Unity Of Processing From The Lateralized Brain Varieties Of Interhemispheric Interaction

One of the most striking and consistent observations in studies that involve functional brain imaging is that many areas within both brain hemispheres are activated by even very simple tasks. This reflects the fact that the behavior of neurologically intact individuals is virtually always the result of processing in both hemispheres as well as in a variety of subcortical structures. In this section, I consider a variety of ways in which the left and right hemispheres interact and the biological mechanisms that support those interactions.

The corpus callosum, with at least 200 million nerve fibers, is the largest fiber tract that connects the two cerebral hemispheres. Comparison of split-brain patients with intact individuals provides a clear indication that the corpus callosum is critical to normal interhemispheric interaction, especially interaction that requires the transfer of information about the identify or name of a stimulus. Although there are no cortical landmarks that divide the corpus callosum, it is generally the case that different regions of the corpus callosum contain fibers originating in different cortical areas. That is, anterior portions of the corpus callosum contain primarily fibers that originate in premotor and frontal regions of the cortex, middle portions contain fibers that originate in motor and somatosensory, regions, and so forth. In addition, many callosal fibers are homotopic; that is, they connect homologous areas of the two hemispheres. An interesting hypothesis is that these fibers produce a type of homotopic inhibition at the computational level, thereby producing mirror-image patterns of activation and inhibition in the two hemispheres, which could be described as a kind of complementarity and that might contribute to the development of hemispheric asymmetry. However, the corpus callosum also contains fibers that originate in a specific region of one hemisphere and terminate in a completely different region of the opposite hemisphere, creating a mechanism whereby neural activity within one hemisphere could have more generalized excitatory or inhibitory effects on neural activity within the other hemisphere.

At a functional level, we have seen that the two hemispheres are often dominant for different task-relevant processing components. In such cases, each hemisphere is likely to take the lead for those components of processing that it handles best. In order for the two hemispheres to coordinate their activities effectively, the results of processing must be integrated across the two hemispheres. At the same time, many hemispheric asymmetries seem to involve complementary analyses of the sort that depend on incompatible neural computations, and it would seem useful to insulate those computations from each other in order for them to proceed efficiently at the same time.

With this in mind, it is not surprising that the corpus callosum has been hypothesized to play two important but very different roles: transferring information between the hemispheres and creating a kind of inhibitory barrier that minimizes maladaptive cross talk between the complementary processes for which each hemisphere is dominant.

Although the corpus callosum is certainly critical for certain forms of interhemispheric interaction, it is also clear that some types of information can be transferred subcortically. This includes information about the categories to which an object belongs, contextual information about an object, and certain aspects of information about spatial location. Subcortical structures can also play a role in producing unified behavioral responses. Although the two hemispheres of the intact brain are capable of sharing many types of information, cooperation at all levels does not necessarily occur all the time. Studies of perceptual processing in normal individuals provide important insights about the factors that determine when it is more efficient for the two hemispheres to operate collaboratively than to operate independently. Laterality techniques described earlier have been modified to include trials that demand interhemispheric collaboration by presenting each hemisphere with only a portion of the total information needed to perform a task (the across-hemisphere condition) and comparing performance to conditions that present all the relevant information to one hemisphere (the within-hemisphere condition). One general conclusion suggested by this research is that distributing information across both hemispheres becomes more beneficial as the task becomes more demanding of attentional resources. That is, when the processing demands are minimal (such as indicating whether two letters are physically identical), there is often a within-hemisphere advantage. However, when the processing demands are increased (such as indicating whether two letters of different case have the same name or whether both are vowels), there is typically an across-hemisphere advantage. Dividing input between the two hemispheres is also beneficial when it permits them to engage in mutually inconsistent processes and at earlier compared to later stages of practice, although these may simply be additional ways of manipulating overall processing demand.

A somewhat different aspect of interhemispheric interaction has been studied in experiments that include bilateral redundant trials in which exactly the same information is presented simultaneously to both hemispheres. When normal observers attempt to identify printed consonant-vowel-consonant letter trigrams, there is a right visual field (left hemisphere) advantage, and the pattern of errors is different for the two visual fields (and hemispheres). On right hemisphere trials there are many more third-letter errors than first-letter errors, as if individual letters are processed in order one at a time. On left hemisphere trials this difference is reduced, even when the error types are normalized to compensate for the left hemisphere advantage. This may reflect the left hemisphere's ability to treat the trigram as a single pronounceable unit and thereby spread attention more evenly across the letters. When the same three-letter stimulus is presented simultaneously to both hemispheres, performance is even better than it is on left hemisphere-only trials, indicating the benefit of in cluding both hemispheres in processing. In view of the very good level of performance on redundant bilateral trials, one might expect the error pattern to be like that obtained on left hemisphere-only trials. In fact, the error pattern on redundant bilateral trials is intermediate between the left and right hemisphere patterns (and often more similar to the right hemisphere pattern), again suggesting processing contributions from both hemispheres.

The processing strategy on redundant bilateral trials is not always a mixture of the different strategies used on unilateral trials. In some cases, the bilateral strategy has been identical to the strategy associated with one hemisphere or the other. Interestingly, when this happens it is not always the strategy of the more efficient hemisphere that emerges on bilateral trials. In fact, it is not always possible to derive the processing strategy on bilateral trials from knowledge of the strategies utilized on unilateral trials. For example, in experiments that required normal observers to make rhyming judgments, certain effects of letter font and case on bilateral trials could not be predicted at all from the complete absence of such effects on unilateral trials. This suggests that interhemispheric collaboration can also have emergent properties that are impossible to deduce from the sum of the parts provided by the two individual hemispheres. It will be important in future studies to identify the factors that determine which of these different types of functional interaction will be observed and the biological mechanisms that underlie the different types of interaction. Uncovering the mechanisms of interhemi-spheric interaction may also have more general implications for how it is that unified processing emerges from a brain consisting of highly specific, modular subsystems.

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