Overshadowing: a silver lining to a dark cloud in horse training.

Overshadowing is a process known in behavioral science that occurs when two stimuli of different strengths are applied simultaneously to a nonhuman animal. Typically, the stronger stimulus overshadows the weaker one, resulting in attenuation of the weaker stimulus. This phenomenon explains ways in which the decreased responsiveness and consequent conflict behaviors (and possibly learned helplessness and wastage) in some performance horses can result from the application of two concurrent aversive stimuli. Despite some adverse consequences in the context of ridden horses, overshadowing can have serendipitous benefits because it offers an efficient method of desensitization for certain stimuli that are sometimes highly aversive: the saddle/girth pressure, clippers, aerosols, and needles. Desensitization with concurrent overshadowing appears to be comparatively rapid, particularly with highly aversive stimuli, possibly because attentional mechanisms are diverted to the more salient stimulus. It is important to note that, following the overshadowing procedure, the effects appear to be retained when assessed on subsequent days. Using 4 examples, this article presents a preliminary exploration of the beneficial use of a poorly understood, underutilized--yet promising--phenomenon that warrants further investigation.