The Garcia Effect: A Tailored Taste for Survival - eScholarship

T he G arcia E ffect A T ailored T aste for S urvival B S J Chang Liu On the surface, taste seemingly only serves as a factor of enjoyment in response to various foods, but it commands a much stronger persuasion on the human subconscious than any of the other four senses. Taste is the only sense that can unconsciously and permanently plant a memory of disgust and repulsion within animals without requiring any conscious thought or mental notation. This phenomenon is scientifically known as “conditioned taste aversion,” and was first researched by Dr. John Garcia, a notable psychologist who conducted most of his studies during the 1950s at Hunter’s Point Radiological Defense Laboratory in San Francisco. Garcia and his colleagues were originally studying the effects of ionizing radiation on the behaviors of laboratory rats when they stumbled upon the curious phenomenon of taste aversion (Garcia, 1988). They discovered that radiated rats would eventually stop drinking the saccharin solutions supplied to them for hydration during radiation tests, opting instead for thirst. Garcia switched his research to hone in on this peculiar finding and eventually demonstrated that it had a psychological basis. He postulated that the rats must have unwittingly attributed ingesting saccharin water— the only physical cue that they had—with the aversive symptoms that were actually due to undetectable radiation (Garcia, 1988). A NOVEL IDEA Garcia’s discovery of taste aversion led to a plethora of questions from the scientific community. While the phenomenon seemed to follow a basic pattern of Pavlovian conditioning, it broke many of the key principles proposed by Pavlov and Thorndike regarding learning theory. For instance, the rats in Garcia’s studies retained their distaste for saccharin-flavored water long after radiation was removed; also, they learned to avoid the solution despite the fact that they only drank the water sporadically while under radiation. All of these factors violate many key tenets of Pavlovian conditioning, such as the need to consistently pair a conditioned stimulus with a conditioned response over multiple trials in order to cement an association. Taste aversion also defies the “Law of Exercise” from Thorndike’s learning theory, since the rats did not require repetition to learn or strengthen the association; in fact, the learning itself was spontaneous and the response lasted for much longer than it should have without the requisite practice. Psychologists noticed that a number of factors including sex, age, testing procedures, deprivation level, and drug history all affected the acquisition rate and terminal strength of taste aversion, but no one was quite able to put a finger on how the phenomenon could be implanted within an organism’s memory so instantly and irrevocably (Parker, 2003). Furthermore, conditioned taste aversion is encountered at all levels of evolution, with similar forms of food aversion learning found in vertebrate and invertebrate species whose ancestral lines diverged more than 500 million years ago (Bures, 1998). Thus, the acquisition of taste aversion is not a simple Pavlovian-conditioned event, but a unique psychological phenomenon with deep evolutionary roots. THE PSYCHOLOGICAL & PHYSIOLOGICAL BASIS Many preliminary and subsequent in-depth studies were conducted in relation to taste aversion. While earlier studies aimed to uncover the physical effects of taste aversion, more recent studies are focused on the psychological framework and physiological mechanics behind the phenomenon. Linda A. Parker, a research psychologist at Wilfrid Laurier University in Waterloo, Ontario, discovered that the emetic system of the midbrain and brainstem was responsible for the nausea that results as a consequence of taste aversion. The emetic system is not a group of organs in the traditional sense, such as the digestive or nervous system. However, it is so named because “emetics” have traditionally been used as substances that induce vomiting to purge the body of toxins and harmful parasites; conditioned disgust reactions are established by the association between a flavor and the activation of the emetic system (Parker, 2003). Although rats are incapable of vomiting, they were observed to display conditioned disgust reactions such as gaping and chin rubbing when they were exposed to a flavor previously paired with drug-induced nausea. Time and the ability to form sensory memories play important roles in the acquisition of taste aversion as well. Baby rats could only acquire an aversion to a particular food if it was eaten within sixty minutes of nausea onset; the association failed to appear with longer time gaps (Stephenson, 2001). Baby rats also forgot taste information more easily than adult rats, pointing to the prominent tie between memory formation and taste aversion. Lastly, experiments showed that severing one of the vagus nerves in rats (see 1 • B erkeley S cientific J ournal • S cience of F ood • V olume 16 • I ssue 1

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