BENEFITS OF EXERCISE FOR MENTAL HEALTH

The Transient Hypofrontality Hypothesis Arne Dietrich, Ph.D. Department of Social and Behavioral Sciences American University of Beirut, Lebanon Presented by Roland Carlstedt, Ph.D. Introduction BENEFITS OF EXERCISE FOR MENTAL HEALTH STRESS, ANXIETY, DEPRESSION COGNITIVE FUNCTIONS PARAMETERS MECHANISM OF ACTION NO SATISFACTORY FRAMEWORK CURRENT EXPLANATIONS REMAIN HEAVILY FOCUES ON NEUROTRANSMITTER SYSTEMS (DOPAMINE, SEROTONIN, NOREPINEPHRINE, ENDORPHINS, ENDOCANNABINOIDS) BROADER FRAMEWORK NEEDED Important Note on Neural Resources • Misconception: Blood flow to brain increases during exercise • This is incorrect: Global cerebral blood flow to the brain is constant, approximately 750 ml/min, regardless of mental or physical activity • At rest: brain gets ca. 15% of total cardiac output per beat • As exercise intensity increases, cardiac output is redistributed (mostly to muscles) and the brain receives a lower percentage per beat. • At maximal intensity, the brain gets ca 4% of cardiac output per beat • However, this reduction is precisely offset by the overall increase in total cardiac output (the heart beats ca 4 times faster), resulting in a steady perfusion rate. • Global oxygen and glucose uptake is also constant Important Note On Neural Resources Important Note On Neural Resources The Transient Hypofrontality Hypothesis 133Xe Washout, Radioactive Microsphere, and Autoradiography Motor Structures Cortex (Primary, Supplementary and Premotor) Basal Ganglia Cerebellum Midbrain and Brainstem Nuclei (Substantia Nigra, Red Nucleus, etc) Various Thalamic Nuclei Pathways Sensory, Perceptual and Autonomic Structures Cortex (Primary, Secondary, Association) Sensory Thalamus Pathways Brainstem Nuclei Hypothalamus The Transient Hypofrontality Hypothesis Brain structures not showing LCGU after 30 min 85%max O 2 (Vissing et al., 1996) Prefrontal Cortex Frontal Cortex Cingulum CA3 (Hippocampus) Medial Nucleus of the Amygdala Nucleus Accumbens Various Hypothalamic Nuclei Median Raphe Interpeduncular Nucleus Nucleus of the Solitary Tract Inferior Olive Given that frontal regions are the only sizable amount of brain mass not activated, aerobic exercise can be regarded as a state of generalized brain activation with the notable exception of the executive system The Transient Hypofrontality Hypothesis Theoretical Framework • Most brain tissue is devoted to basic motor, perceptual, and autonomic processes • Large-scale bodily motion (e.g., running or tennis) recruits these neural regions • Endurance exercise recruits this large amount of neural tissue for a prolonged time • There are costs and benefits associated with efficient information processing • The brain has to operate on a fixed and quite limited amount of metabolic resources • Fundamental principle: processing in the brain is competitive, which is a direct consequence of the principles of evolution (Neural Darwinism, Edelman, 1993) Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256 Dietrich and Sparling (in press). Brain and Cognition The Transient Hypofrontality Hypothesis Theoretical Framework Because the brain has to make due with a finite amount of neural resources, humans posses a limited information processing capacity, which is not only true at the bottleneck of consciousness, but there also exists a total cap on all neural activity, including unconscious, parallel information processing. In other words, because the brain cannot maintain activation in all neural structures at once, the activation of a given brain structure must come at the expense of other brain structures. Thus, the massive and sustained neural activation during aerobic exercise must result in the temporary inhibition of brain structures not essential to the exercise, that is, brain regions the individual can afford to disengage. The transient hypofrontality hypothesis states that these regions are the higher cognitive centers of the prefrontal cortex and, perhaps, the amygdala. Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256 The Transient Hypofrontality Hypothesis Supportive Evidence • Numerous lines of evidence showing decreased prefrontal activity • Human EEG studies showing alpha and theta increases • Single cell recordings in exercising cats • Blood flow and metabolism in exercising rats • Testing of prefrontal-dependent cognitive abilities during exercise • Improvements in neuroimaging allowing for motion are needed Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256 Application to Mental Health ANXIETY DISORDERS (OCD AND GENERALIZED ANXIETY DISORDER) VMPFC: WIDESPREAD HYPERMETABOLISM (ALSO IN THE AMYGDALA) THIS CREATES A STATE OF HYPER-VIGILANCE & HYPER-AWARENESS (STRESS) (Baxter, 1990; Baxter et al., 1987) DEPRESSION VMPFC: HYPERMETABOLISM (right hemisphere, which mediates neg. emotions) DLPFC: HYPOMETABOLISM (PERSEVERATION ON NEGATIVE EVENTS) THINKING SAD THOUGHTS INDUCES A SIMILAR ACTIVATION PROFILE (Mayberg, 1997; Mayberg et al., 1995; Damasio et al., 2000) A NEW MECHANISTIC EXPLANATION The transient hypofrontality hypothesis offers a novel neural mechanism for the beneficial effects of exercise on mental health The massive and sustained neural activation during exercise (especially aerobic exercise that is maintained for a longer time and involves large muscle groups), causes a strain on the brain’s metabolic resources. This makes it impossible to maintain the excessive neural activity in the PFC that is characteristic of the neural profile of depression, anxiety and stress. In other words, the brain is forced to run the PFC on “safe mode”, which results in phenomenological subtraction and provides relief from life’s worries. Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256 A NEW MECHANISTIC EXPLANATION The transient hypofrontality hypothesis also offers a novel approach to the phenomenon known by the unfortunate descriptor the “Runner’s High” The hallmark of this subjective state is a subtle change in mental status: Analgesia Euphoria (disinhibition) A sense of timelessness A sense of well-being Floating or ephemeral attention Reduced awareness of surroundings Unity with self or nature These descriptions are consistent with a state of prefrontal hyopfunction Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256 The Transient Hypofrontaltiy Hypothesis Duration and Intensity The brain codes intensity by the rate of neuronal firing. An increased rate of firing increases a neuron’s metabolic needs. Thus, low-intensity exercise is less likely to force the brain to shift its limited resources away from the PFC. Conversely, exercise of high intensity introduces a second limiting factor, as intense exercise cannot be maintained long enough by the cardiovascular system to tax the brain’s resources. However, a moderate workload would be associated with a considerable increase in neuronal firing rates in a majority of brain tissue that can also be sustained for a long period of time. Thus, exercise intensity at the anaerobic threshold, is the effort most conducive to force a reallocation of resources at the expense of higher cognitive and emotional structures such as prefrontal areas. This fits well with clinical data showing that exercise in the moderate, aerobic range is most beneficial to mental health, a fact that has yet to be adequately addressed by neurochemical theories. Dietrich, A. (2003). Consciousness and Cognition, 12, 231-256