Introduction. Stereopsis is the primary cue underlying our ability to make fine depth judgments. In adults, depth discriminations are supported largely by relative rather than absolute binocular disparity, and depth is perceived primarily for horizontal rather than vertical disparities. Although human infants begin to exhibit disparity-specific responses between three and five months of age, it is not known how the relative disparity mechanisms underlying stereopsis develop. Methods. We used a sweep steady state visual evoked potential (SSVEP) paradigm to measure disparity-tuning functions for horizontal and vertical disparity by incrementally increasing disparity magnitude through 10 values. Cyclopean gratings alternating between flat and disparate at 2 Hz were presented while neural responses were recorded with a 128-channel EEG system. We extracted the maximally reliable components of the 128-channel data (Dmochowski et al., NeuroImage, 2015) and applied spectral analysis to examine neural responses at the harmonics of the disparity modulation frequency. Eight adults and 18 four- to six- month-old infants participated (mean age ?± SD: 4.7 ?± 0.4 mos.). Results & Conclusion. Adult SSVEP responses were characterized by two features: 1) nearly 10x more sensitivity to horizontal rather than vertical disparity (thresholds: horizontal = 0.7 arcmin; vertical = 6 arcmin), and 2) the maximally reliable component was dominated by the first harmonic (4.6x higher amplitude than the second harmonic's amplitude). Infant responses were less sensitive and also exhibited large qualitative differences: 1) the maximally reliable component was much less dominated by the first harmonic (only 1.3x higher amplitude than the second harmonic) and 2) horizontal sensitivity was only 2.5x better than vertical sensitivity. The results indicate that the specialization for horizontal, relative disparity, which is characteristic of adult stereopsis, is not yet mature in infants four to six months of age.
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