Are rhythms of human cerebral development “traveling waves”?

Psychopathology of early frontal lobe damage: Dependence on cycles of development

A new theory of frontal lobe development is presented in which the role of the human frontal lobes during normal development and the psychopathological consequences of early frontal lobe injury are explored. Analyses of the development of human electroencephalograph (EEG) coherence indicate that there are oscillations and cyclic growth processes along the mediolateral and anterior-posterior planes of the brain. The cycles of EEG coherence are interpreted as repetitive sequences of increasing and decreasing synaptic effectiveness that reflects a convergence process that narrows the disparity between structure and function by slowly sculpting and reshaping the brain's microanatomy. This process is modeled as a developmental spiral staircase in which brain structures are periodically revisited resulting in stepwise increases in differentiation and integration. The frontal lobes play a crucial role because they are largely responsible for the selection and pruning of synaptic contacts throughout the postnatal period. A mathematical model of cycles of synaptic effectiveness is presented in which the frontal lobes behave as gentle synaptic “predators” whereas posterior cortical regions behave as synaptic “prey” in a periodic reorganization process. The psychopathological consequences of early frontal lobe damage are discussed in the context of this model.

Morphogenesis and mental process

Parcellation and heterochrony (neoteny) reflect the pattern and rate of a growth mechanism in morphogenesis. Structure (morphology) and function (behavior) are staged realizations of morphogenetic process. This process continues into adult cognition in the actualization of the mind/brain state. Parcellation obtains in the pruning of cells and connections in early growth, whereas inhibition obtains in a relatively stable morphology with constraints on context: item transforms in microgeny. Selective retardation in process (neoteny) leads to growth at earlier (juvenile) phases. This accounts for the specification of the language areas and elaboration at preliminary phases in mind — for example, dominance, introspection, and creativity.

Timing of puberty and EEG coherence during photic stimulation

Selective elimination of a large proportion of cortical synapses constitutes the last major stage in brain development which is linked to sexual maturation. Various neurodevelopmental theories have proposed an association between schizophrenia and late brain maturational events. Based on epidemiological and psychopharmacological data, Saugstad hypothesised a relationship between timing of puberty, synaptic density and psychosis proneness. Early puberty should result in an earlier end to synaptic pruning, a larger number of surviving connections and an increased risk for affective psychosis. In contrast, in late maturers prolonged synapse elimination should lead to lower synaptic density which constitutes a diathesis for schizophrenia. We used EEG coherence as a neurophysiological measure of cortico-cortical connectivity. Late maturers were expected to show higher coherence, especially for short-distance connections where lower synaptic density should lead to decreased local differentiation. EEG was recorded from 12 bipolar derivations in 19 early and 19 late maturing subjects of both sexes under conditions of rest and photic stimulation at different flicker frequencies. Whereas there were no coherence differences between early and late maturers during rest, both groups differed during flicker stimulation. As hypothesised, late maturers were found to have higher short-distance coherence. Late maturers were also found to have higher intrahemispheric long-distance coherence. These findings were restricted to delta, alpha and beta 2 bands and were more pronounced in males. Our findings are interpreted in terms of a structural difference in neuronal connectivity between extreme early and late maturers.

Cyclic cortical reorganization during early childhood

EEG coherence was computed from 8 left and 8 right intrahemispheric electrode pairs from 253 children ranging in mean age from 6 months to 7 years. The first derivative of mean coherence was computed in order to study growth spurts or rapid changes in mean coherence over the early childhood period. Growth spurts in EEG coherence were approximately 6 months to 1 year in duration and involved a cyclical process composed of a sequential lengthening of intracortical connections in the left hemisphere and a sequential contraction of intracortical connections in the right hemisphere. Each growth spurt cycle had a period of approximately 2 to 4 years and involved both a rostral-caudal expansion and contraction as well as a lateral-to-medial rotation. Data support the view that the functions of the left and right hemisphere are established early in human development through complementary developmental sequences and that these sequences appear to recapitulate differences in adult hemispheric function.