7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
In the dark, without visual stimuli that capture your attention, you can appreciate your sense of hearing, or audition. Evolutionarily, being able to hear approaching predators or prey in the dark, or behind one’s back, helped increase chances of survival. Hearing is the primary sensory modality for human language. How do you hear? Sound waves result from the mechanical vibration of molecules from a sound source such as your vocal cords or the strings of a musical instrument. The vibrations move in a medium, such as air, outward from the source, first compressing molecules and then letting them move apart. This compression and expansion is called one cycle of a sound wave. The greater the compression, the larger the amplitude or height of the sound wave and the louder the sound. The amplitude is measured in logarithmic units of pressure called decibels (dB). Established by Fechner, every increase of 10 dB corresponds to a 10-fold increase in volume. The absolute threshold for hearing is 0 dB. Normal conversations measure about 60 dB. Differences in the frequency of the cycles, the number of complete wavelengths that pass a point in a second (hertz or Hz), determine the highness or lowness of the sound called the pitch. The shorter the wavelength, the higher the frequency and the higher the pitch. The longer the wavelength, the lower the frequency and the lower the pitch. People are sensitive to frequencies between about 20 and 20,000 Hz. You are best able to hear sounds with frequencies within the range that corresponds to the human voice. You can tell the difference between the notes of the same pitch and loudness played on a flute and on a violin because of a difference in the purity of the wave form or mixture of the sound waves, a difference in timbre.
Parts of the Ear
Your ear is well adapted for converting sound waves of vocalizations to the neural impulses you perceive as language (see Figure 7.2). Your outer ear consists of the pinna, which is the visible portion of the ear; the auditory canal, which is the opening into the head; and the eardrum or tympanum. Your outer ear channels sound waves to the eardrum that vibrates with the sound waves. This causes the three tiny bones called the ossicles (the hammer, anvil, and stirrup) of your middle ear to vibrate. The vibrating stirrup pushes against the oval window of the cochlea in the inner ear. Inside the cochlea is a basilar membrane with hair cells that are bent by the vibrations and transduce this mechanical energy to the electrochemical energy of neural impulses. Hair cells synapse with auditory neurons whose axons form the auditory nerve. The auditory nerve transmits sound messages through your medulla, pons, and thalamus to the auditory cortex of the temporal lobes. Crossing of most auditory nerve fibers occurs in the medulla and pons so that your auditory cortex receives input from both ears, but contralateral input dominates.
Figure 7.2 The ear.
How do you know where a sound is coming from? With ears on both sides of your head, you can locate a sound source. The process by which you determine the location of a sound is called sound localization. If your friend calls to you from your left side, your left ear hears a louder sound than your right ear. Using parallel processing, your brain processes both intensity differences and timing differences to determine where your friend is. The location of a sound source directly in front, behind, above, or below you is harder for you to pinpoint by hearing alone because both of your ears hear the sound simultaneously at the same intensity. You need to move your head to cause a slight offset in the sound message to your brain from each ear.
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 Békésy’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 that hearing intermediate-range pitches involves some combination of the place and frequency theories.
Why do hearing aids help only 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.