[Studies of directional sensitivity of neurons in the cat primary auditory cortex]

A set of impulsive transient signals has been synthesized for earphone delivery whose waveform and amplitude spectra, measured at the eardrum, mimic those of sounds arriving from a free-field source. The complete stimulus set forms a "virtual acoustic space" (VAS) for the cat. VAS stimuli are delivered via calibrated earphones sealed into the external meatus in cats under barbiturate anesthesia. Neurons recorded extracellularly in primary (AI) auditory cortex exhibit sensitivity to the direction of sound in VAS. The aggregation of effective sound directions forms a virtual space receptive field (VSRF). At about 20 dB above minimal threshold, VSRFs recorded in otherwise quiet and anechoic space fall into categories based on spatial dimension and location. The size, shape and location of VSRFs remain stable over many hours of recording and are found to be shaped by excitatory and inhibitory interactions of activity arriving from the two ears. Within the VSRF response latency and strength vary systematically with stimulus direction. In an ensemble of such neurons these functional gradients provide information about stimulus direction, which closely accounts for a human listener's spatial acuity. Raising stimulus intensity, introducing continuous background noise or presenting a conditioning stimulus all influence the extent of the VSRF but leave intact the gradient structure of the field. These and other findings suggest that such functional gradients in VSRFs of ensembles of AI neurons are instrumental in coding sound direction and robust enough to overcome interference from competing environmental sounds.