As Doty mentions, the very interesting findings of Lewine and colleagues (1994) suggest that even though forebrain commissurotomy seems to create two independent and disconnected cognitive systems, one in each hemisphere, some aspects of mnemonic (as well as at-tentional) processing are in fact shared between the hemispheres, possibly at the brain stem level. I would like to mention a recent paper by Mascetti (1997) on in-terhemispheric transfer in split-brain cats, the results of which go along with the above suggestion. He trained two groups of cats with section of the optic chiasm and the forebrain commissures on two visual pattern discriminations. Training was monocular, and the eye used for training was changed from one session to the next in both groups. With each discrimination the contingencies of reinforcement were consistent for both eyes in one group, while the other group was reinforced for choosing one of the two discriminanda when using the right eye and the other discriminandum when using the left eye. The learning curves of the two eyes of the group with consistent contingencies of reinforcement were generally superimposable, in agreement with the notion of an independent, parallel acquisition of the discriminations by the two hemispheres. In the other group of cats, learning was much slower with either eye, presum
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