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Abstract:
Objective:
The segregation of visual processing into a dorsal and a ventral stream is well established for posterior cortex. It is unclear, however, whether this segregation can also be found in frontal cortical areas, involved in working memory function. Most of the previous imaging experiments comparing activation patterns for memory of objects and memory for spatial locations failed to demonstrate a clear dissociation in prefrontal areas. An explanation for these failures could be that the objects used in these studies may not be specific enough for the ventral stream, due to their spatial properties being processed in the dorsal stream too. To avoid this confusion, we set up an experiment comparing working memory for colors, as a specific indicator for the ventral stream, with working memory for spatial information.
Method:
In a delayed-match-to-sample task, subjects were presented with four dots of different colors (first modality) in a certain spatial configuration (second modality). To suppress verbal coding of the colors, letters were shown within the dots and had to be memorized as a third modality. Before each trial, subjects were instructed as to which two of the three modalities they had to memorize. Event-related fMRI data were analysed with BrainVoyager on an intra- and intersubject level.
Result:
A conjunction analysis identified areas that were specifically active while subjects had to maintain colors and different areas of spatial locations. Areas active during color memory were mainly located in the medial frontal gyrus while premotor areas were activated during spatial memory. A connectivity analyis showed that activation profiles from a color area covaried with activity in ventral occipito-temporal regions, whereas activation profiles from a spatial area covaried with activity in superior-parietal regions.
Conclusion:
Our data support the hypothesis of a domain-specific segregation of frontal areas involved in working memory.
