Central to the theory of equine learning are the concepts of associative learning and non-associative learning. A stimulus is something which causes a response physiologically or behaviourally. Non-associative learning occurs as a response to a single stimulus, whereas associative learning takes place when a link is made between two stimuli.
Early equine learning is derived through imprinting. Observed in a number of species, the term 'imprinting' describes a phase-sensitive early learning period. Whilst equine research has yet to define the exact boundaries of this period, imprinting is thought to occur in very young foals and influences the development of social preferences. For example, during the filial imprinting period, a foal learns to follow its mother and siblings. In a similar vein, adult mating preferences are influenced by early sexual imprinting. Indeed, research studies include examples of how animals raised by a foster species later show a sexual preference for the foster species rather than their own.
Horses also learn through habituation. During habituation, a stimulus is repeatedly presented until the responsiveness to it is decreased. Habituation can occur naturally within a horse's environment or as part of the training process. An example of habituation might include a horse turned out in a field next to a train track. Initially, the horse might be highly responsive to a passing train, however after repeatedly experiencing the passing trains, the horse's responsiveness is reduced. In free-ranging equids, habituation is vital to survival as it creates a balance between the need to respond to threatening stimuli and that of over-responsiveness which is costly in both time and energy. A study by Leiner and Fendt examined the relationship between the behavioural and physiological responses in horses before and after habituation. Eighteen stallions were individually exposed to an umbrella and a tarpaulin whilst heart rate and behavioural responses were recorded. Over the following 6 days, the horses were habituated to the umbrella via twice daily exposure, but not to the tarp. On the 7th day, the stallions were re-exposed to both objects and responses were recorded. The team found that, whilst the number of behavioural responses correlated with the heart rate, habituation training reduced both the behavioural and physiological indicators of fear. This reduction only related to the umbrella, the item to which horses had been habituated, demonstrating that habituation was specific to the object itself.
In contrast to habituation, horses also learn by sensitisation. This describes how a horse can become more reactive to a particular stimulus after repeated presentation and it is often seen when an individual cannot escape or avoid an unpleasant experience. For example, if a horse became tangled in live electric fencing, it might become highly sensitised to the presence of electric fencing following that incident.
Classical conditioning involves linking a previously neutral stimulus with physiological response or an innate behaviour. The most well know example of classical conditioning concerns the work of Ivan Pavlov. Pavlov performed a series of trials using dogs, whereby a buzzer would be sounded prior to feeding. Because the sound of a buzzer seemed to predict the arrival of food, the dogs began to salivate upon hearing the noise alone. Dressage is good example of the application of classical conditioning in horse training, since when the previously neutral seat aid is employed prior to the application of the leg or hand aids, consequently the horse learns to respond to the seat as an aid in itself.
Operant conditioning differs from classical conditioning in that it uses punishment and reward to alter behaviour. The term reinforcement refers to a reward, which makes behaviour more likely to occur, and punishment which refers to something aversive, making a behaviour less likely to happen again. The terms 'positive' and 'negative' describe the addition and removal, respectively, of either a punishment or reward. An example of positive reinforcement is a horse on restricted grazing that learns to escape from its paddock. In this case, the horse associates the breaking of the fence with the reward of grass, increasing the likelihood that the horse will try to escape again occur in future. In a training context, using leg pressure when riding is a form of negative reinforcement since the release of that pressure following an upwards transition rewards the horse. This makes the horse more likely to give the same response in future. It also highlights how timing is critical in the application of operant conditioning.
A recent study by Bierke and colleagues explored a direct comparison between positive and negative reinforcement techniques. The researchers used 16 horses, split into 2 groups, to compare the use of grain (positive reinforcement) and whip taps (negative reinforcement) to train the horses to load into a trailer. There was no difference between the two groups in time taken to load, heart rate or body temperature. However, horses in the negative reinforcement group were seen to display more avoidance behaviour and those in the positive reinforcement group showed more investigative behaviours.