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Efficiency of the forebrain commissures: memory for stimuli seen by the other hemisphere

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Kavcic V., Doty R. W.

Acta Neurobiologiae Experimentalis

2002

vol. 62

issue 4

235-242

How well will one cerebral hemisphere recognize items viewed initially via the other? Nonverbalizable images or words were presented to one visual field and memory for them tested in the same or the other visual field. The initially viewing hemisphere subsequently had no secure advantage in accuracy, and only for images was there a 30-ms (ca 3%) penalty in reaction time for viewing with the ?other? hemisphere. Interhemispheric mnemonic communication is thus highly reliable. At longer retention intervals (1-2 min vs. 4-30 s, with accumulating added stimuli), however, recognition of words was asymmetric as to hemisphere, in that initial viewing via the right hemisphere was subsequently (and paradoxically) much better recognized via the left (other) hemisphere than was the converse situation. This suggests that the initial engram with right hemispheric viewing of words ultimately becomes established in the left, and that the right has less accurate access to a previous ?left hemispheric view?.

Unity from duality

100%

Doty R. W.

Acta Neurobiologiae Experimentalis

2003

vol. 63

issue 3

163-170

When, in the primeval sea, creatures first began to crawl, 'right' and 'left' came into being, yielding neuronal nets to control response to the sidedness of stimuli. In the half billion years of moving and sensing, two brains have evolved, the right and the left; and human experience now shows them to be roughly equivalent, potentially independent, conscious entities. This dramatic fact is evidenced by 'split-brain' patients and by numerous cases of therapeutic removal of either hemisphere. Equally dramatic, of course, is that there is not the slightest sign of this duality in everyday experience, the right and left visual fields are seamlessly knit, and cross purpose is absent in the moment to moment operation of the two cerebral hemispheres. This unity is constantly synthesized by the 100,000,000 fibers passing from each hemisphere to the other; the vastness of that interchange emphasized upon comparison with the mere 1,000,000 fibers conveying all the visual world from each eye. With the large distances in the human brain some 100+ ms may commonly transpire for one hemisphere to send to and receive a response from the other. Efficiency thus demands that most neuronal calculation occur within rather than between hemispheres, thereby promoting differences in the characteristic capabilities of each alone, i.e., 'hemispheric specialization'. Despite this there is a bewildering bilaterality of activation revealed by fMRI for most cognitive tasks. In the absence of the forebrain commissures brainstem systems can be shown, in macaques, also to participate in the unification of behavioral result from the actions of the separated hemispheres. The system favors synthesis from congruent (visual) input to the two hemispheres; but in the face of incompatible hemispheric input, the two hemispheres are able to work out an accommodation in their control of subcortical systems.

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Efficiency of the forebrain commissures: memory for stimuli seen by the other hemisphere

Unity from duality