Riley Wiland¹, Enoch Ansah Asiedu¹, Kimberly Meier¹; ¹University of Houston

Frequency-tagged SSVEP can isolate brain responses to signals sent to each eye, by flickering the two eye images at different frequencies. Previous research using this approach in a binocular rivalry paradigm found that greater imbalance in the amplitude of the eye-specific responses was associated with weaker intermodulation (IM) responses in amblyopia (Hu et al., Heliyon 2024;10:e39358). One proposed explanation was that weaker IM responses could partly reflect less time in a ‘mixed’ percept state if one eye dominates perception, rather than the amblyopic deficit itself. To test this hypothesis, we systematically reduced contrast in one eye of healthy controls to determine if unequal input reduces time spent seeing mixed percepts, and whether reduced mixed percepts are associated with weaker IM responses. Participants (n=7) viewed a red patterned stimulus in one eye and green in the other during 30-sec trials. Contrast was fixed in the left eye (80%) and varied in the right eye (0-100%) across trials. Observers continuously reported whether they saw a green, red, or mixed image. We extracted the amplitudes of the second harmonics for left-eye (6 Hz) and right-eye (7.5 Hz) frequencies, and the IM component (13.5 Hz), from the SSVEP responses. We found that mixed percept reports did not decrease monotonically as contrast was reduced in one eye: during rivalry, the main effect of right-eye contrast on mixed percepts was not significant (p = 0.17), and the mean within-subject slope (0.02) was not different from zero (p = 0.74). Time spent seeing mixed percepts did not significantly predict IM amplitude within observers (p = 0.26). Thus, reducing contrast in one eye did not produce the predicted decrease in mixed percepts or relationship with IM amplitude. Together, these results suggest unequal input alone does not fully account for the weaker intermodulation responses previously reported in amblyopia.

Acknowledgements: NIH Center Core Grant P30 EY007551