Effects of Aging on Working Memory Performance and Prefrontal Cortex Activity: A Time-Resolved Spectroscopy Study

Abstract

Objective This study aimed to employ time-resolved spectroscopy (TRS) to explore age-related differences in prefrontal cortex (PFC) activity while subjects performed a working memory task. Methods We employed TRS to measure PFC activity in ten healthy younger and ten healthy older subjects while they performed a working memory (WM) task. All subjects performed the Sternberg test (ST) in which the memory-set size varied between one and six digits. Using TRS, we recorded changes in cerebral blood oxygenation as a measure of changes in PFC activity during the task. In order to identify left/right asymmetry of PFC activity during the working memory task, we calculated the laterality score, i.e., Δoxy-Hb (right Δoxy-Hb—left Δoxy-Hb); positive values indicate greater activity in the right PFC, while negative values indicate greater activity in the left PFC. Results During the ST, statistical analyses showed no significant differences between the younger and older groups in accuracy for low memory-load and high memory-load. In high memory-load tasks, however, older subjects were slower than younger subjects (P < 0.05). We found that the younger group showed right lateral responses with a stronger right than left activation in the frontal pole, whereas the older group showed bilateral responses (P < 0.05). Conclusions The present results are consistent with the hemispheric asymmetry reduction in older adults (HAROLD) model; working memory tasks cause asymmetrical PFC activation in younger adults, while older adults tend to show reduced hemispheric lateralization.