A model-based analysis of the impairment of semantic memory.

We use cognitive models to evaluate three theories of the change in semantic memory caused by Alzheimer's disease. We use data from 14,096 clinical assessments of 3602 Alzheimer's patients and their caregivers. Each patient completed a semantic memory task involving the odd-one-out comparison of animal names. Each patient was also independently evaluated to determine their level of impairment. Our cognitive models assume a feature-based representation of the animals and odd-one-out choice probabilities based on common-feature similarities. We find no evidence for the restructured representation hypothesis, which claims that impairment causes changes in the features used to represent stimuli. We also find no evidence for the attention change hypothesis, which claims that impairment causes greater attention to be given to concrete features at the expense of more abstract features. We do find evidence for the noisy access hypothesis, which claims that odd-one-out choices become less determined by semantic similarity and more prone to the simple response strategy of choosing the last option. We conclude that the noisy access hypothesis provides a simple account of odd-one-out choice behavior throughout the progression of Alzheimer's disease. More elaborate theories involving changes to underlying mental representations and attention processes need to provide evidence they are superior to the noisy access account.