The representation of peripheral neural activity in the middle-latency evoked ¢eld of primary auditory cortex in humans 1 Andre¤ Rupp a ;  , Stefan Uppenkamp b , Alexander Gutschalk a , Roland Beucker a , Roy D. Patterson b , Torsten Dau c , Michael Scherg a a Section of Biomagnetism, Department of Neurology, Universita«t Heidelberg, 69120 Heidelberg, Germany b Centre for the Neural Basis of Hearing, Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK c AG Medizinische Physik, Fachbereich Physik, Universita«t Oldenburg, 26111 Oldenburg, Germany Received 16 April 2002; accepted 23 July 2002 Abstract Short sweeps with increasing instantaneous frequency (up-chirps) designed to compensate for the propagation delay along the human cochlea enhance the magnitude of wave V of the auditory brainstem responses, while time reversed sweeps (down-chirps) reduce the magnitude of wave V [Dau, T., Wegner, O., Mellert, V., Kollmeier, B., J. Acoust. Soc. Am. 107 (2000) 1530^1540]. This effect is due to synchronisation of frequency channels along the basilar membrane and it indicates that cochlear phase delays are preserved up to the input of the inferior colliculus. The present magnetoencephalography study was designed to investigate the influence of peripheral synchronisation on the activation in primary auditory cortex. Spatio-temporal source analysis of middle- latency auditory evoked fields (MAEFs) elicited by clicks and up- and down-chirps showed that up-chirps elicited significantly larger MAEF responses compared to clicks or down-chirps. Both N19m^P30m magnitude and its latency are influenced by peripheral cross-channel phase effects. Furthermore, deconvolution of the empirical source waveforms with spike probability functions simulated with a cochlear model indicated that the source waves for all stimulus conditions could be explained with the same unit-response function, i.e. a far field recorded cortical response of a very small cell assembly along the medio-lateral axis of Heschl’s gyrus that receives input from a small number of excitatory fibres. The conclusion is that (i) phase delays between channels in the auditory pathway are preserved up to primary auditory cortex, and (ii) MAEFs can be described by a convolution of a unit-response function with the summary neural activity pattern of the auditory nerve. ß 2002 Elsevier Science B.V. All rights reserved. Key words: Primary auditory cortex ; Auditory image ; Magnetoencephalography ; Spatio-temporal source modelling ; Chirp signal; Auditory evoked ¢eld; Cochlear phase delay 1. Introduction Transient stimuli like clicks are commonly used in electrophysiological research of the human auditory system to elicit synchronised auditory brainstem re- sponses (ABRs) and middle-latency auditory evoked ¢elds (MAEFs). In the cochlea, however, the response to a click is not entirely synchronous; the peak of the 0378-5955 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0378-5955(02)00614-7 * Corresponding author. Tel.: +49 (6221) 567537; Fax: +49 (6221) 565258. E-mail address: andre.rupp@urz.uni-heidelberg.de (A. Rupp). 1 Parts of this study were presented during the 24th Midwinter Research Meeting of the Association for Research in Otolaryngology [Rupp, A., Uppenkamp, S., Gutschalk, A., Dau, T., Patterson, R.D., Scherg, M., 658 (2001) p. 186], St. Petersburg Beach, FL, USA. Abbreviations: ABR, auditory brainstem response; AIM, auditory image model; AN, auditory nerve; BESA, brain electrical source analysis; BM, basilar membrane; BMM, basilar membrane motion; CAP, compound action potential; GCV, generalised cross-correlation; MAEF/MAEP, middle-latency auditory evoked ¢elds/potentials; MEG, magnetoencephalography; MRI, magnetic resonance image; NAP, neural activity pattern; PAC, primary auditory cortex; PDF, probability density function; K, exponent determining the rate of change of instantaneous frequency in the chirps; m þ ,m 3 , sign for the phase change in ‘Schroeder phase’ harmonic complex tones Hearing Research 174 (2002) 19^31 www.elsevier.com/locate/heares