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Chapter 2 Thesis Aims and Objectives
2.1
General Aims
The aims and objectives of the thesis were two-fold. The thesis end-point was a planned intervention study to assess the potential for a bovine-derived PL intervention to modulate cognitive performance and stress responses under conditions of stress. Considering the small and inconsistent reported evidence of the potential for PLs to moderate cognitive performance under stress conditions, the examination of a number of design and methodological factors was considered an important prerequisite for this intervention study. The preparatory identification of suitable design and methodological conditions for the PL intervention study constitutes the second aim and objective of the thesis.
The thesis studies preceding the dietary intervention study were undertaken with the aim of identifying the appropriate methodological context in which to undertake an intervention study. These methodological considerations can be summarised as:
1. Identification of an appropriate stress context
2. Identification of specific cognitive tests sensitive to stress exposure
3. Identification of the characteristics of an appropriate sample
2.2
Stress Context
The identification of a reliable stress context for the PL intervention study is a central aim of the thesis. Considering the centrality of cortisol in both the moderation of cognitive performance, and potential stress-buffering effect of PLs, a stressor that reliably provokes the HPA axis was required. The majority of PL intervention studies to date have employed laboratory-based stress contexts. Laboratory protocols offer the benefit of standardisation of the stress faced, systematic and reliable measurement of stress responses, and control of extraneous variables. However, laboratory stress tasks typically employ artificial stimuli rarely encountered in the real-world and less intimately linked to genuine goals and identity. Furthermore, evidence of only a moderate relationship between cortisol responses elicited in the laboratory and real-world contexts have been reported (Van Eck et al., 1996). Hence, the feasibility of employing a real- world or a laboratory stress contexts was explored and reported in Chapters 4 and 5.
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A mixed design comprising a between-subjects random allocation to a PL-drink or placebo condition, and a within-subjects repeated exposure to stress following PL or placebo drink intake was adopted for the PL intervention study (reported in Chapter 7). The increased power and reduction of random effect variation were the main considerations underpinning the decision to examine cognitive performance under stress in the same participants pre- and post-dietary intervention. This was considered important due to the modest to small effects reported for PL intervention studies, combined with the high inter-individual variability in cortisol response and cognitive test performance. The benefits offered by this design create a number of key methodological considerations. Firstly, as habituation is an often reported characteristic of the HPA axis, repeated exposure to a homotypic stressor will likely result in non-comparable endocrine responses across repeated stress exposures. Comparing stress responses and cognitive performance before and after a nutritional intervention without comparable cortisol responsivity across the repeated exposures increases the likelihood that any effect of the nutritional manipulation will be confounded, or overshadowed, by the divergent background endocrine milieu. Therefore the capacity of stress contexts to provoke significant cortisol responses over repeated exposures was explored.
2.3
Cognitive Tests Sensitive to Stress
Previous evidence of the limited capacity of PL intake to moderate cognitive performance under conditions of stress may be partly reflective of the divergent sensitivities of specific cognitive tests to stress provocation. Examination of the effects of stress on cognitive performance, and the potential for a PL dietary intervention to modulate this relationship, requires the use of suitable cognitive tests shown to be sensitive to stress conditions. Specific tests of cognitive domains sensitive to stress exposure were administered under stress and non-stress conditions across the studies in this thesis. This permitted the selection of tests, showing capacity for impaired performance under stress conditions, to explore the potential protective effects of PL intake.
2.4
Selection of Sample
The stress and cognitive performance literature suggests that the modulating effects of stress on cognition may only be demonstrated in those exhibiting robust cortisol responses. Furthermore, evidence of the potential for PL interventions to moderate
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cortisol responsivity, and by extension protect cognitive function, suggests such effects may be limited to individuals with some form of stress vulnerability. Hence, individuals with a tendency towards high cortisol responses to stress provocation were targeted for the PL intervention study. The existence of stable dispositional factors related to cortisol responsivity was explored via the collection of trait personality measures in studies utilising real-world (Chapters 4 and 5) and laboratory contexts (Chapter 6). Emergent predictors of cortisol responsivity were then employed to select a sample for the PL intervention study (Chapter 7).
2.5
Specific Thesis Aims
I. Explore the utility of a real-world or laboratory stressor for a dietary intervention study (Chapters 4 – 6)
II. Assess the capability of the identified stress context to elicit comparable stress (primarily cortisol) responses over repeated exposures (Chapter 6)
III. Identify cognitive tests sensitive to the impairing effects of stress (Chapter 5 and 6)
IV. Identify potential predictors of cortisol responsivity to aid selection of an intervention sample likely to elicit robust cortisol responses (Chapters 4 – 6) V. Examine the effects of a dietary PL intervention on cognitive performance and
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