A collection of studies in 1960's and 1970's on patients with unilateral hippocampal lesions show that the two hippocampi can be functionally dissociated. Studies by Milner, Corkin, Corsi and Petrides show that patients with right hippocampus lesions are impaired in tasks such as maze learning, face recognition, spatial block tapping, spatial position, spatial association, spatial memory and self ordered design recall. On the other hand, patients with left hippocampal lesions are impaired at tasks including recall of nonsense syllables, word lists, recurring digits test, non-spatial association, self-ordered word recall, and recall of consonant trigrams.
The maze learning task is similar to the one applied to H. M. The subject is supposed to learn the path in a model maze using visual or tactile clues. The spatial position test involves marking a circle indicated on an exposed 8-in line, then, after a short delay, reproducing this position on another 8-in line. The recurring digits test first invented by Hebb, is an ingenious test that probes the long term memory. First the digit span of the subject is discovered. Then the subject is given groups of digits that are one more than his span. Every third trial, the subject is given the same set of digits. The subject's performance on this recurring set increases with time whereas it stays constant with the random sets. Corsi devised a spatial version of this test, the block tapping. Subjects see several blocks lying on a table. The examiner taps the blocks in a certain order, and the subject is supposed to reproduce this order. Just like the previous test, subjects have a span for blocks. Thus the same experiment can be performed by giving the subjects a repeating sequence every third trial. This test appears to be the best available noninvasive test for right hippocampal function. The patients with right hippocampal lesions do not learn the repeating sequence or do so very slowly.
Studies on patients with temporal neocortex lesions show that this region also plays an important role in memory. Although patients with temporal neocortex lesions do not show global deficits of the sort H. M. does, they do have more selective impairments. Milner and her colleagues have double dissociated the effects of damage to the temporal neocortex of each hemisphere on several memory tasks. Patients with lesions in the right temporal lobe were impaired in the tests of geometric recall, paired-associate nonsense figures, recognition of nonsense figures, recurring nonsense figures, recognition of faces, unfamiliar melodies and tunes. Patients with left temporal lobe lesions were impaired at recall of stories, paired-associate words, recognition of words, numbers, and recurring nonsense syllable tests.
Jones found that patients with left hemisphere lesions could improve their verbal memory by encoding verbal information with the assistance of visual imagery. Similarly, patients with right temporal lesions benefited from the use of verbal encoding. H. M. did not benefit from any of these strategies, indicating that these encoding schemes only help in the existence of one intact hippocampus.