Science Update January 25, 2010
From Neurons to Thought: Coherent Electrical Patterns Observed Across the Brain
Amidst the background hum of electrical signaling generated by neurons in the brain, scientists have found that local groups of neurons, firing in coordination, sometimes create a signal that is mirrored instantaneously and precisely by other groups of neurons across the brain. These transient episodes of coherence across different parts of the brain may be an electrical signature of thought and actions.
One of the goals of neuroscience research is to identify how thoughts and actions are encoded in the activity of neurons. A challenge has been to extract meaningful patterns from the ongoing tumult of electrical activity in the brain. This global electrical activity is built from the firing of individual neurons. A single neuron responds to a stimulus in an all or nothing manner—if the stimulus reaches a certain threshold, the neuron “fires” an electrical signal. Groups of neurons firing in a coordinated way create a local electrical field that is in itself a signal that can vary in pattern. These local field potentials (LFPs) have been a target of research.
In this research, Dietmar Plenz and colleagues at NIMH and Duke University pinpointed LFPs in the cortex that surpassed a minimal size threshold, and then searched the rest of the cortex to see what was occurring at the same time. In each case, they found other answering LFPs across the brain that mimicked each other with high precision: there was no degradation or loss of power (amplitude) in the signal. Unlike what is observed after dropping a stone in a pond—with wavelets getting smaller farther from the stone—the intensity of the LFPs was the same across the brain. The investigators call these LFPs coherence potentials. Although LFPs that occur during these transient episodes of coherence are identical to each other, they are also multidimensional and potentially infinitely diverse, providing a means to encode information. Most LFPs do not reach the threshold that characterizes a coherence potential but with those that do, propagation of the LFPs across the brain is extraordinarily rapid. The authors note that the rapid dispersion of such a signal mimics the spread of ideas and behaviors in social networks; a sufficiently provocative idea can spread very swiftly through a population.
Coherence potentials simultaneously engage groups of neurons in different parts of the brain with diverse functions. This is consistent with the multi-faceted nature of mental associations and memories—a memory focused on a person or object might conjure various kinds of sensations and thoughts—visual, tactile, auditory, and emotional, for example.
These findings emerged from recent work that demonstrated that, like other systems in nature, the cortex exists at a critical state between stability and instability. A characteristic of this state in the brain is the presence of neuronal avalanches—if a stimulus reaches a certain threshold, it will set off cascades of neuronal firing. This dynamic is analogous to when the slope of a sandpile reaches a point at which adding one more grain will trigger an avalanche. The adherence of the cortex to this critical state ensures that the brain can respond to a wide range of stimuli, but not lapse into a chaos of excess activity (such as the too-synchronous firing during epilepsy). Coherence potentials emerge predominantly when the cortex is critical, that is, when it displays neuronal avalanches. Nudging the cortex away from this point, by inhibiting neuronal signaling with medications for example, disrupts these dynamical patterns.
Coherence potentials were present in cells in culture as well as awake monkeys, a robust demonstration that they occur in the functioning cortex. Future studies will be aimed at monitoring coherence potentials in the context of behavioral function with the ultimate aim of making a connection between specific coherence potentials and behaviors.
Thiagarajan, T.C., Lebedev, M.A., Nicolelis, M.A., and Plenz, D. Coherence potentials: loss-less, all-or-none network events in the cortex. PLoS Biology 2010, doi:10.1371/journal.pbio.1000278.
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