Are separate theories of conditioning and timing necessary?

Conditioning and timing studies have evolved under separate traditions, which is exemplified in both traditional theories (e.g. the Rescorla-Wagner model of conditioning vs. Scalar Timing Theory) and in a dual process model (Gibbon, J., Balsam, P., 1981. In: Autoshaping and Conditioning Theory. Academic Press, New York.). Other lines of theoretical development in both timing and conditioning fields have resulted in the emergence of 'hybrid' theories in which conditioning and timing processes are integrated. Simulations were conducted with a recent hybrid theory of timing (Machado, A., 1997. Psychol. Rev. 104, 241-265). The simulations were of classical conditioning procedures in which the local or global predictability of food was varied by manipulating the variability of the CS-US relationship, variability of the CS duration, and variability of the intertrial interval. The hybrid model provided good qualitative fits to indices of conditioning (discrimination ratios) and timing (local rates of responding), indicating that it may be possible to model both conditioning and timing results with a single process in which an internal representation of time and a strength of association are integrated. However, the failure of the model to provide good quantitative fits of the data indicates the need for a consideration of alternative perceptual representations of time and/or principles of association within the framework of the hybrid model.