer. Clearly, all symbols, except one, fall in the first and third quadrants, indicated that SED_combo and SED_inhibition are highly correlated. The data points are also plotted in different symbols, with all the open symbols regardless of their shapes representing the observers whose SED could not be explained by an interocular difference in contrast threshold (IDCT) for horizontal and/or vertical grating. The relationship between SED and IDCT will be further elaborated in Figure 5 where the differently shaped symbols will be defined.

Figure 4

Relating sensory eye dominance to stereo threshold. (a) SED_combo versus crossed/front depth threshold; (b) SED_combo versus uncrossed/back depth threshold. (c) SED_inhibition versus crossed/front depth threshold; (d) SED_inhibition versus uncrossed/back depth threshold. Generally, a larger interocular imbalance results in a higher stereo threshold for both types of SED.

Figure 5

Relating sensory eye dominance to the interocular difference in contrast threshold (IDCT). (a) SED_combo versus IDCT for horizontal grating. (b) SED_inhibition versus IDCT for vertical and horizontal gratings. Filled circles represent observers whose SED_combo and SED_inhibition were consistent with IDCT. Open circles represent observers whose SED_combo and SED_inhibition were both opposite from the prediction made based on IDCT. The triangle represents the observer whose SED_inhibition, but not SED_combo, was consistent with IDCT. The squares represent observers whose SED_combo, but not SED_inhibition, was consistent with IDCT. Taken together, the data suggest that both monocular and binocular channels can cause interocular imbalance.

Figure 6

Relating sensory eye dominance (SED) to motor eye dominance (MED). (a) SED_inhibition versus MED. (b) SED_combo versus MED. Each bar along the x-axis represents the MED data of an individual observer. The data for observers with RE MED are plotted with open bars while the data for observers with LE MED are plotted with filled bars. The length of the bar is determined by his/her SED, whose magnitude is represented along the y-axis, with data for the observers with RE dominance being plotted as positive values. Therefore, should SED and MED be consistent, all filled bars should have negative values. Our data show that this is not the case. Graphs (c) and (d), respectively, plot the average SED_inhibition versus MED and average SED_combo versus MED. Although each graph shows the mean SED was consistent with the mean MED, statistical analyses fail to show a significant relationship.