An important part of the human brain has to work harder to actively forget a memory than it does to remember it, according to the results of a newly-published study. The research is a step towards understanding how and why the brain is able to discard an experience, and could one day lead to a treatment designed to remove painful memories.

We all have memories that we want to forget. For some of us, it's a catastrophically bad social interaction, the recall of which will make you involuntarily kick the sheets at night. For others, the memory of a traumatic experience can have a deeper, more damaging effect on their health and well being. We often try to forget these memories, with varying levels of success.

"Decades of research has shown that we have the ability to voluntarily forget something, but how our brains do that is still being questioned," says Jarrod Lewis-Peacock, the study's senior author and an assistant professor of psychology at the University of Texas at Austin (UT Austin). "Once we can figure out how memories are weakened and devise ways to control this, we can design treatment to help people rid themselves of unwanted memories."

Earlier research on the processes that govern forgetting had focused on the control regions of the brain, and those that deal with long-term memory. They sought to determine whether the brain was attempting to redirect attention away from the painful memory, or to prevent its retrieval. The new study took a different approach, and observed a region of the brain called the ventral temporal cortex, which is associated with the processing of sensory and perceptual information, including memories of complex visual stimuli.

During the study, participants were shown a series of images that they were asked to commit to memory, or actively forget. During the exercises, the brain activity of the volunteers was recorded using functional magnetic resonance imaging (fMRI). This data was then interpreted using machine learning.

Counterintuitively, the team discovered that individuals being asked to actively forget an image exhibited an increased level of activity in the ventral temporal cortex relative to those who were asked to remember a picture.

Whilst the study findings confirmed that humans do indeed possess the ability to direct the brain to forget, they also suggested that not all sensory information was equally easy to dismiss. The researchers found that participants were more likely to successfully forget scenes than they were faces. According to the researchers, this is likely because the latter images convey a greater amount of emotional information.

"A moderate level of brain activity is critical to this forgetting mechanism. Too strong, and it will strengthen the memory; too weak, and you won't modify it," explains Tracy Wang, a psychology postdoctoral fellow at UT Austin, and lead author of the study. "Importantly, it's the intention to forget that increases the activation of the memory, and when this activation hits the 'moderate level' sweet spot, that's when it leads to later forgetting of that experience."

The paper has been published in the Journal of Neuroscience.