Dissociated pattern of neural correlates for verbal and non-verbal coding strategies in visual working memory

Abstract

Dorsolateral prefrontal cortex (DLPFC), parietal areas and extrastriate visual cortex have been identified as the neural correlates of visual working memory (WM). The involvement of DLPFC in the maintenance and/or manipulation of memory representations remains disputed, as is the issue of the cortical dissociation of verbal and non-verbal encoding strategies in WM, as well as how these processes depend on the delay period. We sought to explore the effect of explicit encoding strategies in a delayed discrimination task for grating orientation. Method: We used functional MRI in a 3-T head scanner to determine the neural correlates of the delayed discrimination of eccentrically presented Gabor stimuli differing in orientation. On each trial, subjects were cued to code the relative grating orientation either verbally (tilted left or right of vertical, covert speech) or non-verbally (visual discrimination between two oblique orientations) for short (2 s) or long (8 s) memory delay periods. They responded in a 2AFC whether the test and reference gratings had the same or different orientation. Results: Performance in the verbal encoding condition was significantly higher (p [[lt]] 0.01) and reaction times were lower (p [[lt]] 0.01) compared to the results of the non-verbal encoding condition. Blood-oxygen level dependent (BOLD) fMRI revealed significant activity bilaterally in prefrontal, anterior cingulate, posterior parietal, superior temporal and precuneus cortex. A random-effects analysis revealed that the right DLPFC was significantly more active in the non-verbal encoding condition for the long delay period, whereas the left superior temporal gyrus was more active in the verbal encoding condition for both short and long delays. Our results suggest that the coding strategy used by the subject has a significant effect on the resultant pattern of BOLD activation during delayed discrimination tasks requiring WM.

Footnotes

 Funded by Bavarian Research Foundation