Our days are full of things to remember, and they don't always arrive in an orderly fashion. Perhaps you begin your commute home and remember that you need to pick up milk. But then immediately, another to-do springs to mind: You never called back your friend last week. You may try to hold both in your head, but in the end the milk, the phone call or both still sometimes fall away, forgotten.

A new scientific model of forgetting is taking shape, which suggests keeping multiple memories or tasks in mind simultaneously can actually erode them.

Neuroscientists already knew that memories can interfere with and weaken each other while they are locked away in the recesses of long-term memory. But this new model speaks to what happens when multiple memories are coexisting front and center in our minds, in a place called "working memory."

It argues that when we let multiple memories come to mind simultaneously, those memories immediately lock into a fierce competition with each other. The milk and the phone call fight to each be remembered more than the other.

"When these memories are tightly competing for our attention the brain steps in and actually modifies those memories," says Jarrod Lewis-Peacock, a neuroscientist at UT Austin.

The brain crowns winners and losers. If you ended up remembering the milk and forgetting the phone call, your brain strengthens your memory for getting milk and weakens the one for phoning your friend back, so it will be easier to choose next time you're faced with that dilemma.

Previous research has demonstrated this competition-based weakening of memories over very short periods of time, but Lewis-Peacock and his colleagues recently put it to the test again, to see if it could cause long-term forgetting. They decided to force two memories to compete: pictures of human faces and pictures of scenes.

First they used an advanced type of MRI technology to get a window into the minds of the study's participants.

"We're starting to get to the point where we can pretty reliably sort of read out what a person is thinking about, seeing [and] trying to remember. And we're doing this on a moment-to-moment basis," says Lewis-Peacock. His team's MRI machine learned to recognize the unique pattern that emerges when each participant thinks of faces, scenes or both at the same time.

Then, while participants were loaded in the MRI, they were shown pictures of faces and scenes. They were then repeatedly asked to recall the pictures — in most cases just the images of scenes. "Most of the time, I'll show you both [then] test you on the scene. You can basically forget the face at that point," Lewis-Peacock told them.

The minds of participants were now presumably focusing on the memories of the scenes alone. "But occasionally I'm going to sort of trick you and say 'Aha, no: On this trial I'm actually going to test your memory for the face item,' " which forces your brain to quickly pull the face item back to mind, Lewis-Peacock says.

For many participants this meant suddenly both the scene and the face memories existed in their heads at once — competing with each other.

The research team used their MRI to verify that both memories were present at once, and 30 minutes later they did another test for memory of that scene. Indeed, in the trials where competition had taken place, memory for scenes weakened significantly. The upshot: people had more trouble recalling a memory when it had earlier been simultaneously active with another one.

Jarrod Lewis-Peacock cautions that more testing is required before researchers can strongly recommend certain memory-enhancing techniques. Still, he says one interpretation of this is that "switching between thoughts cleanly or efficiently is a good thing."

"When you're done thinking about something you totally pack it away. Don't let it sit in the back of your head," he says. "Because if you do, it might thrust it accidentally into competition with what you're moving on to think about."

Lewis-Peacock also says this competition theory of forgetting points to the limitations of our own minds.

"I think what this data is suggesting is that there might be these unintended consequences to the way that we're juggling thoughts in our head," he says. "Maybe it's not just a whole big free lunch that you can try to do as many things as you can try to without any repercussions."

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