Abstract
Studies of anatomical changes in early brain development have provided evidence of a hierarchical progression in which primary sensory areas mature first and prefrontal cortex matures last. Whether these sequential structural and physiological changes directly limit the emergence of complex stimulus specificity and the functional specialization of cortical areas is not known. In the present study we recorded high-density, steady-state visual evoked potential responses to intact and scrambled images of objects (Kourtzi and Kanwisher, 2000) and comparable images of human faces, both of which are known to activate areas within lateral occipital cortex in adults. We compared two conditions: alternating (1 Hz) scrambled versus intact pairs of images which maintained the same low-level properties, but differed in global configuration, and alternating scrambled-scrambled pairs which were equated in low-level properties. A difference in the response waveform between these two conditions was used to isolate object- and face-selectivity. Such selectivity was found in both infants and adults for both object categories. However, adults showed a larger response to the onset versus the offset of both object and face images, but infants showed a larger onset response only to faces. Object responses in infants were dominated by offset rather than onset responses and the onset response was weak and delayed for objects compared to that for faces. The topographies of the adult onset responses to objects and faces were both bilaterally symmetric over lateral occipital cortex and were also present over frontal cortex. In contrast, the infant responses were restricted to medial and lateral occipital electrodes, and were not found over frontal electrodes. These findings demonstrate that while at least a basic form of selectivity is present in 4- to 6-month-old infants for both objects and faces, the qualitative pattern of responses to faces is more mature than that of objects.