The ‘Zwicker tone’ (ZT) is an auditory after-image that can be evoked most effectively when a band-suppressed noise (relative width of gap 1/3 octave) presented for a certain period of time has been switched off. The sensation of this purely monaural phenomenon is that of a pure tone with a frequency corresponding to the center frequency of the gap and an equivalent level of 10-15 dB above auditory threshold. The sensation decays gradually; it may last as long as 10 s depending on how long the evoking noise was presented. The search for a physiological correlate has been futile so far, probably because the search was confined to more peripheral levels of the auditory system (inferior colliculus). A neuromagnetic study was performed in normal-hearing subjects in order to look for a neurophysiological correlate of the ZT in the auditory cortex. With a stimulation paradigm especially designed for this study, we have been able to isolate poststimulus activity which appears to be related to the ZT and which originates in the supratemporal auditory cortex. It is a sustained neuromagnetic activity that shows a clear-cut dipolar field distribution, and it appears that this activity has certain similarities with the tone-evoked auditory sustained response. The hypothesis is put forward that during the sensation of the ZT a process takes place in the auditory cortex which is similar to that underlying the sustained response, and which gives rise to the sensation of the ZT. In contrast to the sustained response, however, which is due to neural activity evoked by an external acoustic stimulus, the sustained activity associated with the ZT is due to a temporary absolute or relative reduction of neural activity originating from those regions in which the ZT exciting stimulus caused an adaptation. These differences in neural activity cannot be distinguished by the auditory system from a corresponding external acoustic signal. Preliminary studies in patients suffering from tonal tinnitus yielded results which exhibit a certain similarity with those obtained in the ZT experiment.

A neuromagnetic study in tinnitus patients and normal-hearing controls was performed with a modified contingent negative variation (CNV) paradigm. While the warning stimulus S1 was a tone burst at an intensity well above threshold, the imperative stimulus S2 was presented at a near threshold intensity because, in the majority of cases, the perceived loudness of tinnitus is very close to the threshold for a pure tone of the same frequency. Subjects had to respond to S2 by pressing a button until its offset was detected. In this case, instead of the usual sudden cut-off of the CNV after the perception of S2, a slow negative deflection develops, the post-imperative negative variation (PINV). Its initial portion probably indicates the development of a second initial CNV because the subject had to attend also to the offset of S2. The neuromagnetic data were analysed both in the time domain and in the frequency domain (short-time spectral analysis of the classical EEG bands). The time domain waveform as well as the spectrotemporal patterns of the MEG bands exhibited deviations from the normal pattern in several tinnitus subgroups, depending on the characteristics of tinnitus (tonal vs. noisiform, monaural vs. binaural) and on the stimulation conditions (tinnitus side vs. non-tinnitus side).

The auditory afterimage is a sensation which occurs for several seconds after the exciting acoustic signal has been switched off, and which roughly corresponds to the inverse of the spectrum of the exciting signal. In contrast to the well-known visual afterimage, the physiological mechanism generating the auditory afterimage has been questionable so far. Neuromagnetic source imaging revealed that the source of cortical neural activity which coincides with the sensation of the afterimage is located in the auditory cortex and exhibits a tonotopic organization similar to that of the sustained response which occurs during continuous presentation of an acoustic stimulus. It is concluded that the neural processes leading to the generation of the two phenomena -sustained response and auditory afterimage - are similar.

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