Differential neural responses during performance of matching and nonmatching to sample tasks at two delay intervals

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 cer tain common mechanisms, there are behavioral differences between them, and neuroimaging provides a means of assessing explicitly whether this is under pinned by differences at a neural level. Findings of delay-dependent defici ts, after lesions in humans and animals, suggest that the neural implementa tion of these tasks may also critically depend on the delay interval. In th is study we determined whether there were differential neural responses ass ociated with DMTS and DNMTS tasks at two different delay intervals using fu nctional magnetic resonance imaging. Ten healthy volunteers were studied un der four test conditions: DMTS and DNMTS at 5 and 15 sec delay. The main ef fect of DMTS compared with DNMTS across both delay intervals was associated with significant activation in bilateral head of caudate and medial orbito frontal cortex, By contrast, DNMTS compared with DMTS was associated with s ignificant activation in mediodorsal thalamus, bilateral lateral orbitofron tal cortex, and left premotor cortex. The main effect of short compared wit h long delay, across both tasks, was associated with significantly greater activity in occipital and parietal cortices. By contrast, long compared wit h short delay was associated with significantly greater activity in tempora l and ventrolateral frontal cortices. We conclude that DMTS and DNMTS are n ot equivalent and furthermore that the precise neural implementation of the se tasks is a dynamic function of delay interval.