The response of the brain to a sensory stimulus may present itself in the electroencephalogram (EEG) as evoked and/or induced activity. While the evoked response is given by peaks and troughs in the signal, time-locked and phase-locked to the stimuli, the induced response is time- but not phase-locked, and can be considered as an increase or a decrease in the power of EEG in a specific frequency band at a specific time range with regard to the stimulus onset. The induced response does not have the same phase following successive stimuli. It is believed that cognition and perception of a stimulus present themselves primarily as the induced response in the EEG. In this paper, the induced response of the brain to auditory speech stimuli is investigated and different approaches to detect induced activity are compared. It is shown that there is an increase in theta and delta power in response to words compared to the baseline, starting around 500ms after their onset. During this time, there is also an increase in pairwise coherence between the posterior electrodes. In response to tone bursts, a change in pairwise coherence was observed in the beta band starting around 200ms. To the best of our knowledge, this is the first time such responses have been described using simple protocols without complex stimulus manipulations being involved. Responses in the EEG to speech rather than the more conventional tone-bursts or clicks suggests that it may be feasible to use the EEG as an objective means to demonstrate brain activation to salient real world stimuli. This would be of particular benefit in investigating access to speech in patients who are unable or unwilling to reliably respond to conventional subjective experimental protocols, such as infants. 
Induced activity in EEG in response to auditory stimulation