An Automated Olfactory Discrimination Procedure with Rats

Although rats are commonly used as subjects in psychological research, few researchers have utilized their olfactory capabilities. An automated chamber capable of presenting multiple odor stimuli to 5 locations was used to train rats on 3 simple dis crimination and reversal tasks. Unique odor pairs were used for each discrimination and reversal task (Sets A, B, & C). All rats readily learned each task, taking longer to learn the reversal than the initial discrimination. Stimulus location did not influence accuracy, with only two exceptions. Moreover, rats showed no position biases for any of the 5 locations. These data demonstrate the feasibility of testing rats using olfactory cues in an automated apparatus with as many as 5 stimulus locations. This apparatus and procedure have potential applications to many areas of psychological investigation, including extra-dimensional shifts, compound stimulus training, and conditional discriminations. Key words: discrimination, olfactory, stimulus location, nose poke, rats ********** Rats are commonly used subjects in psychological research that are frequently tested on tasks that require bar pressing or maze running. Moreover, these tasks are typically used to examine associative, or discrimination, learning in rats. Few researchers, however, have supplanted these methodologies reliant on pressing and running behavior with those that take advantage of rats' superior sense of smell. For example, a search of the abstracts on PsychINFO returned over 23,000 references using the keyword "discrimination," but only 627 when the keywords "rat" and "discrimination" were combined. Utilizing odor cues in discrimination tests involving rats is advantageous because this species possesses a biological preparedness for learning about odors. For example, Nigrosh, Slotnick, and Nevin (1975) demonstrated that when responding was brought under control of redundant odor and tone cues, control of behavior was more effective with the odor cue (but see Neill & Harrison, 1987, for a discussion on the variables influencing auditory discrimination learning in rats). Moreover, developing reliable behavioral tests that take advantage of rodents' olfactory sense has the potential to increase the utility of animal models of conditions, such as mental retardation, that are partially defined in terms of their behavioral effects (e.g, McIlvane & Cataldo, 1996). Nonetheless, the absence of utilizing olfactory stimuli in studies of discrimination learning remains notable. One possible reason is the inherent difficulty of working with them. Despite this, some researchers have attempted the challenge. Bunsey and Eichenbaum (1996) trained rats on matching to sample conditional discriminations using olfactory stimuli. Everyday household spices were mixed into sand and placed in small cups. Rats were required to dig into the cups to obtain reinforcement (a piece of chocolate). This procedure was effective in establishing reliable matching behavior. Moreover, the basic procedure has been replicated (Dudchenko, Wood, & Eichenbaum, 2000; Wood, Dudchenko, & Eichenbaum, 1999) and has been successfully modified for simple discrimination training with both rats and mice (Dusek & Eichenbaum, 1997; Mihalick, Langlois, Krienke, & Dube, 2000, respectively). Despite the success of this procedure, there are some disadvantages: a large number of scented cups need to be mixed prior to each session, the researchers have to manually present each stimulus during the session, and data have to be recorded by hand. Pena, Pitts and Galizio (2006) have taken this innovative idea further by conducting sessions in a standard operant chamber and streamlining the sand-cup delivery process. Baited sand cups are presented to the rats by sliding a tray that fits underneath the floor in and out of the chamber. This procedure involves less contact with the subjects during the session but still requires manual data recording. …