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Many motor commands in the nervous system are associated with corollary discharges which alter the excitability in both sensory and motor systems. These discharges may assist in the distinction between self-generated and externally produced movements; they also allow (or represent) monitoring of the motor commands before the effector response has occurred. Here, I hypothesize that this mechanism of control and integration is also present in thinking, which as Hughlings Jackson pointed out, may be considered the highest and most complex form of motor activity. I speculate that if corollary discharges are normally part of the motor mechanisms of thought, their derangement could produce many of the symptoms of schizophrenia.

It is now recognized that extensive maturational changes take place in the human brain during adolescence, and that the trajectories of these changes are best studied longitudinally. We report the first longitudinal study of the adolescent decline in non-rapid eye movement (NREM) delta (1-4 Hz) and theta (4 -8 Hz) EEG. Delta and theta are the homeostatic frequencies of human sleep. We recorded sleep EEG in 9-and 12-year-old cohorts twice yearly over a 5-year period. Delta power density (PD) was unchanged between age 9 and 11 years and then fell precipitously, decreasing by 66% between age 11 and 16.5 years (P < .000001). The decline in theta PD began significantly earlier than that in delta PD and also was very steep (by 60%) between age 11 and 16.5 years (P < .000001). These data suggest that age 11-16.5 years is a critically important maturational period for the brain processes that underlie homeostatic NREM EEG, a finding not suggested in previous crosssectional data. We hypothesize that these EEG changes reflect synaptic pruning. Comparing our data with published longitudinal declines in MRI-estimated cortical thickness suggests the theta age curve parallels the earlier maturational thinning in 3-layer cortex, whereas the delta curve tracks the later changes in 5-layer cortex. This comparison also reveals that adolescent declines in NREM delta and theta are substantially larger than decreases in cortical thickness (>60% vs. <20%). The magnitude, interindividual difference, and tight link to age of these EEG changes indicate that they provide excellent noninvasive tools for investigating neurobehavioral correlates of adolescent brain maturation.adolescence ͉ brain development ͉ EEG ͉ sleep T he human brain undergoes pervasive maturational changes during the second decade of life. Cross-sectional data show dramatic and parallel declines of about 50% in cortical synaptic density, cortical metabolic rate, and non-rapid eye movement (NREM) delta wave amplitude between age 10 and 20 years (1). We have hypothesized (1, 2) that the synaptic pruning of adolescence (3) drives the metabolic and EEG changes. We speculate that this late brain change is the final ontogenetic manifestation of the recurrent motif of overproduction and regression that sculpts vertebrate nervous systems. The potential clinical importance of adolescent brain reorganization is suggested by the frequent onset of schizophrenia and other psychiatric disorders in the second decade of life (2, 4).Longitudinal studies can discern the timing and trajectories of maturational development more efficiently than cross-sectional studies. For this reason, longitudinal studies of adolescent brain maturation are now being pursued in several laboratories, many using structural and functional MRI (5, 6). For example, the National Institutes of Health has initiated a multi-institutional longitudinal study of developmental changes using structural MRI (7), and a recent National Institute of Mental Health position paper emphasized the importance of using longitudin...

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