Abstract
In primates, shape discrimination improves in early development. Postnatal changes in the early visual pathway (primary visual cortex, V1) cannot account for this improvement. Thus, our ability to integrate lower-level information about forms, contours and curvatures into global shape percepts likely depends on areas downstream of V1. Neurons in mature V4 are sensitive to intermediate-level shape cues in ways that V1 neurons are not, but it is not known whether this selectivity is present in infancy. We have studied both behavioral and neurophysiological development of form sensitivity in 2 infant macaque monkeys, aged 5 to 12 mo. We used circular targets whose radii are modulated sinusoidally (radial frequency stimuli, RFS) to assess the development of form sensitivity in infant macaques. We tested the animals’ ability to discriminate RFS with different frequencies of deformation (4, 8, 16 per circle) from circles as a function of depth of modulation using a 4-alternative oddity task. Behavioral sensitivity improved with age: sensitivity to higher frequency modulations improved relatively more. We also recorded neuronal responses to RFS from the same subjects at the same ages using two 96-channel “Utah” arrays, one in area V1 and one in V4. RFS responses in V1 were generally inconsistent, suggesting that they were driven primarily by local contrast. In V4, more sites showed consistent tuning for particular radial frequencies and amplitudes. We used linear discriminant analysis to determine each population’s ability to discriminate between a circle and RFS stimuli. As expected, the V4 neuronal population was more selective to RFS than the V1 population. We also found that relative sensitivity for different radial frequencies was similar for behavior and the population of V4 neurons, suggesting that this kind of shape selectivity may reflect the activity of area V4 even in infancy.