Glucose and memory: The influence of drink, expectancy, and beliefs
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ORIGINAL INVESTIGATION
Glucose and memory: The influence of drink, expectancy, and beliefs Brian Stollery & Leonie Christian
Received: 10 July 2012 / Accepted: 18 March 2013 / Published online: 5 April 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Rationale An increasing number of studies suggest that glucose can enhance aspects of memory and the central methodology is the use of the glucose–placebo design. One critical issue therefore is separating the pharmacological effects of glucose from the expectancies created by consuming a drink that might contain glucose. Objective A modified balanced placebo design examined the role that expectancy and belief about the drink consumed has on the pharmacological changes observed following glucose consumption. Method Ninety-three participants, allocated according to a drink (glucose, placebo) × message (told glucose, told nothing, told placebo) unrelated design, were administered tasks assessing immediate and delayed verbal free recall, spatial recognition and semantic verification. Each task has some evidence for hippocampus involvement, and variations in task difficulty were used to assess the idea that glucose effects are sensitive to task difficulty. Results While the messages biased drink judgements in the expected direction, judgements of drink content were at chance and glucose only enhanced delayed free recall. The subtle effects of the messages did not modify the glucose enhancement. However, believing glucose had been consumed showed an independent improvement in delayed free recall. There was no evidence that task complexity enhanced the glucose effect. Conclusions The findings indicate that expectancy effects are unlikely to be confused with glucose enhancements, but beliefs about consuming glucose can augment performance on delayed free recall. The discussion considers the hippocampus and complexity hypotheses of glucose’s mode of B. Stollery (*) : L. Christian School of Experimental Psychology, University of Bristol, 12a Priory Road, Bristol BS8 1TU, UK e-mail: [email protected]
action and proposes the routine collection of drink beliefs in future studies. Keywords Glucose . Episodic memory . Semantic memory . Spatial memory . Expectancy . Beliefs . Balanced placebo design
Introduction The consumption of a drink containing glucose is typically associated with relatively selective improvements on tasks of learning and memory (Smith et al. 2011). One feature common to human research exploring this influence of glucose on cognition is the placebo design. In this design, participants are administered a glucose drink or a placebo drink that has been equated for sweetness (e.g. using aspartame or saccharine). With the establishment of informed consent, it is usually necessary for participants to be made aware that the drink they will be asked to consume might contain glucose (but see Foster et al. 1998; Sünram-Lea et al. 2002b). As the participant’s expectancies or beliefs about the drink might create its own effects or modify those of glucose
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