Introducing the brain: looking at Libet's time gap (Introduction)

by David Turell , Wednesday, March 23, 2022, 20:41 (974 days ago) @ David Turell

Dopamine study:

https://www.quantamagazine.org/brain-chemical-helps-signal-to-neurons-when-to-start-a-m...

"Every time you reach for your coffee mug, a neuroscientific mystery takes shape. Moments before you voluntarily extend your arm, thousands of neurons in the motor regions of your brain erupt in a pattern of electrical activity that travels to the spinal cord and then to the muscles that power the reach. But just prior to this massively synchronized activity, the motor regions in your brain are relatively quiet. For self-driven movements like reaching for your coffee, the “go” signal that tells the neurons precisely when to act — instead of the moment just before or after — has yet to be found.

"In a recent paper in eLife, a group of neuroscientists led by John Assad at Harvard Medical School finally reveals a key piece of the signal. It comes in the form of the brain chemical known as dopamine, whose slow ramping up in a region lodged deep below the cortex closely predicted the moment that mice would begin a movement — seconds into the future.

Dopamine is commonly known as one of the brain’s neurotransmitters, the fast-acting chemical messengers that are shuttled between neurons. But in the new work, dopamine is acting as a neuromodulator. It’s a term for chemical messengers that slightly alter neurons to cause longer-lasting effects, including making a neuron more or less likely to electrically communicate with other neurons.

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"But whenever the movement occurred, the researchers found that it followed almost immediately after the rising level of dopamine in the fluid-filled space around neurons seemed to reach a certain threshold. When dopamine rose very quickly, the movement happened early in the response period; when dopamine rose slowly, the movement happened later.

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"Dopamine was a major component of the signal that told the mice exactly when to move in this case, but other neuromodulators and neural activity that play a role in the “go” signal for movement still need further investigation.

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"The new paper shows that dopamine levels are also slowly evolving over many seconds to directly influence the decision about not just whether to move but exactly when to do it. It could help explain why patients with Parkinson’s disease — a movement disorder in which dopamine levels are reduced — have trouble initiating movements with proper timing: Their slowly evolving dopamine levels may rarely reach the critical threshold.

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"As for why a neuromodulator like dopamine would be involved in deciding when to move, it’s possible that slowly varying neuromodulatory signals could allow the brain to adapt to its environment. Such flexibility wouldn’t be afforded by a signal that always led to movement at the exact same time. “The animal is always uncertain, to some extent, about what the true state of the world is,” said Hamilos. “You don’t want to do things the same way every single time — that could be potentially disadvantageous.” (my bold)

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"The neuromodulatory system [is] the most brilliant hack you can imagine,” said Mac Shine, a neurobiologist at the University of Sydney. “Because what you’re doing is you’re sending a very, very diffuse signal … but the effects are precise.” (my bold)

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"It was widely believed that acetylcholine always increased alertness by making neurons more independent of the activity in their circuits. Cardin’s team found that this holds true in small circuits with only hundreds to thousands of neurons. But in networks with billions of neurons the opposite occurs: Higher levels of acetylcholine lead to more synchronization of activity patterns. Yet the amount of synchronization also depends on the region of the brain and the arousal level, painting the picture that acetylcholine does not have uniform effects everywhere. (my bold)

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"Researchers are also looking at evidence that that some neuromodulators modulate one another. For example, endocannabinoids, the neuromodulators that bind to the same receptors as the active component in marijuana, seem to help keep the amount of neuromodulators released by individual neurons within an optimal range.

"That’s why endocannabinoids are “crucial to our survival,” said Joseph Cheer, a neuroscientist at the University of Maryland School of Medicine who has been studying their impact on dopamine for nearly 20 years. “We have these little molecules that are fine-tuning most synapses in our brain.”

"To Marder, studying neuromodulators in isolation is “akin to looking under the lightbulb for your keys just because that’s where there’s light,” she said. “Nothing about modulation is ever linear or simple.'”

Comment: note my bolds. The brain is designed to help us, not restrict our free will. The complexity requires a designer.