Aloha! You're speaking to Dr. Michael Salkin
I'm sorry that your question wasn't answered in a timely manner. Unless there were supportive evidence of a medical problem - and there isn't in Reggie's case - I would consider his hiding a fear response. It's a rather complex physiologic response involving a variety of areas of the brain
. When a threatening or fear-evoking situation is perceived, a dog must react instinctively to survive; it either readies for confrontation or runs away as Reggie is doing. An intensely unpleasant or aversive event can lead to an intensely fearful and lasting "memory" of the stimulus ("one-event learning"). Although the pet may only become fearful of specific stimuli ("discrimination") such as a noise (e.g., gun fire), place (e.g., veterinary clinic), or person (e.g., neighbor's child), some pets may generalize to many similar stimuli (e.g., all novel noises or all unfamiliar children). In addition, the pet may become fearful of events that preceded the unpleasant situation (e.g., the car ride that precedes the veterinary visit).
It's much too early to consider retraining but that can be done if necessary. Owners need to be counseled to use appropriate behavioral modification methods which include flooding, habituation, systematic desensitization, counterconditioning, shaping, positive reinforcement, and techniques that teach the pet an alternate acceptable response that can be reinforced (response substitution, differential reinforcement).
Please respond with further questions or concerns if you wish.