We designed a melody perception experiment involving eight harmonic complex tones of missing fundamental frequencies (hidden auditory object) to study the short-term neuronal plasticity of the auditory cortex. In this experiment, the fundamental frequencies of the complex tones followed the beginning of the virtual melody of the tune “Frère Jacques”. The harmonics of the complex tones were chosen so that the spectral melody had an inverse contour when compared with the virtual one. Evoked magnetic fields were recorded contralaterally to the ear of stimulation from both hemispheres. After a base line measurement, the subjects were exposed repeatedly to the experimental stimuli for 1 hour a day. All subjects reported a sudden change in the perceived melody, indicating possible reorganization of the corticalprocesses involved in the virtual pitch formation.After this switch in perception, a second measurementwas performed. Cortical sources of theevoked gamma-band activity were significantlystronger and located more medially after a switchin perception. Independent Component Analysisrevealed enhanced synchronization in the gamma-bandfrequency range. Comparing the gamma-bandactivation of both hemispheres, nolaterality effects were observed. The resultsindicate that the primary auditory cortices areinvolved in the process of virtual pitchperception and that their function is modifiableby laboratory manipulation.
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