Sensitivity to direction and velocity of fast frequency chirps in the inferior colliculus

Neurons in the mammalian inferior colliculus (IC) are selective for the direction of fast frequency sweeps (chirps) contained in Schroeder-phase harmonic complexes (SCHR). This selectivity points to a feature sensitivity to fast-chirp direction and velocity in the IC potentially shared by all mammals. Here, we introduced a novel stimulus consisting of Aperiodic Random Chirps (ARC) to distinguish neurons’ sensitivity to chirp direction and velocity from the confounding periodicity of SCHR stimuli. Extracellular, single-unit recordings were made in the IC of Dutch-belted rabbits. SCHR stimuli had a range of velocities and periodicities. ARC stimuli with matched velocities and sinusoidally amplitude-modulated (SAM) noise were also presented. The prevalence of direction sensitivity and direction bias to chirps in SCHR and ARC responses was quantified, revealing that the majority of IC neurons (90.5%) were selective to SCHR-chirp direction and that sensitivity to the direction of aperiodic chirps was nearly universal (99.6%). Rate-velocity functions were constructed from ARC responses, revealing that neurons were often sensitive to the direction of the lowest-velocity chirps used and were insensitive to the direction of the highest-velocity chirps used. ARC and SAM noise responses were combined using a generalized linear model (GLM) to predict SCHR response rates. The GLM analysis revealed that both velocity and periodicity cues were required to explain SCHR responses. In general, velocity cues have a stronger effect on responses for stimuli with lower F0s. This result has implications for neural encoding of complex sounds that combine envelope periodicities and frequency-sweeps.