Lynch, J. J., Fregin, G. F., Mackie, J. B. et al. 1974. Heart rate changes in the horse to human contact. Psychophysiology 11(4), 472-478.

The effects of human social contact on the electrocardiogram (EKG) and general behavior of 2 horses were explored. Petting elicited a slowing of heart rate, while a person entering and exiting elicited transient, but often marked heart rate increases. In both horses transient periods of T wave inversion occurred in the EKG during human contact. In one of the horses the frequency of dropped heart beats increased dramatically during successive trials of human petting. In this horse a total of 29 dropped heart beats were observed of which 23 occurred while a person was with the horse, although this contact occupied only 15 percent of the total observation period.These findings parallel previous observations made with dogs. Similar observations of changes in the heart rate and rhythm of patients in coronary care units and a shock-trauma unit during social contact with other humans underscore the need to develop empirical analogues of these reactions. The horse appears to offer an ideal model for understanding the psychosocial, hemodynamic, and central nervous system mechanisms involved in these reactions.