Analysis of non-EEG evidence for preparatory activity in the human brain: pre-contraction impedance changes in...

Precontraction impedance changes (PICs) are known to occur during isometric contractions of the human forearm finger flexors. Without reliance on electroencephalogical data these are construed as evidence for the construction of a 'template of instructions' in the brain for muscle contractions to be performed once a 'go' instruction is received, as widely expected from EEG measurements A model is proposed to explain earlier impedance results, based on a PIC-associated time-dependent inhibitory constraint and a modified role for the 'go' command. A principal result is that (for appropriate choice of parameters) the time difference between occurrence of a PIC and the onset of force generation can be independent of the timing of the 'go' command, as has in fact been reported earlier. Independent of the detailed model, it is argued that there is no reproducible relationship between the time of occurrence of a PIC and the emergence of force, even for repeat tests on the same subject. However, we propose that results for a wide range of circumstances are nevertheless governed by a common probability distribution, based on data for 1200 PICs in a cohort of 11 healthy young subjects.