The Visual Control of Vertical Eye Alignment
Purpose: Maintaining proper binocular vision requires correcting misalignments of the eyes in the horizontal, vertical, and torsional directions. Although horizontal vergence has been extensively studied, a significant knowledge gap exists regarding the visual input and neurological pathway of the vertical eye alignment reflex. Our studies examined the role of spatiotemporal frequency, contrast, and visual periphery in the control of vertical eye alignment. Methods: We measured the contrast sensitivity of the vertical eye alignment reflex to sinewave gratings and bandwidth filtered noise patterns at various spatial frequencies [0.125-16 cycles per degree (cpd)] and contrast levels [0-64%] using an eye-tracking device and compared with psychophysical contrast sensitivity. We also measured the contrast sensitivity of vertical disparity-driven vergence eye movements in response to bandwidth filtered steady and 6 Hz counterphase flickering noise in a nonius task and compared with psychophysical contrast detection sensitivity. We tested seven spatial frequencies [0.25–16 cpd] and three vertical disparities [5, 10, and 30 arcmin] to obtain the contrast sensitivity functions for each disparity. Additionally, we determined the vertical disparity detection thresholds at various eccentricities up to 30 degrees of eccentricity using spatially scaled sinewave grating patches as stimuli in a nonius task. Lastly, we compared the contrast sensitivity of the eye alignment reflex near the center and periphery and compared with psychophysical contrast sensitivity. Results and Conclusion: The contrast sensitivity of the vertical eye alignment reflex is robust for the middle spatial frequency (0.50 to 4 cpd). However, the response was weaker for the lower and higher spatial frequencies. The addition of counterphase flicker did not enhance the contrast sensitivity of the eye alignment reflex, while the perceptual system showed a significant improvement. The spatial frequency showing the highest contrast sensitivity increased as the disparity decreased, consistent with the size disparity correlation of disparity processing. The vertical eye alignment reflex showed comparable disparity and contrast thresholds in the center and periphery but lower overall contrast sensitivity than the psychophysical method. These results suggest that eye alignment uses the same neural substrate as disparity processing for depth perception.