Motor-sensory and visual behavior after hemispherectomy in newborn and mature rats

Abstract

Immature mammals are widely believed to compensate functionally for nervous system alterations better than adults with comparable disorders. Embryos restitute huge losses, but plasticity, remodeling, or use of alternate mechanisms said to underly compensation in injured infants are not understood. Toward understanding these, the effects of ablating or altering parts of the nervous system in infant and mature rats are being studied. In these experiments one lateral half of the forebrain and diencephalon was largely removed at birth or maturity and the consequences to nervous system structure and motor-sensory and visual behavior were observed. Similarities between animals operated on as adults or infants were loss of tactile placing opposite the ablation, ability to discriminate visual patterns, and gauge variable jumping distances visually. Some subjects performed the visual tasks using the eye opposite the hemispherectomy alone, which was exclusively supplied with uncrossed retinogeniculate fibers. Differences were: loss of tactile placing after operation in infants was delayed until the seventeenth day; stride was impaired in animals operated on as adults but was spared in infant subjects; with appropriate ablations, Fink-Heimer-Nauta stains showed that after hemispherectomy, infants, but not adults, developed a small, uncrossed corticospinal tract. The stride component in locomotion seemed dependent on the corticospinal tract system, and was partially dissociated from the placing reaction essential for locomotion on rough terrain. The possibility was considered that the small remodeled corticospinal tract spared the stride component.