Neuronal basis of neuroimaging in motor networks

Abstract

In attempting to understand the physiology of the brain, it is most helpful to combine a number of imaging and physiological techniques. A multimodality approach with fMRI, EEG and transcranial magnetic stimulation (TMS) allows a comprehensive non-invasive approach. There needs to be particular caution using the BOLD technique for fMRI since it is so far removed from neuronal events. Recent demonstrations are reassuring that show perfusion technique results to be similar to BOLD results.1 The main executor of movement in the cortex is the contralateral primary motor cortex (M1) and generally it shows the largest BOLD signal in fMRI experiments. The timing of M1 activity can be seen with the latter part of the movement-related cortical potential (MRCP) with EEG measurements. M1 activity is greater with more frequent movements, movements with greater amplitude and with greater force. In no-go trials of go/no-go experiments, there is no fMRI activity in M1 suggesting that nothing is happening.2 However, there is similar EEG activity compared with go trials, and TMS studies show active inhibition,3} suggesting that there is inhibitory activity in M1, but that this does not reveal itself with fMRI. The failure of fMRI to show change in this circumstance has been interpreted as due to the fact that inhibition does not take as much metabolic energy as excitation. On the other hand, prolonged suppression of a motor action does show a negative BOLD response (as well as TMS inhibition), suggesting that with inhibition over a longer time, the activity in the network is diminished.4 Some fMRI studies show an increased activity in the ipsilateral motor cortex while others show a decrease. EEG shows similar activity contralaterally and ipsilaterally in the early part of the MRCP and less activity ipsilaterally in the latter part of the MRCP. TMS can show excitation or inhibition depending on the motor task. Detailed analysis of the fMRI signal shows that increases likely derive from premotor cortex and decreases from M1.5 Hence, ipsilateral M1 may be inhibited and ipsilateral premotor cortex excited.