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Abstract:
Visual short-term memory requires the maintenance of objects
or locations, or their conjunctions. We investigated visual
short-term memory for objects, places, and their conjunctions in
humans using event-related time-resolved functional magnetic
resonance imaging (fMRI). FMRI was performed at 1.5T (Siemens
Magnetom Vision). 3D-data sets in Talairach-space were analysed on
a single trial basis using the BrainVoyager software. For each task
and subject, 3D-correlation maps were obtained. On this basis, the
amount of overlap between the task-related activations could be
determined. Comparison of correlation maps for the component
processes revealed a segregation into different inter-individually
variable neocortical compartments. The activation during the
conjunction task showed considerable overlap with the cumulative
map of the isolated tasks, but recruited additional areas,
including pre-SMA and the anterior insula. We suggest that the
retention of spatial and non-spatial cues from identical visual
stimuli engages different processes during the delay interval
rather than different encoding strategies during stimulus
presentation. The cortical activation pattern associated with tasks
requiring retention of conjunctions between spatial and non-spatial
cues was not simply the sum of the patterns associated with
retention of the individual cues, suggesting the existence of a
distinctive network for the retention of these conjunctions.
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