The Dynamic Brain: An Exploration of Neuronal Variability and Its Functional Significance

First-year medical students are taught that the action potential is the fundamental unit by which neurons convey information. Instructors tell students that action potentials are robust and faithful signals: when a bar of light is presented, relevant neurons in the visual cortex produce a series of action potentials that represent the bar. Implicit in this idea is that every time the bar is presented, the number and frequency of the action potentials should stay the same and reflect the constancy of the stimulus. However, in reality this assumption is not quite accurate. Experiments show that the response of a neuron to a specific stimulus is not constant but varies considerably. This variability is present at any level of analysis: from the single neuron to neuronal populations to whole-brain activity and behavior. In a real sense, variability is a fundamental property of neural systems. Investigators have traditionally overcome this problem by presenting a stimulus multiple times and averaging recordings to extract the “real” signal from the background noise and variability.