Although the gross somatotopic organization of the posterior bank of the precentral gyrus is well established, a fine scale organization of the representations of the digits of the hand has not been fully characterized. Previous neuroimaging studies have failed to find clear evidence for a specificity of digit representations in motor cortex, but rather report a distributed network of control. Reported here are the results of two experiments; in Experiment 1 a sequential finger tapping task produced strong blood oxygen level dependent (BOLD) responses in the contralateral precentral gyrus, but there was a lack of specificity for distinguishing individual representations. A randomly ordered task did accomplish this goal. In the second experiment, a randomly ordered finger-tapping task was used and the findings demonstrated BOLD responses in clusters of voxels specific to movement of a single digit. The region of interest defined for each digit comprised several noncontiguous clusters. A "selectivity index" was developed to quantify the magnitude of the BOLD response to the movement of a specific digit, relative to BOLD response associated with movement of other digits. Strong evidence of BOLD selectivity (albeit not exclusivity) was found in the hemisphere contralateral to the cued digit; however, there was no evidence for an orderly spatial topography. These findings demonstrate that a selectivity of activation is quantifiable, supports a theory of noncontiguous distribution of control, and provides a method for comparing between healthy and impaired populations and investigating changes following training or intervention.
- Finger tapping
- Primary motor cortex