People who are stuck in the present reveal a strange interplay between memory and body

New Scientist, 8 October 2012

 HM's Brain.  Credit: UC San Diego School of Medicine

HM's Brain. Credit: UC San Diego School of Medicine

 

TO THE casual observer, there would have been nothing unusual about Henry Molaison as he tucked into dinner at his usual slow-and-steady pace. But to the group of psychologists at the Massachusetts Institute of Technology who were observing him, his behaviour was astonishing: just 60 seconds earlier, he had polished off an identical three-course meal. Yet Molaison was no glutton. Instead, part of his brain had been removed in an attempt to cure his epilepsy. From then on, he was unable to form new memories and became stuck in the present for perpetuity.

Scientists usually consider feelings of hunger to arise from hormonal signals in the gut, but Molaison's behaviour suggested that our memories of what we have just eaten may be more important in curbing our appetite. The idea found further support a decade later, in 1998, when Morris Moscovitch at the University of Toronto, Canada, replicated this experiment using two people with a similar memory condition. Not only did these people eat a second meal, just 15 minutes after finishing the first, but in some trials they unquestioningly ate a third.

There is always the possibility that the brain damage may have brought on complications besides the memory loss that interfered with the gut's signals to the brain, but a recent experiment by Suzanne Higgs at the University of Birmingham in the UK suggests otherwise. She tapped into "sensory specific satiety" - the familiar sensation that our liking for a given food decreases the more we eat of it, whereas a different dish will feel more appetising; it is the reason that we can find extra space for pudding. Higgs found that people with amnesia retain such preferences. After a hearty lunch of sandwiches they will prefer crisps or cookies to further sandwiches, even though they couldn't tell you what they had just eaten. She concludes that the digestive signals are reaching the brain, and that the amnesiacs' lack of memory lies behind their seemingly insatiable appetite.

Incredible endurance

The unexpected effects of memory on our feelings and behaviour might not stop with food. Diane Van Deren is one of the world's elite ultra runners. In one race this year she ran more than 1500 kilometres over 22 days. On some of those days, she ran for as long as 20 hours. Van Deren had always been good at sport, but her incredible endurance seems to be down in part to her poor short-term memory, again the result of brain surgery for epilepsy.

Often, she just cannot remember how long she has been running for, underestimating the time by as much as 8 hours. "Most people with amnesia suffer a tyranny of the present," says Adam Zeman, a neurologist studying memory and epilepsy at the University of Exeter, UK, but Van Deren's inability to remember how long she has been running seems to free her from the feelings of fatigue that plague other runners. Perhaps, while others get caught up in the details of where they have been and where they are going, Van Deren gets into a more zen-like state that lets her run for longer without feeling so much strain. Of course, it could also be that after the challenges in her life Van Deren has a higher threshold for discomfort than most people.

For the rest of us, losing track of time on a long run is difficult, but there are certainly ways in which these findings affect us all. Higgs has found that simple distractions such as watching TV can stop people from forming good memories of what they are eating. As a result, they tend to snack more after the meal than control groups who were not distracted.

Imagination can play a powerful role too. Thanks perhaps to its close link to memory, simply imagining the process of eating something can lead people to feel more satiated, causing them to eat less. Which all goes to show that in the fight against overeating, memory could be your biggest ally, even if at times it would be more palatable to forget.