Conclusions

A hunting animal depends on adequate sensory systems in order to detect its prey and to pinpoint the prey's location in space. Bats and owls have auditory systems that are dedicated to the task of detecting and localising prey by means of sound. In both groups, specialisations are found at many levels of the auditory pathway that enable spatial information about the prey to be extracted from measurement of the intensity, frequency and time of arrival of sound at the ears. Hearing in owls is adapted to process the noises made by their prey because owls depend on listening only, but hearing in bats is adapted to process the echoes of their own cries because bats hunt and navigate by echolocation.

Bats employ two basic kinds of echolocation signal, which reflect different strategies for extracting information about the prey from the returning echoes. Frequency-modulated sounds are used for prey description and for estimating the distance to the prey, both of which are based on accurate analysis of a wide range of frequencies. Distance is estimated from the delay between sound emission and echo arrival: this time interval is encoded by sharply tuned neurons that respond promptly to a given frequency in the outgoing pulse or in the returning echo. This enables higherorder neurons to be tuned to particular echo delays and so to transform the peripheral responses into a neural map of target range.

Constant-frequency sounds are used for prey detection, whether in open spaces or among dense clutter by picking out fluttering prey from the background. Analysis of CF echoes is based on an acoustic fovea, consisting of a large number of auditory neurons that are very sharply tuned to a small range of frequencies, combined with a behaviour pattern, Doppler-shift compensation, which clamps the CF echo within this range. This combination makes the auditory system extremely sensitive to small Doppler shifts, the acoustic glints, imposed on the echo frequency by the wing beats of a flying insect.

Barn owls are able to detect the angular direction of prey from sounds made by the prey. The angle in azimuth is determined from the time of arrival of the sound at the two ears, and the sound's angle of elevation is determined from intensity differences between the ears. This cue for elevation is made possible by the specialised arrangement of non-neural accessory structures, the external ear openings and associated feathers. On the basis of input from the two ears, auditory interneurons are tuned to particular combinations of time and intensity differences that correspond to particular locations in space. These neurons are arranged to form a neural map of acoustic space, which underlies the owl's skill in open-loop localisation of the prey's sounds.

In both bats and owls, the higher levels of the auditory pathway tend to be organised functionally into discrete populations of interneurons, each specialised for encoding a small set of acoustic parameters. Each set of parameters represents a particular category of information that is directly relevant to the animal's hunting or navigational behaviour, such as echo delay and intensity representing target range. Within each of these discrete populations, the neurons are arranged topographically so that a particular value of an acoustic parameter is encoded at a particular place in the brain.

Brain areas that encode sound frequency are arranged tonotopically, an arrangement that is produced by simple topographical projection from the array of receptors in the cochlea. But the representation of acoustic space or target range is synthesised by neuronal interactions and does not arise by topographical projection of the receptor array. Discrete parallel pathways extract different types of information from the simple frequency and intensity coding carried out by the receptors. Individual neurons at the higher levels then respond only to a specific combination of parameters synthesised by neuronal interactions. Thus, they act as feature detectors for specific values of behaviourally relevant information, such as angular location or target range.

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Essentials of Human Physiology

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