A novel index to measure pre-planning in the Tower of London task: Test-retest reliability and known-group validity

The Tower of London (TOL) is a planning task frequently used in clinical settings and research. Planning and execution times are the most common outcome variables despite yielding lower effect sizes in clinical group comparisons and lower test-retest reliability than planning accuracy. Here, it is proposed that planning time be analysed not in isolation, but in relation to the combined duration of planning and execution, yielding a novel pre-planning index (PPI). In N = 179 healthy participants, test-retest reliability analyses yielded higher absolute agreement and less intra-individual variability over two sessions for PPI than for planning and execution times. The clinical validity of PPI was probed by comparing patients known to exhibit planning deficits and healthy controls. Stroke and Parkinson's patients showed significantly lower PPI than controls, driven by reduced planning and longer execution times. There was no difference in PPI between patients with mild cognitive impairment and controls. Consistently across healthy participants and patients, the positive correlation of PPI with planning accuracy exceeded that of planning times with accuracy. Thus, this pre-planning index can enhance both the reliability and clinical validity of TOL latency variables and represents a useful complement to accuracy for measuring planning performance in health and disease.