Complete bilateral removal of the auditory cortex does not prevent a cat or monkey from detecting sounds or reacting in a crude manner to sounds. However, it does greatly reduce or sometimes even abolish the animal's ability to discriminate different sound pitches and especially patterns of sound. For instance, an animal that has been trained to recognize a combination or sequence of tones, one following the other in a particular pattern, loses this ability when the auditory cortex is destroyed; furthermore, the animal cannot relearn this type of response. Therefore, the auditory cortex is especially important in the discrimination of tonal and sequential sound patterns.
Destruction of both primary auditory cortices in the human being greatly reduces one's sensitivity for hearing. Destruction of one side only slightly reduces hearing in the opposite ear; it does not cause deafness in the ear because of many crossover connections from side to side in the auditory neural pathway. However, it does affect one's ability to localize the source of a sound, because comparative signals in both cortices are required for the localization function.
Lesions that affect the auditory association areas but not the primary auditory cortex do not decrease a person's ability to hear and differentiate sound tones or even to interpret at least simple patterns of sound. However, he or she is often unable to interpret the meaning of the sound heard. For instance, lesions in the posterior portion of the superior temporal gyrus, which is called Wernicke's area and is part of the auditory association cortex, often make it impossible for a person to interpret the meanings of words even though he or she hears them perfectly well and can even repeat them. These functions of the auditory association areas and their relation to the overall intellectual functions of the brain are discussed in more detail in Chapter 57.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.