Abstract
Introduction: Immaturities exist at multiple levels of the developing human visual pathway, starting with immaturities in photon efficiency and spatial sampling in the retina and on through immaturities in early and later stages of cortical processing. Here we use Steady-State Visual Evoked Potentials (SSVEPs) and controlled visual stimuli to determine the degree to which sensitivity to horizontal retinal disparity is limited by the visibility of the monocular half-images, the ability to encode absolute disparity or the ability to encode relative disparity. Methods: Responses were recorded from male and female participants at average ages of 5 months, 5 and 25 years. SSVEPs were recorded in response to contrast and blur modulation of dynamic random dot patterns to measure sensitivity to the spatio-temporal content of the monocular half-images. Disparity sensitivity was measured using planar stereograms that modulated absolute disparity and in stereograms portraying disparity gratings that additionally had relative disparity in them. Results: Disparity thresholds derived from SSVEP amplitude vs disparity response functions for planar stimuli modulating absolute disparity changed little over development, but those for grating stimuli modulating relative disparity changed by a factor of ~10. Equating subjective contrasts between infants, children and adults did not equate disparity sensitivity. Disparity sensitivity at age 5 was adult-like, but disparity tuning at supra-threshold levels was not. Conclusion: The protracted developmental sequence for relative disparity coding shown in our measurements is not simply inherited from immaturities in encoding absolute disparity, but rather reflects immaturities in the computations needed to represent relative disparity that likely involve extra-striate cortical areas where relative disparity is first extracted.