| Résumé |
Natural sounds display strong intensity fluctuations over time. However, we currently understand better auditory processing in the frequency domain than in the temporal domain. Recently, auditory psychophysics showed that humans perceive a sound whose intensity increases as louder than a sound whose intensity decreases over time, although the overall intensity of the sounds are the same. The underlying neuronal mechanisms of this striking perceptual asymmetry are still elusive. To test if the direction of intensity variation is asymmetrically processed by the auditory system, we have recorded the activity of large populations of neurons in the auditory cortex of awake mice while playing sounds of ramping-up and ramping-down intensities with various durations. We observed that long ramps (> 250ms) produce complex cortical population dynamics with different sets of neurons firing at the beginning compared to the end of the ramps. This indicates that the coding of intensity variations coding is strongly distributed in the auditory cortex. More interestingly, we observed that population firing rate is overall larger for increasing ramps than for decreasing ramps, suggesting that, also for mice, sounds ramping up could be perceptually more salient than sounds ramping down. To test this hypothesis, we performed behavioral experiments in which the saliency of a sound is measured through associative learning speed. We observed that increasing ramps are more rapidly associated to a correct behavior than decreasing ramps, showing that increasing sound intensities are more salient than decreasing sound intensities for mice. Altogether, these novel observations indicate that strongly non-linear processes in the auditory system shape both the perception and the cortical representation of time-varying sounds, to eventually reinforce rising and thus potentially approaching sound sources. |
