ERPs are voltage fluctuations in
the
Electroencephalogram (EEG) elicited by the presentation of a controlled
stimulus. The latencies of different positive and negative components
in an ERP reveal the time course of activation of the neuronal
populations that are recruited during the processing of that stimulus.
Given its exquisite temporal resolution (within milliseconds), the ERP
technique is ideal to investigate fast and transient processes such as
the ones that mediate word recognition. Although ERPs have a low
spatial resolution and do not allow for a precise characterization of
underlying electric generators,
components characterized by different scalp distributions are assumed
to reflect the activation of different populations of neurons and,
therefore, non-identical brain regions. The figure above shows how we can chart the time course of semantic analysis of words while we read single words presented on a computer monitor. The electrode shown in the figure is placed over the parietal areas. Here, we are comparing the brain activity elicited by target words preceded by related (e.g., cat-DOG, blue line) and unrelated words (e.g., sun-DOG, red line). The two lines overlap during the first 300 ms and then diverge: words that are not congruent with the prime word elicit a negative-going component that peaks around 400 ms, called the N400. The time course of this effect indicates that, in these reading conditions, it takes around 300 ms for the brain to retrieve the meaning of the target word and decide whether or not the word is related to the prime word. Therefore, the N400 is an index of semantic processing.
