Pattern Separation in Major Depressive Disorder

Pattern separation is the process of separating overlapping sensory information, contexts, or experiences into distinct neural representations. In humans, deficits in pattern separation are believed to underlie overgeneralization seen in depression and post-traumatic stress disorder (PTSD). Cross-sectional data support the effects of SSRI medications on pattern separation. Specifically, a recent paper demonstrated patients with depression who responded to SSRI medication had improved pattern separation performance (mnemonic discrimination) of emotionally neutral stimuli than SSRI non-responders, and that the findings were reversed with respect to emotionally negative stimuli (Phillips et al., 2023). These data are consistent with the negative cognitive bias in depression (Beevers et al., 2019), and provide indirect evidence of SSRI effects on pattern separation in humans. However, there is a dearth of longitudinal, within-subject analyses of SSRI effects on pattern separation performance, and associated neural activity quantified by BOLD signal from fMRI. In the current study, we seek to examine the individual effect of SSRI treatment (in combination with placebo) as compared to the combined effect of treatment with SSRI and memantine, an NMDA-antagonist, on pattern separation, both in terms of behavioral performance and activity in the dentate gyrus as assessed by fMRI, and on depression severity. We propose to study individuals with major depressive disorder in a current major depressive episode who are currently unmedicated, who will undergo baseline medical and psychiatric assessment and fMRI scanning, followed by standardized treatment with escitalopram combined with either placebo or memantine in the context of a double-blind, placebo-controlled, randomized clinical trial.