To make sure the subjects were paying attention, they were asked a question about each image as it came up, like whether they liked a certain celebrity, how much they wanted to visit a certain place or how often they used a certain object.
Later, without any images and while their brains were still being scanned, the subjects were asked to recall as many of the images as they could.
The researchers found that the patterns of brain activity associated with each picture "reinstated" themselves seconds before the people could verbally recall the memories. On average, the time between beginning brain activity associated with the memory and the subjects verbally stating the memory was about 5.4 seconds.
"When you have an experience, that experience is represented as a pattern of cortical activity," explained Sean Polyn, a postdoctoral researcher at the University of Pennsylvania and leader of the study. "The memory system, which we think lives in the hippocampus, forms a sort of summary representation of everything that's going on in your cortex."
Googling your brain
The process can be compared to the way web crawlers work to browse and catalogue web pages on the Internet. Web crawlers are automated programs that create copies of all visited pages. Search engines like Google then tag and index the pages.
In the same way, as we're trying to remember something, our brains dredge up the memory by first recalling a piece of it, scientists say.
When trying to remember a face you saw recently, for example, you might first think broadly about faces and then narrow our search from there, enlisting new details as you go, Polyn explained. It's like adding more and more specific keywords to a Google search, until finally we find what we want.
Scientists call this process "contextual reinstatement."
"The memories that came up would be hits and the ones that most match your queries would be the ones that came up first," Polyn told LiveScience.
The researchers were even able to do a little mind-reading by watching the search in progress.
By comparing the brain scans of the subjects while they tried to remember the images they'd seen with those collected when they first viewed the images, the researchers were able to correctly conclude whether the people were going to remember a celebrity, place or object.
"We can see some evidence of what category the subject is trying to recall before they even say anything, so we think we're visualizing the search process itself," Polyn said.
A similar mind-reading effort was announced earlier this year, when researchers found they could predict where a patient would move his hand based on brain activity the instant prior.
Scientists think that contextual reinstatement is unique to memories that involve personal experiences, so-called "episodic" memories, but that similar processes might be at work in other forms of memory.
The study was detailed in the Dec. 23 issue of the journal Science.
