MEG responses to rippled noise and Huggins pitch reveal similar cortical representations Ingo Hertrich, 1,CA Klaus Mathiak, 2,3 Hans Menning, 1,2 Werner Lutzenberger 2 and Hermann Ackermann 1 1 Department of General Neurology, Hertie-Institute for Clinical Brain Research, University of Tˇbingen, Hoppe-Seyler-Str. 3, D-72076 Tˇbingen; 2 MEG Center, University of Tˇbingen, Otfried-Mˇller-Str. 47, D-72076 Tˇbingen; 3 Department of Psychiatry and Psychotherapy, University of Aachen, Pauwelsstr. 30, D-52074 Aachen, Germany CA Corresponding Author: ingo.hertrich@uni-tuebingen.de Received 29 November 2004; accepted 2 December 2004 The onset of pitch within an ongoing noise signal evokes a particu- lar brain activity, the pitch onset response (POR). Using whole- head MEG, PORs to iterated rippled noise (IRN) and Huggins pitch (HP), representing prototypical pitch-in-noise signals, were mea- sured in twenty subjects during a pitch identi¢cation task (333 Hz, 400 Hz, randomized). HP and IRN yielded similar responses, later- alized to the left hemisphere and peaking about 180 ms after pitch onset. The initial phase (140 ms) showed stronger activations to 400 than to 333 Hz whereas later stages (200^300 ms) showed tar- get vs nontarget e¡ects. These results suggest, ¢rst, that di¡erent pitches converge into a common cortical representation and, sec- ond, that the POR encompasses various successive processing stages. NeuroReport 16:193^196 c 2005 Lippincott Williams & Wilkins. Key words: Lateralization; Magnetoencephalography; Pitch perception INTRODUCTION Pitch reflects a distinct aspect of auditory perception mapping an acoustic signal onto a low/high scale [1]. The human auditory system is assumed to encompass two different pitch extraction mechanisms, relying either on tonotopic representations of the incoming signal [2–4] or on the temporal distribution of neural activity [5–8]. Iterated rippled noise (IRN), a signal produced by adding delayed copies onto an original stretch of white noise (Fig. 1b), has been considered a prototypical probe of temporal pitch encoding [9–11]. Recent MEG recordings were able to document that a sudden shift from white noise to IRN elicits a distinct pitch onset response (POR), characterized by the same polarity as the M100 field but a slightly different magnetic source location [12]. Besides IRN, Huggins pitch (HP) represents a further paradigm of a tonal percept derived from noise signals in the absence of any spectral envelope changes [13]. HP is generated by shifting within a narrow frequency band the interaural phase by 1801 and, thus, reflects binaural interac- tion effects (Fig. 1a). While the periodic structure of IRN extends across the entire spectrum, the HP effect arises from signal manipulations within a narrow frequency band only. Perceptually, HP resembles narrow-band filtered noise. Thus, HP and IRN exhibit quite different acoustic signatures posing, most presumably, different demands on the neural system with respect to temporal and spectral pitch processing. To further characterize the POR, the present study measured evoked responses both to HP and IRN using a whole-head MEG device. If HP and IRN elicit similar PORs, this activity should reflect a common format of pitch representation subsequent to acoustic feature processing. Furthermore, if POR reflects perceptual aspects of pitch encoding, this response must be expected to exhibit a significant correlation with perceived pitch. In order to test these assumptions, subjects were asked to perform a pitch recognition task during MEG recordings. MATERIALS AND METHODS Subjects: Twenty paid volunteers (age 20–40 years; 11 females) participated in the experiment. Right-handedness was confirmed by means of a questionnaire addressing a variety of activities of daily life (modified Edinburgh handedness inventory). None of the subjects reported a history of neurological disorders and all had normal hearing thresholds. The study has been approved by the ethics committee of the University of Tu¨ bingen. Composition of the stimuli: All stimuli comprised an aperiodic noise segment of a duration of 0.7 s followed by a 0.3 s HP or IRN interval. The length of the initial duration of noise was chosen near the minimum required to obtain a stationary sustained field that can be used as a baseline interval prior to pitch onset [14]. HP and IRN signals with two different pitches each (333 and 400 Hz, i.e., IRN delays of 3.0 and 2.5 ms, respectively) were generated (Fig. 1). In order to achieve an IRN pitch strength that is comparable to HP under the conditions of the air-tube headphones used within the MEG booth, the number of IRN iterations was determined in an informal perceptual study based on five subjects (5 iterations were necessary). In order to avoid COGNITIVE NEUROSCIENCE AND NEUROPSYCHOLOGY NEUROREPORT 0959-4965 c Lippincott Williams & Wilkins Vol 16 No 2 8 February 2005 193 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.