Development of spatial frequency tuned "covariance" channels: individual differences in the electrophysiological...

We investigated the spatial frequency tuned channels underlying the contrast sensitivity functions (CSFs) of adults and infants. CSFs were measured in adults and in 8-, 14-, 20-, and 32-week-old infants, using the swept-contrast visual evoked potential (sweep-VEP). At each age, 8 to 21 subjects provided complete data. Subjects viewed achromatic sine wave gratings (0.3 to 8 c/deg) on a 20 degrees field presented on a CRT. Gratings were counterphased at 12 reversals/s (6 Hz). The second harmonic response was used to interpolate thresholds. We computed statistical "sources" of individual variability (or factors) underlying CSFs, then calculated the number, nature (discrete vs. continuous) and frequency tuning of "covariance" channels. CSFs from adults each contained three spatial frequency tuned covariance channels, consistent with psychophysical results spanning a similar spatial frequency range. Covariance channels in infants shifted upward in spatial frequency with age, with rapid shifts occurring between 8 and 14 weeks. The change in scale coincided with, and was probably determined by, developmental cone migration into the fovea and growth in eye size.