HEMISPHERIC DISTRIBUTION OF VISUAL MEMORY

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These experiments address two deeply fundamental questions: the nature of mnemonic operations; and the degree to which the forebrain commissures, collectively or separately, are able to unify accessibility of a mnemonic store. Macaques, in addition to man, can perform the serial probe recognition task of Sternberg, in which a subject is required to identify as rapidly as possible whether a visual image is or is not a member of a previously specified set of target items. The intriguing feature of this task is that the time required for the decision is linearly related to the number of items in the target set, and it has therefore been proposed that the brain processes the task in a serial fashion, searching through the complete memory store each time for each of the target items. It has also been proposed that the great functional difference between the right and left hemispheres in man is a reflection of a difference in parallel versus serial processing. While the merit of this speculation is yet to be defined, the serial probe recognition task is an ideal tool for assessing this possibly elementary difference between the surgically separable hemispheres in macaques; other indications of right-left functional asymmetries in the macaque brain adumbrating those in man still being somewhat problematical. Of even greater interest, however, is the opportunity this task affords to assay how the corpus callosum, the anterior hippocampal commissure, and the anterior commissure operate following transection of the optic chiasm, when the target items can be parcellated between the hemispheres. Is there uniform access, hemispheric specialization, or simple doubling of the access time? In other words, how does each of these commissures contribute to the accessibility of a mnemonic store to either hemisphere for information initially given to the other|? Finally, the experiments offer some possibility of testing whether retrieval from memory might be separable from mechanisms of memory formation.