Why must the entire imprinting procedure be repeated on the other side of the foal?

Repeating the entire imprinting desensitization procedure on the other side of the foal — after the first side has been completed to criterion — is not a redundancy but a neurological necessity that reflects an important and often underappreciated feature of the horse's brain: horses have limited transfer of learning between the two sides of the body, and what is learned on one side is not automatically available on the other. This limitation is rooted in the anatomy of the horse's brain and visual system. Unlike humans, whose brain hemispheres share information rapidly through a large corpus callosum, horses have a proportionally smaller interhemispheric connection that limits the speed and completeness of information transfer between the left and right brain. Each eye sends information primarily to the opposite hemisphere, and stimuli processed through the right eye — which is connected to the left hemisphere — are partially segregated from those processed through the left eye, which connects to the right hemisphere. This means that a horse that has been thoroughly desensitized to something approaching from the left side of the body may react with genuine alarm to the same stimulus approaching from the right side, not out of stubbornness or inconsistency but because the right hemisphere did not fully share in the learning that the left hemisphere established. In practical terms, this means a foal imprinted only on its left side will have genuine foundational desensitization on that side but only partial desensitization on the right side, which will show up throughout the horse's life as asymmetric reactivity — the horse is calmer on the left, more reactive on the right. This asymmetry is not only an inconvenience; it can be a safety issue when the horse is worked from both sides, loaded from either direction, or approached by veterinarians and farriers from the right. The procedure for the second side is identical to the first — all the same areas, worked to the same criterion of stillness — but may proceed more quickly because the foal has already been through the protocol once and arrives at the second side with some generalized reduction in arousal from the first side's work. The handler should nonetheless treat the second side as a complete procedure rather than a quick review, because the criterion must be reached at each area on the right side independently for the right-hemisphere filing to occur. Imprinters who skip the second side or rush through it in significantly less time than the first side typically produce horses with the characteristic left-right asymmetry that reveals the incomplete procedure: a horse that is easier to handle from the left, more reactive to bridling from the right, harder to load when the ramp is on the right side, and more spooky about things approaching from the right throughout its life. Completing both sides thoroughly is what produces the symmetric, whole-body desensitization that makes an imprinted horse genuinely different from one that was not.