Using cognitive models to understand how semantic representations change with cognitive impairment

Understanding how semantic memory changes because of cognitive impairment is a basic challenge for cognitive science, and an important question for society. A rich source of real-world behavioral evidence to address this challenge is provided by memory tests routinely administered in clinical care settings. We use tens of thousands of test results from the triadic comparison task in the Mild Cognitive Impairment Screen (MCIS). This task requires people to identify the "odd one out" of a set of three animal names, which provides information about how they represent the semantic relationships between the animals, and allows inferences about the underlying mental representations. We develop a novel cognitive model of the task, using classic theories from mathematical psychology including Tversky's contrast model and Luce's choice rule. This model-based approach allows us to test different hypotheses about whether and how semantic memory changes as impairment increases. Contrary to previous claims, we find no evidence that the semantic representation of the animals changes. Instead, changes in performance can be explained in terms of worsening access to memory and the use of compensating response strategies. We emphasize how the use of cognitive models increases the theoretical insight into the changes in semantic memory, and provides a fine-grained clinical measurement capability that can be used in detection, diagnosis, and treatment.