Lecture 14: Hearing
Lecture 14: Hearing NSC 3361
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This 7 page Class Notes was uploaded by Rachael Couch on Thursday March 3, 2016. The Class Notes belongs to NSC 3361 at University of Texas at Dallas taught by Van S Miller in Summer 2015. Since its upload, it has received 33 views. For similar materials see Behavioral Neuroscience in Neuroscience at University of Texas at Dallas.
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Date Created: 03/03/16
Lecture 14: Hearing Basics of sound Sound is vibrating air that the brain can detect Amplitude (valley to peak) o = Intensity/loudness o Measured in decibels (dB) Frequency = pitch o Cycles per second o Measured in hertz (Hz) o Measured peak to peak o Increased wavelength = decreased frequency Pure tone tone of a single frequencyof vibration o Not interesting to the brain because they don’t occur in nature – electronic only Musical tone – modulated pure tones with repetition (rhythm); involves a pattern Noise – random sounds; unpatterned o Most sounds are just noise Fourier analysis Any complex sound is a collection of simple sine waves added together The brain breaks down the sound into individual sine waves Distinguishes whether something is just noise or not by the number of sine waves o “Cat” has about 10 patterns/waves The brain processes this as more than just noise – it has repetition o More waves (hundreds) = probably noise Different frequencies Cats hear higher frequencies than humans o Useful because mice (prey) emit high frequency sounds Elephants can hear very low pitch sounds o Elephants have large ears to hear lower frequencies o Useful because they can hear other elephants footsteps half a mile away Mothers can hear frequencies no one else can hear Human hearing Broad range (about 20 Hz – 20K Hz) Small range that is very good – can hear sounds at a low volume Conversation range is less than 2000 Hz o Hearing evolved first – speech was tweaked to be in the best hearing mode Ears Each part of the ear has a function Many steps involved because this makes the process less likely to break down Outer ear: The external ear and ear canal collect low frequency sound waves Middle ear: Up to tympanic membrane Inner ear: cochlea 1) Air molecules vibrate 2) Come in via external ear then through the ear canal 3) Vibrate the ear drum (= tympanic membrane) 4) Ear drum vibrates bones 5) Cause the oval window to vibrate o Oval window is in the “membrane” of the cochlea 6) Basilar membrane in the cochlea vibrates 7) This causes the stereocilia on hair cells to bend o Hair cells are stuck in the tectorial membrane so that when the basilar membrane moves, they bend 8) Stereocilia bending causes ion channels to open o Thin fibers called tip links connect the stereocilia on a hair cell to one another o When the stereocilia bend – the ion gates are pulled open by the tip links (mechanically opened) 9) The hair cell depolarizes (ions come in), and calcium influx at the base of the cell causes glutamate release Two muscles in the middle ear protect hearing o When activated too much, these muscles stiffen to dampen loud sounds o When you can’t hear temporarily after hearing loud sounds it’s because these muscle retract so that they don’t vibrate the oval window even when sound is coming in to the eardrums o Protection because loud sounds kill hair cells which cannot be repaired Auditory hair cells o 2 kinds Inner Involved in understanding speech Transducers Die over time with age Outer Hearing support – not as important Amplifiers o Adjust the volume (sensitivity to sound) by increasing the volume of the signal and the noise o Compensate inner hair cell loss by turning everything up – makes it difficult to distinguish sounds o Neurons run to the ear from the brain and back (feedback) Unsure what the neurons running from brain ear do o Hair cells do not have axons so therefore do not generate action potentials Cause graded potentials and EPSPs but not action potentials Don’t need an action potential because they don’t have an axon – don’t have to communicate over long distances Auditory pathway Auditory nerve cochlea cochlear nucleus superior olivary (SO) nuclei inferior colliculus medial geniculate nucleus (in the thalamus) auditory cortex o SO is very close in proximity to the cochlear nucleus o SO nuclei is the first place with bilateral input (receives input from both ears) o Most right ear sounds are processed by the left hemisphere but the process is bilateral Auditory cortex Processes sounds o 1 – Distinguish noise from patterned sound Looks for patterns/repetitions o 2 – Break down patterned sound into individual sine waves (fourier analysis) Auditory cortex cells each have a ‘preferred frequency’ (receptive field) o Cells specialize in what frequency they fire to o Some cells are less picky than others – larger receptive field Trying to pick up any sound Good at detecting whether sounds are changing Auditory neurons have tonotopic organization (arranged by tone) o Cells that detect high frequency are on the inside, in the back Well protected – most important o CQ: Low frequency sounds are processed by the anterior auditory cortex Anterior = front o Neurons next to each other are similar frequencies Two ways that we hear pitch (frequency) 1) Frequency coding o Encodes pitch by the firing rate of auditory neurons o 50 Hz sound causes an auditory cell to fire 50 times a sec (max) o Only works for lower pitches because the cell cannot fire 20,000 times/sec o Volley principle Each fiber only fires at a certain point in the cycle but does not respond to each cycle Each fiber fires in a different cycle, when all pooled together in the auditory cortex it’s a “complete transcript” combined is neurons firing at the same point in every cycle just like it did in the case of lower frequencies Ex: One neuron gets all the odd cycle; the other neuron gets the even cycles – combined output = complete 2) Place coding o The cochlear apex is shaped like a triangle o In the basilar membrane, all the “strings” are skinny at the close end and thick at the other end o Each place on the basilar membrane has a resonant frequency (based on the thickness of the string) o High pitch sounds vibrate the thin parts of the strings o The brain knows frequency by knowing which part of the basilar membrane vibrated o Not as accurate as frequency coding Place and volley mechanisms act together to code frequency o 1. Low frequencies use frequency coding o 2. High frequencies use place coding o 3. Intermediate frequencies (10005000 Hz) use a combination of volley & place mechanisms Detecting sound location Binaural cues detect sound location Compare the ears o Intensity differences – louder in closer ear o Latency differences – arrives at closer ear first o Processing includes both for accurate localization Superior olive (SO) is our main sound localization nucleus For high frequency sounds, location is coded by intensity differences in the SO o Uses lateral SO (lateral SO compares loudness) o High frequencies travel too fast – not enough difference in arrival times For low frequency sounds, location is coded by latency differences in the SO o Uses medial SO (medial SO compares meeting times) o Notes where the input from the left ear and right ear meet in the MSO o If the sound is from the far right side, the right ear is much farther along in processing – sounds end up meeting closer to left Right makes it farther because it gets a headstart o If the sound is right in the middle, they will meet in the middle of the MSO Analyzing sounds Auditory cortex analyzes complex sounds in two streams Dorsal stream is in the frontoparietal lobe o Where is it? Involved in sound location Ventral stream is in the temporal lobe o What is it? Analyzes components of sound Speech is analyzed much more than random noise o There is more activity in the brain when hearing speech than environmental noises Left hemisphere processes sounds that you do know Right hemisphere processes sounds you don’t know Trained shift of an auditory cell’s receptive field If a cell is normally tuned to 1K Hz, it can be recruited to a more important frequency o Plasticity the tonotopic map can be reorganized as needed Unconscious hearing Involves three systems: sleep, limbic system, and hypothalamus Separate pathway than conscious hearing Hearing loss There are 3 categories of deafness: central, conduction, and sensorineural Central deafness Rare; hardest to treat Caused by problems (lesions, injuries) in the brain Associated with neurologic disorders (multiple sclerosis, tumors) Involves CNS (cortex, brainstem, or ascending auditory pathways) 2 types: o Cortical deafness Pure word deafness Can speak normally Can’t understand speech Can identify nonverbal sounds o San hear a car starting and know it’s a car Auditory agnosia Can only hear pure tones Inability to recognize verbal or nonverbal sounds o Auditory hallucinations Hearing things (music or speech) that isn’t there Common in schizophrenia; injury to secondary auditory cortex; or during a temporal lobe seizure Auditory hallucinations are more common than visual hallucinations in schizophrenia Usually auditory hallucinations are negative towards themselves o Hearing danger/warnings or negativity (insults) Damage to brainstem structures such as the superior olive can sometimes cause auditory hallucination Conduction deafness Disorders of outer or middle ear (pinna up to but not including cochlea) that prevent sounds from reaching the cochlea Causes o Otitis Media – swelling of the eardrum o TM Perforation torn tympanic membrane o Ossicular arthritis affects the small bones in the ear they have trouble moving/conducting properly Sensorineural deafness = SNHL (sensorineural hearing loss) Problem with cochlea or auditory nerve Dysfunction of the hair cells or auditory nerve Characteristics o Inappropriately loud voice They can’t hear themselves so they don’t know they’re raising their voice o High frequency loss common o Speech sounds distorted Because speech tends to be at higher frequencies o Background noise makes listening more difficult Can be caused by viral infections (measles and CMV) that kill developing auditory hair cells NIHL (noise induced hearing loss) Loss can be sudden (explosion) or gradual (working with power tools) that goes unnoticed as there is accumulated damage over time The #1 preventable cause of deafness Presbycusis Gradual onset hearing loss Starts at age 30 As age increases, higher frequencies have to be louder to be able to hear them Tinnitus 15% of the population Can be moderate and people live with it or can be so severe that people become suicidal Although damage to the cochlea causes hearing loss and often initiates tinnitus, CNS maintains chronic tinnitus Not much is known and is difficult to treat Brain problem, not a hearing problem o Brain makes a maladaptive memory and replays it repeatedly o = an auditory hallucination Example: o Outer hair cells ‘turn up the volume’ via efferent connections in response to loss of hearing (from death of inner hair cells) o Auditory cortex, inferior colliculus, cochlear nucleus all contribute Case Studies Kulesha Infant girl born underweight and with a left eye cataract Newborn otoacoustic emission test found severe bilateral hearing loss (deaf) Also born with microcephaly Development since has been delayed Mom was carrying a virus that was transmitted to the womb and affected the babies developing hair cells Auditory hallucinations 25 year old man admitted for psychiatric evaluation After breakup, had auditory hallucinations consisted in hearing voices of people who were not present, or who had died They were short sentences of insults directed to him: ‘Get lost’, ‘You’re mad’, ‘You’re an idiot’, ‘Drop dead’
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