What does Warwick Schiller say about the difference between a horse that is calm and a horse that is shut down?

Warwick Schiller's distinction between a calm horse and a shut-down horse is one of his most important contributions to the understanding of desensitization work, and it has practical implications for how desensitization is evaluated and conducted. A calm horse is genuinely relaxed and unafraid. Its nervous system is regulated, it processes its environment normally, it blinks, breathes deeply, and shows interest in its surroundings. When something new or mildly startling happens, it notices, assesses, and returns to calm fairly quickly. The calm horse's response to a genuinely frightening stimulus is a normal fear response — its heart rate rises, it may spook or move away — but the response is proportionate to the stimulus and recovery is rapid. A shut-down horse presents deceptively like a calm horse. It stands still for frightening stimuli. It does not visibly react when objects are waved around it. From the outside, it looks desensitized. But the stillness of the shut-down horse is not relaxation — it is learned helplessness. The horse has learned through repeated overwhelming experiences that responding to threats does not help, so it has stopped responding externally. Internally, its cortisol levels may be extremely high, its heart rate elevated, and its body preparing for explosion. Schiller teaches that the way to distinguish calm from shut-down is to look for the processing signals — licking, chewing, blinking, sighing, head lowering. These physiological markers of nervous system regulation are present in a genuinely calm horse and absent in a shut-down horse. A shut-down horse that looks still will often have a fixed, unblinking eye and a held, shallow breath. The significance for desensitization is that working a shut-down horse with increasing pressure is dangerous, because the horse has reached its threshold already — it simply has no more response to show. The explosion that comes when a shut-down horse finally releases its suppressed fear response can be much larger and more sudden than anything the horse showed leading up to it.