McFadden thinks that the EM field could influence the brain in a number of ways. Redistribution of ions could modulate neuronal activity, given that voltage-gated ion channels are a key element in the progress of axon spikes. Neuronal firing is argued to be sensitive to the variation of as little as one millivolt across the cell membrane, or the involvement of a single extra ion channel. Transcranial magnetic stimulation is similarly argued to have demonstrated that weak EM fields can influence brain activity.