Neurophysiology for MedicineTutis Vilis Auditory Physiology http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/ Hearing P314 Our sense of hearing is often under appreciated. If you had to choose, which would you give up, hearing or vision? People who lose hearing feel very isolated from the world, and most importantly the company of people. Imagine what life would be like if at dinner you were unable to participate in conversation with your friends and family. What is sound? • Sound is produced when something vibrates, like the speaker in your stereo. When the speaker pushes on the air, it compresses it. • The vibrating speaker produces a series of pressure waves • The waves travel to the ear causing the ear drum to vibrate. • But notice that you did not hear the speaker vibrate, nor the sound waves move through the air. It was only when the sound waves moved your ear drum and activity reached your cortex that you perceived sound. How this occurs is the subject of this lecture. • A loud sound is produced when the speaker produces a large vibration. The large vibration produces large waves of compressed air. Soft sounds are produced by a small vibration. • A high frequency sound is produced when the speaker vibrates rapidly. This produces a closely spaced series of air pressure waves. Low frequency sounds are produced when the speaker vibrates slowly. • The normal range of frequencies audible to humans is 20 to 20,000 Hz (the number of vibration per second). A range of 200 to 2000 Hz is required to understand speech. pressure waves travelling at the speed of sound low frequency high frequency soft sound loud sound Reference to Neuroscience Purves et al 4th Edition 1 revised 2011 • If sound waves struck the oval window directly. 2) The vibration is conveyed to across the air-filled middle ear by bones called ossicles. • The ossicles then act like a lever system. neurons are activated. • These muscles are activated a) before you speak (a preprogrammed response ) or b) after a sustained loud noise such as a rock concert (a reflexive response). 3) Finally vibrations reach the fluid-filled inner ear where. they would mostly bounce off. • The ear drum picks up weak vibrations over a large area. air/ outer air/ middle ossicles fluid/ inner ear fluid oval window cochlea ear drum The function of the middle ear The ossicles transmit vibrations from the ear drum to a smaller drum called the oval window.3 1) Sound passes down the air-filled outer ear canal striking the ear drum. concentrating vibrations over the smaller area of the oval window 2. • Muscles in the middle ear are able to reduce the transmission efficiency of the ossicles in order to protect the inner ear from loud sound. The middle ear has two functions: 1. inside a coiled shaped cochlea. Gating. 2 . • The fluid in the cochlea is much harder to vibrate than air.The three parts of the ear F13. Impedance matching. Box13C 3 . the sound will be heard loudest by the good ear because even conduction through the skull cannot activate damaged cells. It would be heard loudest by the most affected ear (Weber Test). Conductive loss can be tested by touching the top of skull with a vibrating tuning fork.Conduction deafness Conduction deafness is any blockage of sound waves from reaching the hair cells including putting your finger into your ear. by conduction of sound through the skull bone: Beethoven had conduction deafness and used conduction through the skull by placing his head against the piano. In this case. This is probably because ambient sounds mask that of the tuning fork in the good ear. Sensorineural hearing loss Sensorineural hearing loss occurs when the hair cells or the nerve are affected. but poorly. Conduction deafness can often be treated with external hearing aids. One can hear without the ossicles. Sensorineural hearing can be treated with a cochlear implant. • When the oval window is pressed in.8 K+ K+ 4 . allowing K+ to enter the hair cell causing it to depolarize. the oval window could not move. • Neurotransmitter is released. • When the basilar membrane bends. oval window round window basilar membrane hair cell How is fluid motion transduced into neural firing? • Hair cells are located on the basilar membrane. 8th nerve F13. This problem is solved by the round window. • A filament between adjacent hairs opens ion channels. Without it. • This in turn produces a bulge in the round window into the middle ear. the hairs on the hair cells are also bent. But this membrane is surrounded by fluid which cannot be compressed. the initial portion of the pliable basilar membrane also bulges.What is the function of the round window? • The idea is to displace the basilar membrane. • Thus pressure waves are transmitted across the basilar membrane to the round window which acts as a pressure outlet. increasing the firing rate in the 8th nerve neurons. • The basilar membrane is narrow and stiff (like the high E string) near the oval window. each portion of the basilar membrane vibrates maximally for a particular frequency of sound.Different parts of the basilar membrane are sensitive to different frequencies. and wide and floppy (like the low E string) near the other end. Why? • A traveling wave sweeps down the basilar membrane starting near the round window and ending at the opposite end.5 basilar membrane 5 . • Because of this. the wave becomes larger and then smaller. • Low frequency sounds maximally displaced hair at the other end. • The figure below shows a “snapshot" of such a wave taken an instant apart. floppy stiff high frequencies low frequencies F13. • Notice that as it sweeps down the basilar membrane. • High frequency sounds maximally displace the hair cells near the oval window. Loud sounds break parts in the ear. kills cells. Inner ear infections can damage hair cells. Four Major Causes of Hearing Loss Box13A 1. not by its firing rate. Blockage in the blood supply. • This is like labeled lines in the sense of touch. Extremely loud sounds from explosions and gun fire can rupture the ear drum. • Most every day sounds are complex because they contain multiple frequencies. Toxins and some antibiotics can enter hair cells through the open channels and poison them. or tear the basilar membrane. Middle ear infections can fill it with fluid and in rare cases rupture the ear drum. Old Age. • The large vibration produces more activation of the hair cells. Infections.How does the basilar membrane code the frequency of a sound? • Thus sound frequency is topographically represented on the basilar membrane (place coding). high low frequencies which is active determines frequency how much determines loudness low Auditory Charts frequency high loss low frequency loss high frequency loss 6 . Toxic drugs. • Frequency is coded by which neuron is activated. 3. • Loud sound vibrates the basilar membrane more than soft sounds. The parts simply wear out with time. 2. Auditory charts help locate which part of basilar membrane is affected. each hair cell encoding the loudness of a particular frequency. break the ossicles. • The hair cells decompose this sound into its different frequencies. 4. Loud sounds common around the house (eg lawnmowers or loud music) can shear the hairs off the hair cells. • This is what makes one sound of one musical instrument different from that of another. • Thus loudness is encoded by the frequency of ap's in a particular afferent fiber. • Also. The auditory system performs two comparisons. team up with audition to localize the sound source. 1) Sound intensity differences. • The difference in time is very small. 7 . especially vision. such as when we think we hear a ventriloquist’s dummy speak. Because we have two ears. striking the ear facing away from its source. • The peak of a sound wave strikes the ear facing the source before it strikes the other ear.05 msec. about . other sensory modalities. • The sound. • Sometimes vision can mislead us. intensity difference shadow more intense timing difference 1st 2nd earlier • The shape of the ear lobe. its direction. the auditory system can compare the sound each receives. • This method works best for high frequencies. • Low frequency sounds wrap around the head and do not cast a shadow. • One can also localize sound by turning your head to where the sound is loudest.What are the cues to sound direction? Besides frequency and loudness the auditory system computes a third sound quality. besides amplifying sound. • Timing differences are best for low frequency sounds. helps in distinguishing sounds coming from in front vs those coming from behind (and perhaps above vs below). • Think of the head as casting a shadow in the airborne sound waves. will be muffled by the head. 2) Timing difference. Cells in the medial superior olive look for particular timing differences. Cells in the lateral superior olive look for differences in sound intensity.Describe the role of the superior olive in sound localization. primary auditory cortex (superior temporal gyrus) thalamus (medial geniculate) inferior colliculus F13. The superior olive is the first place where signals from the two ears come together and can be compared. bipolar cell cochlear n. midline hair cell right 8th nerve Auditory information is then sent 1) to the inferior colliculus which through the superior colliculus mediates orienting movements of the eye and head towards a sound 2) via the medial geniculate nucleus of the thalamus to the primary auditory cortex which is the first relay involved in the conscious perception of sound.12 superior olivary nucleus left cochlear n. 8 . 15 primary auditory cortex low f 9 . high f F13. • The basilar membrane is mapped topographically in a strip of columns (low frequencies at one end and high frequencies at the other).The columnar structure in primary auditory cortex • The primary auditory cortex has a columnar organization. pa) which are the elementary parts of words. Patients with lesions of Wernicke's area cannot understand language but can say words although these are often nonsense words. Move your tongue back a forth to change one into the other. These are called phonemes (e.Where are the higher auditory language regions? F27. 4) Broca’s area is responsible for language production Compare the language disorders (aphasias) caused by lesions in Broca's and Wernicke's areas. A2. And how does one ask babies whether they can differentiate between ba and pa? In 1974 Peter Eimas found that babies habituated to the repetition of one sound (ba. These filters act like magnets which 1) attract sounds that are slightly different to make them sound like familiar sounds 2) produce a clear boundary between different familiar sounds. Patricia Kuhl and her students took recorded phonemes and a similar pacifier around the world testing babies of different ages and backgrounds. Patricia Kuhl found that after the age of 6 months. A1. ba. Likewise someone raised in a English environment will not distinguish sounds indigenous only to Japan. pa etc). ba. 10 . Because of this filter. is activated by all sounds. 2) The secondary auditory area. on an electronically monitored pacifier. 3) Wernicke’s area is responsible for word comprehension. We are born citizens of the world. Newborns. Patients with lesions of Broca's area cannot say the right word or use the correct grammar but can understand language. Note that because of this filter you hear either and ‘r’ or an ‘l’ not some third sound. Make ‘r’ sound and then an ‘l’. initially all have the ability to distinguish a common set of phonemes.g. regardless of where they are born. etc) and that they started sucking much more rapidly. This is a true association area which is activated by hearing words. when the sound changed (pa. we start losing the ability to distinguish phonemes that are not part of our culture. is best activated by word-like sounds. the auditory system starts to filter familiar sounds. in Japan. adults have difficulty distinguishing between 'r' and 'l'.2 W P690 B 4) Broca’s area A1 A2 3) Wernicke’s area 1) A1 2) A2 1) The primary auditory area. reading or touching brail. For example. Experiment. Primary Primary visual Auditory Higher order auditory Wernicke's area Higher order visual areas detect complex features. In the PTO association area there is convergence of visual. Facial area of motor cortex P-T-O association Higher order visual Broca's area Wer nicheGeschwind model The primary visual cortex detects simple features such as the line elements of a letter. Wernicke's area is involved in verbal understanding and associating the object with the sound of a word. Perhaps this occurs when one is reading without understanding. Broca's area is involved in verbal expression. Note. Working memory is required to order words in a sentence. Lesion of Wernicke's Broca's Subject cannot understand language say the right word or grammar Subject can say words(often nonsense) understand language 9-11 . Here the left and right side of the word is put together and the object is recognized. auditory and tactile information.The probable sequence of activity that occurs when a person repeats a written word. Finally the facial area of motor cortex contracts the right muscles to produce the required sound. the production of the sound. When reading out loud. Use this model to compare the deficits caused by lesions in Broca's and Wernicke's areas. This is part of frontal working memory. Wernicke's area may be by passed and information sent directly from PTO to Broca's area.