When the Pressure Is On: Stress and Learning in Horses

A study recently published in PLoS One reported that extrinsic stress, reinforcement type, and personality differentially affect learning performance in horses. “The present study,” the researchers wrote, “contributes to a better understanding of the influence of stress on learning performance by showing the importance of the nature of the stress (related or unrelated to the task) and personality.”

To date, studies evaluating the effect of stress on learning performance have been conducted primarily in primates and rodents. In these studies, researchers have analyzed whether stress negatively or positively affects cognitive performance.

Extrinsic stressors (unrelated to the learning task) could impair learning; intrinsic stressors (task-related, such as negative reinforcement), on the other hand, could enhance learning by redirecting attention toward the task.

Previous studies have reported the influence of personality, particularly traits associated with emotional reactivity, on learning performance variations between individual horses. For example, one study reported that fear negatively affected learning when extrinsic stressors were present.

Study Design

Sixty female horses first underwent behavioral assessments for 5 personality traits, including fearfulness and locomotor activity. Next, the horses were placed into 4 testing groups:

• Positive reinforcement (PR)
• PR + Extrinsic stressor (ES)
• Negative reinforcement (NR)
• NR + ES

For the learning procedure, an individual horse stood in a start box and learned to enter 1 of 2 compartments by following visual cues, which became increasingly difficult through the study’s 4 learning stages. Horses in the PR groups received a food reward immediately after entering the correct compartment; those in the NR groups received aversive tactile stimulation until they entered the correct compartment. Audience horses stood near the testing area to avoid stress from social isolation.

Learning performance was determined by the number of trials needed to reach stage 1 and the highest stage achieved.

To assess stress, each horse was led into a “stress” box before performing the learning procedure. Those in the ES groups could not see the audience horses and received 3 stressors (eg, loud noise) in 10-second intervals. Horses were then led back to the start box and their behavior was observed. Researchers also measured cortisol in salivary samples obtained before and after the learning procedure.

Results and Discussion

Without extrinsic stressors, reinforcement type did not significantly influence learning performance, observed behaviors, or salivary cortisol concentration. With extrinsic stressors, researchers noted several important observations:

• Learning performance decreased and salivary cortisol concentration increased for both reinforcement types, but more so with positive reinforcement
• Several behaviors, including alert postures and startle reactions, occurred more frequently regardless of reinforcement type

In the presence of extrinsic stressors, learning performance likely decreased less with negative reinforcement because this reinforcement type redirected attention toward the task, the researchers noted. Cortisol levels may have been higher in the PR + ES group than the NR + ES group because of a lack of stress-reducing coping mechanisms; researchers proposed further investigation of equine stress measurements.

Fear played a major role in learning performance. When extrinsic stressors were absent, the most fearful horses performed best with negative reinforcement and worst with positive reinforcement; negative reinforcement may have increased focus on the task, while horses receiving positive reinforcement may have become more distracted.

With extrinsic stressors present, the most fearful horses demonstrated the worst learning performance, regardless of reinforcement type. These horses likely reacted most strongly to these stressors, causing a marked disruption in cognition and attention.

Overall, the researchers believed their study’s findings on stress, personality, and learning performance in horses can “provide important clues to more personalized training according to each animal’s needs.”

Dr. JoAnna Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.