The visual system undergoes major developmental changes in infancy and continues to mature throughout childhood. This study was designed to investigate the developmental change in the contrast response function and the neural mechanisms that contribute to this change.
Participants were 29 infants from 15 to 28 weeks of age, two children, one adolescent, and nine adults. Visual evoked potentials were elicited by horizontal square-wave gratings contrast-reversed at 7.5 Hz. Spatial frequencies of 0.75 and 1.5 cpd were used, and contrast was swept in seven octave steps from 1 to 64% with an initial step at 0%. There were 10 runs of each condition (8.5 s each). Fourier analysis was used to derive amplitude and phase of the dominant (second harmonic) frequency component in the response, which were then plotted vs. contrast. The empirical contrast response functions were fitted using a nonlinear model, which generates estimates of shunting inhibition
, conductance, and the integrative time constant in the system.
Typically, in comparison to older observers, contrast response functions in infants are relatively linear with increases in contrast, and they exhibit little if any contrast gain control
(amplitude compression and phase advance with increasing contrast). Time constants in infants are longer than in adults, and infants demonstrate less decrease in time constant values with increasing contrast than do adults.
These results are consistent with greater shunting inhibition
in the visual cortex of older observers.