Late development of sensory thresholds for horizontal relative disparity in human visual cortex in the face of precocial development of thresholds for absolute disparity

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. Responses were recorded from male and female human participants at average ages of 5.3 +/- 1.6 months, 4.7 +/- 1.3 years, and 25.3 +/ -6 years. Horizontal disparity sensitivity was measured using planar stereograms that modulated absolute disparity and in stereograms portraying disparity gratings that additionally contained relative disparity. Disparity thresholds for absolute disparity changed little over development, but those for relative disparity changed by a factor of ∼10. SSVEPs were also 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. Equating subjective contrast and blur levels between infants, children and adults based on these measurements did not equate disparity sensitivity. The protracted developmental sequence for horizontal relative disparity coding shown in our measurements is not simply inherited from immaturities in encoding absolute disparity or retinal image contrast, 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.