The ability to learn CS-UCS associations may change during development, as the organism acquires new capacities to encode details of stimuli in particular sensory modalities. For example, associative learning of olfactory and gustatory CS occurs earlier than auditory and visual CS in the rodent as these modalities mature differentially with development (for review see Richardson & Hunt, 2010). Additionally, the expressions of learned associations may continue to change as further development supplies the maturing organism with an increasingly complex behavioral repertoire. For instance, rats as young as 16 days can express learned associations between olfactory or visual CSs and a shock-UCS via freezing behavior and heart rate; however, the presence of such associations are not expressed at this age using fear potentiated startle, but do manifest at 23 days of age (for review see Richardson & Hunt, 2010). These complex processes influence the inferences that can be drawn about development and fear conditioning. The degree to which fear conditioning might appear mature or immature may depend on the particular stimuli used during learning and the behavioral modality through which learning is probed.