Contrast Sensitivity of Vertical Vergence Eye Alignment

Abstract

Purpose: The purpose of the study was to measure the response of vertical vergence eye movement to bandpass filtered noise of various spatial frequencies and contrasts. The results of our previous experiment in which we measured the contrast sensitivities using vertical vergence response to sine-wave gratings suggested that vertical vergence responses were robust for middle spatial frequencies (0.5 to 4 cycles per degree). However, the responses to higher and lower spatial frequencies were weaker. The weak responses at higher spatial frequencies could have been due to matching ambiguity of the gratings in the two eyes. So, to eliminate this ambiguity we used bandpass filtered noise as the stimulus to measure the contrast sensitivity of vertical vergence eye alignment. Methods: Subjects were 2 adults with normal vision. A Dual Purkinje image eye tracker was used for eye tracking. Vertical vergence was measured in response to full-screen bandpass filtered noise patterns of all orientations with one-octave bandwidth. A block of trials comprised combinations of 8 contrasts [0%, 1%, 2%,4%,8%,16%,32%,64%] and 8 spatial frequencies [0.125, 0.25, 0.50, 1, 2, 4, 8 ,16 cpd] presented in shuffled order. Various Oculometric functions (AUC, Percent Correct, Weibull fit, Vergence amplitudes) were created to determine the oculometric CSFs. The psychophysical CSF was obtained with a 2AFC staircase method for the same spatial frequencies using bandpass filtered noise patterns as stimuli. Results: Vertical vergence responses were robust for middle spatial frequencies. The responses for higher spatial frequencies were weaker or absent, similar to the results with sine-wave gratings. Conclusion: The results of this study suggest that matching ambiguity was not the primary limiting factor in the response of vertical vergence at higher spatial frequencies, despite being easily detected psychophysically. Further research is needed to understand why vertical vergence does not show a response at higher spatial frequencies.