The complexity of the environment is such that even a snapshot in time from a single modality can exceed the processing capacity of the human brain (Tsotsos,1990). This means that selection of some incoming stimuli for more detailed analysis (while ignoring other stimuli) is essential for efficient cognitive processing. Without such filtering we would be inundated with competing sensory impressions. Thus notions of stimulus salience and relevance have assumed prominence in a number of domains, from visual attention (Fecteau & Munoz, 2006; Li, 2002; Morris, Friston & Dolan, 1997) to animal cognition (Mackintosh, 1975).

But what exactly is meant by the term stimulus salience? There is no straightforward answer to this question, as the apprehension of salience is likely to be determined by a variety of factors. First, the inherent features of the stimulus will contribute to its effective salience, for example its physical intensity (what we will term intrinsic salience). More physically intense stimuli will be learned about more easily. A stimulus's physical intensity cannot be modulated, but the perception of its intensity can be. Second, associative learning theories tell us that past experience with the stimulus can also influence the amount of attention that is paid to it (what we will term acquired salience). This affects what is known as its associability – how easily it may be learned about – prototypically demonstrated in latent inhibition (LI). Thus past experience can also modulate the effective salience of a stimulus.