The term refers to a gradual habituation to an arousing stimulus. Systematic desensitisation is a commonly used behaviour modification technique for the alleviation of behaviour problems caused by inappropriate arousal. In a controlled situation, the animal is exposed to low levels of the arousing stimulus according to an increasing gradient, and rewarded when it remains relaxed or shows an appropriate response. An increase in the level of the stimulus is not made until the animal reliably fails to react to the previous level. In this way the technique aims to raise the threshold for a response.

Example: The horse is fearful of aerosols. As a first step a handler brings an aerosol close to the horse and strokes it on the body with the bottle (no spraying). This is to habituate the horse to the visual characteristics of the aversive stimulus. When the horse shows no avoidance responses, a next step is to stand some meters from the horse and spray in the opposite direction, preferably with water, i.e. a fluid with no smell. This is to gradually habituate the horse to the aural characteristics of the aversive stimulus. The handler gradually steps closer to the horse, and when the horse shows no responses to the handler standing next to it and spraying in the other direction, the handler can gradually spray closer to the horse. Before spraying directly on the horse’s coat, the handler should stroke the horse with a hand and spray gently on the hand. At all stages, it is important to ensure that the horse is only rewarded for appropriate responses, i.e. the aerosol should be removed or spraying terminated when the horse stands still. Positive reinforcement (e.g. food, wither scratching) can be used as an additional reinforcer for appropriate behaviour.

To ‘condition’ means to train, and ‘counter’ means opposite. Counter-conditioning refers to training an animal to show a behaviour (e.g. eating) which is counter to the one that the trainer wishes to eliminate (e.g. flight), i.e. producing an incompatible response to a stimulus that is expressed in preference to the undesired response. This can be done by forming an association between the feared stimulus and a pleasant stimulus so that the fear-eliciting stimulus comes to predict something good to the horse. As soon as the horse observes or encounters the fear-eliciting stimulus, we introduce something that produces a pleasant emotional reaction (e.g. food). Over many repetitions, the animal learns that whenever the fear-eliciting stimulus appears, something pleasant happens. Eventually, the process produces a neutral or positive emotional reaction to the sight of the previously feared or disliked event, person, place or object. This technique is widely used in combination with systematic desensitisation. By ensuring that the preferred behaviour is more rewarding, the animal learns to perform the new behaviour when exposed to the problematic stimulus.

Example: A horse becomes anxious when it hears the bell before the competition, which compromises good performance. Through counter-conditioning, positive outcomes such as a food reward or termination of work are associated with the sound of the bell so that it becomes a predictor of a positive event. Once the horse shows a neutral or positive emotional state at the sound of the bell, it will no longer be tense before a competition.

Overshadowing describes the phenomenon whereby habituation to the least salient stimulus takes place when two or more competing stimuli are presented concurrently. In the practical setting of horse training, overshadowing provides an effective method of desensitising horses to aversive stimuli such as clippers, needles or other invasive procedures. These aversive and invasive procedures frequently elicit a withdrawal response in the horse. Similarly, when a horse learns to respond to lead rein signals of forward or reverse, they are initially acquired because they too produce a withdrawal response, which through the refinement of further training, diminish to light lead rein cues. Therefore outcompeting the withdrawal response elicited by the clippers through the use of the lead rein cues of forward and reverse can be a useful overshadowing protocol. The horse must therefore be reliably trained to step forward and back from lead rein cues via operant conditioning. Overshadowing differs from systematic desensitisation and counter-conditioning principally because of the use of the lead rein mobility responses.

Example: The horse is needle–shy and when it sees the person approaching with the syringe it becomes hyper-reactive and pulls against the handler’s lead rein pressures in its attempts to escape. The needle pricking the horse’s skin also induces a severe flight response. The solution in terms of overshadowing involves the horse being trained to step back and forward from lead rein pressure so that the horse’s reaction is elicited from the lightest of lead rein cues. Next the person with the syringe approaches the horse with the syringe and as soon as the horse displays even the smallest of fear responses, the person with the syringe stops and remains immobile so that the distance between the horse and the syringe stays constant. The horse is then signalled to step back one step and perhaps then forward a step. Initially the horse is delayed and its reaction to light pressure is ignored because his attentional mechanisms are overshadowed by the syringe so the handler then increases the motivational pressure of the lead rein so that in a few repetitions the horse is now responding to light signals of the lead rein. The horse’s fear reaction to the syringe has, at this distance, decreased. The syringe is now brought closer to the horse and as soon as the horse shows the slightest fear reaction, the process is repeated. This process continues until the horse’s response to the syringe has diminished. Positive reinforcement at each increment enhances the acquisition of the lowered arousal. These lead rein signals and their associated mobility responses soon achieve stimulus control of the horse’s locomotion and thus overtake the syringe or clippers for salience. The less salient stimulus either no longer elicits or greatly diminishes withdrawal from the original, more salient stimulus. The procedure however is most successful if the process is begun at the lowest levels of arousal.

This method exploits the natural tendency of horses to explore and approach unknown objects, in combination with systematic desensitisation. The horse is stimulated by the rider or handler to approach the object of its fear, which is retreating as the horse approaches. The horse may then be signaled to stop before it reaches its fear threshold, so that the object retreats even further. The horse is then signaled to catch up. As soon as the horse slows its approach it is deliberately stopped and this is repeated until the horse becomes as close as possible to the object. The method has been successfully applied to horses that are afraid of tractors and diggers, motorbikes and trams.

Example: the horse is fearful of tractors, motor bikes or trams and attempts to escape, in order to lower its fear. However if the process is reversed whereby the horse approaches the retreating machine, this can have the opposite effect: its fear is lowered because the machine itself escapes. This technique has been used in training police horses. In best practice, when the horse closes in on the machine it is stopped, thus allowing the machine to increase its distance from the horse. The horse is then stimulated to approach again and each time it draws closer to the machine before it is stopped. Stopping the horse apparently increases its motivation to approach. This is continued until the horse actually makes contact with and investigates the machine.

The method uses a stimulus to which the horse has already habituated to systematically desensitise the horse to the original fear-inducing stimulus. The fear-inducing stimulus is applied gradually at the lowest threshold of fear simultaneously as the known, non-fear-inducing stimulus is applied, and then systematically increased in intensity. For example a horse may be afraid of aerosol sprays but unafraid of being hosed. The aural and tactile characteristics of the aversive stimulus (e.g. aerosol) are gradually mixed with the habituated one (e.g. the hose) making identification of the formerly aversive one difficult and perceptually different. The old benign stimulus can then be diminished and finally terminated after which the horse will show habituation also to the new stimulus.

Example: the horse is fearful of aerosols. In this technique, a stimulus to which the horse has already habituated is used to blend with the problem stimulus. If the horse is used to hosing on its body, the aerosol is introduced during hosing and on the hosed patches of skin. The sound and feeling of the usual water on the horse’s body will blend with the novel sound and tactile feeling of the aerosol, making it less distinct. The hosing can then be terminated while the spraying continues. Adopted from From McLean & Christensen, 2016. The application of learning theory in horse training. Appl. Anim. Behav. Sci. (in press)