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Neural Processing: Echolation

            The purpose of this paper is to present in a concise manner the intricate process of echolocation in bats, specifically Pteronotus parnelli rubiginosus (the mustached bat). The way in which sound signals emitted by bats allow them to visualize and target an object will be explained. Also, the roles of neural pathways and tonotopic maps in the bat brain with respect to this biosonar computation will be discussed. .
             Though bats have a perfectly functional visual sensory system, they have evolved an extremely accurate way of detecting their surroundings in complete darkness: echolocation. The need for this system arises from the fact that bats are nocturnal and therefore hunt at night. Most bats are insectivorous, meaning that their prey consists of insects such as moths. Bats give off biosonar pulses, chirps that are inaudible to the human ear, which then bounce off of objects, sending an echo back to the bats" ears. Bats use several aspects of both the pulses and the returning echoes to approximate the velocity, distance from the approaching bat, shape, size and the wing-beat frequency of the soon to be captured moth (Kruse, 1996). .
             The biosonar pulse is a high frequency vocalization, usually produced via the mouth, or sometimes the nose of the bat. The pulses emitted are exceeding 30kHz, much higher than the human threshold of hearing. There can be three types of these pulses: constant frequency (CF), frequency modulated (FM), and combined CF-FM. CF pulses are made up of a single frequency, or tone, whereas FM pulses sweep downward through different frequencies. Combined CF-FM pulses are just made up of a CF pulse finished off by an FM burst (Suga, 1990). .
             Mustached bats use CF-FM pulses. The CF part is a perfect signal for target detection (as long as the size of the object is larger than the wavelength of the signal) and measurement of velocity, while the FM part is better suited to ranging, localization and characterization of a target.

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