Post-natal development of the cerebello-cerebral response was investigated in 126 kittens from birth to 142 days of age by analysis of laminar field potentials in the cerebral cortex; the thalamocortical projection mediating the cerebello-cerebral response was examined on four new-born and three one-month-old kittens by means of anterograde axonal transport of horseradish peroxidase. A marked response was evoked in the frontal motor cortex from birth and an appreciable response could be evoked in the parietal association cortex at 2 days after birth. The latency of response in the frontal cortex decreased sharply from birth till 3 weeks of age whereas that in the parietal cortex remained almost unchanged until 2 weeks of age. Maturation of the cerebello-cerebral projection, in every respect, proceeds earlier in the frontal cortex than in the parietal cortex. The cerebello-cerebral response in kittens at any age, like in adult cats, consisted of two types of elementary responses: one which is characterized by a surface positive-depth negative (s.p.-d.n.) wave and the other which is characterized by a surface negative-depth positive (s.n.-d.p.) wave. The response in the frontal cortex was a sequential occurrence of the two waves while the responses in the parietal cortex was a pure form of the s.n.-d.p. wave. Two types of thalamocortical projections corresponding to the two types of elementary responses were revealed: one is the projection mainly onto layer I which appears to mediate the s.n.-d.p. wave and the other is the projection mainly onto layer III which appears to mediate the s.p.-d.n. wave. Development of the cerebello-cerebral response and changes in the terminal distribution of the thalamocortical projection during maturation are consistent with the principle of ontogenesis of the mammalian neocortical organization, i.e. ascending sequential maturation.
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