Why Can’t We Stimulate The Surface of the Brain Non-Invasively?

No doubt a naive question, but something I’m super-curious about.

Certain neurosurgeries require patients to remain awake in order to communicate with the surgeon. In this paper,  Cingulum stimulation enhances positive affect and anxiolysis to facilitate awake craniotomy, the authors demonstrate that stimulation of “the left dorsal anterior cingulum bundle was discovered to reliably evoke positive affect”. A low current stimulation evoked a smile and a “really good feeling”.

Many years ago, perhaps even as a boy, I saw this clip of Wilder Penfield eliciting memories from brain surgery patients, and it really stuck with me. Probably the notion of one’s entire stream of consciousness existing somewhere in the brain waiting to be unlocked with a bit of stimulation is incorrect, but nevertheless, to hear these actual recordings of patients suddenly recalling events from their past is fascinating.

My question is, Why can’t non-invasive brain stimulation, like tDCS, evoke similar responses to those outlined in these two videos? If it’s that tDCS stimulates too broadly, then perhaps something like Temporal Interference (where stimulation occurs at the point where two frequencies meet) could be used to stimulate these areas. Certainly an experiment worth thinking about.

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