Virtual-space receptive fields of single auditory nerve fibers

1. Sounds reaching the tympanic membranes are first modified by the acoustic properties of the torso, head, and external ear. For certain frequencies in the incident sound there results a complex, direction-dependent spatial distribution of sound pressure at the eardrum such that, within a sound field, localized areas of pressure maxima are flanked by areas of pressure minima. Listeners may use these spatial maxima and minima in localizing the source of a sound in space. The results presented describe how information about this spatial pressure pattern is transmitted from the cochlea to the central auditory system via single fibers of the auditory nerve. 2. Discharges of single fibers of the auditory nerve were studied in Nembutal-anesthetized cats [characteristic frequencies (CFs) ranged from 0.4 to 40 kHz]. Click stimuli were derived from sound-pressure waveforms that were generated by a loudspeaker placed at 1,800 locations around the cat's head and recorded at the tympanic membrane with miniature microphones. Recorded signals were converted to acoustic stimuli and delivered to the ear via a calibrated and sealed earphone. The full complement of signals is referred to as "virtual acoustic space," and the spatial distribution of discharges to this array of signals is referred to as a "virtual-space receptive field" (VSRF). 3. Fibers detect both pressure maxima and pressure minima in virtual acoustic space. Thus VSRFs take on complex shapes. 4. VSRFs of fibers of the same or similar CF having low spontaneous rates had the same overall pattern as those from high-spontaneous rate (HSR) fibers. For HSR fibers, the VSRF is obscured by the high background spike activity. 5. Comparison of the VSRF and isolevel contour maps of the stimulus derived at various frequencies revealed that auditory nerve fibers most accurately extract spectral information contained in the stimulus at a frequency close to or slightly higher than CF.