evokedvelden

Evokedvelden, or evoked fields, refer to time-locked brain responses elicited by discrete sensory, cognitive, or motor events. The term is primarily used in magnetoencephalography (MEG) to describe the measured magnetic fields generated by synchronized neuronal activity in cortex. The corresponding electric potentials measured with electroencephalography (EEG) are usually called evoked potentials; together these responses are often described as event-related responses.

These responses are typically obtained by presenting many repetitions of a stimulus and averaging the recorded signals. Averaging enhances the signal-to-noise ratio by canceling out random brain activity and non-brain noise. Common stimulus modalities include auditory, visual, and somatosensory stimulation. The evoked response contains a sequence of deflections at characteristic latencies, such as M50, M100, and M170 in MEG, or P50, N100 (N1), and P200 in EEG, reflecting different stages of sensory processing in cortical areas.

Analysis often involves preprocessing to remove artifacts, epoching around stimulus onset, and averaging across trials. In addition to temporal waveforms, researchers use source localization methods to estimate the neural generators, employing models such as equivalent current dipoles or distributed inverse solutions. Multivariate statistics and sensor- and source-space analyses enable comparisons across conditions, ages, or clinical groups.

Applications of evokedvelden include basic research on sensory and cognitive processing, language and attention studies, and clinical uses such as presurgical functional mapping (e.g., somatosensory and language areas) and assessment of sensory deficits. MEG’s high temporal resolution and good spatial specificity for certain sources, alongside EEG’s broader accessibility, make evokedvelden a foundational tool in neurophysiology. Limitations include the ill-posed inverse problem, susceptibility to artifacts, and variability in latency and amplitude across individuals.