Maturation of evoked potentials and visual preference in 6-45-day-old infants: effects of check size, visual acuity, and refractive error.

Visual evoked potentials (VEPs) and the percentage time fixated (PTF) were investigated in response to checkerboard light flashes in 10 human infaed as a function of the size of check in the evoking stimulus (diffuse light, 11, 22, 45, 90 and 180 min of arc), the refractive lens strength the checkerboards were viewed through (-6 to +6 diopters), and the age of the infants (6-26 or 27-45 days). Check size significantly influenced VEP amplitude in infants as young as 6 days. The 11' checks evoked greater responses that diffuse light suggesting a visual acuity of better than 20/220. Only the 27--45-day-olds behaviorally discriminated the checks, PTF indicating an acuity of 20/120. Evoked potential refraction with spherical lention between VEP amplitude and check size measured from different VEP components at different ages indicated the function contained two modes or components. The first mode was inverted "U-shaped" and was obtained in response to check sizes less than 45'. It was primarily due to changes in amplitude of the early VEP components (less than 210 msec after the evoking stimulus) and was poorly correlated with the behavioral PTF measure. It was proposed that this mode reflected subcortical activity. The second mode was a linear increase in amplitude as check size was increased from 45' to 180'. It was primarily due to changes in the amplitude of late VEP components (240--400 msec after the evoking stimulus) and was highly correlated with the percentage time the infants fixated the various check sizes. It was proposed that this mode reflected cortical activity. Age selectively influenced the late VEP components and the PTF behavioral measure, these measures being influenced by check size only in the 27--45-day-old infants. This change in responsivity of late VEP components and the transition from passive to more active and discriminating visual preference, suggest the onset of increased cortical function between 28 and 45 days of age.