A few years ago we discussed a fascinating study which appeared to show that the main reason we stop eating at the end of a meal isn't because we "feel" full. Instead, we simply see that we've finished eating the food in front of us, so we stop. We don't eat more an hour later because we remember we just ate.
In that study, led by Paul Rozin, experimenters provided two amnesic patients with two meals separated by just 15 minutes. They both did not recall eating the previous meal due to their medical condition, and each of them ate both meals as if they hadn't had anything to eat.
But maybe amnesia has additional subtle effects. The condition is usually caused by severe brain trauma, so it's possible that amnesics (or the particular amnesics in the study) have also lost their ability to detect fullness. Could that explain their seemingly bizarre behavior?
A team led by Suzanne Higgs identified two new patients who had a similar type of amnesia: They could not form new long-term memories, although their old memories and short-term memory were intact. They repeated Rozin's multiple-meal study and found the same result: while people with normal memory refused a second lunch, both amnesic patients ate two full meals, consuming nearly 2,000 calories while the others ate only about 700.
But in a separate experiment, volunteers (including the two amnesics) were presented with small samples of four different foods (a cookie, potato chips, rice pudding, and sandwiches) for tasting. They were asked to rate each food for taste, texture, and desire to eat. Next they were given a meal-sized portion of the sandwiches and asked to eat as much as they liked. Finally, they rated each item once more on the same scale. Here are the results:
The graph shows the change in ratings, so a negative value here means that the rating went down after eating, while a positive value means the rating went up. People with normal memory consistently rated the sandwiches as significantly less appealing than the other foods after having eaten lunch, along all three dimensions. The ratings for taste and texture of the other foods didn't change significantly.
The amnesics showed a similar result: They rated the sandwiches significantly lower than the other foods after having the sandwiches for lunch, even though they had no recollection of ever having eaten the sandwiches. As with the other volunteers, amnesics showed no significant change in their ratings of the other foods after lunch.
The researchers say this shows that a difference in ability to detect fullness is unlikely to be the reason amnesics are willing to eat a second meal immediately after the first one. And memory of having eaten a food doesn't appear to be the reason we find it less appealing a few minutes later -- otherwise the amnesics' ratings of the sandwiches wouldn't have decreased after lunch.
But this decrease in taste and desirability doesn't appear to affect our willingness to eat: otherwise the amnesics wouldn't have consumed so much more in the first experiment. While there may be some other explanation for the eating behavior of the amnesics in these two studies, the combined results suggest that the social component of eating -- the the memory of having consumed an entire meal -- is one of the key reasons we decide to stop, and to avoid eating until the next socially proscribed mealtime.
Suzanne Higgs, Amy C. Williamson, Pia Rotshtein, Glyn W. Humphreys (2008). Sensory-Specific Satiety Is Intact in Amnesics Who Eat Multiple Meals Psychological Science, 19 (7), 623-628 DOI: 10.1111/j.1467-9280.2008.02132.x
I am troubled by the ethics of these experiments. How is it okay to "trick" amnesics (a vulnerable population) into consuming 2000 calories in the course of an hour? Granted, a one-time experiment of this type is unlikely to cause an adverse impact on the subject's health - but it still seems rather uncool.
Then why do we sometimes stop eating before all the food is gone? And it can't be just social situations, since it happens when we're eating alone too. At the local diner, I notice a lot of people ordering large breakfasts, and then not finishing them. I don't like ordering the omelets because I know they'll be too big to finish.
I can certainly appreciate your concern. The amnesics have normal intelligence (one was a musician and the other was bank manager prior to their illnesses [the amnesia was caused by herpes simplex encephalitis]) and gave informed consent to participate.
My understanding is that overeating is a significant problem among amnesics and this type of research may help uncover strategies for dealing with the issue.
There are some immediate physical sensations of fullness, along with the progressive decrease in pleasure as we continue to eat. And we're also not immune from social pressures when we're alone (why do we close the bathroom door when we're home alone?). But clearly memory may play an important role too, as this study demonstrates.
I've always found it very hard to distinguish 'feeling hungry' from 'feeling sick'. If I start feeling sick, I do a mental back-check and often realise it's been a few hours since I've eaten - at which point a (savoury) meal will stop me feeling queasy.
On the other hand, if I eat too much for dinner (I don't like leaving food, so I will struggle to finish what's on my plate if I order too much!) then I sometimes can't eat even the next morning. That can't be to do with social mealtimes, because socially I should have breakfast.
Interesting topic :)
It's an interesting study but I don't know what to make of the results. It doesn't seem very definitive when the amnesiacs may have other brain damage affecting feeding behavior (perhaps cell-specific damage of ghrelin or orexin neurons that wouldn't show up on any scan, etc.) Did they control for any other variables?
There's no way to really know for sure that what caused their amnesia is completely separate from other behaviors. The mechanisms of feeding behavior are complicated and not fully understood. While this may help in the treating other amnesiacs, it seems premature to make assumptions about overall feeding behavior in the general population based on this study alone. Especially if you're going to go around saying "the *only* reason you didn't eat two dinners is memory!" That seems just silly. Shoot, last Saturday I had two dinners. I fully remembered the first one, but just still felt hungry.
It is interesting tho...
Reminds me a bit of the Bottomless Soup Bowl, which won an Ig Nobel Prize: http://www.mindlesseating.org/ignobel.htm
I'm a diabetic and I take the typical medication Metformin. My doctor explained to me when I began to take it that it had an "anorexic effect." This was very, very welcome because one of the common signs of untreated diabetes is being hungry as a wildfire, all the time, even when you've just eaten.
So I took the medication, and it had the effect as promised. (Never fear, I'm a long way from actual anorexia.) How it affects me is quite interesting, though. It makes me feel like I've just eaten a lot of whatever it is I'm thinking about eating. If I force myself to cook a meal anyway, because other people are hungry and waiting, I find I eat a normal portion quite easily... I'm not full; I'm just not hungry.
I would be interesting to see differences in behavior if at one time, the amnesiac patient was informed about having just eaten a meal 15 minutes prior at the time the second meat is offered (both in case of that actually being true and not) and see whether it changes his or her decision on the second (or presumed second) meal..
Ooh I like that idea, Ksenia!
I have always been fascinated by the difference in appetite I notice in close relatives. I have seen very little discussion anywhere of the differences in the appetite we are all born with. As a child, I had two younger sisters. As in many families, the youngest child was born the heaviest and in this instance also grew to be taller than my mother and almost as tall as my father. The middle sister was the lowest birthweight and has grown to be the shortest. I weighed a few hundred grams more than the middle sister and have grown to be slightly taller than her, but we are both shorter than both our parents. Both our parents have always been a healthy weight. As children, myself and my youngest sister were always 'looking for food'. AS preschoolers we were looking for lunch at 11am. I can remember as a primary/grade school child looking forward to breakfast from the night before! Due to the vigilance of our mother and deliberate emptiness of the pantry, we were all healthy weights as children in the 70's and in our early 20's were all UK size 10 (US size 6). However the middle sister always more likely to leave food on her plate and always slightly thinner than myself and the younger sister. Unlike myself and my younger sister who started dieting as 14 year olds in order to avoid gaining weight, the middle sister never had to diet until after the birth of her first child when she was 29. I have always had a great deal of problem in following any diet as feelings of hunger interfere with my concentration and prevent me from continuing with whatever activity I am doing, even if I am interested in it, and don't want to stop to eat. I am now a mother and have a child (now 10) who is not biologically mine. I have been fascinated to observe the lack of appetite of this child who never cried for food as a baby, and who as a picky toddler, was able to delay food for hours in order to carry out activities he was interested in, or 'hold out' for food that was more to his liking. Instead of spending my life, as I feared I would, trying to keep my child out of the refrigerator, as my mother did with me, I have spent years throwing away uneaten school lunches (playtime is more important than eating!) and feeding dinner to a child who simply can't be bothered to eat unless the food is a favourite.
Therefore, having lived with people who are quite thin and 'eat what they like' it is clear to me why they are thin - their natural tendency is to eat less than other people! Conversely, people who are obese as children and remain that way tend to say they 'don't eat much' but I think they perceive this because they are feeling hungry much more than most people and almost certainly eat more than most people but never really feel full.
I thought this was an interesting topic! It is true when a person who is thinner than you is most likely to eat more but in lesser portion or meals. But as for someone who is big and can eat more they always say that they don't eat much because their portion is much bigger and they don't spread it out toward they day which makes them feel much more hungrier then others.
I agree with the person who posted above about her thin sister. Genes are key. I have one sister who is thin and one who is heavy, and I am in between. My heavy sister looks just like my mother's father, who was also heavy. My thin sister looks more like my father's family, all of whom are slim. Both my father and my thin sister sometimes forget to eat lunch, but by heavy sister, my mother, and I would NEVER forget a meal. Another wrinkle in all this is that my thin sister is generally a non-addictive person. Back when all three of us used to smoke cigarettes, I always HAD to have a cigarette right after my morning coffee and would smoke about a pack a day (just like my mother). My thin sister was much less compulsive about it and would sometimes forget to have a cigarette all day (just like my father). So along with genetic hunger/metabolism differences, genetic psychological differences would seem likely to affect eating behavior as well.