Abstract
We examined the spatiotemporal dynamics of word processing by recording the electrocorticogram (ECoG) from the lateral frontotemporal cortex of neurosurgical patients chronically implanted with subdural electrode grids. Subjects engaged in a target detection task where proper names served as infrequent targets embedded in a stream of task-irrelevant verbs and nonwords. Verbs described actions related to the hand (e.g, throw) or mouth (e.g., blow), while unintelligible nonwords were sounds which matched the verbs in duration, intensity, temporal modulation, and power spectrum. Complex oscillatory dynamics were observed in the delta, theta, alpha, beta, low, and high gamma (HG) bands in response to presentation of all stimulus types. HG activity (80–200 Hz) in the ECoG tracked the spatiotemporal dynamics of word processing and identified a network of cortical structures involved in early word processing. HG was used to determine the relative onset, peak, and offset times of local cortical activation during word processing. Listening to verbs compared to nonwords sequentially activates first the posterior superior temporal gyrus (post-STG), then the middle superior temporal gyrus (mid-STG), followed by the superior temporal sulcus (STS). We also observed strong phase-locking between pairs of electrodes in the theta band, with weaker phase-locking occurring in the delta, alpha, and beta frequency ranges. These results provide details on the first few hundred milliseconds of the spatiotemporal evolution of cortical activity during word processing and provide evidence consistent with the hypothesis that an oscillatory hierarchy coordinates the flow of information between distinct cortical regions during goal-directed behavior.
Highlights
Paul Broca (Broca, 1861) and Carl Wernicke (Wernicke, 1874) were among the most noted scientists to identify critical brain regions responsible for the production and comprehension of speech
Spatial results All subjects showed an increase in high gamma (HG) power following presentation of words relative to acoustically matched nonwords at electrodes located over the posterior superior temporal gyrus, middle superior temporal gyrus, and the superior temporal sulcus (STS)
Event-related power changes were observed in the delta (2–4 Hz), theta (4–8 Hz), alpha (8–12 Hz), beta (12–30 Hz), and low gamma (30–80 Hz) bands in all subjects (p < 0.001, FDR corrected)
Summary
Paul Broca (Broca, 1861) and Carl Wernicke (Wernicke, 1874) were among the most noted scientists to identify critical brain regions responsible for the production and comprehension of speech. Using functional magnetic resonance imaging (fMRI), Binder and colleagues generated a map of functional subdivisions within the human temporal cortex by having subject listen to unstructured noise, frequencymodulated (FM) tones, reversed speech, pseudowords, and words. They demonstrated that cortical regions surrounding Heschl’s Gyrus bilaterally – in particular, the planum temporale and dorsolateral superior temporal gyrus (STG) – were more strongly activated by FM tones than by noise, suggesting that the regions are involved in processing temporally structured auditory stimuli.
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