In a new study published in Neuron titled, “Working memory readout varies with frontal theta rhythms,” researchers from the Picower Institute for Learning and Memory at Massachusetts Institute of Technology (MIT) show that the ability to spot subtle changes in environment could depend on a theta-frequency brain wave (3–6 Hz) that scans through a region of the cortex which maps field of view. The findings explain how the brain implements visual working memory and why performance is both limited and variable.
Prior studies from the lab of Earl Miller, PhD, Picower professor of neuroscience and corresponding author of the Neuron study, supported the theory that the brain employs waves of different frequencies to carry out the analog computations needed for cognitive tasks such as working memory. The study provides a new illustration of the effects of those brain waves on function.
“It shows that waves impact performance as they sweep across the surface of the cortex. This raises the possibility that traveling waves are organizing or even performing neural computation,” said Miller.
To track working memory, non-human primates played a video game in which an array of colored squares appeared and then disappeared on a screen. After about a second, the array reappeared with one square sporting a different color. The animals had to glance at the square that changed as quickly as possible.
Researchers tracked the reaction time and position of the animals’ gaze and measured brain wave power across a broad frequency spectrum and individual neural electrical spikes in a region called the frontal eye fields. This region maps visual information analogously to where it first hits the retina, termed a retinotopic map.
The animals’ accuracy and speed depended on a combination of the phase of a theta frequency brainwave when the changed square appeared and the vertical location on the screen of that target square. Each height on the screen possessed its own phase of the theta wave where performance was maximized, and the lower a target square appeared on the screen, the later the phase of the wave that correlated with peak performance.
Numerous studies by Miller’s lab have shown that alpha and beta frequency waves (~8-25 Hz) impose the brain’s understanding of the rules of a task and regulate when faster gamma frequency waves (30Hz and above), can be used to encode data from the senses. In this study, theta waves seemed to orchestrate that rivalry between beta and gamma. In the excitatory phase of the theta waves, beta was suppressed and visual information was evident in the neural spiking activity. In the inhibitory phase of theta, beta power was stronger and the spiking decreased.
Miller’s lab is working to develop closed-loop analog feedback systems that can strengthen the power of waves at different frequencies for clinical applications.
