When sheep, bees, and 24K magic explain why grading papers puts you to sleep: Latent inhibition

Have you ever had the experience of hearing a new song repeatedly on the radio and when you first heard it, you strongly disliked it, but after your favorite radio station played it over and over and over again, it eventually grew on you?

Bruno Mars’ “24K Magic” is a recent example for me.  As a way to illustrate to my students how classical conditioning works in our everyday lives, I decided to perform a little experiment: I wanted to use “24 carat Magic” to condition myself to get sleepy. To do this, I played this song every night in bed just before I fell asleep. Unfortunately, my experiment failed, and I did not become sleepy when listening to this song. (Actually, I become sleepy whenever I grade my student papers, which I tend to do right before bedtime, such that no matter what time of day, grading papers makes me sleepy!)

The failure to condition sleepiness to “24 carat Magic” is likely due to latent inhibition, or the pre-exposure of an unfamiliar stimulus (i.e., Mars’ song) that is trying to be paired with an unconditioned stimulus (i.e., being sleepy right before falling asleep) and, because of its familiarity, fails to become conditioned.

Unlike Mars’ song, paper grading preceded bedtime and was associated with sleepiness, thus grading student papers induces sleepiness in me no matter what time of day – thanks to classical conditioning.

Sheep, Bees, and Schizophrenia!

Latent inhibition, a phenomenon associated with classical conditioning, was originally found with sheep as reported by Lubow and Moore in 1959 (see Moore, 1965 for a brief review).


However, it has been tested and found in many different species, including bees, rats, fish, and humans, especially adult humans. In all cases, conditioning is impaired for stimuli that have acquired some prior familiarity, compared to novel cues.

One adaptive function of latent inhibition is that it may minimize the development of phobias, such that pre-exposure to a novel stimulus may keep a person from excessively associating that stimulus with a fearful experience.


For example, I experienced bees for 32 years without ever being stung, until one fateful day a bee flew into the soda can from which I drank, and I was promptly stung on my lip during my son’s 6th birthday party.  Although I have been cautious of soda cans when outside after this experience, the sting itself did not lead me to a fear, much less a phobia, of bees. This was most likely thanks to latent inhibition.

Interestingly, people with schizophrenia or people with schizotypal personality are less likely to experience latent inhibition as it requires attention to one’s environment, which is reduced in people with these characteristics (Lubow & Kaplan, 2005).  An interesting aside: One of the underlying mechanisms in schizophrenia is an excess of dopamine in the brain, and when dopamine antagonists (i.e., drugs that block the release/production of dopamine) are administered, latent inhibition appears in these individuals; the reverse occurs when a dopamine agonist (i.e., a drug that increases the release/production of dopamine) is given to individuals without schizophrenia.

So how does all this relate to rats and conditioned taste aversions?


Procedural manipulations, of course. . . A common activity for learning and conditioning laboratories is to refine different conditioning procedures and techniques to determine their effects on various associative learning phenomena.

A recent study by A. Molero-Chamizo, published in the Psychonomic Society’s journal Learning and Behavior, combined two classical conditioning phenomena in an effort to test the effect of a procedural change on both of them; namely, latent inhibition in a taste aversion learning paradigm.

The overall purpose of the experiment was to determine if changing the time of day between pre-exposure and conditioning (i.e., the temporal context) would affect latent inhibition. 50 Wistar male rats were exposed to a particular context prior to conditioning a taste aversion (I discussed taste aversion in a previous blog post).  This latent inhibition manipulation was examined under two additional conditions: did the rats get to drink the taste stimulus as much as they wanted (n = 26 rats) or did they experience restricted access, in this case, 5ml of the taste stimulus (n = 24 rats)?

The flow chart below summarizes the experimental design. The groups that should have been susceptible to latent inhibition were the pre-exposed to taste groups in both access conditions (with vs. without restriction).


The table below provides more information about the procedure used by Molero-Chamizo. The letter codes in the Group column are explained in the preceding figure, and the other columns refer to pre-exposure on Days 1 and 2 (PE1-2/Wpm; water or saline), the conditioning day (CTA), and the 5 testing days (T1-T5pm), respectively.


The results of the study are shown in the figure below for the unrestricted group, using the amount of saline water consumed as an indicator of conditioning—the less the animals consumed after conditioning (CTA), the more they had paired the saline with an aversive stimulus. The labels on the X-axis refer to the events explained in the preceding table.


The figure shows that changing the context of time of day during pre-exposure and conditioning (i.e., pre-exposure in the afternoon/conditioning in the morning, PE-D) disrupted the effect of latent inhibition and this group developed a conditioned taste aversion like the two non-exposed groups (blue circle in the figure). The group that received the pre-exposure and conditioning at the same time of day (PE-S) experienced latent inhibition and continued to consume the saline water despite the taste-aversion conditioning trial (red circle).

These results mean that when the rats had unlimited access to the taste substance (saline, a neutral stimulus), latent inhibition occurred only when the pre-exposure to the saline was at the same time of day as the taste-aversion conditioning to the saline paired with a LiCl injection (the aversive unconditioned stimulus; see my earlier post on taste aversion).

In contrast, when access to the taste stimulus was restricted, the change in temporal context between pre-exposure and conditioning did not disrupt latent inhibition from occurring. This is shown in the next figure below.

Rather, under these restricted conditions, both pre-exposure groups showed the effect of latent inhibition (red circle in the figure below) whereas both non-pre-exposure groups developed the conditioned taste aversion (blue circles).


The implications of these results are that contextual change between experiencing a novel stimulus in the absence of an unconditioned stimulus, and then in its presence, may be enough of a detectable difference when the novel stimulus is freely accessible that rats experience latent inhibition that overrides a conditioned taste aversion.

However, as the author noted, this release from taste aversion requires a long exposure period and appears to only be experienced under unlimited access conditions. Manipulating temporal contexts and availability of stimulus access may help researchers to better understand the specific mechanisms that underlie the development of conditioned taste aversions and the factors that facilitate latent inhibition.

To drink or not to drink, or maybe it should be, when to drink?

So what does this all mean for people who like to drink soda outside? When sodas are limited access (i.e., one only drinks them at parties), it won’t matter if you saw or heard bees in the morning or afternoon and then experienced a bee sting in the afternoon or the morning, you will still experience latent inhibition and continue to drink soda at parties! However, if you drink soda all the time, then be sure to drink your soda at different times of day from when you get stung, otherwise drinking soda may become aversive! Then again . . . that might not be such a bad idea health-wise . . . but that is for a another time!


Article focused on in this post:

Molero-Chamizo, A. (2016). Circadian-temporal context and latent inhibition of conditioned taste aversion: Effect of restriction in the intake of the conditioned taste stimulus. Learning & Behavior, 1-7. DOI: 10.3758/s13420-016-0251-0.


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