If you were exposed to ‘white noise’ – a broadband sound signal – that contained a frequency gap, your brain would ‘hear’ a single-frequency sound for several seconds after the real signal had ended. This auditory illusion is known as the Zwicker tone and is surprising because the note ‘heard’ by the brain has a frequency that falls in the ‘gap’ in the original signal.

Scientists agree that the Zwicker tone does not originate in the cochlea or the auditory nerve, but until now they have also struggled to explain the effect in terms of neuron activity. The unusual frequency characteristics rule out the possibility that stimulated neurons are active for a short time after the signal is switched off, which makes the Zwicker tone fundamentally different to the ‘habituation’ of neurons that produces after-images in the visual system.

Now Franosch and co-workers say that anti-noise neurons in the brain could produce the Zwicker tone, which occurs for many sound configurations that contain broadband noise. Imagine a pure tone superimposed onto white noise. According to the researchers, the neurons that respond to pure tones suppress the activity of noise-reducing neurons over a narrow frequency range around the frequency that the particular pure-tone neuron is sensitive to. But this ‘hole-burning’ effect is asymmetrical and the anti-noise neurons on the low-frequency side of the pure tone are still inhibited after the signal is switched off – so the brain ‘hears’ a tone of this lower frequency.

Franosch and colleagues have performed computer simulations that they say can explain all evidence of the Zwicker tone to date. They also hope their model will provide new insights into how the auditory system handles noise. This could help some sufferers of tinnitus, who persistently ‘hear’ noises with frequencies at which they have hearing loss.