A functional magnetic resonance imaging study of working memory and the trail-making test (part B)

The Trail-Making Test part B (TMTB) has often been used to evaluate cognitive set switching in normal and clinical subjects. Lesion studies and fMRI studies have identified the left dorsal lateral prefrontal cortex, medial prefrontal cortex, and superior parietal areas as essential to TMTB performance. In the multi-component working memory model, a central executive area is subserved by a phonological loop and visuospatial sketchpad. The left dorsal lateral prefrontal cortex was associated with the central executive, the left inferior medial prefrontal cortex was associated with phonological loop rehearsal, and the superior parietal lobe was associated with attention aspects and/or the visuospatial sketchpad. The author predicts that in non-clinical subjects, left dorsal lateral prefrontal brain activation and left inferior medial prefrontal cortex brain activation will be higher in subjects who complete TMTB slowly vs. those who complete it quickly. In addition, the author predicts that a second part of TMTB is "harder" than the first part of TMTB and thus will have higher activation of left dorsal lateral prefrontal cortex, left inferior medial, and superior parietal areas than the "easier" first part. The author tested 26 non-clinical individuals using fMRI utilizing a block design of 22 second presentations of TMTB followed by 10 seconds of counting (to 10) in 6 blocks. The data were analyzed statistically using SPM2 to determine the greatest amount of brain activation in regions of interest using random effects analysis. The results show that slow completion of the TMTB does lead to higher activation in the left dorsal lateral prefrontal cortex and left inferior medial prefrontal cortex. The superior parietal areas do not show a difference. The "hard" part of the task does show greater activation of the left dorsal lateral prefrontal cortex and the bilateral superior parietal lobes when compared to the "easy" part. Thus, the TMTB shows differential activation of working memory areas between "fast" and "slow" subjects as well as between "easy" and "difficult" parts of the test.