In trying to explain the classical conditioning process, experiments where two (or even more) CSs are presented with a single US were conducted. By varying aspects of the CSs and comparing the effectiveness of their conditioning to the single US the scientists hoped to discover what properties of the CS (and, eventually the US), as opposed to the CS-US pair, determine the effectiveness of conditioning. Probably the simplest experiment comparing two CSs is a demonstration of the phenomenon of overshadowing. In this experiment two CSs, CS1 and CS2, are always presented together during training. In the test-phase, the strength of conditioning to the stimuli CS1 and CS2 presented individually are measured. The typical outcome shows that the strength of conditioning to each CS depends on their relative intensity. If CS1 is a dim light and CS2 a bright light then, after conditioning to the CS1-CS2 combination, the CR to the bright light is very strong while the dim light alone produces little or no reaction. The general perceived strength of stimuli is commonly referred to as their salience. Although it might be related to the physically measurable intensity of stimuli, salience is refers to the intensity of the subjective experience of stimuli, not of the objective intensity of the stimuli themselves. Salience, as subjective experience, varies between individuals, and, more importantly, between species. Salience is depends on some combination of the physical characteristics of stimuli and of the sensory systems of the perceiver.

This document has been restructured from a lecture kindly provided by R.W.Kentridge.