General to specific development of functional activation in the cerebral cortexes of 2- to 3-month-old infants.

A critical issue in the functional development of the cerebral cortex is whether cortical regions are functionally differentiated in early infancy. Although a growing number of neuroimaging studies have revealed that functional differentiation between early sensory and association regions of the cortex is already present at 3 months of age, it is unclear how functional regions per se emerge in the earlier developmental period. Here, we present 3 possible hypotheses regarding the functional development of the cerebral cortex as follows: (1) functionally differentiated regions are prespecified in the early developmental period; (2) functional activations appear in a hierarchical order from early sensory regions to the association regions; and (3) functional activation patterns change in a general-to-specific manner, thereby increasing the localization of regions activated by a particular stimulus and increasing the exclusivity of the response to specific stimuli within a particular cortical region. In the present study, we used multichannel near-infrared spectroscopy (NIRS) to measure cortical hemodynamic responses to 2 different video images of colorful mobile objects and black-and-white checkerboard pattern reversals over the occipital and prefrontal regions in awake 2-month-old infants. Both visual stimuli produced comparative activations over broad regions of the cortex including the early sensory and association regions, supporting the general-to-specific development (Hypothesis 3). This result suggests that functional cortical regions emerge between 2 and 3 months of age for visual perception.