Eyeblink conditioning

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Eyeblink conditioning is a form of classical conditioning in which animals are trained to blink in response to a tone.

Typically, the tone is played just before a puff of air to the eye. This results in the animal blinking. Eventually, the animal blinks in response to the tone alone. There are two types of conditioning protocols, "Delay Conditioning" and "Trace Conditioning". During delay conditioning, the onset of the tone (or conditioned stimulus, CS) occurs before the air puff (unconditioned stimulus, US). The US is given while the CS is still ongoing. The animal learns to blink in response to the CS at the time when the US would be expected. That is, it predicts the delay between the CS and US. During trace conditioning, the CS is presented entirely before the US. After the CS is over, there is a time interval before the US is presented. While the differences between delay and trace conditioning seem almost trivial, evidence shows that trace conditioning requires activation of the hippocampus and cerebellum, while delay conditioning depends on the cerebellar circuitry.

Functional neuroanatomy of acquisition & retention in delay conditioning: Cerebellar learning induced changes are observed in the principle cells of the dorsolateral interpositus nucleus, ipsilateral to the conditioned eye, and Purkinge cells in discrete cortical regions. The afferent pathway of the conditioned stimulus is believed to be the mossy fiber system, emanating from pontine neucleus. The afferent pathway of the unconditioned stimulus is believed to be the climbing fiber system, emanating from the inferior olives. In the cortex, it is hypothesized that the plasticity correlated with delay conditioning is long-term depression of the synapse between parallel fibers and Purkinje neurons. (DS Woodruff-Pak, JT Green, SI Levin, & MH Meisler (2006). Behavioral Neuroscience, v120, 229-240.)

Most eyeblink conditioning studies have been performed in rabbits and cats, however humans can also exhibit this conditioned behavior. For example, many optometrists have machines which put a puff of air in the eye that patients are required to have done to both eyes. There is often a soft ticking sound in the machine prior to the puff of air. The patient sits, leaning on the chin rest and is instructed to refrain from blinking until the air has been blown. The puff of air causes the patient to blink. However, because the ticking sound precedes the air, patients will soon learn to blink when the ticking sound is heard. In this scenario, the original puff of air is the unconditioned stimulus and the blinking from that air is the unconditioned response. The ticking is originally a neutral stimulus; however when placed in association with the air, the ticking soon becomes a conditioned stimulus that elicits a conditioned response, the blinking.

"Extinction" occurs when, after training, the tone is played alone. Eventually the animal stops responding to the conditioned stimulus.

The vestibulo-ocular reflex has similar circuitry. Both forms of motor learning depend on the cerebellum.