Pitch Perception

As described earlier, encoding information leading to the perception of pitch relies initially on the tonotopic organization of hair cells, an organizational plan that is maintained by all subsequent components of the auditory pathway. The tonotopic map of the auditory cortex is composed of groups of adjacent cells that respond maximally to a specific frequency (characteristic frequency) that is determined by the specific area of the basilar membrane providing the input. Cells with similar characteristic frequencies are arranged in a series of cortical bands called isofrequency bands, with lower frequencies represented most anteriorly within the primary cortical region and higher frequencies located more caudally.

In addition to receiving information from similar regions of the basilar membrane, cells within isofre-quency bands share other similarities. Auditory neurons tend to fire at the same phase of any given sound wave. Thus, their response is phase-locked to some location on the wave and their firing rate reflects the frequency of the original sound. The firing rate then becomes an additional code for the perceived pitch of the sound to augment the code represented by the position of the cell within the tonotopic map. Neurons within isofrequency bands tend to fire in synchrony at a rate indicative of their characteristic frequency range.

afferent neuron to the brain

FIGURE 5 Structure of the organ of Corti. (A) Cross section of the cochlear ducts showing the placement of the organ of Corti. (B) Organ of Corti enlarged. The sensory hair cells and endolymph are shown in blue. Reissner's membrane is very flexible and does not contribute to the mechanical properties of the system. Its major function is to separate the scala media from the scala vestibuli. The scala media is separated from the scala tympani by the stiff basilar membrane that establishes the properties of the system. The reticular lamina overlies the cells of the organ of Corti and provides a barrier between the endolymph of the scala media and the perilymph of the scala tympani. (C) The auditory hair cell is similar to the vestibular cell except it is responsive to much higher frequencies. Bending the stereocilia in the direction of the longest stereocilia opens potassium channels and depolarizes the hair cell, causing release of neurotransmitter.

A. Lateral view of cerebral cortex

A. Lateral view of cerebral cortex

B. Coronal section of cerebral cortex

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