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
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
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
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
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.”