Development of contrast mechanisms in humans: a VEP study.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
These results are consistent with greater shunting inhibition in the visual cortex of older observers.
Department of Psychology, New York University, Center for Neural Science, New York, NY 10003, USA. lgarciaquispe@earthlink.net
Leticia A
LA
García-Quispe
James
J
Gordon
Vance
V
Zemon
D000293:Adolescent; D041923:Adolescent Development; D000328:Adult; D000375:Aging / Q000502:physiology; D002648:Child; D002657:Child Development; D002675:Child, Preschool; D015350:Contrast Sensitivity / Q000502:physiology; D005074:Evoked Potentials, Visual; D005583:Fourier Analysis; D006801:Humans; D007223:Infant; D011930:Reaction Time