In partial fulfillment of the requirements for the degree of 

Doctor of Philosophy in Applied Physiology

in the 

School of Biological Sciences

Matt Wittbrodt

will defend his dissertation

Do body water deficits and exercise-heat stress affect brain structure, function, and neuromotor performance?

Thursday, March 22th, 2018

8:00 AM

555 14th Street, NW, Atlanta, GA 30318

Room 1253

Thesis Advisor:

Mindy L. Millard-Stafford Ph.D. (advisor)

School of Biological Sciences

Georgia Tech

Committee members:

Audrey L. Duarte Ph.D.

School of Psychology

Georgia Tech

J. Chris Mizelle Ph.D.,

Department of Kinesiology

East Carolina University

Michael N. Sawka Ph.D.

School of Biological Sciences

Georgia Tech

Lewis A. Wheaton Ph.D.

School of Biological Sciences

Georgia Tech

Abstract

There is reason to believe body water deficits (dehydration) adversely impact the central nervous system despite protective physiological mechanisms to maintain brain homeostasis. Early animal models suggested severe dehydration (10% body mass loss) does not alter brain volume while recent findings in humans are conflicting regarding the impact of moderate levels (2-3% body mass loss) on cognitive-motor performance. Magnetic resonance imaging (MRI) allows real-time in vivo measurement of human brain morphology and localized hemodynamic responses during cognitive-motor tasks. Of the few MRI studies investigating effects of dehydration on brain morphology, most concur brain volume is unchanged, but specific structures (e.g., lateral ventricles) may be impacted. Furthermore, the relationship between structural changes (e.g., thalamus, cerebellum, cortical grey matter) and cognitive-motor functional decrements are not well-established. Most computerized tests assessing cognitive-motor performance require a motor response (e.g., button press) via activation of the visuomotor system. Visuomotor system function has two major phases: motor planning (i.e., from visual perception through determination of a movement goal) and motor execution (i.e., pressing a button after movement goal determination). Disruption to motor planning and/or execution have significant consequences such as degraded driving performance, increased navigation errors, and occupational accidents. The goal of this dissertation is to investigate whether changes in brain structures and activation due to dehydration impairs cognitive-motor functions and visuomotor performance. My overall hypothesis is dehydration-mediated plasma hypertonicity will elicit fluid shifts out of intracellular compartments (grey and white matter crenation) into extracellular spaces (expanding brain ventricles) and alter brain activations coupled with degraded visuomotor performance. In Aim 1, a systematic review of literature with meta-analysis determined the factors influencing the effect of dehydration on cognitive-motor performance. In Aim 2, changes in brain structures, activation, and performance during a cognitive-motor task emphasizing motor execution were quantified using MRI following moderate dehydration. In Aim 3, the effects of moderate dehydration on motor planning performance and function as measured by visual evoked potential amplitude using electroencephalography (EEG). The results of these experiments indicate dehydration impairs cognitive-motor and visuomotor performance (motor execution and motor planning). During motor execution, dehydration increased brain activations in the bilateral thalamus, basal ganglia, and anterior cingulate cortex without successfully preserving visuomotor performance. During motor planning, dehydration decreased attentional gating towards the stimulus which was also associated with impaired task performance. Changes in plasma tonicity were associated with volume changes in brain ventricles and adjacent periventricular structures. These studies provide novel insight regarding mechanisms by which dehydration alters specific brain structures and visuomotor functions commonly needed to complete tasks in a variety of occupations and daily life.