Researchers showed 100 volunteers either a small or large portion of tomato soup before lunch and then used a hidden pump to refill or empty the bowl without the diner noticing.
"For the first time, this manipulation exposed the independent and important contribution of memory processes to satiety," Jeffrey Brunstrom of the University of Bristol's nutrition and behaviour unit and his co-authors concluded in the December issue of the journal PLoS One, published by the Public Library of Science.
"Opportunities exist to capitalize on this finding to reduce energy intake in humans."
Immediately after the meal, self-reported hunger was influenced by the amount of soup consumed, the researchers said.
But two and three hours after the meal ended, the pattern switched and hunger was predicted by the perceived amount in the bowl at the beginning of the meal rather than the actual amount.
Those who were shown 500 ml of soup experienced greater satiety or fullness than those shown 300 ml.
On the second day, researchers looked at expected satiation for the two groups. They all received 400 ml of soup.
Participants were also asked an open-ended question about the purpose of the study. The six who answered that the soup was artificially refilled or drained were excluded.
No need to cue memory of meal
Previous research by another team showed that reminding people of a recent meal can reduce how much is eaten at a later meal, an effect that lasts for hours but only when the memory of a very recent meal is recalled.
Brunstrom's team said the new findings suggest that the role of memory is substantial and can be triggered without explicitly cueing a memory of a recent meal. In the latest experiment, participants were never told to try to remember how much they ate.
Scientists have also observed that eating while distracted, which could interrupt memory, also seems to hinder how we respond to signals of feeling full.
But Brunstrom's team couldn't draw a cause-and-effect relationship between hunger and memory.
Other factors could be involved, such as if someone who saw a 300 ml portion was disappointed by the small size and responded negatively by rating their hunger higher than those who saw 500 ml of soup.
The researchers suggested measuring mood and satisfaction with portions in future experiments to take that possibility into consideration.
The study was funded by the UK's Biotechnology and Biological Sciences Research Council.Suggest a correction