Presentation rate of stimulus modulates neuronal responses to frequency modulated sweeps in the mouse inferior colliculus

Abstract

Presentation rate (PR) represents the number of sound stimuli within the unit time. It is one of the essential indexes reflecting acoustical energy and information received by auditory system in a given time. Our recent investigations have shown that the temporal properties of inferior collicular (IC) neurons responding to the pure tone bursts are closely correlated to PR. The frequency modulated sweeps (FMs) share major dynamic parts in both human languages and animal communicating sounds. The modulation range (MR) and sweep directions of FMs, which are important temporal features of FMs, play key roles in speech distinguish in human. Although neuronal selectivity of MRs and sweep directions has been intensively studied in echolocating bats, whether the PR could modulate the neuronal selectivity of the FM remains unclear. In present study we addressed it in the mouse inferior colliculus (IC). 20 mice ( Mus musculus KM) were used in the experiments. Under the condition of free field stimulation and by using normal electrophysiological techniques, we recorded the responses of 90 IC neurons to FMs with different modulation ranges (2, 5, 10, 15, 20 and 25 kHz; the center frequency was always set at the CF of each recorded) at different presentation rates (0.5, 1.0, 5.0, 10.0 or 20.0 Hz). The main results were as follows: 1) When the PR was fixed at a mediate value (2.0 Hz), the majority of neurons responded best to narrower MRs (narrow-pass, up-sweep: 63.33%; down-sweep: 60.00%). Other types such as band-pass (up-sweep, 17/90, 18.89%; down-sweep, 12/90, 13.33%), all-pass (up-sweep, 15/90, 16.67%; down-sweep, 17/90, 18.89%) and wide-pass (up-sweep, 4/90, 4.44%; down-sweep, 4/90, 4.44%) also be found; 2) When different PRs were used, from 0.2 Hz to 10 Hz, the percentage of the narrow-pass neurons decreased, while the percentage of band-pass and all-pass neurons increased slightly and that of wide-pass neurons changed little; 3) As the PR increased, the recorded neuron's firing rate decreased, the latency delayed and the firing duration shortened; 4) The directional selectivity of the IC neurons was modulated by the MR and this influence was PR-dependant. Also, the PRs could affect the directional selectivity of the neurons alone and displayed MR-dependant. Our data demonstrated that PR could modulate the neural coding of MR and directional selectivity of the IC neurons responding to FMs stimuli, and the PR and MR have the co-modulated effect. It reveals that the temporal representation of dynamic sounds in the central auditory system is correlated with the repetition rate of naturally occurring sound.