Auditory enhancement at the absolute threshold of hearing and its relationship to the Zwicker tone

Auditory enhancement describes an improvement in the detection of a tonal signal in a broad-band masker with a spectral gap at the signal frequency if the signal is delayed in its onset relative to the masker. This auditory enhancement may be based on an increase of the effective signal level instead of a decline in the effective masker level. In order to evaluate whether this signal enhancement also exists at the threshold of hearing, we measured the absolute threshold for pure-tone pulses of different frequencies with and without preceding band-rejected noise. Such noise also causes the sensation of the Zwicker tone-a faint pure tone lasting for a few seconds immediately after the noise presentation. The pitch of this sensation is a complex function of the noise parameters but always lies at a frequency within the rejected band. During the Zwicker tone sensation, auditory sensitivity for tone pulses at frequencies adjacent to the Zwicker tone was improved by up to 13 dB instead of being reduced which might be expected due to the presence of the simultaneously audible Zwicker tone. The failure to influence this threshold shift with low-frequency tones and measurements of the ear's acoustical response indicate that this threshold improvement may be produced through neuronal disinhibition rather than through a release from mechanical suppression in the cochlea.