A New Measure of Mnemonic Discrimination Applicable to Recognition Memory Tests with Continuous Variation in Novel Stimulus Interference

Abstract

Background: Mnemonic discrimination (MD) involves distinguishing new stimuli from memories of highly similar “lure” items or events, and is a putative indirect probe of dentate gyrus functioning. MD is impaired in the elderly and in individuals with hippocampal lesions, schizophrenia, major depressive disorder, and Alzheimer’s disease. The gold-standard MD test, called the mnemonic similarity task (MST), is rarely used in clinical research. We therefore aimed to validate a novel analysis method that extracts information about MD and recognition memory in widely clinically used recognition memory tests which do not have categorical distinctions between “lures” and “foils.”Methods: By fitting a logistic function to the relationship between stimulus interference and the probability of classifying a stimulus as novel, at the single participant level, we derived participant-level indices of MD (λ) and overall recognition memory performance (Δ). We applied the novel measures to MST data from two independent datasets (N=18; N=67). Using linear mixed-effects modelling, we sought to confirm that λ predicts the MST’s lure discrimination index (LDI), while Δ predicts the MST’s overall recognition memory index (REC). Results: Across both datasets, λ predicted LDI (β=0.76, 95% CI [0.62-0.91], p<0.001), but not REC (β=-0.06, 95% CI [-0.20-0.09], p=0.438), while Δ predicted REC (β=0.93, 95% CI [0.83-1.02], p<0.001), but not LDI (β=0.06, 95% CI [-0.03-0.15], p=0197). The λ and Δ indices were not correlated.Conclusion: Our novel measure accurately indexes MD, without correlating with overall recognition memory performance. Future studies should apply it to large clinical datasets with widely used recognition memory tests, such as the California Verbal Learning Test.