New methods, measures, and models for analyzing memory impairment using triadic comparisons.

We study the effect of memory impairment on triadic comparisons of animal names in a large clinical data set. We define eight groups of subjects in terms of their delayed free recall performance, and present standard analyses of the triadic comparison and free recall data that provide little insight into the effect of memory impairment on semantic structure. We then develop and apply two new methods for analyzing the data, based on cognitive models and using Bayesian statistical inference. The first new method focuses on modeling changes in semantic representation, by inferring multidimensional scaling (MDS) representations for each group based on their triadic comparisons. These representations reveal a successive decrease in semantic cluster structure and increase in uncertainty with increasing impairment. We propose a measure of spatial organization as a means of quantifying the visually evident changes in semantic organization, and demonstrate its usefulness. The second new method focuses on modeling changes in memory access with impairment, inferring the extent to which each individual makes triadic comparisons consistent with a common semantic representation. Although these inferences are based on just 12 comparisons per subject, we show that they vary systematically with memory impairment group. We conclude by discussing the potential for clinical application of our new models, measures, and methods.