Responses to diotic tone-in-noise stimuli in the inferior colliculus: stimulus envelope and neural fluctuation cues

•Envelope cues have been proposed as a solution to limitations of the power-spectrum model in explaining masked detection, especially for paradigms in which energy cues are unreliable. The envelope of a masker stimulus is affected by the addition of a tone. Fluctuations in the responses of auditory neurons to noise maskers are further affected by added tones, due to nonlinearities in the inner ear.•Discharge rates of Inferior colliculus (IC) neurons, which are sensitive to amplitude modulation, reflect the combined effects of stimulus envelope and neural fluctuations.•Here we tested the ability of IC responses to explain masked-detection thresholds for diotic tone-in-noise stimuli. The amplitude-modulation transfer function for each neuron was included in the analysis.•We used two types of noise masker with different envelope statistics: Gaussian noise (GN) and Low-noise Noise (LNN). The envelopes of these maskers change differently upon addition of a tone: GN envelopes are flattened, whereas LNN fluctuations increase.•Positive or negative changes in discharge rates of IC neurons supported the hypothesis that changes in fluctuations of the stimulus envelopes and peripheral responses contribute to diotic detection thresholds, especially for higher masker levels. Slopes of neural modulation transfer functions were significantly correlated to absolute changes in rate upon addition of a tone to the noise maskers. Human detection thresholds in tone-in-noise (TIN) paradigms cannot be explained by the prevalent power-spectrum model when stimulus energy is made less reliable, e.g., in roving-level or equal-energy paradigms. Envelope-related cues provide an alternative that is more robust across level. The TIN stimulus envelope is encoded by slow fluctuations in auditory-nerve (AN) responses - a temporal representation affected by inner-hair-cell (IHC) saturation and cochlear compression. Here, envelope-related fluctuations in AN responses were hypothesized to be reflected in responses of neurons in the inferior colliculus (IC), which have average discharge rates that are sensitive to amplitude-modulation (AM) depth and frequency. Responses to tones masked by narrowband gaussian noise (GN) and low-noise noise (LNN) were recorded in the IC of awake rabbits. Fluctuation amplitudes in the stimulus envelope and in model AN responses decrease for GN maskers and increase for LNN upon addition of tones near threshold. Response rates of IC neurons that are excited by AM we