Healthy aging is accompanied by declining sensory function in all five senses and
subtle deficits in higher order cognition. Among the age-related changes that have been
observed is an increase in multisensory integration. In the young brain, multisensory
integration is governed by a combination of stimulus characteristics (spatial location,
timing, and intensity) and higher-order cognitive constraints (semantic congruence,
attention). Given previous research suggesting that executive functioning and attention
might be impaired in healthy aging, it was hypothesized that increased integration
resulted from a failure to properly restrict incoming sensory information with crossmodal
attention.
Behavioral testing comparing reaction times and accuracy during voluntary and
involuntary attentional tasks demonstrated that older adults maintained the ability to
successfully engage cross-modal attention. Therefore, another behavioral experiment was
undertaken to investigate the ability of older adults to restrict multisensory processing
using cross-modal attention. The amount of multisensory integration while participants
divided their attention across vision and hearing was compared to that when they
attended selectively to vision or hearing. Greater multisensory integration was observed
with aging in all attentional conditions, suggesting that older adults were processing more
to-be-ignored stimuli than younger adults. However, older adults were able to attenuate
multisensory integration equivalently to younger adults, supporting the findings from the
first study that cross-modal attentional mechanisms are functional in healthy aging. The
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reason for increased integration during selective attention appeared to be that there was
an overall increase in the amount of sensory information processed in all conditions. This
suggests that the reason for increased multisensory integration with aging is not due to a
failure of attention, but rather greater sensory processing at baseline.
A functional neuroimaging study was undertaken to investigate the neural
mechanisms of a baseline shift in sensory processing. Average quantitative perfusion
maps were used to assess functional brain activity in older and younger adults for resting
state and visual steady-state conditions. Previous research shows that cross-modal
attention acts primarily by suppressing processing of stimuli in the unattended modality.
Therefore, relative activity in auditory and visual cortices was examined, as well as the
ratio of visual to auditory activity. Results of the imaging experiment supported
behavioral observations. Older adults showed relatively greater auditory activity than
younger adults at resting baseline. While participants in both age groups showed
expected task-related reductions during the visual condition, older adults still exhibited
greater activity to background auditory stimuli.
Finally, it was hypothesized that the observed baseline increase in processing of
unattended auditory stimuli was due to increased activity in the default mode network.
Evidence did not support this hypothesis. The default mode was found to have lower
resting activity in older adults than younger adults, and default mode activity did not
correlate with auditory activity. However, the observation of reduced metabolic activity
in the default mode network is novel and important in its own right, as is the suggestion
from the data that the default mode network may become decoupled during visual tasks
in healthy aging adults.