Working memory for letters, shapes, and locations: fMRI evidence against stimulus-based regional organization in human prefrontal cortex

Investigations of working memory (WM) systems in the frontal cortex have re vealed two stimulus dimensions along which frontal cortical representations may be functionally organized. One hypothesized dimension dissociates verb al from nonverbal WM processes, dividing left from right frontal regions. T he second hypothesized dimension dissociates spatial from nonspatial WM, di viding dorsal from ventral frontal regions. Here we used functional magneti c resonance imaging to probe WM processes associated with three different t ypes of stimuli: letters (verbal and nonspatial), abstract shapes (nonverba l and nonspatial), and locations (nonverbal and spatial), In a series of th ree experiments using the "n-back" WM paradigm, direct statistical comparis ons were made between activation patterns in each pairwise combination of t he three stimulus types. Across the experiments, no regions that demonstrat ed responses to WM manipulations were discovered to be unique to any of the three stimulus types. Therefore, no evidence was found to support either a left/right verbal/nonverbal dissociation or a dorsal/ventral spatial/nonsp atial dissociation. While this could reflect a limitation of the present be havioral and imaging techniques, other factors that could account for the d ata are considered, including subjects' strategy selection, encoding of inf ormation into WM, and the nature of representational schemes in prefrontal cortex. (C) 2000 Academic Press.