The relation of learning and retention to the extent of cerebral lesions in the rat.

In a preliminary study of the mass factor in neural function, lesions of various sizes were produced in the visual areas 1 of the cerebral cortex of fourteen rats. The animals were then trained in a habit of brightness discrimination, and the amount of practice required for learning, as indicated by the total number of errors made before the habit was perfected, was correlated with the locus and extent of the lesions. The destructions involved from 13 to 40 per cent of the entire cerebral cortex. The operated animals required an average of 148.4 trials for learning, with an average of 45.6 errors. The average for 60 normal animals in the same problem is 139.7 trials with 41.5 errors. The difference is not significant. All parts of the occipital third of the cerebral cortex were explored in this series, and the rate of learning is apparently unaffected by the locus of the injury. The rank-order correlation between the amount of cortex destroyed and the number of errors made in learning is 0.084. The rate for initial learning of brightness discrimination, after lesions to the visual area, is independent of the extent of the lesion.

In a second series, twenty-three normal animals were trained in the same habit. When learning was completed, lesions of various loici and magnitude were produced in the occipital cortex. Seven days after operation the degree of retention of the habit was measured in terms of the numbers of errors made before accurate discrimination was re-established. This amount of practice required for relearning was correlated with the locus and extent of the lesions. The injuries involved from 1.5 to 31.6 per cent of the total cortex; the errors made in relearning ranged from 0,to 83.