The Synaptic Representation of Sound Source Location in Auditory Cortex

A key function of the auditory system is to provide reliable information about the location of sound sources. Here, we describe how sound location is represented by synaptic input arriving onto pyramidal cells within auditory cortex by combining free-field acoustic stimulation in the frontal azimuthal plane with in vivo whole-cell recordings. We found that subthreshold activity was panoramic in that EPSPs could be evoked from all locations in all cells. Regardless of the sound location that evoked the largest EPSP, we observed a slowing in the EPSP slope along the contralateral-ipsilateral plane that was reflected in a temporal sequence of peak EPSP times. Contralateral sounds evoked EPSPs with earlier peak times and consequently generated action potential firing with shorter latencies than ipsilateral sounds. Thus, whereas spiking probability reflected the region of space evoking the largest EPSP, across the population, synaptic inputs enforced a gradient of spike latency and precision along the horizontal axis. Therefore, within auditory cortex and regardless of preferred location, the time window of synaptic integration reflects sound source location and ensures that spatial acoustic information is represented by relative timings of pyramidal cell output.