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