Functional dynamics of spoken and signed word production: A case study using electrocorticographic spectral analysis

Background: Event-related changes in the EEG power spectrum have recently been used to study functional brain activation in humans. Like traditional event-related potentials, event-related spectral changes provide information about the temporal evolution of cortical activation that is difficult to obtain with fMRI or PET. These spectral changes include event-related suppression of power in the alpha band (8-13 Hz) and even-related augmentation of power in the gamma band (>30 Hz). Most studies of the gamma band have focused on frequencies in and around 40 Hz. Using electrocorticographic (ECoG) signals recorded with subdural electrodes implanted for the surgical management of epilepsy, we have also observed a broadband augmentation of power in higher gamma frequencies (>80 Hz). This index of cortical activation has compared favourably with alpha suppression and with electrical cortical stimulation mapping in studies utilising simple motor and auditory processing, but such a comparison has not been done for tasks requiring more complex language processing. Aims: We used two different spectral indices of cortical activation - alpha power suppression and gamma power augmentation, to study the spatial and temporal patterns of human cortical activation during different word production tasks. Methods: Subdural ECoG electrodes were implanted for clinical purposes in a patient with normal hearing, who had learned sign language as an adult and later developed intractable epilepsy. We measured event-related spectral changes during language tasks with different input and output modalities. Picture naming and word reading (visual inputs), and word repetition (auditory input) were performed with spoken responses (oral-articulatory output) and with signed responses (manual-gestural output). In addition we used a visually guided motor task to map tongue and hand representations independent of language. Results: Speech and signing differentially activated tongue and hand regions of sensorimotor cortex. Although posterior superior temporal gyrus was activated early during auditory word repetition, picture naming and word reading also activated this region, albeit slightly later, consistent with phonological and/or lexical-semantic processing. In contrast, basal temporal-occipital cortex was activated earlier and with greater magnitude during picture naming and word reading than during word repetition, suggesting semantic processing or object and word recognition. In all of the tasks, event-related gamma augmentation occurred in more discrete spatial and temporal patterns than event-related alpha suppression. Conclusions: Event-related alpha suppression and gamma augmentation provide complementary information about task-related cortical activation. This study demonstrate the utility of ECoG spectral analysis for studying the functional anatomy of human language with excellent temporal resolution.