Grading odor similarities in a Go/No-Go task.

Recent studies show that some features of odor perception are predicted by olfactory receptor biophysics and olfactory bulb physiology. Those studies used a behavioral assay in which rodents dig in a dish of scented cage bedding after pretraining to associate a buried reward with an odorant. The advantage of the digging task is an intensity measure of similarity (number of seconds spent digging). The method has the disadvantages of odorant contamination and low control over concentration and timing, making it difficult to use in electrophysiology. We describe an operant task that avoids these disadvantages and provides a reliable intensity-based similarity measure. Odorants can be delivered with a standard air dilution olfactometer, and rats learn to lever press to one odorant and avoid pressing to another in a Go/No-Go (CS+/CS-) task with a partial reinforcement protocol. Generalization tests substitute a portion of the unrewarded CS+ trials with test odorants. The number of generalization trials on which a subject responds to a test odorant is the measure of odor similarity intensity. We present validation tests using mixture component recognition, which show high repeatability, little variability across subjects and no decrease in responding across sessions. The results match those obtained with the digging task in four of five mixtures tested. This method allows optimal control over stimulus parameters and is compatible with simultaneous electrophysiological recording.