Abstract

Visual short-term memory in humans and animals is frequently assessed using delayed matching to sample (DMTS) and delayed nonmatching to sample (DNMTS) tasks across variable delay intervals. Although these tasks depend on certain common mechanisms, there are behavioral differences between them, and neuroimaging provides a means of assessing explicitly whether this is underpinned by differences at a neural level. Findings of delay-dependent deficits, after lesions in humans and animals, suggest that the neural implementation of these tasks may also critically depend on the delay interval. In this study we determined whether there were differential neural responses associated with DMTS and DNMTS tasks at two different delay intervals using functional magnetic resonance imaging. Ten healthy volunteers were studied under four test conditions: DMTS and DNMTS at 5 and 15 sec delay. The main effect of DMTS compared with DNMTS across both delay Delayed matching to sample (DMTS) and delayed nonmatching to sample (DNMTS) paradigms are widely used to study visual memory in humans and animals. Both tasks require subjects to hold a visual stimulus “on line ” over a delay interval before responding to a choice of stimuli. In DMTS, subjects must select the familiar stimulus; in DNMTS they must select the novel stimulus. Successful performance of both tasks depends on the integrity of inferior temporal regions (Gross, 1973; Mishkin, 1982; Horel et al., 1987), posterior perceptual cortex, and medial temporal lobe structures (Zola-Morgan and Squire, 1985;

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