Auditory event-related responses are generated independently of ongoing brain activity.
For researchers and clinical practitioners alike, evoked and event-related responses measured with MEG and EEG provide the means for studying human brain function and dysfunction. However, the generation mechanism of event-related responses remains unclear, hindering our ability to formulate viable theories of neural information processing. Event-related responses are assumed to be generated either (1) separately of ongoing, oscillatory brain activity or (2) through stimulus-induced reorganization of ongoing activity. Here, we approached this issue through examining single-trial auditory MEG data in humans. We demonstrate that phase coherence over trials observed with commonly used signal decomposition methods (e.g., wavelets) can result from both a phase-coherent state of ongoing oscillations and from the presence of a phase-coherent event-related response which is additive to ongoing oscillations. To avoid this problem, we introduce a method based on amplitude variance to establish the relationship between ongoing oscillations and event-related responses. We found that auditory stimuli do not give rise to phase reorganization of ongoing activity. Further, increases in spectral power accompany the emergence of event-related responses, and the relationship between spectral power and the amplitude of these responses can be accounted for by a linear summation of the event-related response and ongoing oscillation with a stochastically distributed phase. Thus, on the basis of our observations, auditory event-related responses are unique descriptors of neural information processing in humans, generated by processes separate from and additive to ongoing brain activity.
Mäkinen
Ville
V
Apperception and Cortical Dynamics (ACD), Department of Psychology, PO Box 9, FIN-00014 University of Helsinki, Finland. ville.makinen@biomag.hus.fi
Tiitinen
Hannu
H
May
Patrick
P
D000161:Acoustic Stimulation; D001303:Auditory Cortex / Q000502:physiology; D001921:Brain / Q000502:physiology*; D004585:Electrooculography; D005071:Evoked Potentials / Q000502:physiology; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D006801:Humans; D015225:Magnetoencephalography; D013997:Time Factors