D4 Evoked potentials

Topic

An evoked response is the specific neuronal activity generated by the brain in response to a sensory stimulus; it is extracted by averaging multiple trials of background electroencephalogram (EEG) activity—the same EEG activity used elsewhere to record the brain's general signal. This averaging process works because random signals uncorrelated with the sensory stimulus tend to cancel each other out, whereas the evoked response—being consistent across trials—is preserved and reinforced with each additional averaged trial.

The improvement in the evoked response's signal-to-noise ratio is not proportional to the number of averaged trials; instead, it increases more slowly—specifically, in proportion to the square root of the number of trials. Thus, doubling the number of trials does not double the signal-to-noise ratio but rather improves it to a lesser degree.

Once extracted, the evoked response is clinically characterized by two features of each waveform peak: latency (the time required for the response to reach a specific component's peak) and the amplitude of that peak.

In brainstem evoked potentials—a specific type of evoked response—the interval between successive peaks allows for the estimation of central conduction velocity. This concept extends the notion of axonal conduction velocity, which describes signal propagation along an axon in other contexts. The longer the interval between peaks in this brainstem evoked response, the lower the estimated central conduction velocity. When the latency or the interval between peaks of the evoked response shows a delay relative to the normative values ​​expected for that sensory pathway, this delay suggests an alteration in the pathway under study—as occurs, for example, with the demyelination associated with multiple sclerosis, which slows signal conduction along the sensory pathway.

Unlike the latency and amplitude of the early components of the evoked response—which depend primarily on the physical properties of the sensory stimulus—there are later cognitive components of the evoked response that depend instead on the subjective probability of the stimulus (i.e., how expected or unexpected the stimulus is to the individual) rather than solely on its physical properties. Ultimately, it is this evoked response—extracted from the background electroencephalogram through trial averaging—that enables both the characterization of the integrity of the brain's sensory pathways and the study of more complex cognitive processes that extend beyond the physical properties of the stimulus.

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