A Pilot Validation of a New Task Taxing Asemantic Mnemonic Similarity—The Role of the Hippocampus

A central feature of episodic memory is the ability to transform similar and overlapping experiences into distinct representations, a process known as pattern separation. This function is critically linked to the hippocampus, particularly the dentate gyrus (DG) and its projections to CA3. Here, we introduce the Visual Sensory Memory Task (VSMT) as a novel paradigm for assessing mnemonic similarity processes in healthy adults. The VSMT uses asemantic visual stimuli derived from abstract pink-noise images and allows controlled manipulation of stimulus similarity while minimizing learning effects. Behavioural results showed reliable discrimination between targets and foils, whereas low-similarity lures were more difficult to identify than high-similarity lures, providing initial evidence for sensitivity to mnemonic similarity. Using 3 T fMRI, we measured BOLD responses during recognition of asemantic stimuli. Robust bilateral hippocampal activation was observed across all stimulus categories. Exploratory atlas-guided localization suggested distributed activation for foils, whereas activation peaks for targets and high-similarity lures were spatially localized to regions encompassing CA1 and CA4/DG, respectively. These localizations should not be interpreted as evidence of subfield-specific activation given the spatial resolution of the functional data. Consistent with previous studies using explicit memory paradigms such as the Mnemonic Similarity Task, the VSMT differences in hippocampal BOLD response magnitude were associated with mnemonic similarity processing. These univariate activation differences are compatible with predictions derived from pattern separation accounts but do not directly demonstrate representational pattern separation. Together, these findings provide initial evidence for the feasibility and potential utility of the VSMT for investigating hippocampal mechanisms related to mnemonic similarity and pattern separation.