>> Hemispheric Interactions and Aging

	Interview with Patricia Reuter-Lorenz, University of Michigan From Studying The Mind, VHS © 2003, W. W. Norton

What role does the corpus callosum play in normal brain function?

You can almost think of the corpus callosum as being a route by which a copy of information that arrives in one hemisphere is related to the other hemisphere. And because it's a copy there may in fact be some degradation or compromise in the signal when it has to be transferred across the corpus callosum. And this has been one of the mechanisms that's been used to explain visual laterality effects in normal subjects. So, for instance, you might find that normal young individuals are better at making a lexical decision such as deciding whether a string is a word or not a word when it's presented to the right visual field, left hemisphere as opposed to the left visual field, right hemisphere. And the explanation for this is that in the case of right hemisphere presentation the information has to be transferred, which degrades quality. So you'll get better performance when you present the word directly to the left hemisphere.

The corpus callosum undoubtedly does a lot more than just transfer information. It is critical to attentional functioning. And so if we think about different aspects of attention, the corpus callosum probably serves as an insulating function. That is, operations that take place within neurosystems that are lateralized to one hemisphere can occur somewhat independently from those in the other hemisphere. This may be particularly advantageous developmentally and in certain stages in the life cycle. It also may serve to increase the neurocomputational power of the hemispheres. So, for example, in split brain patients you have an opportunity to study the specialization of each hemisphere. And, remarkably, many functions studied, in fact, are represented to some extent in both hemispheres. It's rare that you will find that one hemisphere can do something and the other hemisphere can't do it at all. So there is some degree of competence across many, many domains bilaterally.

What this means, then, is that if you can harness that bilateral competence—and there may be situations in which you have to–then the corpus callosum actually becomes a structure that allows for the recruitment of resources when the going gets rough. And we think that this relates back to aging, and it may help to explain why older adults tend to show bilateral activation at levels of task difficulty where young adults can still get away with just using a single hemisphere.

How do working memory and attention functions differ in young and older adults?

The work that I've done on aging has focused primarily on two broad age groups, younger adults who range in age from 18 to 28 years, and older adults who are between the ages of 62 and 75. We compared two different types of working memory that are well-lateralized in young adults—verbal working memory and spatial working memory. Our initial question focused on the task demands within working memory—in other words, if we were to make the task more challenging, would we begin to see age differences that weren't there in the less challenging tasks?

To our surprise, the aging effects were really robust on the less challenging tasks. And what's exciting about using neuroimaging to study ageing effects is that we could see changes in the brain that are not evident in performance. So in the verbal task in young adults we found left lateralized activity in a circuit that includes frontal, parietal, several different regions of frontal cortex, and pre-motor cortex. And each of these areas has been associated with a different aspect of working memory, to include rehearsal and storage of information. In older adults we found activation in those same sites, but we also found activation in homologous sites within the right hemisphere. And we found these differences between age groups to be primarily limited to frontal activation differences, with older adults showing bilateral activity, and younger adults showing left lateralized activity.

It became obvious that it was really important to see whether this was just a verbal thing or whether we would find a complementary result in the spatial domain. So we did the next study which compared spatial working memory performance (memory for locations across a three-second interval), and there we found right frontal activation in the young adults in addition to other sites as well, and the older adults again showed bilateral activation. And that I think captures the main phenomena that we and others are trying to understand.