Friday, April 27, 2012

Conditioned Cues Elicited Feeding in Sated Rats

    This is the second week of my blog and I have another exciting article to talk about.  For anyone with access, the article is here. The title of the article is Conditioned Cues Elicited Feeding in Sated Rats: A Role for Learning in Meal Initiation.  The article talks about Pavlovian conditioning influencing rats to eat even while they are full (satiatied), if they have learned the cues that signal food.  The procedure for this experiment was to take seven male rats, housed individually and carry out two phases: conditioning and testing.  During conditioning a 4.5 minute conditioned stimulus (CS+) consisting of a buzzer and a light presented together, was presented before every meal.  The '+' sign in this experiment, as professor Blaisdell taught us, means that the US (the food) will actually be appearing in this experiment, and is signaled by the CS.  The meal, which was composed of 8ml of a liquid diet (some type of evaporated milk) was delivered to the mice during the last 30 seconds of the CS+.  Rats received six meals in a day, 3.5 hours apart.  A CS- of a pure tone was presented at the midpoint of every inter-meal interval.  As we learned the '-' sign in this case signals that no food will be paired with the stimulus, and it was meant to condition the rats to learn that feeding had ended, at least for some period of time.  Once the rats had been conditioned to a stimuli and knew which stimuli were meant to signal food, they were tested in a subsequent experiment.  The conditioned simuli were presented and the responses of the rats measured.  The most important observation of this experiment was that feeding responses were made to the CS+ during testing on all 21 days, EVEN WHILE THE RATS WERE SATIATED!   In further testing, where rats were tested with the conditioned stimuli and then later allowed to feed on a food bottle for a free period of time, the experimenters discovered some very shocking results.  The rats would eat when presented with the previously conditioned stimuli, but they would limit their food intake from a food bottle when the signal was not present.  This goes to show that rats had learned to eat only in the presence of specific signals, and did not eat simply because food was readily available; the rats also ate while satiated if the signals were presented.  As the text itself has explained:  Once an association has been learned between signals for food and a meal, stimuli retain their ability to influence feeding for protracted periods and even under a state, satiation, that might be expected to minimize the impact of such events.
   Although it is possible for people to eat when hungry (this experiment has not directly disproved that explanation), it is more logical to think of eating as a complex process that incorporates learned procedures and extenuating circumstances that influence our ability to ingest foods.  It is interesting to see that an energy deficiency is NOT the dominating factor in causing someone to eat, contrary to what the major thinking has been for as long as I have known.  I mean, most people that I talk to would say that they eat because they think they are hungry, but little do they know that there are external cues and conditioned stimuli that can influence their conditioned response to eat, without them even knowing.  In the experiment with rats, the conditioned stimuli to signal food were a light and a buzzer, but in real life the stimuli that have conditioned many of us takes on different forms.  Think about when we decide to eat.  If you are out with friends, the social cues around us, such as friends ordering food, the smell of food in a restaurant, people enjoying their food, will all elicit cues for you to order food and eat, even if you are not hungry (much like the rats in this experiment).
     The example that I think of with my own life, is the time of day that elicits cues for eating. I have associated the time of 12-1 o'clock with eating lunch and 5'oclock with eating dinner.  Regardless of what time I wake up, when those times roll around, my brain will be alerted and want to eat a meal... To clarify this point, during the summer I often wake up around 12 o'clock and will start my day off with eating lunch, and then eat a dinner around 5 p.m.  On the other hand, during the school year, I wake up at a more respectable time of 10 o'clock and begin my day with a breakfast, eat lunch around 3 p.m., but when 5 p.m. rolls around I immediately begin thinking about eating again even though I am full from eating just two hours ago (much like the satiated rats in the experiment).  The same processes are at work with conditioning rats to eat at the sight of a light and the tone of a buzzer, with conditioning humans to eat at cues of time or external signals.  Sometimes we both do things that don't make sense based on needs, we do things because we have become accustomed and learned to do them by association.
     In rats, much like in real life, once an association has been made, it is hard to counteract those effects and unlearn or extinguish those associations.  We often do not eat when we have an energy depletion, mostly because we have associated quicker, easier cues to the desire to eat.  I imagine this to be a biological effect, because food was very limited in our ancestors.  In other words, since food was not always available for our predecessors, it is easier to learn associations and cues that will make us want to eat even when we are not particularly hungry.  Our genetic makeup predisposes us to want to eat and take in nutrients for the possibility (usually a reality in our ancestors) that food would not be available at a later time.  In other words: we are predisposed to take advantage of the food that is available now at an earlier time.  This explanation, however, is not supported by the finding that rats limit their food intake when presented with a feeding bottle.  If it was an evolutionary limitation explanation, as I proposed, then we probably should have seen the rats eat a lot of food, unconditionally, while sated, but even when the learned cues were not presented.
    This is just my opinion on the matter.  Going out on a limb and saying that there is a biological and genetic basis to the ease of association and learning may be completely incorrect or it may be right on point.   That is up to you to let me know in the comments.  I would love some feedback.

Thanks for reading! Look out for my next post very shortly!

1 comment:

  1. Thanks for revising to include a description of the experiment (though the details were not as clear as I would have liked--we can discuss this during our meeting today).

    You seem to pose hunger as one factor that may induce feeding, but that there are others, such as conditioned responses to eat elicited by food-associated cues. These may not be incompatible causes. It may be that the food-related cues induce hunger (through an incentive motivation process), which in turn causes the individual to eat even when they were initially sated. The interactions among causal variables can be very complex with eating and hunger, so it's important to keep that complexity in mind when deriving conclusions and inferences.

    Good post!