Complex tone processing and critical band in the human auditory cortex Yoshiharu Soeta * , Seiji Nakagawa Institute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 14 April 2006; received in revised form 15 September 2006; accepted 24 September 2006 Abstract Psychophysical experiments in humans have indicated that the auditory system has a well-defined bandwidth for resolution of com- plex stimuli. This bandwidth is known as the critical bandwidth (CBW). Physiological correlates of the CBW were examined in the human auditory cortex. Two- and three-tone complexes were used as the sound stimuli with all signals presented at 55 dB sound pressure level (SPL). The duration of stimulation was 500 ms, with rise and fall ramps of 10 ms. Ten normal-hearing subjects took part in the study. Auditory-evoked fields were recorded using a 122-channel whole-head magnetometer in a magnetically shielded room. The laten- cies, source strengths, and coordinates of the N1m waves, which were found above the left and right temporal lobes approximately 100 ms after the onset of stimulation, were analyzed. The results indicated that N1m amplitudes were approximately constant when the frequency separation of a two-tone complex or the total bandwidth of a three-tone complex was less than the CBW; however, the N1m amplitudes increased with increasing frequency separation or total bandwidth when these were greater than the CBW. These findings indicate critical band-like behavior in the human auditory cortex. The N1m amplitudes in the right hemisphere were significantly greater than those in the left hemisphere, which may reflect a right-hemispheric dominance in the processing of tonal stimuli. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Auditory-evoked fields (AEFs); Critical band; Critical bandwidth; Magnetoencephalography (MEG); N1m 1. Introduction Using several methods, psycho-acoustic studies in humans and animals have identified a fundamental percep- tual unit that defines the frequency resolution of the audi- tory system – the critical bandwidth (CBW). The critical band (CB) concept has been used to explain a wide range of perceptual phenomena involving complex sounds. In 1940, Fletcher first demonstrated the existence of the CB phenomenon in studies of masked thresholds. Greenwood (1961) later reported that tones and noises were effective in masking tonal signals only when their spectra lay within the same CB as the signal. It was reported that perceived loudness remained constant provided the bandwidth of a sound is less than the CBW; however, loudness increased with increasing bandwidth if the bandwidth was increased beyond the CBW (Zwicker et al., 1957; Scharf, 1961; Sato et al., 2002). Physiological correlates of the CBW have been described in the primary auditory cortex (A1) of cats and monkeys, and in several studies examining short- and middle-latency components of the surface-recorded (scalp-recorded) auditory-evoked potential (AEP) or audi- tory-evoked magnetic field (AEF) in humans. Frequency resolution and spectral filtering in the cat AI have been assessed by extracellular recordings of tone responses in white noise of various bandwidths (Ehret and Schreiner, 1997). It has been reported that the tone-evoked response of single A1 neurons in cats increase once the width of a noise masker was narrowed to less than the CBW. It has 0378-5955/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.heares.2006.09.005 Abbreviations: A1, primary auditory cortex; ACF, autocorrelation f- unction; AEF, auditory-evoked field; AEP, auditory-evoked potential; AM, amplitude modulation; ANOVA, analysis of variance; CB, critical band; CBW, critical bandwidth; ECD, equivalent current dipole; ERB, equivalent rectangular bandwidth; MLR, middle-latency response; QFM, quasi-frequency-modulation; SPL, sound pressure level * Corresponding author. Tel.: +81 72 751 9496; fax: +81 72 751 8416. E-mail address: y.soeta@aist.go.jp (Y. Soeta). www.elsevier.com/locate/heares Hearing Research 222 (2006) 125–132 Hearing Research