Directional properties of the auditory periphery in the guinea pig.

The directional sensitivity of the outer ear of the guinea pig was determined by recording changes in the amplitude of the cochlear microphonic to frequencies between 1 and 20 kHz as the location of the sound source was changed throughout 360 degrees of horizontal auditory space. The directional responses to frequencies below 3 kHz were almost omnidirectional. The directional responses for frequencies between 3 and 12 kHz were progressively more directional toward the anterior midline. The responses for frequencies above 12 kHz were highly directional along the ipsilateral interaural axis. In contrast, the directional responses to all frequencies in animals whose pinnae had been removed were orientated along the ipsilateral interaural axis. The observations suggest that the orientation and strength of the directional response of the auditory periphery in the guinea pig are dependent on frequency and that this dependence is attributable, at least in part, to the acoustic properties of the pinna. The observations also indicate that there is a substantial change in the interaural intensity difference at various frequencies and in the spectral transfer function of the ear according to the location of the sound source in the ipsilateral hemifield. The observation that these changes are asymmetrical about the interaural axis for a substantial part of the auditory range of the animal is consistent with the hypothesis that the frequency dependent directionality of the auditory periphery provides a spectral cue for the localization of broad band sounds in the free field.