Differential development of form and motion perception in monkeys

Visual acuity and contrast sensitivity develop to adult levels over the first 6-9 months in macaque monkeys. This developmental period for basic spatial vision was thought to represent the period over which V1 receptive field structure and local connectivity mature. However, recent data from our lab show that postnatal neuronal development in V1 is minimal in comparison to behavioral development. Thus we suggest that the important limitations on visual performance in infants lie in extrastriate cortex. Relatively little is known about cortical development beyond V1. To study extrastriate development we used two tasks that reflect processing in either the form or motion pathways. We compared the development of form and motion perception in individual monkey subjects (Macaca nemestrina). Motion discrimination was tested by detection of coherent motion in random dot kinematograms. Form discrimination was tested by detection of coherent organization in Glass patterns. In each case the monkey's task was to choose one of a pair of circular targets that had coherent motion or structure; the comparison stimulus had random motion or lacked structure. For the form test we used concentric, or radial, and linear Glass patterns. Contrast sensitivity functions were measured for comparison. Animals were tested at ages ranging from 8 weeks to 4 years. The results show that motion discrimination is demonstrable by 8 weeks, but develops over an extended time course up to about 3 years of age. Infants younger than 5-6 months were unable to perform the form discrimination task, and became adept only at much older ages. These tasks require integration over space and time and develop over extended time periods compared to contrast sensitivity. We conclude that form perception develops late in comparison to basic spatial vision and motion perception. Our results suggest a difference in maturation of dorsal and ventral stream extrastriate areas.