Monday, March 28, 2016

Updated Neural Model for Working Memory

Neuroscientists at the Massachusetts Institute of Technology have provided evidence opposing the current model for how working memory operates at the cellular level. The current model says the cellular basis for working memory lies in consistent, sustained activity by brain cells, or neurons. Results from the MIT study, published in the March 17 issue of the scientific journal Neuron, shows the story is more complex, that brain cells involved in working-memory tasks are activated discretely and sporadically.

What is Working Memory?

Working memory is sometimes confused with short-term memory. These separate processes, however, involve different subsystems both located within the brain's prefrontal cortex. Working memory also serves a different function than short-term memory; the former only involves the passive storage or recall of newly acquired information, while the latter involves the active manipulation and organization of this information. “Your short-term memory might help you to remember what someone has just said to you, for example, but your working memory would allow you to recite it to them backwards or pick out the first letter of each word,” neuroscientist and author Jonathan K. Foster said in the Dec. 3, 2011 issue of New Scientist.

Earlier experiments provided evidence that the cellular basis for working memory involves specific prefrontal cortex neurons firing continuously. These previous studies, which began in the early 1970s, have not been discredited so much as more clearly understood. When looking at what's going on more closely, it's revealed that working memory on the cellular level isn't a continuous activity but a discrete series of events. Those early studies on working memory averaged brain activity observed during task performance. The time spans for those activities ranged from seconds to sometimes minutes, Earl Miller, the study's senior author and a professor at MIT’s Picower Institute for Learning and Memory and the Department of Brain and Cognitive Sciences, said in a March 17 MIT press release. “The problem with that is, that’s not the way the brain works,” he said. “We looked more closely at this activity, not by averaging across time, but from looking from moment to moment.”

It's important to note that the earlier studies weren't wrong per se, just limited. When the results of the MIT study were averaged over several trials, it also appeared as continuous neuronal firing—just like the earlier studies suggested was happening.

Action Potentials and Neurotransmitters

Before continuing it's important to understand what it means for a neuron to “fire.” Neurons are considered “excitable” and when sufficiently stimulated, there is a change in the electrical potential between within and just outside a neuron. Neurons typically have a resting potential of -70 millivolts (mV). When sufficiently stimulated, ions flow through specialized channels making the neuron's interior more positive. When the sum of ions reaches a positive-charge threshold, a large electrical signal is generated hitting a charge of +50 mV before the charge momentarily drops below the resting potential and then returns to it. This process, called the action potential, takes approximately 3-5 milliseconds.

Reaching the action potential often triggers the release of signaling molecules within the neuron called neurotransmitters. These neurotransmitters then travel from one neuron to another neuron at junctions called synapses. This is how information is sent from neurons along the nervous system.

The release of neurotransmitters from one neuron to another at the synapse
Image Credit: Chico State University
Before the MIT study, Picower Institute postdoc and study author Mikael Lundqvist developed his own working-memory model, the MIT press release said. Lundqvist's model hypothesizes that while working memory is active information is stored discretely via rapid changes in the quantity of neurotransmitters released. Brief neuronal bursts help “imprint” bits of information into the physical and chemical structures of our brains, while repetition of bursts strengthens information recall.

Artist's conception of a neuron firing in sporadic, coordinated bursts
Image Credit: Jose-Luis Olivares, MIT

Lundqvist's theory aligns well with the MIT study's findings, which linked brief bursts of gamma waves (45–100 Hz) with the “encoding and re-activation of sensory information,” according to the Neuron paper. Furthermore, these higher-frequency gamma waves interrupted slower-frequency beta waves (20–35 Hz). The beta waves appeared to be the “default state” of those neurons.

This implies that these bursts of neurons in the prefrontal cortex help code and reinforce working-memory information, but that it may also hinder other cellular-level activity from interfering with working memory. “By having these different bursts coming at different moments in time, you can keep different items in memory separate from one another,” Miller said in the press release.

Robert Knight, a professor of psychology and neuroscience at the University of California at Berkeley who did not participate in the study, believes this study has implications beyond the neural basis underlying working memory. “The work calls for a new view of the computational processes supporting goal-directed behavior,” Knight said in the press release. “The control processes supporting nonlinear dynamics are not understood, but this work provides a critical guidepost for future work aimed at understanding how the brain enables fluid cognition.”

—Clinton Parks

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