The directional sensitivity of cochlear microphonics (CM) was studied inthe quail by rotating a free-field sound source (pure tones, 160-10 kHz)through 360° in the horizontal plane, under anechoic conditions. Sound diffraction by the head was monitored simultaneously by a microphone at the entrance to the ipsilateral (recorded) ear canal. Pressure-field fluctuations measured by the microphone were non-directional (≤ 4 dB) up to 4 kHz; the maximum head shadow was 8 dB at 6.3 kHz. In comparison, the CM sensitivity under went directional fluctuations ranging up to 25 dB for certain low, mid and high frequency band widths. There was noticeable variation between quail for frequencies producing maximum directional effects, although consistently poor directionality was seen near 820 Hz andto a lesser extent near 3.5 kHz. Well-defined CM directivity patterns reflected the presence of nulls (insensitive regions) at critical positions around the head and the number of nulls increased with frequency. Five major types of directivity patterns were defined using polar co-ordinates: cardioid, supercardioid, figure-of-eight, tripartite and multilobed. Such patterns were largely unrelated to head shadow effects. Blocking the ear canal contralateral to there corded ear was shown to effectively abolish CM directionality, largely by eliminating regions of insensitivity to sound.
It is inferred that the quail ear functions as an asym metrical pressure gradient receiver, the pressure gradient function being mediated by the interauralcavity. It is proposed that the central auditory system codes directional information by a null detecting method and computes an unambiguous (i.e.intensity independent) directional cue. This spatial cue is achieved by the difference between the directional sensitivities of the two ears, defined as the Directional Index (DI). The spatial distribution of DI values (difference pattern) demonstrated ranges and peaks which closely reflected the extent and position of nulls determined from monaural directivity functions. Large directional cues (up to 25 dB) extended throughout most of the audible spectrum of the quail and the sharpness of difference patterns increased with frequency. Primary ‘best’ directions, estimated from peaks in difference patterns, tended to move towards the front of the head at higher frequencies; rearward secondary peaks also occurred. From the properties of directional cues it is suggested that the ability of birds to localize sound need not necessarily depend on frequency; however, spatial acuity may be both frequency and direction dependent, and include the possibility of front-torearerrors. The directional properties of bird vocalizations may need to bere assessed on the basis of the proposed mechanism for directional hearing.
Biophysical Aspects of Directional Hearing in the Tammar Wallaby, Macropus Eugenii
