| |
Abstract:
We used event-related fMRI to measure brain activity in
younger and older adults while they performed an item-recognition
working memory task. Each trial was composed of (1) a 4-second
encoding period in which subjects viewed 2 or 6 letters, (2) a
12-second retention period and (3) a 2-second retrieval period in
which subjects decided if a single probe letter was or was not part
of the memory set. There was a greater volume of activity in
encoding and retrieval periods than in the retention period. Age
differences in the two memory load conditions were observed in
dorsolateral prefrontal cortex in the encoding period. Age
differences in the two memory load conditions were observed in
ventrolateral prefrontal cortex in the retrieval period. Minimal
age differences occurred during the retention period. Regression
analyses of individual subjects' reaction time and cortical
activity indicated that response rate accounted for considerable
variance in fMRI activity for both age groups in the retrieval
period but not at other task periods. Age differences in the
anatomical locus of this relationship were observed. These results
are consistent with models of cognitive slowing in which response
rate is related to processing efficiency. The present results
suggest that individual differences in processing speed may have
different neural substrates for older and younger adults.
|
