Neurophysiological correlate of the auditory after-image ('Zwicker tone')

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.