NEURAL PREDICTORS OF EXERCISE ADHERENCE IN OLDER ADULTS by Swathi Gujral Bachelor of Science in Psychology, Indiana University, 2009 Submitted to the Graduate Faculty of The Dietrich School of Arts and Sciences in partial fulfillment of the requirements for the degree of Master of Science University of Pittsburgh 2015
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NEURAL PREDICTORS OF EXERCISE ADHERENCE IN OLDER ADULTS
by
Swathi Gujral
Bachelor of Science in Psychology, Indiana University, 2009
Submitted to the Graduate Faculty of
The Dietrich School of Arts and Sciences in partial fulfillment
of the requirements for the degree of
Master of Science
University of Pittsburgh
2015
ii
UNIVERSITY OF PITTSBURGH
Dietrich School of Arts and Sciences
This thesis was presented
by
Swathi Gujral
It was defended on
December 9th, 2014
and approved by
Committee Member: Peter Gianoros, Professor, Department of Psychology
Committee Member: Anna Marsland, Associate Professor, Department of Psychology
Thesis Advisor: Kirk Erickson, Associate Professor, Department of Psychology
likely operate in an interactive manner to initiate, maintain, and ultimately achieve goals. An
unexpected association also emerged between grey matter integrity in the cerebellum and
adherence in the present study, with 30% volume in the cerebellum predicting adherence. This
finding is consistent with evidence from a recent meta-analysis of 350 functional neuroimaging
studies showing that the cerebellum is implicated in metacognitive processing that involves high
levels of abstraction (Van Overwalle, Baetens, Marien, & Vandekerckhove, 2014).
Regions in which grey matter integrity predicted adherence independent of self-efficacy
were also explored, given that self-efficacy is presently the most studied predictor of exercise
33
adherence (Young et al., 2014). This examination tested the functional utility of using objective
neuroimaging methods to understand exercise adherence, and indeed demonstrated that structural
MRI methods help us to learn about adherence in ways that cannot be captured by subjective
self-efficacy ratings. Most gray matter regions initially predicting adherence remained significant
after controlling for self-efficacy. However, controlling for self-efficacy reduced the percentage
of gray matter predicting adherence from 22% to 7% across the whole-brain. Interestingly, a
disjunction of regions predicting adherence before and after controlling for self-efficacy revealed
that volume in medial regions of the frontopolar cortex no longer predicted adherence after
controlling for self-efficacy. This is consistent with the proposed role of medial aPFC as central
to metacognition, which closely maps on self-efficacy (Baird et al., 2013). Lateral regions of the
frontopolar cortex remained predictive of adherence after controlling for self-efficacy. This
disjunction analysis overall suggested that gray matter associations with adherence may partially
rely on associations with self-efficacy, as well as uniquely predict adherence independent of self-
efficacy. However, the statistical approach used in the present study cannot confirm the extent to
which self-efficacy truly mediates the relationship between gray matter volume in these regions
and adherence.
The broader implications of this study include its contribution to the emerging field of
neuroimaging research using the ‘brain as a predictor’ approach to understanding real-world
behavioral phenomenon (Berkman & Falk, 2013). The aim of this new methodological approach
is to leverage objective measures of neural structure and function using neuroimaging to predict
long-term, ecologically valid outcomes that extend beyond laboratory testing. The advent of
neuroimaging technology affords the possibility to link objective neurobiological markers to
behavior in a variety of domains, including cognitive function, health, economic decision-
34
making, and clinical and neurological outcomes (Berkman & Falk, 2013). Berkman and
colleagues have outlined guidelines for using this methodological approach to understand real-
world outcomes. An important assumption underlying this approach is that neural markers serve
as objective summary measures of psychological constructs and behavioral outcomes. Using this
approach, the present study aimed to tap into neural substrates of exercise adherence and self-
efficacy.
The findings from the present study have shown that older adults with greater grey matter
volume in regions relevant to self-efficacy and self-regulation demonstrate better adherence to a
yearlong exercise intervention. Importantly, these associations may be heightened in this elderly
sample, given that older adults are known to have greater gray matter atrophy and greater
variability in exercise adherence (Conn et al., 2003; Resnick & Nigg, 2003) The implications of
these grey matter associations may also extend beyond exercise adherence, to include the
adoption and maintenance of other healthy lifestyle behaviors that are protective against physical
and cognitive health decline. In turn, grey matter integrity in these regions may broadly influence
quality of life in older adults.
Understanding the relationship between gray matter volume prior to the intervention and
exercise adherence is also the first step to understanding individual differences in exercise-
induced improvements in gray matter volume (reduction in atrophy). The next step will be to
examine the extent to which regions predictive of adherence show intervention-induced
volumetric changes. This will help us to understand whether this relationship between brain
health and adherence impacts exercise-induced improvements in gray matter as a function of
poor adherence. To address this, interventions can be tailored to focus on improving self-efficacy
during the initial phases of the intervention and target improving self-regulatory skills, such as
35
planning and goal setting. On the other hand, individuals with greater gray matter atrophy in
these regions may show similar levels of improvement in brain health as those with less atrophy.
This could indicate that those with poorer brain health have ‘more to gain’ from the exercise
intervention, relative to those with better brain health, who may show a ‘ceiling effect’. Future
research can also expand on this study by examining the relationship between gray matter
volume and adherence after controlling for additional psychological predictors of adherence (i.e.
self-regulatory strategies, executive functions). This will help to distinguish which brain regions
are implicated in each psychological factor, as well as to understand the extent of overlap
between regions implicated in each psychological factor. Future studies can also statistically
examine the extent to which self-efficacy and other psychological factors mediate the
relationship between gray matter volume and exercise adherence.
Limitations
There are several limitations to the present study. This is the first examination of the
neural substrates predicting exercise adherence, therefore regions specifically predictive of
adherence relative to those supporting behavioral goal-pursuit more generally cannot be
distinguished from this study. Also, a comprehensive explanation for grey matter regions
predictive of exercise adherence is yet to be determined; this study did not include adequate
measures to test several possible explanations for these associations. Next, this was a 12-month
intervention, and it is unclear whether these same effects would occur for shorter or longer trials
or trials of a different type, duration, or intensity (e.g., resistance training). This study was also
conducted using a mostly Caucasian sample of highly educated healthy older adults from a small
Midwestern town; therefore, these results may not be easily generalizable to more culturally
diverse, younger, and clinical populations. There are a number of additional limitations related to
36
the MRI analysis methods used in this study. First, voxel-based morphometry only provides
estimates of tissue type, and thus in drawing conclusions from our data, it must be taken into
account that the data is probabilistic rather than absolute. Also, using VBM techniques, brain
images are forced into registered space prior to assessing volumetric maps, limiting the accuracy
of these volumetric findings. Estimates using VBM are also not on a cellular level, so it is
difficult to ascertain true “volume” from this segmentation technique. Nonetheless, VBM has
been used as a standard method for estimating gray matter volume in a number of studies, and
allows for examining relationships between gray matter volume and outcomes on a voxel-wise
basis. Finally, estimates of effect size are difficult to ascertain using bootstrap regression
methods with neuroimaging data; therefore, extracting values into SPSS only allows for a rough
approximation of effect size.
In summary, I found that gray matter volume in a broad array of prefrontal, cingulate,
temporal, parietal, subcortical, and cerebellar regions predicted exercise adherence in older
adults. Most of these associations remained after accounting for the relationship between self-
efficacy and adherence. Gray matter regions associated with self-efficacy were similarly
widespread across cortical and subcortical regions, with significant overlap with regions
predictive of adherence. These findings provide preliminary support for neural substrates
underlying exercise adherence, as well as self-efficacy. Future research will need to expand on
these findings by examining neural substrates of other social-cognitive factors, as well exploring
how these associations impact exercise-related improvements in brain health.
37
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