Abstract
Working memory (WM) is a limited-capacity cognitive system that allows the storage and use of a limited amount of information for a short period of time. Two WM processes can be distinguished: maintenance (i.e., storing, monitoring, and matching information) and manipulation (i.e., reordering and updating information). A number of studies have reported an age-related decline in WM, but the mechanisms underlying this deterioration need to be investigated. Previous research, including studies conducted in our laboratory, revealed that age-related cognitive deficits are related to decreased millisecond timing, i.e., the ability to perceive and organize incoming events in time. The aim of the current study was: (1) to identify in the elderly the brain network involved in the maintenance and manipulation WM processes; and (2) to use an fMRI task to investigate the relation between the brain activity associated with these two processes and the efficiency of temporal information processing (TIP) on a millisecond level reflected by psychophysical indices. Subjects were 41 normal healthy elderly people aged from 62 to 78 years. They performed: (1) an auditory verbal n-back task for assessing WM efficiency in an MRI scanner; and (2) a psychophysical auditory temporal-order judgment (TOJ) task for assessing temporal resolution in the millisecond domain outside the scanner. The n-back task comprised three conditions (0-, 1-, and 2-back), which allowed maintenance (1- vs. 0-back comparisons) and manipulation (2- vs. 1-back comparisons) processes to be distinguished. Results revealed the involvement of a similar brain network in the elderly to that found in previous studies. However, during maintenance processes, we found relatively limited and focused activations, which were significantly extended during manipulation. A novel result of our study, never reported before, is an indication of significant moderate correlations between the efficiency of WM and TIP. These correlations were found only for manipulation but not for maintenance. Our results confirmed the hypothesis that manipulation in the elderly is a dynamic process requiring skilled millisecond timing with high temporal resolution. We conclude that millisecond timing contributes to WM manipulation in the elderly, but not to maintenance.
Highlights
Working memory (WM) is involved in almost every cognitive task and plays a crucial role in complex cognition in humans
Our results indicate the contribution of the ventrolateral prefrontal cortex (vlPFC) to maintenance, which is congruent with the findings of Owen (1997) and D’Esposito et al (1998)
In agreement with the traditional functional division between maintenance and manipulation, our results suggest that activity in the dorsolateral prefrontal cortex (dlPFC) (BA 9 and 46), but not in vlPFC (BA 44, 45, 47), is involved in manipulation
Summary
Working memory (WM) is involved in almost every cognitive task and plays a crucial role in complex cognition in humans. It allows one to successfully keep and manipulate information in mind over a short period of time (Baddeley, 2000; Cowan, 2008). Many people with advancing age report problems in processing information in rapidly changing contexts. Evidence indicates that WM consists of two different processes: maintenance for temporary storage in a readily accessible state and manipulation for the online information processing required for the guidance of subsequent behavior. Maintenance has been defined as storage (including rehearsal), monitoring, and matching information in WM, whereas manipulation refers to the reorganization and updating of each memory set (Fletcher and Henson, 2001; Veltman et al, 2003)
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