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The Journal of Neuroscience, 2002, 22:RC211:1-5

RAPID COMMUNICATION
Distinct Gamma-Band Evoked Responses to Speech and Non-Speech Sounds in Humans

Satu Palva^{1, 3}, J. Matias Palva², Yury Shtyrov^{1, 4}, Teija Kujala¹, Risto J. Ilmoniemi^{1, 3}, Kai Kaila², and Risto Näätänen^{1, 3}

¹ Cognitive Brain Research Unit, Department of Psychology and ² Department of Biosciences, Division of Animal Physiology, FIN-00014 University of Helsinki, Finland, ³ BioMag Laboratory, Engineering Centre, Helsinki University Central Hospital, FIN-00029 HUS, Finland, and ⁴ Cognition and Brain Sciences Unit, Medical Research Council, CB2 2EF, Cambridge, United Kingdom

To understand spoken language, the human brain must have fast mechanisms for the representation and identification of speech sounds. Stimulus-induced synchronization of neural activity at gamma frequencies (20-80 Hz), occurring in humans at 200-300 msec from stimulus onset, has been suggested to be a possible mechanism for neural object representation. Auditory and visual stimuli also evoke an earlier (peak <100 msec) gamma oscillation, but its dependence on high-level stimulus parameters and, thereby, its involvement in object representation has remained unclear.

Using whole-scalp magnetoencephalography, we show here that responses evoked by speech and non-speech sounds differed in the gamma-frequency but not in the low-frequency (0.1-20 Hz) band as early as 40-60 msec from stimulus onset. The gamma-band responses to the speech sound peaked earlier in the left than in the right hemisphere, whereas those to the non-speech sound peaked earlier in the right hemisphere. For the speech sound, there was no difference in the response amplitude between the hemispheres at low (20-45 Hz) gamma frequencies, whereas for the non-speech sound, the amplitude was larger in the right hemisphere. These results suggest that evoked gamma-band activity may indeed be sensitive to high-level stimulus properties and may hence reflect the neural representation of speech sounds. Consequently, speech-specific neuronal processing may commence no later than 40-60 msec from stimulus onset, possibly in the form of activation of language-specific memory traces.

Key words: evoked gamma oscillation; speech sound; representation; human; magnetoencephalography (MEG); lateralization

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Copyright © Society for Neuroscience  0270-6474//$05.00/0

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