Spatiotemporal dynamics of word processing in the human brain

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.

Oscillatory phase coupling coordinates anatomically dispersed functional cell assemblies

“…Dynamic interactions between multiple assemblies may then give rise to the large-scale functional networks found in mammalian brains (6-8). Despite the theoretical appeal of Hebb's idea (9) and growing empirical evidence of assemblies (10-12), it remains unclear how diverse groups of neurons spanning several cortical regions transiently coordinate their activity to form cell assemblies or how multiple coactive assemblies regulate their interactions to form larger functional networks.Brain rhythms may play a key role in coordinating neuronal ensembles (13-15), with a dynamic hierarchy of neuronal oscillations modulating local computation and long-range communication (16)(17)(18). This hypothesis is supported by evidence that spiking activity depends on the local field potential (LFP) in both hippocampus (19-21) and neocortex (22,23).…”

“…Brain rhythms may play a key role in coordinating neuronal ensembles (13-15), with a dynamic hierarchy of neuronal oscillations modulating local computation and long-range communication (16)(17)(18). This hypothesis is supported by evidence that spiking activity depends on the local field potential (LFP) in both hippocampus (19-21) and neocortex (22,23).…”

Signal quality of simultaneously recorded invasive and non-invasive EEG

“…Ball et al, 2004Ball et al, , 2008Canolty et al, 2006Canolty et al, , 2007. To ensure that the results of our study are not restricted to the case of a CA reference, we also analyzed our data using a local average reference, where each non-invasive EEG and ECoG channel was referenced against its neighboring channels.…”

“…E-mail address: tonio.ball@uniklinik-freiburg.de (T. Ball). emphasized is that it is less susceptible to artifact contamination Canolty et al, 2007;Engel et al, 2005;Lachaux et al, 2003;Leuthardt et al, 2004;Schalk et al, 2007). There are, however, only very few studies so far that investigated the possibility that similar types of artifacts as in non-invasive EEG might also be present in invasive recordings.…”

Brain-Computer Interfaces in Medicine

“…12,[92][93][94] Most recently, ECoG signals measured over speech cortex during overt or imag-ined phoneme and word articulation were used for online cursor control 95 and were also accurately decoded off-line for potential application to direct speech synthesis. 96,97 It has been shown that epidural ECoG can provide BCI control, 98 that ECoG-based BCI performance with fixed parameters is stable over at least 5 days, 99 and that motor imagery-based BCI control using locations over motor cortex can produce ECoG changes exceeding those produced by actual movements. 100 A study in monkeys found that ECoG recordings and the performance of the model used to decode movement remained stable over several months.…”