Stimulus Generalization, Discrimination Learning, and Peak Shift in Horses

Abstract

Using horses, we investigated three aspects of the stimulus control of lever-pressing behavior: stimulus generalization, discrimination learning, and peak shift. Nine solid black circles, ranging in size from 0.5 in. to 4.5 in. (1.3 cm to 11.4 cm) served as stimuli. Each horse was shaped, using successive approximations, to press a rat lever with its lip in the presence of a positive stimulus, the 2.5-in. (6.4-cm) circle. Shaping proceeded quickly and was comparable to that of other laboratory organisms. After responding was maintained on a variable-interval 30-s schedule, stimulus generalization gradients were collected from 2 horses prior to discrimination training. During discrimination training, grain followed lever presses in the presence of a positive stimulus (a 2.5-in circle) and never followed lever presses in the presence of a negative stimulus (a 1.5-in. [3.8-cm] circle). Three horses met a criterion of zero responses to the negative stimulus in fewer than 15 sessions. Horses given stimulus generalization testing prior to discrimination training produced symmetrical gradients; horses given discrimination training prior to generalization testing produced asymmetrical gradients. The peak of these gradients shifted away from the negative stimulus. These results are consistent with discrimination, stimulus generalization, and peak-shift phenomena observed in other organisms.

Cognition and learning in horses (Equus caballus): What we know and why we should ask more

“…When matching stimuli were placed farther apart or when the background stimulus was changed from the initial experimental conditions (from an uneven wooden surface with beams to a flat gray panel), horses could generalize information to these novel environments and presentations (Flannery, 1997). Dougherty and Lewis (1991),asked horses to discriminate between a black circle 2.5 inches in diameter (the positive or rewarded stimulus) and a black circle 1.5 inches in diameter (the negative stimulus). Interestingly, when horses were presented with intermediate stimuli Dougherty and Lewis (1991) found that, unlike pigeons and humans, horses showed symmetrical generalization gradients (they chose stimuli that were closest in size to the originally rewarding stimulus on average), as opposed to asymmetrical generalization (where there is a tendency to choose exaggerated versions of the initially rewarded stimulus, i.e.…”

“…Dougherty and Lewis (1991),asked horses to discriminate between a black circle 2.5 inches in diameter (the positive or rewarded stimulus) and a black circle 1.5 inches in diameter (the negative stimulus). Interestingly, when horses were presented with intermediate stimuli Dougherty and Lewis (1991) found that, unlike pigeons and humans, horses showed symmetrical generalization gradients (they chose stimuli that were closest in size to the originally rewarding stimulus on average), as opposed to asymmetrical generalization (where there is a tendency to choose exaggerated versions of the initially rewarded stimulus, i.e. choosing circles larger than the original positive stimulus on average, representing an avoidance of the negative stimulus).…”

“…While research would suggest that many horses can be successful on a wide range of discrimination tasks, variability in performance has made it difficult to draw conclusions about the species as a whole or applicability to horses kept in different settings (Dougherty and Lewis, 1991;Sappington and Goldman, 1994;Flannery, 1997).…”

Artificial neural networks as models of stimulus control

“…A compound case would be changing the size of the signal in s + , involving thus both a change of total intensity and different sets of units involved by different test stimuli. This latter case will not be investigated in this paper (but see Grice & Saltz, 1950;Mednick & Freedman, 1960;Dougherty & Lewis, 1991, for experimental data).…”

Conditioning Methods for Animals in Agriculture: A Review

“…Horses tend to be easy to study because even well-fed horses respond well to food reinforcement and studies indicate that horses can discriminate between people, shapes (34) , and colors (35) . Furthermore, horses show response matching (36) , some degree of stimulus generalization (37) , respond to different reinforcement schedules (38) and differential rewards (39) .…”