It's important to note that in most cases, desensitization cannot be used alone. This is because it is not enough for the animal to be exposed to something where "nothing bad" happens. One reason for this is perhaps the animal didn't have a bad experience this time he was brushed for example, but had a painful tangle pulled 6 months ago. Fear is an emotion which becomes quickly and deeply ingrained, so the dog can still experience emotional arousal when he merely sees the brush due to the anticipation of something bad happening. This emotional arousal causes stress hormones to be released throughout the animal's body which, in turn, causes the physiological symptoms we see such as panting, increased heart rate, and other such symptoms. This negative feeling causes the animal to create a negative association with the experience or object. The same applies even if the animal is being introduced to something for the first time. You may see absolutely nothing wrong, but your dog may be perceiving something very different. For some reason, your dog may be experiencing fear. Thus, a negative association can be created. For example, I am terrified of spiders. I've never been mortally wounded by one, and one might say that in my every exposure to spiders "nothing bad" has ever happened. However, that doesn't alter the fact that I am terrified of spiders. Now, if I was a comfortable distance away from one and every time it crossed my path a $100 bill fell from the sky, I would feel very differently about spiders. I would likely even get to the point where I could get very close to one, and maybe eventually even hold one. Granted, it would be a process and wouldn't happen overnight, but it could certainly happen. Therefore, we should make every effort to help "something good" happen when we are using desensitization. This is why it must be paired with classical conditioning.
-- Show the dog the hair brush and offer food, then hide the brush again.
--Reach a small bit towards the dog with the hairbrush and feed.
--Get closer and closer, feeding the dog every time.
--Barely touch the dog with the brush for a split second and feed.
--Do a one inch stroke with the hairbrush on your dog's favorite "scratch me" spot and feed.
--Do longer and longer strokes, feeding every time.