Age Differences in Behavior and PET Activation Reveal Differences in Interference Resolution in Verbal Working Memory

Older adults were tested on a verbal working memory task that used the item-recognition paradigm. On some trials of this task, response-conflict was created by presenting test-items that were familiar but were not members of a current set of items stored in memory. These items required a negative response, but their familiarity biased subjects toward a positive response. Younger subjects show an interference effect on such trials, and this interference is accompanied by activation of a region of left lateral prefrontal cortex. However, there has been no evidence that the activation in this region is causally related to the interference that the subjects exhibit. In the present study, we demonstrate that older adults show more behavioral interference than younger subjects on this task, and they also show no reliable activation at the same lateral prefrontal site. This leads to the conclusion that this prefrontal site is functionally involved in mediating resolution among conflicting responses or among conflicting representations in working memory

Age Differences in the Frontal Lateralization of Verbal and Spatial Working Memory Revealed by PET

Age-related decline in working memory figures prominently in theories of cognitive aging. However, the effects of aging on the neural substrate of working memory are largely unknown. Positron emission tomography (PET) was used to investigate verbal and spatial short-term storage (3 sec) in older and younger adults. Previous investigations with younger subjects performing these same tasks have revealed asymmetries in the lateral organization of verbal and spatial working memory. Using volume of interest (VOI) analyses that specifically compared activation at sites identified with working memory to their homologous twin in the opposite hemisphere, we show pronounced age differences in this organization, particularly in the frontal lobes: In younger adults, activation is predominantly left lateralized for verbal working memory, and right lateralized for spatial working memory, whereas older adults show a global pattern of anterior bilateral activation for both types of memory. Analyses of frontal subregions indicate that several underlying patterns contribute to global bilaterality in older adults: most notably, bilateral activation in areas associated with rehearsal, and paradoxical laterality in dorsolateral prefrontal sites (DLPFC; greater left activation for spatial and greater right activation for verbal). We consider several mechanisms that could account for these age differences including the possibility that bilateral activation reflects recruitment to compensate for neural decline

Order and item information in working memory: Neuroimaging, patient, and behavioral evidence.

Working memory is thought to include a mechanism that allows for the coding of order information. One question of interest is how order information is coded, and how that code is neurally implemented. This dissertation is divided into two parts. The first is an empirical study using functional magnetic resonance imaging (fMRI) to examine the neural regions involved in order processing. It includes behavioral and fMRI findings from an experiment which involved comparing two tasks, an item-memory task and an order-memory task. In each case, five letters were presented for storage, followed after a brief interval by a set of probe letters. In the case of the item-memory task, the two letters were identical, and the subject responded to the question, Was this letter one of the items you saw?. In the case of the order-memory task, the letters were different, and subjects responded to the question, Are these two letters in the order in which you saw them?. Behaviorally, items that were farther apart in the sequence elicited faster reaction times and higher accuracy in the Order task. Areas that were significantly more activated in the Order condition included parietal and prefrontal cortex. Parietal activations overlapped those involved in number processing, leading to the suggestion that the underlying representation of order and numbers may share a common process, coding for magnitude. The second part is a review of the behavioral, neuroimaging, and patient evidence for the separability of item and order memory and potential cognitive and neural mechanisms of the latter. The mechanisms discussed are inter-item associations, direct codes, dual codes, and magnitude codes, and it is suggested that different coding mechanisms may underlie order operations in different tasks. Furthermore, patient and neuroimaging evidence suggests that the prefrontal cortex and, perhaps, the parietal cortex are involved in order processing.Ph.D.Biological SciencesCognitive psychologyNeurosciencesPsychobiologyPsychologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132833/2/9990935.pd

Neurocognitive Ageing of Storage and Executive Processes

Converging behavioural and neuropsychological evidence indicates that age-related changes in working memory contribute substantially to cognitive decline in older adults. Important questions remain about the relationship between working memory storage and executive components and how they are affected by the normal ageing process. In several studies using positron emission tomography (PET), we find age differences in the patterns of frontal activation during working memory tasks. We find that separable age differences can be linked to different cognitive operations underlying short-term information storage, and interference resolution. Some operations are associated with age-related increases in activation, with older adults displaying bilateral activations and recruiting prefrontal areas more than younger adults. Other operations are associated with age-related decreases in activation. We consider the implications of these results for understanding the working memory system and potential compensatory processes in the ageing brain