Practice effects in nutrition intervention studies with repeated cognitive testing

BACKGROUND: There is growing interest in the use of nutrition interventions to improve cognitive function. To determine intervention efficacy, repeated cognitive testing is often required. However, performance on tasks can improve through practice, irrespective of any intervention. OBJECTIVE: This study investigated practice effects for commonly used cognitive tasks (immediate and delayed recall, serial subtractions, Stroop and the Sternberg task) to identify appropriate methodology for minimising their impact on nutrition intervention outcomes. METHODS: Twenty-nine healthy young adults completed six repetitions of the cognitive battery (two sessions on each of three separate visits). Subjective measures of mood, motivation and task difficulty were also recorded at each repetition. RESULTS: Significant practice effects were apparent for all tasks investigated and were attenuated, but not fully eliminated, at later visits compared with the earlier visits. Motivation predicted cognitive performance for the tasks rated most difficult by participants (serial 7s, immediate and delayed recall). While increases in mental fatigue and corresponding decreases in positive mood were observed between test sessions occurring on the same day, there were no negative consequences of long term testing on mood across the duration of the study. CONCLUSION: Practice effects were evident for all investigated cognitive tasks, with strongest effects apparent between visits one and two. Methodological recommendations to reduce the impact of practice on the statistical power of future intervention studies have been made, including the use of alternate task forms at each repetition and the provision of a familiarisation visit on a separate day prior to data collection.