Different Modes of Pitch Perception and Learning-InducedNeuronal Plasticity of the Human Auditory Cortex

Abstract

We designed a melody perception experimentinvolving eight harmonic complex tones ofmissing fundamental frequencies (hidden auditoryobject) to study the short-term neuronal plasticityof the auditory cortex. In this experiment, thefundamental frequencies of the complex tonesfollowed the beginning of the virtual melody ofthe tune “Frère Jacques”. The harmonics of thecomplex tones were chosen so that the spectralmelody had an inverse contour when comparedwith the virtual one. Evoked magnetic fields wererecorded contralaterally to the ear of stimulationfrom both hemispheres. After a base line measurement,the subjects were exposed repeatedly to theexperimental stimuli for 1 hour a day. All subjectsreported 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.Peer Reviewe