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Functional cortical mapping of auditory illusion: An fMRI investigation of “Scale Illusion”

We studied cortical activation using 1.5 T fMRI during "Scale Illusion", a kind of auditory illusion, in which eighteen subjects (20±1.3 years old) perceived smooth melodies while listening to dichotic irregular pitch sequences consisting of scale tones, in repeated phrases composed of eight tones. Twelve male and six female subjects listened to different stimuli that included an illusion-inducing tone sequence (I), monaural tone sequence (S) and perceived pitch sequence (P) with a control of white noises (N) delivered to the left and right ears in random order. Thirty two scans (TR 3 sec.) with a between 3 second interval for each type of the four stimuli were performed. BOLD signals were analyzed by the standard procedure using SPM8 software. In group analysis, activation of the P-N contrast was observed in the left motor area and premotor area. Activations common to both P-N and I-N were observed in the superior temporal gyrus (primary auditory cortex), and these activations showed strong left-hemisphere dominance. Activation of the I-N contrast extended to the anterior part of the superior temporal gyrus, and in the other case (I-N) the activated area extended into the posterior direction from the communal activating area. These findings are consistent with the notion that the hemispheres are differentially specialized for processing auditory stimuli even in the absence of linguistic information and also identifying an acoustic stimulus and its spatial location are processed in separate streams in the auditory cortex.