Frequency sensitivity and directional hearing in the gleaning bat,Plecotus auritus (Linnaeus 1758)
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The neural audiogram of the common long-eared bat,Plecotus auritus was recorded from the inferior colliculus (IC). The most sensitive best frequency (BF) thresholds for single neurones are below 0 dB SPL between 7–20 kHz, reaching a best value of -20 dB SPL between 12–20 kHz. The lower and upper limits of hearing occur at 3 kHz and 63 kHz, respectively, based on BF thresholds at 80 dB SPL. BF threshold sensitivities are about 10 dB SPL between 25–50 kHz, corresponding to the energy band of the sonar pulse (26–78 kHz). The tonotopic organization of the central nucleus of the IC (ICC) reveals that neurones with BFs below 20 kHz are disproportionately represented, occupying about 30% of ICC volume, occurring in the more rostral and lateral regions of the nucleus.
The acoustical gain of the external ear reaches a peak of about 20 dB between 8–20 kHz. The gain of the pinna increases rapidly above 4 kHz, to a peak of about 15 dB at 7–12 kHz. The pinna gain curve is similar to that of a simple, finite length acoustic horn; expected horn gain is calculated from the average dimensions of the pinna.
The directional properties of the external ear are based on sound diffraction by the pinna mouth, which, to a first approximation, is equivalent to an elliptical opening due to the elongated shape of the pinna. The spatial receptive field properties for IC neurones are related to the directional properties of the pinna. The position of the acoustic axis of the pinna and the best position (BP) of spatial receptive fields are both about 25° from the midline between 8–30 kHz but approach the midline to 8° at 45 kHz. In elevation, the acoustic axis and the BP of receptive fields move upwards by 20° between 9–25 kHz, remaining stationary for frequencies up to 60 kHz.
The extremely high auditory sensitivity shown by the audiogram and the directionality of hearing are discussed in terms of the adaptation of the auditory system to low frequencies and the role of a large pinna inP. auritus. The functional significance of low frequency hearing inP. auritus is discussed in relation to hunting for prey by listening and is compared to other gleaning species.
KeywordsInferior Colliculus Directional Sensitivity Good Frequency Inferior Colliculus Neurone Acoustic Axis
central nucleus of the inferior colliculus
sound pressure level
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