This is a really great article that explains how horses learn "chemically", if I may call it that. :)
From:
http://www.thehorse.com/articles/36130/dopamine-and-horses-learning-stereotypies-and-more?utm_source=Newsletter&utm_medium=behavior&utm_campaign=08-02-2015

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Got dopamine?

Yes, you do. And so does your horse. One British equine behavior specialist says that’s a good thing: Without dopamine, horses wouldn’t learn and they could become depressed.

However, it’s also possible to have too much of a good thing. When your horse produces too much dopamine in his brain, for instance, he can develop stereotypies such as crib-biting, said Sebastian McBride, PhD, a researcher at the University of Cambridge.

What creates a stereotypy-inducing overdose of dopamine? Essentially, stress.

“Stereotypies appear to be related to some dysregulation of the neurophysiology of the striatum (a learning center in the brain), most likely the result of stress,” McBride said during his plenary lecture on dopamine in equine brains at the 2014 International Society of Equitation Science conference, held Aug. 6-9 in Denmark.

Within the striatum are three well-organized structures designed for the learning process. The ventral striatum (or the nucleus accumbens) is involved in “initial stamping” of learning—in other words, the “light bulb” step. When your horse suddenly realizes that taking a step backwards means you will release the pressure on his halter, he gets a good shot of dopamine in his ventral striatum.

Later, once the horse recognizes the association among the aid (halter pressure), the action (stepping backwards), and the reward (pressure release), it’s the dorsomedial striatum, also known as the caudatus, that takes over. Dopamine is released during this part of the learning process, too, McBride said, but with one distinct difference. In this part of the brain and in this phase of learning, it’s no longer the reward that stimulates the dopamine release. It’s the aid.

“That’s the interesting thing about dopamine—it’s only released when the reward is unexpected or greater than expected,” he said. “Once you’ve learned something and you get that reward regularly, you stop getting a dopamine signal at the point of reward.”

The learning process enters this phase in the caudatus where dopamine is released when the horse gets the cue we give him. And the greater the dopamine response, the greater the strength of the horse’s behavior in response—what we know as motivation, McBride said.

“What’s going on in the medial striatum is all about action and outcome,” he said. “It’s about monitoring actions and adjusting behavior accordingly.”

In these two sections of the brain—the nucleus accumbens and the caudatus—dopamine acts as the “concrete” that ensures the brain structures’ function, McBride said. It serves the critical role of linking sensory information (what the horse feels, sees, or hears) to motor output (what he does). Without the dopamine release, the learning wouldn’t happen.

But a dopamine “overdose” can lead to habit formation, he said, along with a similar phenomenon: stereotypies, such as cribbing.

Stereotypes are a manifestation of what scientists call “hypermotivation,” which can occur in both humans and horses. While motivation is a good thing, hypermotivation exceeds the limits of being healthy or useful. For example, a hypermotivated horse might be hypermotivated to chew—and ultimately become a cribber—or hypermotivated to move—and start weaving or stall-walking.

Dopamine overdoses are related to actual structural changes in the striatum, McBride said, which, unfortunately, are permanent. That’s why when stereotypies develop, they’re usually there for life.

Horses with stereotypies tend to have other signs of habit formation as well, he added. This is part of their structural brain change—they get “stuck” in habits and have a hard time accepting change. Studies have shown that horses with and without stereotypies vary in their understanding of a changed task. For example, if two horses learn that there’s food in one of two buckets, and then the food is placed in the other bucket, the horse with the stereotypy will have more difficulty learning that the food has moved than one without.

“There’s clearly some dysregulated neurophysiology in the striatum, causing these learning anomalies,” McBride said.

Those dysregulations are usually the result of stress, bad training, or both, he said.

“What we’ve really got to think about are these windows of opportunity when the brain is most susceptible to stress physiology,” said McBride. “We’re putting the animal on a path for the rest of its life. Early life experiences are critical in forming the neurophysiological state in that horse. So things like weaning can be really important. Eating, locomotion, and social contact are also really important as the horse gets older.”

By contrast, when too little dopamine is released into the striatum, it can lead to depression in the horse, as it can in humans, McBride said. And that’s where the third part of the striatum comes into play. When this part, the putamen, takes over the learning process, it’s all about mechanization, and “there appears to be little dopaminergic modulation at this stage of learning,” he said. When dopamine is low or absent, depression and learned helplessness can set in.

Depressed horses have little reaction to stimuli, such as humans approaching a stall. They can also fall into a state of learned helplessness, in which the horse no longer makes an effort to learn, understand, or give natural responses to stimuli. He becomes “like a machine,” accepting that he has no control over his environment, McBride said.

Like stereotypy development, depression and learned helplessness can result from stress and poor training, McBride said.

“We know what the horse needs in terms of natural behaviors even in a domestic setting,” he said. “Now we need to be aware of the consequences of not facilitating those behaviors.”