Some researchers have made the discovery that a diet high in saturated fat could impair rats' ability to inhibit responding to stimuli that are reinforced in some circumstances but not in others. This article can be found at this link. This is basically proposing a solution to a problem I had blogged about two weeks ago ("Learning and the persistence of appetite") where if something is extinguished in one context, the same behavior may renew and appear again in a different context (one that it was not extinguished in). The researchers are suggesting that if one consumes a high fat diet that those behaviors will be extinguished for good and will not reappear in a different context (= renewal will not occur). At first glance this sounds preposterous, as it certainly did for me, but if true, the applications are tantalizing. The main examples I think of with extinction and renewal are fear conditioning, extinction of overeating habits, addictions, etc. Imagine if one could finally extinguish these conditions in one setting and have them permanently extinguished and remain that way in other settings as well.
Rats were trained in two contexts, X and Y. They were trained to consume sucrose pellets as reinforcers in two 64-minute sessions with 16 deliveries of two 45-mg sucrose pellets. Then, the rats received training with an auditory cue; in each of eight 64-minute daily sessions rats received sixteen 10 second reinforced presentations of an 80-db white noise. After the acquisition training, half the rats were fed the high fat diet and half were fed the control diet for 7 days. After this phase, rats were kept on a calorie restriction for 7 days, so that they would be motivated for the extinction training. Finally, the rats received extinction training with 16 non-reinforced presentations of noise; half of the rats received extinction in the same context in which they were trained and half received extinction in a different context. The extinction was done with the introduction of a light, which preceded the tone CS, and signaled no food coming. Finally, all the rats returned to the same context and were given four daily tests of renewal of extinguished food cup responding. Responding by the rats was measured as the amount of time they spent with their head in the food cup before or after the time of reinforcer delivery. The results showed that control rats extinguished in a new context showed renewal back in their training context, while rats who were fed a high-fat diet showed considerably less renewal. Therefore, it was concluded that exposure to a high-fat diet reduced the context-specificity of extinction.
An interesting point that the discussion section brings up is the possibility that a high-fat diet may be interfering with normal hippocampus and medial prefrontal cortex learning. They are suggesting that these areas of the brain may be affected, thus leading to a lack of contextual control. This, in turn, is encouraging people to overeat because they ignore the normal cues such as internal satiety signals or external signals (predators is the example given); just like the rats who ignored the light signals that food was not coming. This point actually counters my original thought that overeating could somehow be extinguished (by extinguishing the cues and CS's that cause humans to expect food) and transferred over to multiple settings. In addition to explaining overeating, this article may be helpful for therapeutic extinction of anxieties and fears as well. Imagine a scenario where a fear or anxiety is extinguished in the therapist's office, but rather than renew and reappear in other contexts as these things tend to do, they remain permanently extinguished.
I want to make some critiques of this article that came to my attention as I was reading it and speaking to my professor. What if the rats are simply nosing around the food cup extensively because they like the high-fat food so much and get addicted to it, just as someone gets addicted to candy or potato chips, etc. which are also high in fat. Maybe they choose to ignore the extinction cue of the light simply because the reward (fatty food) is so tantalizing and they can't get enough of it, so they are always in search of it.
An issue that was brought to light in my discussions with Professor Blaisdell was that rats and humans digest fats very differently, which may be the key to the differences in the effects of fats between the two groups. It is well-known that a diet high in fats can cause all sorts of complications in humans such as cardiovascular disease, obesity and an impairment of cognitive functioning, but apparently in rats it shows some positive effects of countering extinction. Lard (pig fat) has a high concentration (almost 50%) of monounsaturated fats, which can be digested better in rats than in humans.
With careful reading of the 'materials and methods' section you can see that there is a disproportionately larger amount of corn starch in the control group as opposed to the high-fat diet group (452.2g vs. 72.8g). Corn starch is a thickening agent, and I wonder if they had to use extra corn starch in order to counter the lower quantity of lard (maybe to give the two groups' foods similar appearances) and if it had any effects on the results of the experiment. In addition, I noticed that there is 20g of lard in the control group as opposed to the 177.5g in the HFD group, which makes sense considering it is a control for this factor, but the researchers never controlled for the amount of kcal/g in the two groups.
Regardless of the critiques of this article, it is an interesting find and definitely something worth blogging about and bringing to the attention of others. If something along these lines can be replicated in humans then, needless to say, it will change the way we think of high-fat foods and may end up appreciating the benefits, especially in a therapeutic setting.