Does white matter matter? Spatio-temporal dynamics of task switching in aging.

Older adults often encounter difficulties in switching between tasks, perhaps because of age-related decreases in executive function. Executive function may largely depend on connections between brain areas-connections that may become structurally and functionally weaker in aging. Here we investigated functional and structural age-related changes in switching between a spatial and a verbal task. These tasks were chosen because they are expected to differentially use the two hemispheres. Brain measures included anatomical information about anterior corpus callosum size (CC; the major commissure linking the left and right hemisphere), and the event-related optical signal (EROS). Behavioral results indicated that older adults had greater task-switching difficulties, which, however, were largely restricted to switching to the spatial task and to individuals with smaller anterior CCs. The EROS data showed both general switching-related activity in the left middle frontal gyrus (with approximately 300-msec latency) and task-specific activity in the inferior frontal gyrus, lateralized to the left for the switch-to-verbal condition and to the right for the switch-to-spatial condition. This lateralization was most evident in younger adults. In older adults, activity in the switch-to-spatial condition was lateralized to the right hemisphere in individuals with large CC, and to the left in individuals with small CC. These data suggest that (a) task switching may involve both task-general and task-specific processes; and (b) white matter changes may underlie some of the age-related problems in switching. These effects are discussed in terms of the hypothesis that aging involves some degree of cortical disconnection, both functional and anatomical.

Reduced suppression or labile memory? Mechanisms of inefficient filtering of irrelevant information in older adults.

Cognitive aging theories emphasize the decrease in efficiency of inhibitory processes and attention control in normal aging, which, in turn, may result in reduction of working memory function. Accordingly, some of these age-related changes may be due to faster sensory memory decay or to inefficient filtering of irrelevant sensory information (sensory gating). Here, event-related brain potentials and the event-related optical signal were recorded in younger and older adults passively listening to tone trains. To determine whether age differentially affects decay of sensory memory templates over short intervals, trains were separated by delays of either 1 or 5 sec. To determine whether age affects the suppression of responses to unattended repeated stimuli, we evaluated the brain activity elicited by successive train stimuli. Some trains started with a shorter-duration stimulus (deviant trains). Results showed that both electrical and optical responses to tones were more persistent with repeated stimulation in older adults than in younger adults, whereas the effects of delay were similar in the two groups. A mismatch negativity (MMN) was elicited by the first stimulus in deviant trains. This MMN was larger for 1- than 5-sec delay, but did not differ across groups. These data suggest that age-related changes in sensory processing are likely due to inefficient filtering of repeated information, rather than to faster sensory memory decay. This inefficient filtering may be due to, or interact with, reduced attention control. Furthermore, it may increase the noise levels in the information processing system and thus contribute to problems with working memory and speed of processing.