Research PaperNeural representation of octave illusion in the human cortex revealed with functional magnetic resonance imaging
Introduction
Auditory illusion is an illusionary phenomenon wherein altered perception occurs from the presented auditory stimulation. Octave illusion is induced using consecutive dichotic sounds, where two tones that are one octave apart, e.g., 400 and 800 Hz, alternate between the ears at 500-ms intervals (Brancucci et al., 2009). Octave illusion is characterized by perception occurring in only one ear at a time. When presenting a low tone to the left ear and a high tone to the right ear, and vice versa (Fig. 1(a)), they can be perceived in four different ways, as shown in Fig. 1(b) (Brancucci et al., 2011). The percepts involve a single high-pitch tone in one ear alternating with a single low-pitch tone in the other ear.
The behavioral result of typical octave illusion has been explained by the “what” and “where” models (Deutsch, 1975a, Deutsch, 1975b, Deutsch, 2004, Deutsch and Roll, 1976, Lamminmäki and Hari, 2002, Lamminmäki et al., 2012). The perceived pitch, “what”, is determined by the dominant ear, whereas sound location, “where”, occurs in the ear receiving a high tone. The “where” explanation is supported by a neurophysiological study (Lamminmäki and Hari, 2002), in which cortical onset responses to four binaural pairs of 400- and 800-Hz tones, which were presented in isolation, rather than as a continuous sequence, were recorded. The auditory N1m response was stronger in both hemispheres when the 800-Hz tone of the dichotic pair was presented to the contralateral rather than to the ipsilateral ear, resembling responses to monaural stimuli. The results suggested that the ear receiving the high (800 Hz) tone within the octave illusion sequence determines the perceived location of the sound. In addition, Lamminmäki et al. (2012) showed that the modulation of auditory steady-state responses (ASSRs) are related to the perception of “what” in which the observed right ear dominance implies relatively increased ASSR in the ipsilateral (right) hemisphere from the right ear and relatively decreased ASSR in the contralateral (left) hemisphere. The role of right hemisphere dominance is established in pitch and melody processing (Patterson et al., 2002). However, these magnetoencephalographic studies (Lamminmäki and Hari, 2002, Lamminmäki et al., 2012) did not investigate brain regions other than the auditory cortex. Brancucci et al. (2016) studied whole brain regions using functional magnetic resonance imaging (fMRI) and showed that the auditory cortex, superior frontal gyrus, middle frontal gyrus, insular cortex, thalamus, and parahippocampal gyrus were activated in listeners when they received stimuli of octave illusion. However, they did not compare this activation to that in non-illusion listeners, instead they obtained the results from difference maps of two percepts during the illusion.
The aim of the current study is to clarify the brain regions underlying the perception of octave illusion, using fMRI. In conventional block design fMRI, activation differences may be examined in a participant group using stimuli that induce illusory and non-illusory percepts in separate blocks. However, such methods are sensitive to acoustic differences, including temporal characteristics existing between the two stimuli, which may yield undesirable activation. In addition, some participants may recognize the presented illusion-inducing stimulus as it is, without illusory perception. Based on this, we classified participants into two perception groups, i.e., participants who perceived the illusion pitch (ILL group) and those who perceived the dichotic stimulus-pitch (non-ILL group) for the same octave-illusion stimulus. To do this, we performed behavioral tests on more than 50 participants to equalize the less-frequent non-ILL participant group with the ILL group. We performed recordings of block design fMRI for the two participant groups using identical octave illusion stimuli. We hypothesized that, by removing the effect of acoustical stimulation itself, it would be possible to represent the regions specifically related to the octave illusion.
Section snippets
Participants
Since octave illusion is the main percept induced by the illusion stimulus of Fig. 1(a) (Deutsch, 1974), we recruited many participants to equalize the sample size of the ILL and non-ILL groups. We performed preliminary perception tests, and selected a total of 52 healthy volunteers (50 males and 2 females), including both of ILL and non-ILL perception groups, as the participants in this study for subsequent measurements. Data for 43 (41 males and 2 females) of the 52 participants, with a mean
Results
Among the selected 52 participants, 25 reported the percepts as shown in Fig. 1(b), signifying the presence of the octave illusion (ILL). Twenty-two participants perceived the same pitch of tones as the stimulus sequence in each ear (non-ILL). The other five participants reported different perception patterns from the ILL or non-ILL pattern, as follows. Two of the five participants perceived only higher pitches alternately in the left and right ears. Two participants perceived higher pitches in
Discussion
Octave illusions occur when alternating pitches separated by an octave (i.e., 400 Hz and 800 Hz) are presented in sequence to both ears. It has been suggested that a front-temporal brain network, comprising the bilateral superior frontal gyrus, medial frontal gyrus (MFG), superior temporal gyrus (STG), insulae thalamus, and PHG, play an important role in maintaining the perception of octave illusion (Brancucci et al., 2016). In the current study, the differences in perception between the ILL
Conclusion
With the same presentation of dichotic octave illusion stimuli, participants experienced different auditory percepts and were classified into ILL and non-ILL groups. The difference in the perception between the two groups was represented as significantly different brain activities, which were observed using fMRI. These perception-dependent activities were limited to the lateral PP in the auditory cortex, right PMC, and left PHG. We inferred the functions of auditory processing in the octave
Acknowledgments
We thank Prof. Iku Nemoto for the generous help with the discussion of the octave illusion. This work was supported by JSPS KAKENHI [Grant Number JP16K01376] (Grant-in-Aid for Scientific Research (C)).
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