Processing of Pitch by the Auditory System

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Many of the sounds we hear in everyday life, such as musical notes and the vowels of speech, have waveforms that repeat overtime. These tones often give us the sensation of a distinct pitch that corresponds to the repetition rate or fundamental frequency (FO) of the tone. Pitch perception is important not only in music, but also for the perception of speech: it helps us tell the difference between a question and a statement, and, because different voices differ in pitch, it helps us to separate the voices of different people speaking at the same time. Because of this, scientists and engineers are interested in programming computers to perform pitch perception automatically. These computer models can be used to help us understand the human auditory system, and to help us develop machines that mimic aspects of human perception, such as pitch tracking devices for converting the sounds of instruments into musical notation, and automatic speech recognition devices. Current computer models are quite successful, but not nearly as good as a human. We are trying to understand more about the pitch mechanisms in human hearing, in order to improve these computer models. Complex tones, such as those in speech and music, are composed of a set of pure tone components, called harmonics. Each harmonic has a sinusoidal waveform that repeats at a frequency that is an integer multiple of FO. The multiplying factor gives the harmonic number. For example, the first six harmonics of a complex tone with an FO of 100 Hz have frequencies of 100, 200, 300, 400, 500 and 600 Hz. When we listen to a complex tone, the cochlea in the inner ear separates out the individual low-numbered harmonics (less than about harmonic number ten). The brain derives pitch mainly from the frequencies of these low-numbered harmonics. Each of the low-numbered harmonics excites a distinct place in the cochlea. In addition, the frequency of each harmonic is represented by a synchronized pattern of neural activity, in that neurons in the auditory nerve will tend to fire (produce electrical impulses) at the same time during each cycle of the sinusoidal waveform of the harmonic. In other words, the information about the harmonic frequencies is represented by a place code (place in cochlea) and by a temporal code (temporal pattern of neural activity).The most popular computer models propose that the pitch mechanism uses only the temporal code, however this is still very much open to question. Our experiments will provide tests of the assertion, using techniques that require human subjects to make comparisons between the sounds that are played to them. One set of experiments will determine if it is essential that the temporal code for a particular frequency be conveyed by the auditory neurons that are connected to the place in the cochlea that normally responds to the same frequency. In other words, does accurate pitch perception depend on a match between the place and temporal codes? Another study will focus on a peculiar phenomenon known as dichotic pitch . This occurs when a sound is presented simultaneously to the two ears, such that the sound in each ear is identical except for a narrow frequency region, in which the sounds in the two ears are independent. Listeners hear a faint pitch corresponding to this region. The independence between the two ears is processed in the brain by an array of neurons. We will test whether the outputs of these detectors are coded using a place code (i.e., which neurons in the array are active). If so, this will mean that the pitch mechanism can exploit this code when extracting pitch, and that the current temporal models will have to be revised. Finally, we will investigate whether the combination of different harmonics from the two ears is automatic, or depends on the listener rapidly switching attention from one ear to the other. This experiment will help determine how the input to the pitch mechanism is controlled.