Tactile discrimination and somatosensory evoked responses after midbrain lesions in cats and rats

Abstract

Tactile discriminations and somatosensory evoked potentials were investigated before and after lesions in the midbrain lemniscal pathways of cats and rats. Midbrain lesions were placed under electrophysiologic control at sites where brief, shortlatency evoked responses to contralateral stimuli could be recorded. Although the lesions extensively damaged the lemniscal pathways, they did not prevent learning or relearning roughness discriminations to control values. Evoked responses remained normal or near normal postoperatively in the behaviorally tested animals. Cats prepared for chronic electrophysiologic recording showed attenuated cortical evoked responses only if the lesions extended medially from the medial lemniscus, the cervicothalamic tract, and the lateral component of the spinothalamic tract in the midbrain. The spared or recovered function was ascribed to lemniscal fibers known anatomically to travel outside the main body of these tracts, perhaps supported by spared fibers in the damaged region. The data further demonstrate that small fragments of sensory pathways are capable of mediating normal or near-normal performance on some simple discriminations.