Components of aging

Age-related cognitive decline has been linked to a reduction in
attentional resources that are assumed to result from alterations
in the aging brain. A core ability that is subject to age-related
decline is visual attention, which enables individuals to select
the most important information for conscious processing and action.
However, visual attention is considered a conglomerate of various
functions and the specific components underlying age differences in
performance remain little understood. The present PhD project aimed
at dissociating age effects on several (sub-) components that
concur in visual attention tasks within a neurocognitive approach.
Established and theoretically grounded psychological paradigms that
allow separating various attentional components were combined with
event-related potentials (ERPs), which provide a temporally
fine-graded dissociation of cognitive processes involved in a task.
1st Project The first project was designed to determine the
origin(s) of age-related decline in visual search, a key paradigm
of attention research. To pursue this goal on a micro-level,
response time measures in a compound-search task, in which the
target-defining feature of a pop-out target (color/shape) was
dissociated from the response-defining feature (orientation), were
coupled with lateralized ERPs. Several ERP components tracked the
timing of processing stages involved in this task, these being (1)
allocation of attention to the target, marked by the
posterior-contralateral negativity (PCN), (2) target analyses in
vSTM, marked by the sustained posterior-contralateral negativity
(SPCN), (3) response selection, marked by the stimulus-locked
lateralized readiness potential (LRP) and (4) response execution,
marked by the response-locked LRP. Slowed response times (RT) in
older participants were associated with age differences in all
analyzed ERPs, indicating that behavioural slowing accrues across
multiple stages within the information processing stream.
Furthermore, v behavioral data and ERPs were analyzed with respect
to age and carry-over effects from one trial to the next. The
intertrial analyses revealed relatively automatic processes – such
as dimension weighting facilitating the early stage of visual
selection, and response weighting facilitating the late stage of
response execution – to be preserved in older age. By contrast,
more controlled processes – such as the flexible stimulus-response
(S-R) (re-) mapping across trials on the intermediate stages of
response selection - were particularly affected by aging. This
indicates that besides general slowing, specific age decrements in
executively controlled processes contribute to age-related decline
in visual search. 2nd Project The second project explored neural
markers of individual and age differences in attention parameters
formally integrated in Bundesen’s computational Theory of Visual
Attention (TVA). According to the model, two parameters of general
visual attention capacity, perceptual processing speed C and visual
short-term memory (vSTM) storage capacity K are defined and can be
modeled mathematically independently for a particular individual.
More recently, the neural interpretation of the model (NTVA)
suggested that the two functions (at least partly) rely on distinct
brain mechanisms. To test this assumption in an empirical approach,
individual TVA-based estimates were derived in a standard TVA whole
report task, and ERPs of the same participants were recorded in an
adapted EEG-compatible version of the task. In the first study of
the second project, we explored neurophysiological markers of
interindividual differences in the two functions in younger
participants. The results revealed distinct ERP correlates to be
related to the parameters: Individuals with higher compared to
lower processing speed C had significantly smaller posterior N1
amplitudes, suggesting that the rate of object categorization is
associated with the efficiency of early visual processing.
Individuals with higher compared to lower storage capacity showed
stronger contralateral delay activity (CDA) over visual areas,
indicating that the limit of vi vSTM relies on
topographically-organized sustained activation within the visual
system. These results can be regarded as direct neuroscientific
evidence for central assumptions of the theoretical framework. In
the second study of the second project, the same approach was
pursued to investigate whether and how TVA attentional capacity
parameters and their neural markers change with aging. First, the
same ERP correlates of processing speed and storage capacity
indexing individual differences in younger participants (i.e., the
posterior N1 marked differences in processing speed C and the CDA
marked differences in storage capacity K, respectively) were found
to be valid also in the older group. In addition to this, two
further components marked performance differences in the parameters
exclusively within the older group: Older participants with lower
processing speed showed smaller anterior N1 amplitudes relative to
faster older and all younger participants, suggesting a selective
loss of resources supporting early control of attentional guidance.
Older participants with higher storage capacity exhibited a
stronger right-central positivity than older participants with
lower storage capacity and all younger participants. This pattern
is indicative of compensatory recruitment of additional neural
resources in high-functioning older individuals, presumably related
to enhanced executive control fostering sustained activation of
vSTM representations. Again, these findings strongly support the
NTVA framework, proposing distinct neural mechanisms underlying
processing speed and storage capacity. Furthermore, they show that
distinct mechanisms of attentional control determine the two
functions in older age.