Researchers at MIT’s Department of Brain and Cognitive Sciences have evidence that one of the ways the brain stores its massive amount of information is through rhythmic cycles of cell communication. “Everyone knows that you can’t consciously hold two distinct ideas in your head at the same time,” says Earl Miller, Director of the Picower Institute for Learning and Memory. However, Miller’s research has suggested that the brain can instead store multiple concepts by performing a balancing act between the firings of brain cells (1).

Miller’s research presents a possible solution to a controversial viewpoint in cognitive neuroscience: that a specific thought can be reduced to an ensemble, or a selective pattern of neuronal firings. Given how interconnected brain cells tend to be, it is difficult to imagine a scenario in which only one ensemble fires. Many distinct ideas are related enough, like cat and dog, that one would imagine the subset of neurons firing for each one to be relatively similar. As put forth by Miller, “How does the activation of one ensemble not lead to activation across the entire brain?”

By training monkeys to respond accordingly to a picture of stripes that varied in color or orientation, researchers in Miller’s lab identified subsets of neurons that responded preferentially to either change. Electrodes implanted into the lateral prefrontal cortex recorded cell firing. Approximately half of the cells showed synchronous firing for color, while the other half showed the same level of firing for orientation. The greatest difference in frequency between the two was a region in the 19 to 40 Hz range called the beta band. Within this frequency range, the cells encoding each particular modality, color or orientation in this case, were highly distinguished from one another (1). In further pursuit of how this frequency difference manifested itself in different cognitive tasks, researchers designed a second experiment to test activation of particular brain areas during a discrimination task and examine how these structures continue to respond over the course of training.

One researcher noticed that, during association learning tasks, activity in the striatum varied at a much higher rate than it did in the prefrontal cortex. By combining this information with data from learning trials, researchers hypothesized that, during association learning, the prefrontal cortex and the striatum were involved in a rhythmic loop that facilitated learning. The striatum was thought to form “patterns” of activity during presentations of the stimulus, and these patterns could by analyzed by the prefrontal cortex to form archetypes and better discriminate future stimuli (1).

Monkeys were placed into another task, this time to categorize a randomized series of dots as category 1 or category 2. The positions of the dots were randomized using two “template” dot orientations such that the randomized versions had some resemblance to the original. The number of orientations grew until the monkey was able to, with around 80% accuracy, calculate 256 different orientations as belonging to template 1 or 2, over half of which had never been seen before. Additionally, striatal activity decreased and prefrontal cortex activity increased as the number of novel orientations grew. This suggests that the prefrontal cortex was more heavily involved in calculating the “rules” or template behind the dot orientations. They also found an increase in beta synchrony between the cells over the course of the task. This eventually resolved itself such that half of the cells of the prefrontal cortex beta synchronized themselves to the presence of category 1, while the other half did so for category 2 (1).

Current and future research plans for Miller’s lab involve determining how these frequencies can be disrupted when a subject is trying to remember too much, as well as possible atypical synchrony in schizophrenics, who show a wide variety of deficits in mental cognition (2).

References:

  1. Miller, E.K. and Buschman,T.J. (2013) Brain Rhythms for Cognition and Consciousness. Neurosciences and the Human Person: New Perspectives on Human Activities. Pontifical Academy of Sciences. Scripta Varia 121, Vatican City, 2013
  2. M Gazzaniga, R.B., G.M.(2013) Cognitive Neuroscience: The Biology of the Mind. New York, NY: W.W. Norton and Company

    The prefrontal cortex is a key brain region that controls decision making, social behavior, and personality, among other functions. The Prefrontal Cortex, Copyright Wiki Commons