The neuroanatomy of mental retardation in the white rat

A provisional examination of a set of questions pertaining to the neuroanatomical basis of mental retardation was undertaken by assessing the learning ability of 25 different groups of young rats prepared with various cortical and subcortical lesions. The test battery included a visual discrimination, a nonvisual discrimination, a three-cul maze and three separate detour problems. Seven of the 25 groups were impaired in learning all problems (suggestive of a generalized learning impairment) and therefore were viewed as being mentally retarded. One of these groups suffered diffuse multifocal neocortical damage, while the lesions in the remaining six were located either within the parietal cortex, globus pallidus, ventrolateral thalamus, substantia nigra, median raphe or pontine reticular formation. Based upon a variety of observations, it is proposed that the generalized learning impairment seen in our brain-damaged rats, rather than being reducible to a sensory, motor, arousal-motivational-emotional, attentional, inhibitory or recent memory defect, is the product of a defect in "executive" processes.