Spatiotemporal dynamics of word processing in the human cortex.

Understanding language relies on concurrent activation of multiple areas within a distributed neural network. Hemodynamic measures (fMRI and PET) indicate their location, and electromagnetic measures (magnetoencephalography and electroencephalography) reveal the timing of brain activity during language processing. Their combination can show the spatiotemporal characteristics (where and when) of the underlying neural network. Activity to written and spoken words starts in sensory-specific areas and progresses anteriorly via respective ventral ("what") processing streams toward the simultaneously active supramodal regions. The process of understanding a word in its current context peaks about 400 ms after word onset. It is carried out mainly through interactions of the temporal and inferior prefrontal areas on the left during word reading and bilateral temporo-prefrontal areas during speech processing. Neurophysiological evidence suggests that lexical access, semantic associations, and contextual integration may be simultaneous as the brain uses available information in a concurrent manner, with the final goal of rapidly comprehending verbal input. Because the same areas may participate in multiple stages of semantic or syntactic processing, it is crucial to consider both spatial and temporal aspects of their interactions to appreciate how the brain understands words.