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Three signals unlock brain change
Epinephrine, the same chemical as adrenaline, is released across the brain from the locus coeruleus when alertness is high. It makes neurons more likely to fire, so without alertness there is no plasticity, though alertness alone is not enough. Listening to a tape while asleep does not teach anything.
The first acetylcholine source sits in the brainstem and sends axons to the thalamus, the brain's sensory filter. Focusing on one sound, such as a dog snoring across the room, lifts that signal above everything around it. The second source is the nucleus basalis in the forebrain.
In experiments by Mike Merzenich, Gregg Recanzone, Michael Kilgard, and Norman Weinberger, stimulating all three release sites made whatever was being heard or done take over the brain's representation of that area in a single trial. Rapid, massive learning followed. It is now considered a fundamental principle of how the nervous system works.
Why does the thalamus matter for attention?
Almost all sensory information, except smell, passes through the thalamus on the way to the cortex. It works as a relay that can boost or dampen what moves forward. A thin shell of cells around it, the thalamic reticular nucleus, helps decide what gets through. Francis Crick proposed in the 1980s that this region acts as an attentional searchlight.
What does the locus coeruleus do?
The locus coeruleus is a tiny cluster of neurons in the brainstem that makes most of the brain's norepinephrine. Its axons reach almost every part of the cortex, which lets it change the state of the whole brain at once. It is most active during stress, surprise, and alertness, and nearly silent during deep sleep. Because the cluster is only a few thousand cells in each hemisphere, its reach is out of proportion to its size.
What does the nucleus basalis do?
The nucleus basalis sits in the basal forebrain and sends acetylcholine to wide areas of the cortex. Researchers such as Michael Kilgard and Michael Merzenich paired nucleus basalis stimulation with tones in rats and saw the auditory cortex remap toward those tones. The cells in this area are among the ones lost in Alzheimer's disease. That loss is part of why drugs raising acetylcholine have been used for that condition.
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