Imipramine-induced erection, masturbation, and ejaculation in male horses

While perusing the medical literature on the internet for an unrelated presentation, I came across the following research abstract. I have abridged it for the sake of space:

Imipramine hydrochloride was administered to five male horses: one experienced young stallion, two mature normal breeding stallions, one 5-year-old stallion with erection and ejaculatory dysfunction, and one long-term castrated male horse...... Erection typically occurred within 10 minutes after injection, and the erection and masturbation continued intermittently for 1 to 2 hours. These erections proceeded as during sexual excitement to a normal firmness and eventual engorgement of the glans penis. Two stallions ejaculated while masturbating. Mild ataxia and drowsiness appeared at the higher doses, but the animals remained responsive to auditory, visual, and tactile stimuli. Erection and masturbation were often interrupted by activities about the barn or the approach of the handler, suggesting cortical inhibitory control of the erection.

Three things strike me as comical about this. First and most obviously, if you have ever seen a horse get an erection, I have said enough. Second, how does a horse masterbate? I can't figure that one out and it's not mentioned in the abstract. Third, and more subtle is the comment about the approach of the handler while in the barn. Basically what is happening to this horse is what is happening to you when you all the sudden think of your grandmother for some ungodly and unknown reason in the middle of doing the hibidy dibity. Who knew that horses were so discretionary in their masturbatory fantasies?