PMID-22127300 Direct electrical stimulation of human cortex -- the gold standard for mapping brain functions?
- Fairly straightforward review, shows the strengths and weaknesses / caveats of cortical surface stimulation.
- Axon initial segment and nodes of Ranvier (which has a high concentration of Na channels) are the most excitable.
- Stimulation of a site in the LGN of the thalamus increased the BOLD signal in the regions of V1 that received input from that site, but strongly suppressed it in the retinotopicaly matched regions of extrastriate cortex.
- To test the hypothesis that the deactivation of extrastriate cortex might be due to synaptic inhibition of V1 projection neurons, GABA antagonists were microinjected into V1 in monkeys in experiments that combined fMRI, ephys, and microstim.
- Ref 25. PMID-20818384
- These findings suggest that the stimulation of cortical neurons disrupts the propagation of cortico-cortico signals after the first synapse.
- Likely due to feedforward and recurrent inhibition.
- Revisit the hypothesis of tight control of excitation and inhibition (e.g. in-vivo patch clamping + drugs). "The interactions between excitation and inhibition within cortical microcircuits as well as between inter-regional connections haper the predicability of stimulation."
- The average size of a fMRI voxel:
- 55ul, 55mm^2
- 5.5e6 neurons,
- 22 - 55e9 billion synapses,
- 22km dendrites (??)
- 220km axons.
- In the 1970s, Daniel Pollen conducted a series of studies stimulating the visual cortex of cats and humans.
- Observed long intra-stim responses, and post-stim afterdischarges.
- Importantly, he also observed inhibitory effects of DES on cortical responses at the stimulation site.
- The inhibitory effect depended on the state of the neuron before stimulation.
- High spontaneous activity + low stim strengths = inhibition;
- low spontaneous activity + high stim strengths = excitation.
- In the author's opinion, there is an equal or greater number of inhibitory responses to electrical microstimulation as excitatory. Only, there is a reporting bias toward the positive.
- Many locations for paresthesias:
- postcentral sulcus (duh)
- opercular area inferior postcentral gyrus (e.g. superior to and facing the temporal lobe)[60]
- posterior cingulate gyrus
- supramarginal gyrus
- temporal lobe, limbic and isocortical structures.
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