Oscillatory cortical activities in the human brain

In the present work, three types of oscillatory cortical activity were investigated: (a) auditory SSR, (b) visual steady-state evoked potentials, and (c) visually induced gamma band rhythms. It was demonstrated that all three responses can be altered by attention thereby providing different information about attentional processes. Auditory steady-state responses have been found to have their generator(s) in the primary auditory cortex. The findings of the present work suggest that in the auditory modality, attentional modulations were already active in the primary auditory cortex. This differs from the visual modality, where attentional modulations have only been found in cortical responses with a latency of about 80 ms, which have their generator(s) in the extrastriate visual cortex. Unlike the auditory steady-state response, the visual steady-state response can be elicited with a large variety of frequencies, suggesting that the steady-state response of the two modalities is generated by different mechanisms. With respect to the steady-state visual evoked potential, it has been shown that the location of an equivalent current dipole changed systematically as a function of frequency, which may be interpreted as a sign that they track different visual pathways. High frequency steady-state visual evoked potentials (20.8 and 27.8 Hz) were altered by visual spatial attention, with a narrow focus of cortical activity over the posterior scalp contralateral to the stimulation side, suggesting extrastriate generators. The continuous cortical steady-state activity is used to track the temporal characteristics of attentional processes. The amplitude augmentation was highly correlated with the time window in which subjects achieved their best performance level, suggesting that the attentional shifting (or attentional blink) period is not a fixed time constant, but related to the features of the task following the shifting cue