Using functional MRI (fMRI) to analyze blood flow activity in the brain, researchers from the University of Oregon (UO) and the University of California (UC), San Diego, said they can see information about what people are holding in their short-term memory, according to research published in the February issue of Psychological Science.
The psychologists, who conducted the experiments at the UO Robert and Beverly Lewis Center for NeuroImaging, reported that they were able to identify the specific color or orientation of an object that was intentionally stored by the observer.
Subjects viewed a stimulus for one second and held a specific aspect of the object in mind after the stimulus disappeared. In 10-second delays after each exposure, researchers recorded brain activity during memory selection and storage processing in the visual cortex, a brain region that they hypothesized would support the maintenance of visual details in short-term memory.
"Another interesting thing was that if subjects were remembering orientation, then that pattern of activity during the delay period had no information about color, even though they were staring at a colored-oriented stimulus," said Edward Awh, an UO professor of psychology. "Likewise, if they chose to remember color, we were able to decode which color they remembered, but orientation information was completely missing."
Researchers used machine-learning algorithms to examine spatial patterns of activation in the early visual cortex that are associated with remembering different stimuli, according to John T. Serences, a professor of psychology at UC San Diego. "This algorithm can then be used to predict exactly what someone is remembering based on these activation patterns," he said.
Increases in blood flow, as seen with fMRI, are measured in voxels, or small units displayed in a 3D grid. Different vectors of the grid, corresponding to neurons, respond as subjects view and store their chosen memories. Based on patterns of activity in an individual's visual cortex, located at the rear of the brain, researchers can pinpoint what is being stored and where, Awh said.
Awh and colleagues found that the sensory area of the brain had a pattern of activity that represented only an individual's intentionally stored aspect of the stimulus. "People choose what is important and relevant to them," Awh said.
"Basically, our study shows that information about the precise feature a person is remembering is represented in the visual cortex," Serences said. "This is important because it demonstrates that people recruit the same neural machinery during memory as they do when they see a stimulus."
The research was primarily funded by a grant from the National Institutes of Health (NIH) to Awh, and by support from the UO's Robert and Beverly Lewis Center for Neuroimaging.