[Functional magnetic resonance and the motor cortex II: measurement of activity]

Functional magnetic resonance (fMR) with BOLD contrast has been shown to be a useful tool for clinical investigation. It is still uncertain which is the best way to establish the measures of activation for subsequent comparisons and their physiological significance.

To explore the activation expressed as a function of different parameters.

We did fMR with BOLD contrast at 3T and a motor paradigm in 14 healthy persons. The activation was expressed as the number of voxels activated within each area; their percentage in relation to the total number of voxels activated; z value; laterality index, speed and degree of temporal variation in the signal of these voxels.

Somatomotor area (SM) activated 121% more voxels than supplementary motor area (SMA), but their variation expressed with their standard variation was very large. The percentage of voxels activated was 163.6% greater in SM than in SMA. The laterality index increased at the expense of activation outside SM. The variation of the two relative measurements was less than the total number of voxels. The z value did not correlate with the degree of activity. The time course of the SMA signal was slightly different to SM.

Very high field fMR is very sensitive to changes in signal. The number of voxels activated varies considerably, so its use as a measurement of activity should be limited. Relative value, such as the index of laterality of the percentage of voxels may show less variability. The centre of mass and z value are stable figures but the physiological significance of the latter is not clear. The study of the temporal variation of the signal seems to be a potentially useful parameter with a certain degree of physiological significance, although better understanding and further analysis is necessary.