Purkinje cell long-term depression in delay eyeblink conditioning
Cerebellar LTD: When a conditioned stimulus (tone, CS) arrives via parallel fibers (→ Purkinje cell) and is shortly followed by an unconditioned stimulus (puff, US) via the climbing fiber (error signal from inferior olive), the parallel fiber synapse undergoes long-term depression (LTD) — weakening so the Purkinje cell fires less. Because Purkinje cells normally inhibit deep cerebellar nuclei (DCN), LTD disinhibits DCN → learned response.
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Trials
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Learned
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PF Synapse
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CR Amplitude
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Marr-Albus-Ito theory: The cerebellar cortex is a perceptron-like supervised learning machine. Climbing fibers carry teaching signals; parallel fibers carry context. This predicts conditioned responses arise from LTD with ~50–100 trials — exactly what behavioral experiments show.
Synapse strength starts at 100%. Each paired CS+US trial induces ~3% LTD. Below ~40% strength, the DCN fires robustly during CS → conditioned response (eyelid closure) appears.