Wingfield and Grossman report in Journal of Neurophysiology their studies of how language comprehension typically remains well preserved in normal aging, in spite of the fact that aging brings with it declines in sensory function, both in vision and in hearing, as well as a general slowing in a variety of perceptual and cognitive operations. They start by reviewing language processing regions of the brain that have been identified both through studies of brain damage and neuroimaging. This figure shows some relevant areas:
Figure 1 description: Within a broad perisylvian region, deficits in language production have been shown to be associated with damage to the ventral inferior frontal cortex (vIFC), which includes Broca's area, and deficits in language comprehension associated with damage to the posterior lateral temporal cortex (PLTC), a part of which is commonly referred to as Wernicke's area. This early picture was completed by discovery of a white-matter tract, the arcuate fasciculus, connecting these two regions. The aphasic syndromes of Broca's aphasia, Wernicke's aphasia, and conduction aphasia were thus neatly accounted for by damage to, or disconnections between, these centers and pathways. These three components of the core sentence processing region colored are shown in blue, with the arcuate fasciculus represented by the connecting blue double-headed arrow. As first implied by the effects of focal brain damage, this core network for sentence processing has been confirmed and more exactly specified by neuroimaging studies in healthy adults. The resource network involves at least several frontal cortical regions and extends to include right hemisphere structures as well as subcortical structures. The left hemisphere structures associated with the recruitment of working-memory and executive resources include dorsolateral prefrontal cortex (dlPFC) and dorsal portion of left inferior frontal cortex (dIFC). Regions in the right hemisphere include PLTC and dIFC. These regions are shown in red.
Winglield and Grossman proceed to present data from their own and other studies that support a compensation hypothesis to explain how declining cell numbers and cortical volumes in these language areas are compensated for by recruitment of other areas. A number of imaging studies contrasting performance by healthy young and older adults have shown a major difference in observed patterns of neural activation when young and older adults are asked to perform the same cognitive task. In general, there is a shift from more focal activation in young adults to more widespread patterns of activation in older adults. This pattern has been observed in a number of cognitive domains, from encoding pictures to studies of episodic memory.
Activity of the core sentence processing network (blue in the above figure) and the resource network (red) during complex sentence comprehension was followed and performance level measured in a number of experiments.
Figure 2 shows regional brain activation contrasts during sentence comprehension for young adults, elderly good comprehenders, and elderly poor comprehenders. A: areas activated by young adults to a greater degree than the elderly good comprehenders. B: areas activated by elderly good comprehenders to a greater degree than young adults. C: areas activated to a greater degree by elderly good comprehenders than by elderly poor comprehenders. D: areas activated to a greater degree by elderly poor comprehenders than by elderly good comprehenders.
In A, young adults were producing a significantly greater degree of activation than the older adults in the posterolateral temporal-parietal cortex in the left hemisphere. This region is thought to support a short-term auditory-phonological buffer that retains information transiently during the course of processing. B. the compensation hypothesis would lead one to expect to see the successful older adults recruit other brain regions to maintain their successful performance. This contrast can be revealed by subtracting the activation levels in the young adults from the older adults' activations. The consequences of this subtraction are shown in the two brain renderings in B, where one sees these successful older adults showing significant upregulation in two areas. One of these is increased activity in the dorsal portion of left inferior frontal cortex. This area is thought to be important for maintaining and rehearsing stored verbal information in working memory. Successful older adults also showed additional activation in the right posterolateral temporal-parietal region. The data illustrate the involvement of a network of cortical areas upregulated in support of sentence processing beyond the left perisylvian core sentence-processing area. In particular, the network upregulates by augmenting working-memory resources needed to support sentence processing.
These studies offer a window on the strategic recruitment of critical brain regions by older adults in response to otherwise limited working-memory resources not seen during sentence comprehension in young adults. They show that older adults augment regional brain activation within the language-processing system both spatially and temporally to help compensate for age-related neuronal changes.