Abstract
The perceptual ability of infant monkeys to integrate local stimulus features over a large area (e.g., contour integration) does not emerge until relatively late in development (Kiorpes and Bassin, 2003). The neural basis of this subnormal perceptual binding in infants is not well understood. In mature monkeys cortical neurons are capable of integrating signals over a large area that surrounds their “classic receptive fields” (CRFs), and this ability to integrate signals over distance is thought to be intimately involved in perceptual “binding”. In this study, we investigated the postnatal development of the cortical circuits underlying long-range signal interactions. Microelectrode recording experiments were conducted in V1 and V2 of 2-, 4- and 8-week-old infant monkeys. To obtain area summation functions of individual neurons, we varied the diameter of drifting circular sine-wave gratings (TF=3.0 Hz; contrast = 80%) that were optimized for the orientation and spatial frequency for each neuron. We found that adult-like center/surround organization was largely present in V1 neurons as early as 2 weeks of age. However, in V2, RF surrounds did not appear until 4 weeks of age and the center/surround organization was still quantitatively immature at 8 weeks of age. These results indicate that the cortical circuitry responsible for the RF center-surround organization matures considerably later in V2 than in V1 and give evidence for the hypothesis that the functional maturation of the primate visual brain proceeds in a hierarchical order.