Research suggests that environmentally-induced (spontaneous) stereotypies arise from dysregulation of the basal ganglia. Basal ganglia dysfunction can also expresses itself as aberrations in learning task performance. As a result, several studies have demonstrated a strong link between inappropriate repeat responding within an extinction learning paradigm and stereotypy performance. In contrast to this, research into the oral equine stereotypy crib-biting has suggested that the aetiology of this behaviour sterns from visceral discomfort as a result of the acidic consequences of eating cereal-based concentrate where, crib-biting is potentially a means of producing additional alkaline saliva to counteract the effect. If crib-biting is an artifact of nutritional and not basal ganglia dysfunction, crib-biting horses should not display the additional aforementioned signs of basal ganglia dysfunction. The aim of this study, therefore, was to test this hypothesis by recording the behavioural response of crib-biting and control animals within an extinction-learning paradigm.
Ten control and 10 stereotypy (crib-biting) horses of similar breed (Thoroughbred and Thoroughbred cross) and sex (control: 5 male [gelding], 5 female; stereotypy: 6 male, 4 female) were shaped to press a 15 cm x 15 cm square button mounted vertically at 1.5 in from the floor in response to a conditioned stimulus (CS) (electronic buzzer [30 Db] and a yellow light bulb [50 W]. All subjects were then placed under extinction conditions whereby button presses following CS delivery would yield no food reward. The following four recordings made: (1) the number of trials to total extinction (zero responses within or outside of the 15 s window of opportunity), (2) the number of trials to extinction within the 15 s window of opportunity, (3) the number of button presses to total extinction and (4) the number of trials taken to re-establish criterion.
Crib-biting horse required significantly more unreinforced trials (P <0.01) and overall button presses (P <0.01) to reach extinction criterion compared to control animals. Thus, crib-biting horses demonstrated characteristics of increased perseveration suggesting alterations in basal ganglia physiology. These results tend to dissuade from the hypothesis that crib-biting is a method of producing additional saliva to ameliorate visceral discomfort, but studies combined might suggest that visceral discomfort has an important role to play in the alteration of basal ganglia activity that then manifests itself behaviourally as oral stereotypy. (C) 2006 Elsevier B.V. All rights reserved.