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ACh suppresses the spread of excitation in the visual cortex

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While ACh has been reported to have confusingly diverse effects, e.g. direct depolarization and hyperpolarization as well as synaptic facilitation and suppression, its effect on the propagation of excitation is very clear; suppression on intracortical connection, leaving thalamocortical inputs rather intact. We postulate that cholinergic innervation enables the afferent input to have a relatively dominant effect in the cortex.



Wiki quote of that study:


Optical recording with a voltage-sensitive dye in rat visual cortical slices demonstrated significant suppression in intracortical spread of excitement in the presence of ACh.


Why is this relevant? 

For the sake of convenience, I'll post a quote from "EEG coherence in post-LSD visual hallucinations":


Coherence, a measure of spectral similarity over time, may estimate cortical coupling. In the eyes-open state in HPPD subjects, widespread reduction of coherence was noted. However, upon eye closure, the occipital region demonstrated augmented regional coherence over many frequencies but with reduced coherence of the occipital region to more distant regions. This occipital coherence increase correlated with previously reported shortened occipital visual evoked potential latency for HPPD subjects. We speculate from coherence and known clinical and psychophysical data that, in HPPD, there is widespread cortical inhibition in the eyes-opened state, but localized and isolated occipital disinhibition upon eye closure, a state known to facilitate hallucinatory experiences.... In HPPD, we speculate that occipital EEG hypersynchrony resulting from increased regional coherence, when coupled with relative isolation of visual cortex, especially upon eye closure, facilitates hallucinations and illusions.

[from the introduction section:]

Coherence values, which range from 0 to 1, are often taken as measures of coupling or connectivity between the brain regions beneath the measuring electrodes, which may be inuenced by factors such as the activity of cortical-cortical ber tracts connecting the two involved brain regions.



Well my brain just pooped out on me, but hopefully you'll be able to see how this is relevant without me trying to disambiguate the context.

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