Responses of inferior colliculus neurons to notched noises in awake mice: putative neural correlates of auditory enhancement and Zwicker tone

Sensory systems are well adapted to constantly changing statistics of the environment and to process specific spectral features of sounds, such as spectral notch (i.e. low energy frequency band) embedded in broadband stimuli. Spectral notches can be added to the stimulus spectrum due to filtering by the outer ear, and can be used as monaural cues related to head or pinna position for localizing sound sources. In addition, broadband sounds with spectral notch are known to produce auditory enhancement, a perceptual phenomenon in which a target within a spectrally notched masker can become salient if preceded by a copy of the masker. Notched noise can also produce an auditory illusion, called Zwicker Tone (ZT), which is perceived immediately after stimulation and whose pitch corresponds to the spectral notch. The present study aimed to further investigate the mechanisms of auditory enhancement, including those of ZT, in the inferior colliculus of awake mice. We show that neural activity can be strongly suppressed during NN stimulation and enhanced immediately after NN stimulation. These effects depend on notch center frequency relative to the best frequency of neurons, stimulus level and notch width. Our results are consistent with the mechanisms described for post-inhibitory rebound in the central auditory system: NN could hyperpolarize the membrane potential, which can then activate several cationic conductances, leading to a rebound of neural activity. We discuss auditory enhancement and ZT as collateral effects of an essential neural mechanism aimed at enhancing the central representation of acoustic spectral contrasts.