Behavioural Effects of Caffeine: the Specificity Hypothesis
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BEHAVIOURAL EFFECTS OF CAFFEINE: THE SPECIFICITY HYPOTHESIS A thesis submitted for the degree of Doctor of Philosophy By WENDY SNOWDEN School of Human Sciences and Law, Faculty of Society and Health, Buckinghamshire New University January 2008 This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author under the terms of the United Kingdom Copyright Acts. No quotation from the thesis and no information derived from it may be published without proper acknowledgement. i Abstract This thesis argues that caffeine use offered a survival advantage to our ancestors and that moderate use continues to offer modern humans benefits. Caffeine ingestion, through the blocking of adenosine receptors, elicits broad elements of the mammalian threat response, specifically from the ‘flight or fight’ and ‘tend and befriend’ repertoires of behaviour: in effect, caffeine hijacks elements of the stress response. If the effects of caffeine had been discovered recently, rather than being available to Homo sapiens since Neolithic hunter gatherer times, it is likely that caffeine would be considered a ‘smart’ drug. More caffeine is being ingested today than ever previously recorded. Caffeine use is found across all age groups, all socio-economic strata, most ethnic groups, and is being used increasingly by the medical and pharmaceutical industries and by the armed forces. Yet despite this wide usage and a substantial body of research literature, there is at present no clear pattern or plausible model for the way caffeine achieves its effects. There is much contradiction in the literature and ambiguity as to why caffeine use should improve performance on some tasks, impair it on others and have no effect on other tasks, for some but not all of the time. The present work argues, through an examination of the specificity of caffeine’s operation, that these effects are not arbitrary but elicited by the nature of the tasks, in particular that caffeine ingestion affects those processes and behaviours which improve the probability of survival under perceived threat or stress. This is argued through the perspective of evolutionary psychology and relies theoretically on Polyvagal Theory. The argument generates testable hypotheses and empirical support for the thesis is garnered from nine experiments on card-sorting, verbal and numerical processing, local and global categorization, field dependence-independence, the Stroop task, tests of visuo-spatial ability, and from a correlational study of caffeine use and personality traits. It is concluded that moderate caffeine use in healthy adults promotes behaviours likely to be adaptive under perceived threat or stress. Limitations of both theory and empirical work and are discussed, together with potential practical applications and suggestions for further work. ii Contents Abstract ………………………………………………………………………………………………….. ii Contents …………………………………………………………………………………………………. iii List of Illustrations ……………………………………………………………………….………...…. viii List of Tables ……………………………………………………………………………………...….... viii Acknowledgements …………………………………………………………..……………………...….. ix Author’s Declaration ……………………………………………………….…………………..……..… x 1. Chapter One: Framing the Research Question Moderate Caffeine Use in Adults Elicits Specific Processes at Physiological, Cognitive and Social Levels, Processes which Transmute into Adaptive Behaviours …………………………….………..……………………………...………… 1 1.1. Overview ……………….…………………………………….………………………………… 1 1.1.1. Rationale for the Thesis ………………………………………………….……………. 5 1.1.2. The Caffeine Paradoxes ……………………………………………….………………. 6 1.1.2.1. Paradox One: Drug Use Without a Perceived ‘Benefit’ ………………………….. 4 1.1.2.2. Paradox Two: Effects of Caffeine Use on the Health of Physical Systems ………. 7 1.1.2.3. Paradox Three: The Effect of Caffeine Use on Sports Performance …………...… 7 1.1.2.4. Paradox Four: The Effect of Caffeine on Behaviours Associated with Different Personality Types ……………………………………………….…………………. 8 1.1.2.5. Paradox Five: Ambiguity of Findings on Caffeine’s Effects on Perception and Cognitive Performance …………………………………………………...………... 8 1.1.2.6. Paradox Six: Lack of a Model of Caffeine’s Effects ………………...…………… 9 1.2. The Thesis ………………………………………………………………………………..…… 10 1.2.1. Empirical Investigation ………………………………….…………………………… 15 1.2.2. Overview: Perspectives and Causal Mechanisms ……………………………….…… 16 1.2.2.1. Perspectives on the Use of the Drug Caffeine ……………………...…………… 16 1.2.2.2. Perspectives from Social Psychology …………………………...…………….… 18 1.2.2.3. The Pharmacological Perspective ……………………………...…………….….. 19 1.2.2.4. Cognitive and Human Information Perspectives …………………………...…… 21 1.2.2.5. The Evolutionary Perspective …………………………………...………………. 24 1.2.3. Inferred Mechanisms through which Caffeine Ingestion Mediates Behaviour ….…... 27 1.2.3.1. Increased Physiological Arousal ………………………………………………… 28 1.2.3.2. Production Systems ……………………………………………………………… 30 1.2.3.3. Polyvagal Theory ………………………………………………………...……… 31 1.2.3.4. Observable Behavioural Shifts Under Caffeine Ingestion ………………………. 32 1.2.3.5. Field Dependence-Independence Shift Under Caffeine Ingestion ………………. 32 1.2.3.6. Visual Attentional Shift ……………………………...………………………….. 33 1.2.3.7. Caffeine and Object Identification Processes ………………………………….... 33 1.2.3.8. Caffeine’s Effect on Categorisation and Speed of Processing …………………... 34 1.2.3.9. Motivational Shift ……………………………………………………………….. 34 1.3. Summary ……………………………………………………………………………………… 35 2. Chapter Two: The Caffeine Model: Caffeine Use Elicits Adaptive Behaviours ...…………….. 38 2.1. The Evolution of Threat Response Strategies ………………………………………………… 38 2.1.1. Fight-or-Flight Behaviours and the Survival Response …………………………….... 38 2.1.2. Development and Dissolution of Complex Threat Responses ………………..……… 39 2.1.3. A Hierarchical Model of Threat Responses ………………………….………………. 40 2.1.4. An Evolutionary Perspective on the Threat Response ………………….……………. 42 2.2. The Model: Caffeine Use Elicits Adaptive Behaviours …………………………………..…... 45 2.2.1. Congruence: Perceptual Changes Elicited by Both Caffeine Ingestion and the Perception of Threat …………………………………………………………………...... 46 2.2.2. Evidence that Caffeine Increases Arousal ………………………………………..…... 47 2.2.3. Evidence that Caffeine Improves Vigilance …………………….…………………… 49 2.2.4. Caffeine Ingestion Affects the Orienting Response …………………..……………… 51 2.2.5. Caffeine Ingestion and the Acoustic Startle Reflex ……………………….…………. 52 2.3. Caffeine and the Survival Response ………………………………………………...………... 52 2.3.1. Caffeine’s Effects on Physical Performance and Endurance ……………….………... 54 2.3.2. Caffeine’s Effects on Motor Co-ordination ……………………………………….…. 55 2.3.3. Caffeine’s Effects on Reaction Time ……………………………………………….... 56 2.3.4. Caffeine’s Effects on Aggression ……………………..……………………………… 57 2.3.5. Caffeine’s Effects on Anxiety ……………………………………………….…...…... 58 iii 2.3.6. Caffeine’s Effects on Subjective Mood …………………………………………….... 59 2.3.7. The Effect of Caffeine Ingestion on Cognitive Performance ……………………..….. 60 2.3.8. The Effect of Caffeine on Creativity and Cognitive Flexibility ………….……..…… 61 2.3.9. Caffeine and Personality …………………………………………….……..………… 62 2.4. Reported Detrimental Effects of Caffeine Use ……………………………………………….. 62 2.4.1. Caffeine’s Effects on Cardio-Vascular Disease …………………………….………... 63 2.4.2. Caffeine Ingestion and Diabetes …………………………………………….……….. 64 2.4.3. Caffeine’s Effects on Reproduction ……………………………………………….…. 64 2.4.3.1. Adaptive Value of Lowered Reproductive Success in a Stressful Environment ... 65 2.5. Summary ……………………………………………………………………………………… 66 2.5.1. The Specificity Hypothesis …………………………………………………….…….. 66 3. Chapter Three: Evolutionary Pressures on Brain, Bodily Systems and Behaviour ..…………. 68 3.1. Introduction …………………………………………………………………………………… 68 3.2. Mechanisms of Adaptation …………………………………………………………………… 68 3.2.1. Survival Selection: Classical Fitness …………………………………………….…... 68 3.2.2. Inclusive Fitness ……………………………………………………………..……….. 69 3.2.3. Sexual Selection ………………………………….…………………………………... 70 3.2.4. Multilevel Selection ……………………………….…………………………………. 70 3.2.5. Mechanisms of Cooperation and Control Amongst Non-Kin …………..……….…… 71 3.2.5.1. Individual Benefit Versus Whole Group Benefit ………………………………... 72 3.2.5.2. Evolution of Moral Emotions ………………………...…………………………. 73 3.2.5.3. Social Contracts …………………………...…………………………………….. 74 3.2.6. Evolution of Fear, ‘Stress’ and the Perception of Threat ………..…….……………... 74 3.3. Evolution of the Primate Brain ………………………………………...………………….….. 75 3.3.1. The ‘Triune’ Brain Hypothesis ………………………………….…………………… 75 3.3.2. Evolutionary Homologies ……………………………………….…………………… 76 3.3.3. Primate Brain Size …………………………………………………………….……... 77 3.3.4. The Social Brain ………………………………………………………..…………….. 78 3.4. Connection and Regulation: the Central Nervous System ……………………………………. 79 3.4.1. Neural Regulation in Response to Threat: the Polyvagal Theory …………………..... 80 3.4.1.1. Mechanisms of Response to Environmental Challenge …………………………. 81 3.5. Theories of Stress and Polyvagal Theory: a Brief Historical Review ……...………………… 84 3.5.1. Early Perspectives on Fear and the Perception of Threat ……………………….…… 84 3.5.2. The Alarm Response or Fight or Flight …………………………………………….... 85 3.5.3. The General Adaptation Syndrome ……………………………………….…………. 86 3.5.4. Learned Helplessness, Depression and Post Traumatic Stress Disorder …………….. 86 3.5.5. The Perception of Control and Retention of the Vagal Brake ………….……………. 88 3.5.5.1. Accessing