How the Brain Learns to Perform Quickly Without Overthinking

Cerebellum highlighted in orange with magnified close-up of cerebellar Purkinje cells. Cerebellar means "relating to the cerebellum."

Source: Kateryna Kon/Shutterstock

In the mid-1970s, my neurosurgeon father was obsessed with how rewiring his cerebellum's Purkinje cells with lots of practice could make him better at brain surgery and improve his tennis game.

At the time, neuroscientists were just beginning to understand how the distinctive, fan-shaped Purkinje cells located in the cerebellum (Latin for "little brain") were key to automatically perfecting well-coordinated muscle movements—such as those used when handling surgical instruments or a tennis racket.

The Seat of Muscle Memory

In the late 1960s and early '70s, three landmark papers (Marr, 1969; Albus, 1971; Ito, 1972) posited that the cerebellum's associative learning capacity—which leads to smooth, fluid motor control and unconscious, automatic muscle memory—relied on "teaching signals" from climbing fibers that provide input to the cerebellum's Purkinje cells during trial-and-error learning tasks.

Over five decades ago, this groundbreaking "climbing fiber hypothesis" of muscle memory encoding was highly speculative and somewhat controversial. Before the early 2000s, scientific lab technologies and tools (such as optogenetics) currently used to test how climbing fibers and Purkinje cells work together during cerebellar learning didn't exist.

Nevertheless, in 1975, when Arthur Ashe—who famously said, "There is a syndrome in sports called 'paralysis by analysis'"—won Wimbledon, my father interpreted this legendary tennis champion's words of wisdom through the lens of that era's fresh ideas about how the cerebellum worked and passed these lessons on to me as a rookie player.

Optimized Purkinje Cells Offset "Paralysis by Analysis"

From Dad's neuroscience-based vantage point, cerebral overthinking often causes an athlete to freeze up and choke (i.e., "paralysis by analysis"). On the flip side, letting go and allowing Purkinje cells to do their thing subconsciously opens the door to frictionless flow (i.e., "superfluidity") and facilitates peak performance.

Decades ago, my father knew that trial-and-error learning hardwired muscle memory into the little brain's Purkinje cells. However, back then, nobody knew exactly how the cerebellum mastered automaticity, the brain's capacity to generate actions spontaneously without cerebral input from the conscious mind.

Until recently, it was also unclear what specific neural mechanisms made it possible for Purkinje cells to perfect finely-tuned motor functions in subcortical brain regions through repeated practice sessions.

Source: Illustration by Rita Felix of the Carey Lab / Used with permission

Climbing Fibers Send Signals That Teach Purkinje Cells Automaticity

New research (Silva et al., 2024) in mice from the Champalimaud Centre for the Unknown's Carey Lab used state-of-the-art neuroscience techniques to establish that climbing fibers provide essential instructive signals to Purkinje cells that reshape these neurons via neuroplasticity during associate learning tasks.

This study's main goal was to use advanced technology to address unknowns related to the decades-old climbing fiber hypothesis of cerebellar learning. "The climbing fiber hypothesis for learning has dominated the cerebellar field for over 50 years, yet definitive proof—or disproof—has remained elusive," the authors write. "Conflicting evidence, competing models, and insufficiently precise tools for neural circuit dissection have sowed substantial controversy and confusion."

For this study, first author Tatiana Silva and colleagues developed an experiment using optogenetics and eyeblink conditioning to test how mice learned to automatically blink when a gentle puff of air into their eyes was coupled with seeing a flash of light.

The same well-conditioned cerebellar reflexes that cause automatic eyeblinks in a science lab facilitate muscle memory in sports and daily life. Purkinje cells that have been conditioned and learned to function automatically through lots of "practice, practice, practice" make it possible for athletes—or anyone who's repeatedly practiced something involving muscle memory—to perform with precision at lightning-fast speeds without overthinking.

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Without Fully Functioning Climbing Fibers, Purkinje Cells Can't Learn

Optogenetics made it possible for Silva et al. to disrupt cell-type-specific communication between climbing fibers and Purkinje cells during their eyeblink experiments. To manipulate climbing fibers with optogenetics, the researchers used a genetic tool called Channelrhodopsin-2 (ChR2), which allowed the Carey Lab's neuroscientists to perturb how these specific neurons function during eyeblink conditioning.

As lab founder and senior author Megan Carey explains in an April 2024 news release, "It turned out that introducing ChR2 into the climbing fibers altered their natural properties, preventing them from responding appropriately to standard sensory stimuli like air puffs. This, in turn, completely blocked the animals' ability to learn."

In typical, unperturbed eyeblink conditioning, Purkinje cells learn to associate the visual cue of seeing a flash of light with a puff of air. Hence, the eyelids automatically blink when the light flashes, even if there isn't a puff of air. However, during this experiment, the researchers blocked climbing fibers' ability to send instructive signals to Purkinje cells via complex spikes, which teach the cerebellum how to perform reflexively with automaticity.

Take-Home Message

The Carey Lab's latest (2024) experiment establishes that if climbing fibers can't communicate with Purkinje cells, the automatic muscle memory associated with eyeblink conditioning isn't encoded into the cerebellum's Purkinje cells.

"When we used optogenetics to selectively silence climbing fibers during the presentation of an actual air puff, the mice completely failed to learn to blink [automatically] in response to the visual cue," Silva emphasized.

Based on these findings, the researchers conclude that climbing fibers are essential for cerebellar learning. "Regardless of possible contributions from additional mechanisms, our findings establish an absolute requirement for climbing fiber instructive signals in associative cerebellar learning," the authors write in their paper's conclusion.

According to Carey, her lab's latest (2024) findings provide "the most compelling evidence to date that climbing fiber signals are essential for cerebellar associative learning." Future research from the Carey Lab will investigate different ways that stimulating climbing fibers might enhance Purkinje cell-based learning in the cerebellum.