A frontal but not parietal neural correlate of auditory consciousness.

Hemodynamic correlates of consciousness were investigated in humans during the presentation of a dichotic sequence inducing illusory auditory percepts with features analogous to visual multistability. The sequence consisted of a variation of the original stimulation eliciting the Deutsch's octave illusion, created to maintain a stable illusory percept long enough to allow the detection of the underlying hemodynamic activity using functional magnetic resonance imaging (fMRI). Two specular 500 ms dichotic stimuli (400 and 800 Hz) presented in alternation by means of earphones cause an illusory segregation of pitch and ear of origin which can yield up to four different auditory percepts per dichotic stimulus. Such percepts are maintained stable when one of the two dichotic stimuli is presented repeatedly for 6 s, immediately after the alternation. We observed hemodynamic activity specifically accompanying conscious experience of pitch in a bilateral network including the superior frontal gyrus (SFG, BA9 and BA10), medial frontal gyrus (BA6 and BA9), insula (BA13), and posterior lateral nucleus of the thalamus. Conscious experience of side (ear of origin) was instead specifically accompanied by bilateral activity in the MFG (BA6), STG (BA41), parahippocampal gyrus (BA28), and insula (BA13). These results suggest that the neural substrate of auditory consciousness, differently from that of visual consciousness, may rest upon a fronto-temporal rather than upon a fronto-parietal network. Moreover, they indicate that the neural correlates of consciousness depend on the specific features of the stimulus and suggest the SFG-MFG and the insula as important cortical nodes for auditory conscious experience.

Asymmetric cortical adaptation effects during alternating auditory stimulation.

The present study investigates hemispheric asymmetries in the neural adaptation processes occurring during alternating auditory stimulation. Stimuli were two monaural pure tones having a frequency of 400 or 800 Hz and a duration of 500 ms. Electroencephalogram (EEG) was recorded from 14 volunteers during the presentation of the following stimulus sequences, lasting 12 s each: 1) evoked potentials (EP condition, control), 2) alternation of frequency and ear (FE condition), 3) alternation of frequency (F condition), and 4) alternation of ear (E condition). Main results showed that in the central area of the left hemisphere (around C3 site) the N100 response underwent adaptation in all patterns of alternation, whereas in the same area of the right hemisphere the tones presented at the right ear in the FE produced no adaptation. Moreover, the responses to right-ear stimuli showed a difference between hemispheres in the E condition, which produced less adaptation in the left hemisphere. These effects are discussed in terms of lateral symmetry as a product of hemispheric, pathway and ear asymmetries.

Ear and pitch segregation in Deutsch's octave illusion persist following switch from stimulus alternation to repetition.

Deutsch's octave illusion occurs when two tones that are spaced an octave apart are repeatedly presented in alternation; the sequence is presented to both ears simultaneously but offset by one tone, so that two dichotic chords are repeatedly presented in alternation. The most common illusory percept consists of an intermittent high tone in one ear alternating with an intermittent low tone in the other ear. The aim of this study was to investigate whether, once the illusory percept has emerged, the illusion will persist when the original sequence is followed by another sequence consisting of the repeated presentation of one of the two dichotic chords. Forty naïve subjects were tested with stimuli consisting first of a priming sequence containing dichotic octaves alternating between ears followed immediately by a test sequence consisting of a single dichotic octave presented repeatedly. The durations of the priming and test sequences were manipulated. The findings showed that the illusory percept is maintained after the switch from alternation to repetition and that the relative length of the priming and test sequences has a negligible influence on the persistence of the illusory percept.