Do discrimination tasks discourage multi-dimensional stimulus processing?: Evidence from a cross-modal object discrimination in rats

Neurobiologists are becoming increasingly interested in how complex cognitive representations are formed by the integration of sensory stimuli. To this end, discrimination tasks are frequently used to assess perceptual and cognitive processes in animals, because they are easy to administer and score, and the ability of an animal to make a particular discrimination establishes beyond doubt that the necessary perceptual/cognitive processes are present. It does not, however, follow that absence of discrimination means the animal cannot make a particular perceptual judgement; it may simply mean that the animal did not manage to discover the relevant discriminative stimulus when trying to learn the task. Here, it is shown that rats did not learn a cross-modal object discrimination (requiring association of each object's visual appearance with its odour) when trained on the complete task from the beginning. However, they could eventually make the discrimination when trained on the component parts step by step, showing that they were able to do the necessary cross-modal integration in the right circumstances. This finding adds to growing evidence that discrimination tasks tend to encourage feature-based discrimination, perhaps by engaging automatic, habit-based brain systems. Thus, they may not be the best way to assess the formation of multi-dimensional stimulus representations of the kind needed in more complex cognitive processes such as declarative memory. Instead, more natural tasks such as spontaneous exploration may be preferable.