A model of signal processing in the cochlear nucleus: Comodulation masking release

Comodulation masking release (CMR) is a psychophysical phenomenon where the masked threshold for a pure tone is reduced for broadband maskers relative to a narrowband masker. This occurs when the broadband masker is modulated coherently across all frequencies. A computational model simulating physiological processes is presented that locates the relevant signal processing in the ventral cochlear nucleus of the auditory brainstem. The model consists of three types of cell: a narrowband, excitatory projection unit, a broadband inhibitory neuron and a narrowband inhibitory neuron. These are tentatively identified with transient choppers (T-stellate), onset units (D-stellate) and vertical (tuberculo-ventral) cells. The model was implemented as a computer program including a multi-channel representation of the auditory nerve response. The same stimuli were used in a comparison of psychophysical (using human listeners), computer modelling and physiological (using guinea pig) approaches to the problem. Sinusoidally amplitude-modulated pure tone maskers were used throughout. An enhancement of the response to target tones was observed in all three studies when comodulated broadband maskers were used. A reduction in response was observed when the flanking tones were modulated out of phase with the masker at the probe tone frequency. It is argued that CMR may be explained by relatively simple signal-processing strategies operating in the auditory brainstem and most probably at the level of the cochlear nucleus.