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Determining Pitch

Do you know someone with perfect pitch? Many musicians can hear a melody, then play or sing it. Several theories attempt to explain how you can discriminate small differences in sound frequency or pitch. According to Georg von Bekesy’s place theory, the position on the basilar membrane at which waves reach their peak depends on the frequency of a tone. High frequencies produce waves that peak near the close end and are interpreted as high-pitched sound, while low frequency waves travel farther, peaking at the far end, and are interpreted as low-pitched sound. Place theory accounts well for high-pitched sounds. According to frequency theory, the rate of the neural impulses traveling up the auditory nerve matches the frequency of a tone, enabling you to sense its pitch. Individual neurons can only fire at a maximum of 1,000 times per second. A volley mechanism in which neural cells can alternate firing can achieve a combined frequency of about 4,000 times per second. The brain can read pitch from the frequency of the neural impulses. Frequency theory together with the volley principle explains well how you hear low-pitched sounds of up to 4,000 Hz, but this theory doesn’t account for high-pitched sounds. It appears hearing intermediate-range pitches involves some combination of the place and frequency theories.

Hearing Loss

Why do hearing aids only help some deaf people? Conduction deafness and sensorineural or neural deafness have different physiological bases. Conduction deafness is a loss of hearing that results when the eardrum is punctured or any of the ossicles lose their ability to vibrate. People with conduction deafness can hear vibrations when they reach the cochlea by ways other than through the middle ear. A conventional hearing aid may restore hearing by amplifying the vibrations conducted by other facial bones to the cochlea. Nerve (sensorineural) deafness results from damage to the cochlea, hair cells, or auditory neurons. This damage may result from disease, biological changes of aging, or continued exposure to loud noise. For people with deafness caused by hair cell damage, cochlea implants can translate sounds into electrical signals, which are wired into the cochlea’s nerves, conveying some information to the brain about incoming sounds.

Touch (Somatosensation)

Just as hearing is sensitivity to pressure on receptors in the cochlea, touch is sensitivity to pressure on the skin. Psychologists often use somatosensation as a general term for four classes of tactile sensations: touch/pressure, warmth, cold, and pain. Other tactile sensations result from simultaneous stimulation of more than one type of receptor. For example, burning results from stimulation of warmth, cold, and pain receptors. Itching results from repeated gentle stimulation of pain receptors, a tickle results from repeated stimulation of touch receptors, and the sensation of wetness results from simultaneous stimulation of adjacent cold and pressure receptors. Transduction of mechanical energy of pressure/touch and heat energy of warmth and cold occurs at sensory receptors distributed all over the body just below the skin’s surface. Neural fibers generally carry the sensory information to your spinal cord. Information about touch usually travels quickly from your spinal cord to your medulla, where nerves criss-cross, to the thalamus, arriving at the opposite sides of your somatosensory cortex in your parietal lobes. Weber used a two-point discrimination test to determine that regions such as your lips and fingertips have a greater concentration of sensory receptors than your back. The amount of cortex devoted to each area of the body is related to the sensitivity of that area. Touch is necessary for normal development and promotes a sense of well-being.

Pathways for temperature and pain are slower and less defined. You probably have a harder time localizing where you sense warmth and pain on your skin than where you sense touch or pressure. Pain is often associated with secretion of substance P, and relief