Introducing the brain: rat whisker codes (Introduction)

by David Turell , Wednesday, September 22, 2021, 23:12 (1156 days ago) @ David Turell

Rat whiskers give their brain great detail:

https://evolutionnews.org/2016/03/by_a_whisker_sc/

"By connecting a motion source to a rat’s whisker when it was anesthetized, and attaching sensors on its brain to a computer, they could observe in real time how the brain responded through a complete phase cycle. A news item posted on Weizmann Wonder Wander explains what they discovered:

"As our sensory organs register objects and structures in the outside world, they are continually engaged in two-way communication with the brain. In research recently published in Nature Neuroscience, Weizmann Institute scientists found that for rats, which use their whiskers to feel out their surroundings at night, clumps of nerve endings called mechanoreceptors located at the base of each whisker act as tiny calculators. These receptors continuously compute the way the whisker’s base rotates in its socket, expressing it as a fraction of the entire projected rotation of the whisker, so that the brain is continually updated on the way that the whisker’s rotation is being followed through.

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"What’s really fascinating is that these mechanoreceptors don’t just blindly send pulses to the brain, leaving the work to the brain to put the information together. They actually run some pre-processing algorithms on the data.

"The discovery that the mechanoreceptors within the whisker follicle were actually calculating the whisker’s motion phase “online” came as a surprise to the researchers, because knowing the phase implies predictive knowledge of how the whisker motion will develop. The assumption was that specialized neuronal circuits would perform this calculation using raw data from both the receptor and the brain’s motion-planning circuits.

“'On second thought,” says Ahissar, “this work division is sensible. The sensory organs are not merely ‘signal converters.’ Rather, they are broad, inclusive interfaces between organisms and their environments, providing everything the brain needs for making sense out of their signals.”

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"The paper describes how the scientists found that the phase angle response was robust, no matter how fast the whisker twitched or how high it moved. This gave them a hint that the cells in the whisker follicle are running a “predictive algorithm” of some sort on the incoming data.

"Using a closed-loop interface in anesthetized rats, we found that whisking phase is already encoded in a frequency- and amplitude-invariant manner by primary vibrissal afferents. We found that, for naturally constrained whisking dynamics, such invariant phase coding could be obtained by tuning each receptor to a restricted kinematic subspace. Invariant phase coding was preserved in the brainstem, where paralemniscal neurons filtered out the slowly evolving offset, whereas lemniscal neurons preserved it. These results demonstrate accurate, perceptually relevant, mechanically based processing at the sensor level.

"The phase angle information appears to get encoded independently of other factors. All this information is coded in neural spikes to the brain, which has to decode the information. The brain can then encode feedback information through the nerves to the muscles that move the whiskers, providing near-real-time response to the data.

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"Our most surprising finding is the ability of mechanoreceptive afferents to represent whisking phases in a reliable and selective manner; we found that most of these cells and their brainstem targets tended to fire at specific phases in the whisking cycle, irrespective of the cycle’s amplitude or duration. As this invariant phase computation is performed while the cycle is on-going, mechanoreceptor coding can be viewed as being equivalent to predicting the future evolution of the whisking cycle. The relative success of the first-order kinematic model in reproducing frequency- and amplitude-invariant phase tuning demonstrates that this capacity relies on the constrained dynamics of natural whisking and requires the cells to respond in a restricted region in the angle-velocity kinematic space. Notably, a recent study found [an] array of club-like mechanoreceptors in the follicular ringwurst structure that surrounds the vibrissal shaft, which, together with the rotation of the follicles during protraction, may enable the kinematic-to-phase transformation along the whisking cycle."

Comment: An extensive design that could not develop by chance. Rats are nocturnal and the whiskers guide them. Baby rats use the whiskers by learning to use them. They were created by design for future use.