- Email: [email protected]
Human cortical activity related to sound localization in the median plane
International Congress Series 1278 (2005) 11 – 14
www.ics-elsevier.com
Human cortical activity related to sound localization in the median plane Yosuke Okamotoa,b,*, Seiji Nakagawaa, Yoh-ichi Fujisakaa, Mitsuo Tonoikea a Institute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology (AIST), Osaka, Japan b Graduate School of Science and Technology, Kumamoto University, 40-1, Kurokami 2-chome, Kumamoto, Japan
Abstract. Recent studies of sound localization in the horizontal plane have revealed that the right hemisphere is dominant in auditory spatial processing of sounds from different directions. In this study, human cortical activity in response to sound sources located in the median plane was investigated. Auditory stimuli, broad-band noises (100–10,000 Hz), were modified by convolutions with headrelated transfer functions (HRTFs) to allow the perception of sound sources outside the head, and were virtually presented from different directions in the median plane. The stimuli were delivered to the ears of subjects using plastic tubes to avoid magnetic interference. Auditory evoked magnetic fields were recorded using a 122-channel whole-head SQUID magnetometer in a magnetically shielded room. Two experiments were conducted with stimuli convolved/unconvolved with HRTFs. Magnetoencephalography (MEG) data obtained from the temporal areas of the left and right hemispheres were chosen and the largest amplitudes of major activity peaks elicited by the auditory stimuli were analyzed. The results showed that the right hemisphere tended to be more sensitive in processing sound sources located in the median plane. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnetoencephalography (MEG); Sound localization; Median plane
1. Introduction Humans are able to localize sound sources based on binaural cues from interaural time and level differences (ITD and ILD) and monaural spectral cues [1]. Spectral cues are * Corresponding author. Graduate School of Science and Technology, 40-1, Kurokami 2-chome, Kumamoto University, Kumamoto, Japan. Tel.: +81 72 751 8526; fax: +81 72 751 8416. E-mail address: [email protected] (Y. Okamoto). 0531-5131/ D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2004.11.001
12
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
derived from head-related transfer functions (HRTFs) and the shape of the HRTF provides information about the location of a sound source, especially in the median plane [2]. Studies on sound localization in the horizontal plane using magnetoencephalography (MEG) have revealed that the auditory cortex is more activated to contralaterally perceived stimuli than to medially or ipsilaterally perceived stimuli [3,4]. Moreover, human cortical activity reflecting processing of spatial sound stimuli was shown to be more pronounced in the right than in the left hemisphere [4,5]. The purpose of this study is to investigate human cortical activity in response to sound sources located in the median plane. 2. Materials and methods 2.1. Stimuli Our stimuli consisted of a broad-band noise (100–10,000 Hz). The duration of a stimulus was 100 ms and the inter-stimulus interval was 1.0 s. The auditory stimuli were presented randomly from four different directions in the median plane (0, +60, +120, +1808). In experiments using MEG, it is impossible to present auditory stimuli with loudspeakers and, therefore, the stimuli were delivered to the subjects’ ears through a plastic tube (Cabot Safety EARTONE 3A) to avoid magnetic interference. To equalize the frequency response at the eardrums between 100 and 10,000 Hz, the impulse response of the system was measured with an ear simulator (Brqel and kj&r type 4157) using the timestretched pulse (TSP) signal [6]. Then, using the impulse response, an inverse filter was calculated and convolved to the noise. Instead of loudspeakers, the stimuli for this study were produced utilizing HRTFs provided by MIT Media Laboratory (http://sound.media.mit.edu/KEMAR.html). Eight paid healthy adults with normal hearing participated in the present study (Fig. 1). 2.2. MEG recording MEGs were recorded using a 122-channel whole-head SQUID magnetometer (Neuromag-122k). The system consists of 61 pairs of sensor units. The MEG responses were filtered from 0.03 to 100 Hz and digitized with a sampling rate of 400 Hz. More than 100 responses were averaged on-line with respect to onset of the stimuli. Averaged data were filtered from 0.1 to 30 Hz and vector sums of the channel pairs were calculated off-line.
Fig. 1. The amplitude spectrum of a broad-band noise convolved with the inverse filter, which was measured at the end of the ear tube.
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
13
Fig. 2. Examples of averaged MEG signals at all 122 channels and vector sums of paired signal (200 ms prestimulus and 500 ms poststimulus).
Experiments were performed in a magnetically shielded room. Subjects were seated in the room with their eyes open and instructed to pay attention to a silent movie. Two types of stimuli were presented to clarify the effect of sound localization on MEG responses. One type included stimuli convolved with HRTFs as mentioned above and the other included stimuli unconvolved with HRTFs. The two types of stimuli had the same energy at each ear and in each direction. Fig. 2 shows an example of the distribution of MEG signals over the scalp using the whole-head system and the vector sums of the paired signal. The figure shows that the amplitudes of the channels in the temporal area are larger than at other areas. 3. Results The peak observed around 100 ms from stimulus onset (N1m) was the main component of the auditory evoked magnetic field. The amplitudes of N1m were calculated and the
Fig. 3. Ratios of N1m amplitude for the stimuli convolved with HRTFs to that for the stimuli unconvolved with HRTFs.
14
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
largest amplitudes in the temporal area of the left and right hemispheres were analyzed in each experiment. Then, the amplitudes of N1m in these experiments were compared. Fig. 3 shows the ratios of N1m amplitude for the stimuli convolved with HRTFs to the N1m amplitude for the stimuli unconvolved with HRTFs. The results show that the ratios of N1m amplitudes in the right hemisphere were greater than those in the left hemisphere. The effects of hemispheric differences (right/left) and the angle of the sound source on these ratios were assessed by two-way analysis of variance (ANOVA), but significant effects were not observed. 4. Discussion The ratios of N1m amplitudes for the stimuli convolved with HRTFs to the N1m amplitude for the stimuli unconvolved with HRTFs in the right hemisphere tended to be greater than those in the left hemisphere. This result might indicate that the right hemisphere is more activated with sound sources located in the median plane. This tendency of the right hemisphere being more sensitive at processing auditory spatial information was also observed in a previous MEG study on human cortical processing of sound localization in the horizontal plane [4,5]. In the present study, however, ANOVA did not show a significant effect of hemispheric difference. There is room for further consideration as to how to calculate magnetic fields of sensors, such as the root mean square across several channels and the area of analysis, besides the temporal area. In previous studies with monaural stimuli, the N1m response was larger and earlier over the contralateral than over the ipsilateral hemisphere; this might have been due to anatomical constraints since the ear projects more strongly to the contralateral than the ipsilateral auditory cortex [7,8]. Cortical activity also increased with sound from a contralateral direction in the horizontal plane in previous studies with binaural stimuli [3,4]. In the present study, the effect of the direction of the sound source in the median plane on N1m amplitude was assessed in each hemisphere, but analysis did not show a significant effect. This might be because among the stimuli in the present study there was no ITD and the change in ILD was not as large as that among stimuli located in the horizontal plane. References [1] J. Blauert, Spatial Hearing: the Psychophysics of Human Sound Localization, MIT press, Cambridge, 1983. [2] W.A. Yost, Fundamentals of Hearing, 4th ed., Academic Press, New York, 2000. [3] L. McEvoy, et al., Human auditory cortical mechanisms of sound lateralization: II. Interaural time differences at sound onset, Hearing Research 67 (1993) 98 – 109. [4] K. Palom7ki, et al., Sound localization in the human brain: neuromagnetic observations, NeuroReport 11 (2000) 1535 – 1538. [5] K. Itoh, et al., Temporal stream of cortical representation for auditory spatial localization in human hemispheres, Neuroscience Letters 292 (2000) 215 – 219. [6] Y Suzuki, et al., An optimum computer-generated pulse signal suitable for the measurement of very long impulse responses, Journal of the Acoustical Society of America 97 (1995) 1119 – 1123. [7] Ch. Pantev, et al., Comparison between simultaneously recorded auditory-evoked magnetic fields and potentials elicited by ipsilateral, contralateral and binaural tone burst stimulation, Audiology 25 (1986) 54 – 61. [8] R. Hari, J.P. M7kel7, Modification of neuromagnetic responses of the human auditory cortex by masking sounds, Experimental Brain Research 71 (1998) 87 – 92.
www.ics-elsevier.com
Human cortical activity related to sound localization in the median plane Yosuke Okamotoa,b,*, Seiji Nakagawaa, Yoh-ichi Fujisakaa, Mitsuo Tonoikea a Institute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology (AIST), Osaka, Japan b Graduate School of Science and Technology, Kumamoto University, 40-1, Kurokami 2-chome, Kumamoto, Japan
Abstract. Recent studies of sound localization in the horizontal plane have revealed that the right hemisphere is dominant in auditory spatial processing of sounds from different directions. In this study, human cortical activity in response to sound sources located in the median plane was investigated. Auditory stimuli, broad-band noises (100–10,000 Hz), were modified by convolutions with headrelated transfer functions (HRTFs) to allow the perception of sound sources outside the head, and were virtually presented from different directions in the median plane. The stimuli were delivered to the ears of subjects using plastic tubes to avoid magnetic interference. Auditory evoked magnetic fields were recorded using a 122-channel whole-head SQUID magnetometer in a magnetically shielded room. Two experiments were conducted with stimuli convolved/unconvolved with HRTFs. Magnetoencephalography (MEG) data obtained from the temporal areas of the left and right hemispheres were chosen and the largest amplitudes of major activity peaks elicited by the auditory stimuli were analyzed. The results showed that the right hemisphere tended to be more sensitive in processing sound sources located in the median plane. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnetoencephalography (MEG); Sound localization; Median plane
1. Introduction Humans are able to localize sound sources based on binaural cues from interaural time and level differences (ITD and ILD) and monaural spectral cues [1]. Spectral cues are * Corresponding author. Graduate School of Science and Technology, 40-1, Kurokami 2-chome, Kumamoto University, Kumamoto, Japan. Tel.: +81 72 751 8526; fax: +81 72 751 8416. E-mail address: [email protected] (Y. Okamoto). 0531-5131/ D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2004.11.001
12
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
derived from head-related transfer functions (HRTFs) and the shape of the HRTF provides information about the location of a sound source, especially in the median plane [2]. Studies on sound localization in the horizontal plane using magnetoencephalography (MEG) have revealed that the auditory cortex is more activated to contralaterally perceived stimuli than to medially or ipsilaterally perceived stimuli [3,4]. Moreover, human cortical activity reflecting processing of spatial sound stimuli was shown to be more pronounced in the right than in the left hemisphere [4,5]. The purpose of this study is to investigate human cortical activity in response to sound sources located in the median plane. 2. Materials and methods 2.1. Stimuli Our stimuli consisted of a broad-band noise (100–10,000 Hz). The duration of a stimulus was 100 ms and the inter-stimulus interval was 1.0 s. The auditory stimuli were presented randomly from four different directions in the median plane (0, +60, +120, +1808). In experiments using MEG, it is impossible to present auditory stimuli with loudspeakers and, therefore, the stimuli were delivered to the subjects’ ears through a plastic tube (Cabot Safety EARTONE 3A) to avoid magnetic interference. To equalize the frequency response at the eardrums between 100 and 10,000 Hz, the impulse response of the system was measured with an ear simulator (Brqel and kj&r type 4157) using the timestretched pulse (TSP) signal [6]. Then, using the impulse response, an inverse filter was calculated and convolved to the noise. Instead of loudspeakers, the stimuli for this study were produced utilizing HRTFs provided by MIT Media Laboratory (http://sound.media.mit.edu/KEMAR.html). Eight paid healthy adults with normal hearing participated in the present study (Fig. 1). 2.2. MEG recording MEGs were recorded using a 122-channel whole-head SQUID magnetometer (Neuromag-122k). The system consists of 61 pairs of sensor units. The MEG responses were filtered from 0.03 to 100 Hz and digitized with a sampling rate of 400 Hz. More than 100 responses were averaged on-line with respect to onset of the stimuli. Averaged data were filtered from 0.1 to 30 Hz and vector sums of the channel pairs were calculated off-line.
Fig. 1. The amplitude spectrum of a broad-band noise convolved with the inverse filter, which was measured at the end of the ear tube.
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
13
Fig. 2. Examples of averaged MEG signals at all 122 channels and vector sums of paired signal (200 ms prestimulus and 500 ms poststimulus).
Experiments were performed in a magnetically shielded room. Subjects were seated in the room with their eyes open and instructed to pay attention to a silent movie. Two types of stimuli were presented to clarify the effect of sound localization on MEG responses. One type included stimuli convolved with HRTFs as mentioned above and the other included stimuli unconvolved with HRTFs. The two types of stimuli had the same energy at each ear and in each direction. Fig. 2 shows an example of the distribution of MEG signals over the scalp using the whole-head system and the vector sums of the paired signal. The figure shows that the amplitudes of the channels in the temporal area are larger than at other areas. 3. Results The peak observed around 100 ms from stimulus onset (N1m) was the main component of the auditory evoked magnetic field. The amplitudes of N1m were calculated and the
Fig. 3. Ratios of N1m amplitude for the stimuli convolved with HRTFs to that for the stimuli unconvolved with HRTFs.
14
Y. Okamoto et al. / International Congress Series 1278 (2005) 11–14
largest amplitudes in the temporal area of the left and right hemispheres were analyzed in each experiment. Then, the amplitudes of N1m in these experiments were compared. Fig. 3 shows the ratios of N1m amplitude for the stimuli convolved with HRTFs to the N1m amplitude for the stimuli unconvolved with HRTFs. The results show that the ratios of N1m amplitudes in the right hemisphere were greater than those in the left hemisphere. The effects of hemispheric differences (right/left) and the angle of the sound source on these ratios were assessed by two-way analysis of variance (ANOVA), but significant effects were not observed. 4. Discussion The ratios of N1m amplitudes for the stimuli convolved with HRTFs to the N1m amplitude for the stimuli unconvolved with HRTFs in the right hemisphere tended to be greater than those in the left hemisphere. This result might indicate that the right hemisphere is more activated with sound sources located in the median plane. This tendency of the right hemisphere being more sensitive at processing auditory spatial information was also observed in a previous MEG study on human cortical processing of sound localization in the horizontal plane [4,5]. In the present study, however, ANOVA did not show a significant effect of hemispheric difference. There is room for further consideration as to how to calculate magnetic fields of sensors, such as the root mean square across several channels and the area of analysis, besides the temporal area. In previous studies with monaural stimuli, the N1m response was larger and earlier over the contralateral than over the ipsilateral hemisphere; this might have been due to anatomical constraints since the ear projects more strongly to the contralateral than the ipsilateral auditory cortex [7,8]. Cortical activity also increased with sound from a contralateral direction in the horizontal plane in previous studies with binaural stimuli [3,4]. In the present study, the effect of the direction of the sound source in the median plane on N1m amplitude was assessed in each hemisphere, but analysis did not show a significant effect. This might be because among the stimuli in the present study there was no ITD and the change in ILD was not as large as that among stimuli located in the horizontal plane. References [1] J. Blauert, Spatial Hearing: the Psychophysics of Human Sound Localization, MIT press, Cambridge, 1983. [2] W.A. Yost, Fundamentals of Hearing, 4th ed., Academic Press, New York, 2000. [3] L. McEvoy, et al., Human auditory cortical mechanisms of sound lateralization: II. Interaural time differences at sound onset, Hearing Research 67 (1993) 98 – 109. [4] K. Palom7ki, et al., Sound localization in the human brain: neuromagnetic observations, NeuroReport 11 (2000) 1535 – 1538. [5] K. Itoh, et al., Temporal stream of cortical representation for auditory spatial localization in human hemispheres, Neuroscience Letters 292 (2000) 215 – 219. [6] Y Suzuki, et al., An optimum computer-generated pulse signal suitable for the measurement of very long impulse responses, Journal of the Acoustical Society of America 97 (1995) 1119 – 1123. [7] Ch. Pantev, et al., Comparison between simultaneously recorded auditory-evoked magnetic fields and potentials elicited by ipsilateral, contralateral and binaural tone burst stimulation, Audiology 25 (1986) 54 – 61. [8] R. Hari, J.P. M7kel7, Modification of neuromagnetic responses of the human auditory cortex by masking sounds, Experimental Brain Research 71 (1998) 87 – 92.