The geometry of stimulus control

Many studies, both in ethology and comparative psychology, have shown that animals react to modifications of familiar stimuli. This phenomenon is ofte n referred to as generalization. The majority of modifications lead to a de crease in responding, but to certain new stimuli an increase in responding is observed. This holds for both innate and learned behaviour. Here we prop ose a heuristic approach to stimulus control, or stimulus selection, with t he aim of explaining these phenomena. The model. has two key elements. Firs t, we choose the receptor level as the fundamental stimulus space. Each sti mulus is represented as the pattern of activation it induces in sense organ s. Second, in this space we introduce a simple measure of 'similarity' betw een stimuli by calculating how activation patterns overlap. The main advant age in this approach is that the generalization of acquired responses emerg es from a few simple principles that are grounded in the recognition of how animals actually:perceive stimuli. Many traditional problems that face the ories of stimulus control (e.g. the Spence-Hull theory of gradient interact ion or ethological theories of stimulus summation) do not arise in the pres ent framework. These problems include the amount of generalization along di fferent dimensions, peak shift phenomena (with respect to both positive and negative shifts), intensity generalization and generalization after condit ioning on two positive stimuli. (C) 1999 The Association for the Study of A nimal Behaviour.