Neural coding of sound in the ascending and descending auditory pathways

Our perception of music, speech and the surrounding soundscape all originate from patterns of vibrations in the cochlea. To encode these high-speed vibrations, evolutionary pressures designed an exquisite hydro/electro/mechanical force transducer, signal amplifier and frequency analyzer – the Organ of Corti – with sensory cells that can individually encode a 1x1012 range of signal amplitudes at rates up to several kilohertz. Neurons in the auditory brainstem synchronize action potential timing to frequency modulations in excess of 1kHz and compute sub-millisecond discrepancies in the timing of sound waves that reach each ear. But then what? While sensory coding processes are well understood in the auditory periphery and brainstem, the auditory computations and feature representations that arise de novo at later stages of processing have proven more difficult to discern. Here, I will present new evidence that sluggish temporal processing at the level of the auditory cortex is more than just an inability for fast processing, but instead reflects an emergent specialization for encoding sound features that unfold on very slow time scales. I will also discuss the massive descending network of projections that originate in the auditory cortex and innervate nearly every level of subcortical sound processing. I will focus on the first-order branch of the descending auditory pathway –corticothalamic neurons – and present new evidence showing their sensitivity to motor inputs that precede anticipated sounds.