Although the neurobiology of autism has been studied for more than two decades, the majority of these studies have examined brain structure 10, 20, or more years after the onset of clinical symptoms. The pathological biology that causes autism remains unknown, but its signature is likely to be most evident during the first years of life when clinical symptoms are emerging. This review highlights neurobiological findings during the first years of life and emphasizes early brain overgrowth as a key factor in the pathobiology of autism. We speculate that excess neuron numbers may be one possible cause of early brain overgrowth and produce defects in neural patterning and wiring, with exuberant local and short-distance cortical interactions impeding the function of large-scale, long-distance interactions between brain regions. Because large-scale networks underlie socio-emotional and communication functions, such alterations in brain architecture could relate to the early clinical manifestations of autism. As such, autism may additionally provide unique insight into genetic and developmental processes that shape early neural wiring patterns and make possible higher-order social, emotional, and communication functions.

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The posterior cingulate and the medial parietal cortices constitute an ensemble known as the posteromedial cortex (PMC), which consists of Brodmann areas 23, 29, 30, 31, and 7m. To understand the neural relationship of the PMC with the rest of the brain, we injected its component areas with four different anterograde and retrograde tracers in the cynomolgus monkey and found that all PMC areas are interconnected with each other and with the anterior cingulate, the mid-dorsolateral prefrontal, the lateral parietal cortices, and area TPO, as well as the thalamus, where projections from some of the PMC areas traverse in an uninterrupted bar-like manner, the dorsum of this structure from the posteriormost nuclei to its rostralmost tip. All PMC regions also receive projections from the claustrum and the basal forebrain and project to the caudate, the basis pontis, and the zona incerta. Moreover, the posterior cingulate areas are interconnected with the parahippocampal regions, whereas the medial parietal cortex projects only sparsely to the presubiculum. Although local interconnections and shared remote connections of all PMC components suggest a functional relationship among them, the distinct connections of each area with different neural structures suggests that distinct functional modules may be operating within the PMC. Our study provides a large-scale map of the PMC connections with the rest of the brain, which may serve as a useful tool for future studies of this cortical region and may contribute to elucidating its intriguing pattern of activity seen in recent functional imaging studies.consciousness ͉ default mode of brain function ͉ emotion ͉ posterior cingulate ͉ precuneus R ecent functional imaging studies have shown an interesting pattern of activity in the posterior cingulate gyrus and the neighboring medial parietal cortex, an ensemble referred to as the posteromedial cortex (PMC). This region includes the posterior cingulate areas 23a, 23b, and 23c, the retrosplenial areas 29 and 30, the mesial parietal area 7m in the precuneus region, and area 31, which lies between the posterior cingulate area 23c and medial parietal area 7m (Fig. 1). The PMC shows deactivation during sleep (1) and propofol-induced anesthesia (2), and it is the first brain region to show an increased activity in patients regaining consciousness from persistent vegetative state (3). Moreover, the PMC is the most active brain area during a baseline state where normal subjects are asked to close their eyes and ''rest'' (4). In this condition, the PMC consumes Ϸ40% more glucose compared to the hemispheric mean (5). Of special relevance is the finding that the PMC (along with the lateral parietal and medial prefrontal cortices) routinely exhibits a decrease of activity during attention-demanding cognitive tasks unless they require a reference to the subject's notion of self (5, 6). For instance, when subjects think about how they would describe their own personality traits and physical appearance, the PMC shows increase of activity, but, ...