The auditory system and echolocation

When echolocation sounds return as echoes to a bat, they are received by an auditory system that conforms to the general mammalian pattern. The sounds are collected by the external ear and enter the ear canal, where they impinge on the tympanum (Fig. 6.10). The vibrations of the tympanum are

Cochle Basilar membrane nerve

Auditi

Cochle Basilar membrane nerve

Auditi

Inner Ear Muscle Bat

Figure 6.10 The middle and inner ear of a bat; diagram based on a horizontal section. The external ear opening is off picture to the right. Structural elements of the middle and inner ear are shown in solid black; the surrounding bone of the skull is shown in stipple. Note that the bones of the middle ear are acted upon by various muscles, known collectively as the middle ear muscles. (Redrawn after Sales & Pye, 1974.)

muscle

Figure 6.10 The middle and inner ear of a bat; diagram based on a horizontal section. The external ear opening is off picture to the right. Structural elements of the middle and inner ear are shown in solid black; the surrounding bone of the skull is shown in stipple. Note that the bones of the middle ear are acted upon by various muscles, known collectively as the middle ear muscles. (Redrawn after Sales & Pye, 1974.)

transmitted by the bones of the middle ear to the oval window of the inner ear. From here, the vibrations travel through the cochlea along the basilar membrane, which forms a helical ribbon, wide at the apex of the cochlea and narrow at its base. Transduction takes place in receptor cells, the hair cells, which are distributed along the length of the basilar membrane. The receptor potentials that are produced in the hair cells by the vibration of the basilar membrane are transmitted across synapses to first-order neurons with axons that carry spikes to the brain along the auditory nerve.

The information that is provided by the traffic of spikes in the auditory nerve is processed through a sequence of levels in the mammalian brain, as it is in other vertebrates. The auditory pathway consists of a rather complex network of brain regions but may be illustrated by three main staging posts. Firstly, there is the cochlear nucleus in the hindbrain, which receives input from the auditory nerve and contains mainly second-order neurons. Secondly, there is the inferior colliculus, which is one of a number of important nuclei in the midbrain containing higher-order auditory neurons.

Thirdly, there is the auditory cortex in the forebrain, which is the final staging post of the auditory pathway. Echolocation is possible because a bat's auditory system has several striking specialisations that enable it to receive and analyse faint echoes. These specialisations start at the peripheral level, as described below.

The first of these specialisations is that a bat's hearing is particularly sensitive to sounds that have similar frequencies to its own echolocation pulses. This is shown by testing bats with sounds of different frequencies and measuring the auditory threshold, which is the lowest sound intensity that elicits a detectable response. The results are expressed as threshold curves, in which the sound pressure level at threshold is plotted against frequency (Fig. 6.11). In bat species that use broadband (FM) signals in the search stage, it is found that the frequencies with the lowest threshold coincide with the dominant frequencies in the echolocation signal (Fig. 6.11b). Apart from this feature, the threshold curve is not very different from that of a non-echolocating fruit bat (Fig. 6.11a). Nor is the absolute threshold of hearing exceptional by mammalian standards. In bat species that use narrow band (CF) signals in the search stage, the threshold curve is much more sharply tuned to a narrow frequency band. In Rhinolophus, for example, hearing is very sharply tuned (Fig. 6.11c) and the echoes are kept close to this best frequency by the Doppler-shift compensation.

A second specialisation of the peripheral auditory system is that echolo-cating bats have highly directional hearing, in contrast to most mammals, which have good all-round hearing. This is shown clearly by behavioural tests carried out on restrained horseshoe bats. A loudspeaker directly in front of the bat's head produces 'echoes' with an electronically shifted frequency following each of the bat's CF pulses, and the bat then responds by compensating for this apparent Doppler shift. The directionality of hearing is tested by presenting sounds at the normal echo frequency from another speaker at different angles around the bat's head. When this second sound is perceived by the bat, it effectively masks the first sound and the bat does not show the compensation response. Hearing proves to be most sensitive directly in front of the head, and sensitivity falls off by about 45 dB from the midline to the side. A similar fall in sensitivity occurs at angles below the horizontal but the drop is less severe above the horizontal. Consequently, returning echoes are useful only if they fall within a narrow cone in front of the head, not more than 30° off the direction of flight (Grinnell & Schnitzler, 1977).

Sound frequency (kHz)

10 20 40 80 100

Sound frequency (kHz)

Figure 6.11 Hearing threshold curves for three species of bat, derived from recordings of summed potentials in the inferior colliculus; thresholds are expressed as sound pressure level in decibels (dB SPL), as a function of sound frequency. (a) Curve for a non-echolocating fruit bat from southern India. (b) Curve for an echolocating bat from the same locality, with a son-agram of the echolocation signal shown above for comparison (rotated through 90°). Note the similarity of the curves in (a) and (b), apart from the tuning of the curve in (b) to the strongest frequencies in the echolocation signal (24 to 26 kHz). (c) Curve for the greater horseshoe bat (Rhinolophus ferrumequinum) showing the sharp tuning to 83 kHz and the notch of insensitivity to frequencies just below this. (a and b modified after Neuweiler, Singh & Sripathi, 1984; c modified after Neuweiler, 1983.)

A third specialisation for echolocation in bats is that the peripheral auditory system shows a reduced sensitivity to the emitted pulse of sound. The echolocation sounds emitted by bats are very intense, with a sound pressure level of around 110 to 120 dB when measured 5 to 10 cm in front of the head. If the bat's auditory system were directly exposed to such intense sounds, it would not be able to recover fully by the time the echo arrived. In bats using brief FM signals, such as Myotis, this problem is overcome by contraction of the middle-ear muscles, which partially uncouples the inner ear from the vibrations of the tympanum (see Fig. 6.10). During echolocation, these muscles begin to contract before each sound pulse and develop maximal tension at the onset of sound emission; they then relax very rapidly, within about 8 ms, so that the response to the echo is not attenuated except at very close range. In Myotis, the attenuation produced by contraction of the middle-ear muscles is some 20 to 25 dB.

In addition, neural attenuation takes place in the midbrain prior to the inferior colliculus. Recording with simple wire electrodes shows that the summed potential elicited by the emitted sounds is smaller in the midbrain than in the cochlea nucleus, but this is not the case for external stimuli. Hence, there must be a central, neural mechanism that attenuates self-stimulation by the emitted sounds. This lasts only for a short time and echoes returning after 4 ms are not affected. The magnitude of the neural attenuation averages about 15 dB in Myotis, and so the total attenuation available from both mechanical and neural mechanisms amounts to some 35 to 40 dB.

Bats that use long CF signals, such as Rhinolophus, cannot exploit these mechanisms because the long outgoing pulse overlaps the returning echo. However, when the bat is on the wing, the emitted sound normally has a lower frequency than the echo, which is kept at a constant frequency by Doppler-shift compensation. Consequently, Rhinolophus is able to solve the problem of self-stimulation simply by being rather deaf at the normal emission frequency, which is a few kiloHertz below the echo frequency (Fig. 6.11c).

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