Abstract:
Age-related differences in information processing have often been explained
through deficits in older adults' ability to ignore irrelevant stimuli and suppress
inappropriate responses through inhibitory control processes. Functional imaging
work on young adults by Nelson and colleagues (2003) has indicated that inferior
frontal and anterior cingulate cortex playa key role in resolving interference effects
during a delay-to-match memory task. Specifically, inferior frontal cortex appeared to
be recruited under conditions of context interference while the anterior cingulate was
associated with interference resolution at the stage of response selection. Related
work has shown that specific neural activities related to interference resolution are not
preserved in older adults, supporting the notion of age-related declines in inhibitory
control (Jonides et aI., 2000, West et aI., 2004b).
In this study the time course and nature of these inhibition-related processes
were investigated in young and old adults using high-density ERPs collected during a
modified Sternberg task. Participants were presented with four target letters followed
by a probe that either did or did not match one of the target letters held in working
memory. Inhibitory processes were evoked by manipulating the nature of cognitive
conflict in a particular trial. Conflict in working memory was elicited through the
presentation of a probe letter in immediately previous target sets. Response-based
conflict was produced by presenting a negative probe that had just been viewed as a
positive probe on the previous trial.
Younger adults displayed a larger orienting response (P3a and P3b) to positive
probes relative to a non-target baseline. Older adults produced the orienting P3a and
3
P3b waveforms but their responses did not differentiate between target and non-target
stimuli. This age-related change in response to targetness is discussed in terms of
"early selection/late correction" models of cognitive ageing.
Younger adults also showed a sensitivity in their N450 response to different
levels of interference. Source analysis of the N450 responses to the conflict trials of
younger adults indicated an initial dipole in inferior frontal cortex and a subsequent
dipole in anterior cingulate cortex, suggesting that inferior prefrontal regions may
recruit the anterior cingulate to exert cognitive control functions. Individual older
adults did show some evidence of an N450 response to conflict; however, this
response was attenuated by a co-occurring positive deflection in the N450 time
window. It is suggested that this positivity may reflect a form of compensatory
activity in older adults to adapt to their decline in inhibitory control.
