The auditory system is a sensory system that is responsible for the sense of hearing. The ability to hear allows animals to be able to detect sounds in their surroundings without direct contact or without seeing the source of the sound. It has been suggested that different frequencies—low-pitched and high-pitched sounds—are mapped to different regions of the auditory cortex, making a tonotopic map. The tonotopic map of the human brain is a method to provide a means of creating a function map of the auditory cortex through the use of acoustic frequencies perceived by the human ear. By interpreting different frequencies, different regions of the auditory cortex increase their activity levels. This change in activity allows researchers to create a function map of the auditory cortex by mapping the specific regions that respond to numerous different frequencies. Scientific research shows that tonotopic maps in the auditory cortex are similar to the retinotopic fields that are mapped in the visual cortex.
Upon receiving a sound wave, an organ within the inner ear known as the organ of Corti transduces the signal of waves into an electrical signal that the brain interprets as sound. Within the organ of Corti are several specialized sensory neurons called hair cells that use mechanotransduction to detect sound waves (movement) near the ear. These hair cells line the organ of Corti across its basilar membrane, which winds throughout the spiral-shaped cochlea. The hair cells form a functional synapse with nerve fibers that make up the vestibulocochlear nerve (cranial nerve VIII). This nerve traverses the brainstem, making a pathway that synapses in the auditory cortex, which is located in the superior portion of the temporal lobe of the brain. The functionality of each of these hair cells lies in their physical layout in the organ of Corti. Specifically, the hair cells are “tuned” to progressively higher frequencies the deeper the hair cells lie. This is due to the fact that the basilar membrane varies greatly in mechanical and physical properties along its length. This difference in the tuning frequencies of the hair cells along the basilar membrane allows the inner ear to act as a sound frequency analyzer, and therefore allows researchers to use neuroimaging to create a map of functional organization within the auditory system.
Tonotopic mapping is useful in creating a functional layout of the auditory cortex of numerous model organisms and humans through the organization of frequency-dependent responses, which are suggestive of specific tonotopically organized regions of the auditory cortex. Studies have shown that the position of the functional response in the auditory cortex varies systematically with differences in frequencies encountered, as shown by functional magnetic resonance imaging (fMRI) of the auditory cortex, located in the superior surface of the temporal lobe of the brain. Through these studies, researchers have identified seven regions in the brain specific to different frequencies encountered, three regions sensitive to high frequencies, and four regions sensitive to lower frequencies. High-pitched sounds have been mapped to the anterolateral aspect of the transverse temporal gyrus (also called Heschl’s gyrus), while low-pitched sounds are mapped to the lateral fissure. The lateral fissure contains the transverse temporal gyrus. Thus, there are multiple tonotopically organized brain regions in humans. Based on these imaging studies, in which they compare differences in anatomical areas and frequency progressions, researchers are able to hypothesize as to the different functional regions within the human auditory cortex.
See also: Age-Related Hearing Loss; Auditory Processing Disorder; Auditory System; Brainstem Auditory Evoked Potentials; Deafness; Sensory Receptors; Vestibulocochlear Nerve
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