Barbour Lecture Notes

Barbour Lecture Notes - Sensory Transduction and the...

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BME140 Dennis L. Barbour September 15, 2010 This lecture will introduce the principles underlying sensation by the nervous system and focus on a specific example of how energy fluctuations in the environment become signals of behavioral relevance to an organism. The auditory system will serve as our example because it makes for an excellent engineering case study. Sounds are transduced (altered in form) in the ear from vibrations of air particles into action potentials propagating down a nerve fiber. In the process of reviewing this sequence of events, we will touch upon aspects of multiple fields, including acoustics, signal processing, mechanics, fluid dynamics, bioelectricity, protein dynamics, genetics, tissue engineering and brain-computer interfaces. In fact, nearly all areas of biomedical engineering have been and are continuing to be applied toward improving our understanding of the early hearing process. Acoustics Sound waves propagate through the air as traveling pressure waves. The air is slightly compressed and slightly rarefied around a mean value, and this pattern of pressure variations propagates across space (i.e., it’s a function of both space and time). The simplest form of vibration is a pure sinusoid, such as the vibrations produced by a tuning fork: ( ) ( ) φ π + = ft A t p 2 cos . The pressure wave p is completely characterized by three parameters: amplitude ( A ), frequency ( f ) and phase ( ). Frequency corresponds to the perceptual descriptor of pitch, and amplitude (or level) corresponds to the perceptual descriptor of loudness. The ear is insensitive to absolute phase, although relative phase is very important for perception. We are neglecting the spatial aspect of the function by assuming we have a free medium (like open air) and that the source and detector are fixed in space (not moving). Under these conditions, the spatial function takes on the same form as the temporal function. This pressure wave is a signal. Other, more complex pressure wave signals convey meaningful information, and it is the job of the ear and auditory system to extract that information by employing meaningful processing techniques. Engineers also must employ signal processing techniques to encode and decode signals, and understanding how the ear performs this task so well provides us with insight about how to go about designing our own signal processors. Fluctuations in pressure cause variations in displacement of molecules and, naturally, in their velocity of displacement. Just as the variations in pressure do not result in a net pressure change, the variations in displacement do not result in a net movement of molecules. In a free medium, the peak pressure and peak velocity vary with the same phase and are related by ( ) ( ) t zv t p = , where z is the impedance of the medium. Peak displacement is described, however, by a relationship that is dependent upon frequency: () ( ) f t v f t d 2 , = Sensation and Audition, p1 September 15, 2010
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Barbour Lecture Notes - Sensory Transduction and the...

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