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Back to Subjects - Psychology

Taste Aversion

Taste Aversion Classical conditioning states that learning is a gradual process, that it is not possible for a subject to be classically condition in only one trial. However, if you eat something and become sick from it, there is a very good probability that you will develop a strong distaste for that food. This effect is known as taste aversion, which has brought up many questions about classical conditioning. It was Garcia and Koelling (1966) who studied the level of conditioning in rats using two conditioned stimulus (CS), an audiovisual stimulus and a salty flavor added to drinking water. The two unconditioned stimulus (UCS), a mild foot-shock and a nausea-producing drug. In the conditioning phase of this experiment, the two CSs (salty flavored and the audiovisual stimulus) always occurred at the same time and were presented to the rats equally. One group of rats received a UCS with a nausea-producing substance in the drinking water (lithium chloride). Another group of rats were shocked in the foot as a UCS 2 seconds after drinking the water (Garcia & Koelling as sited Walker 1995). After many conditioning trials, each rat was tested with each of the two CSs taken separately. In one trial the rats were given access to tasty water, salty flavored water with out the audiovisual CS. In another trial, the rats were given access to “bright noisy water” as stated by Garcia and Koelling: unflavored drinking water that had the audiovisual CS present. If there is an aversion to the stimulus of the foot-shock of the nausea-producing drug then the rats will have become conditioned to the CS. Both UCSs produced an aversion a particular CS; the foot-shock produced a strong aversion to the audiovisual CS but showed little or no avoidance to the salty flavored water. However the group of rats that were given lithium chloride, resulting nausea-produced taste aversion to the salty flavored water, but no aversion to the audiovisual CS. It is seen that the rats promptly learned to associate a taste CS with a UCS of nausea but not with the foot-shock, where the audiovisual CS was conditioned to the foot-shock but not to nausea (Walker 1995). Taste aversion has violated three principles of classical conditioning, the first is that equal associability of stimuli: any CS can be paired with any UCS. This has proven to be untrue because if it were, the rats that became sick would have avoided both the salty water CS and the audiovisual CS. The second principle violated is temporal contiguity: CS and UCS have to be presented close together in time. Again untrue in this case because of such a long delay between drinking the salty water and becoming sick. The third and final principle is that learning is gradual, but in this case only one trial was enough. The studies done by Garcia and Koelling have had much support from many other experiments done in similar or the same situations. A study of ingestional aversion (Gregg, Melanie, Kittrell, Domjan & Amsel, 1978), where 12 and 15 day-old rats were conditioned by infusing a .5% solution of saccharin into the oral cavity. This was then followed by an oral infusion by the injection of lithium chloride. At both ages, subjects that had saccharin exposure was followed by a lithium injection within 2 to 3 minutes drank less when the saccharin solution was again presented by oral infusion 12 hours later. Ingestional aversions were also learned by 12-day-olds when an interval as long as 30 minutes was introduced between saccharin exposure and lithium toxicosis, however there was no sign of aversion to the saccharin solution when the toxicosis was delayed by 120 minutes. In contrast, the 15-day-olds were able to learn aversion with both the 30 minute and 120 minute delay intervals. Although the absence of 120 minute long-delay learning in 12 day olds, further experiments show that ingestional aversion conditioned at 12 days of age were retained for 2 weeks (Gregg, et al, 1978). These results provide more evidence to support taste aversion. To further test the validity of conditioned taste aversion, a study was done in the generalization of conditioned taste aversion in rats (Richardson, Williams & Riccio, 1984). This study further investigates the effects of delayed testing (2, 7, 21 days), the rats were given varying concentrations of sucrose (2.5%, 10%, 32%) and immediately received lithium chloride. Subjects were then tested either 2, 7, or 21 days after conditioning. Findings show that the rats show avoidance in all categories, this further reveals that CTA shares many similarities with the process of classical conditioning, except for the three principles mentioned earlier (Richardson, et al, 1984). A more recent experiment done on taste aversion is a study on the interval between the CS and UCS as a determiner of generalization performance (Land, Harrod & Riccio, 1998). In this experiment rats were trained in a conditioned taste aversion paradigm in order to determine whether a trace interval between the CS and the UCS would result in the forgetting of stimulus attributes. Subjects were conditioned with milk and where given either and immediate or a delayed injection of lithium chloride. They were then tested 48 hours later with either milk of chocolate milk, a generalized flavor. The rats conditioned immediately following the presentation of the milk avoided the milk more than the chocolate milk, pointing out discrimination between the two flavors. Those conditioned after a trace interval avoided both flavors equally, this may suggest that there was a loss of stimulus attributes of the original CS (milk). The delay rats however, still exhibited considerable learning when compared with the control group, which did not receive the UCS. These results once again shows the strength of effects that taste aversion can have on a subject. Although there have been many studies done in support of the permanency of taste aversion, there also has been studies done on extinction cue that can reduce spontaneous recovery of a conditioned taste aversion (Brooks, Palmatier, Garcia & Johnson 1999). The experiment demonstrated that when there was an auditory cue presented during extinction, it influenced the rats’ saccharin consumption by reducing spontaneous recovery. However, it is also noted that cues that had been presented during or prior to conditioning did not reduce spontaneous recovery when presented during the test. In this case, the attempt to reduce spontaneous recovery of a conditioned taste aversion is not supported as well as increasing the strength of a conditioned taste aversion. Taste Aversion has offered theoretical insight, which have practical applications (Walker 1995). For instance, the idea of taste aversion has been utilized by attempting to control the predatory habits of coyotes in the United States. It was found that after feeding the coyotes with the flesh of a lamb treated with lithium chloride, coyotes then were less likely to attack lambs (Walker 1995). Further studies of taste aversion can be beneficial because of the possible applications it may offer, perhaps some techniques in treating such problems as alcoholism (Walker 1995) or to better understand the environment’s affect on animal and human behavior. Bibliography:

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