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Physiology 4710A/B

Lecture 7 – Touch How touch is electrically encoded  As in vision, we can use touch to distinguish edges, feel textures, read letters and recognize objects as complex as faces  We do this with very few receptor types – as in vision  There are five receptors sensitive to touch o There are also receptors that sense pain and temperature Transforming mechanical energy into electrical activity - Pacinian  1. Mechanical stimulus (eg. Pressure) deforms the receptor’s onion like membrane  2. Channels open and Na+ flows through membrane o inside of the receptor depolarizes (voltage becomes more +)  3. If the graded potential summed at the initial segment is above threshold, AP is generated and propagated down the axon o Most touch afferents have myelinated axons in which AP hop from gap to gap, thus speeding up conduction How is the magnitude of the stimulus encoded?  Stimulus magnitude is encoded in part by a frequency code o Greater the pressure, the more the receptor depolarizes o When above threshold, more APs/second are generated  The relationship between number of AP/second and pressure is nonlinear o It saturates at high pressures The response to the stimulus adapts  The # of Aps/sec adapts because the receptor potential adapts  Receptor potential adapts in part because the onion-like laminae slip back, closing the channels o Layers slip back to normal position so the pore closes and potential goes to normal  The adaptation enhances the detection of changes in pressure o Constant pressure, such as that exerted by your clothes is not important Other Receptors for touch  In any one part of your skin you will find 4 receptors: 1. Hair receptors (back of hand) or Meissener (palm of hand) 1 2. Merkel 3. Ruffini 4. Pacinian Which receptors are rapidly adapting (RA) or slowly adapting (SA)? 1. Hair receptors or meissner (RA1 surface) but not both 2. Merkel (SA1 surface) 3. Ruffini (SA2 deep) 4. Pacinian (RA2 deep) receptors  Thus both surface and deep layers of the skin contain both RA and SA receptors  Label 1 = receptors at the epidermis near the skin surface  Label 2 = receptors deeper in the dermis Which receptor has the largest receptive field size?  RF size increases with depth in the skin  Pacinian corpuscles have the largest receptive field  We need a variety of touch receptors to code a large variety of touch stimuli  Without 5 different types of touch receptors touch would be like being color blind Pain and temperature receptors  There are 2 types of free nerve endings that are sensitive to pain stimuli 1. Fast conducting myelinated fiber that signals an early, localized, intense pain a. Also mediates sensation of itching 2. Slow conducting unmyelinated fiber that signals a later, poorly localized, long lasting, dull pain  Also two types of free nerve endings sensitive to temperature stimuli 1. Fast conducting myelinated fiber that fires most for hot but not burning stimuli 2. A fast conducting myelinated fiber that fires for cold but not freezing stimuli  Burning or freezing stimuli activate pain receptors  Touch afferent fibers have large diameters  Pressure first blocks the conduction of AP in large fibers o Your limb “falls asleep”  But the sense of temperature and pain, mediated by small diameter fibers is often preserved  The Gate control theory – Patrick Wall and Ronald Melzack o Suggests that pain sensation is dependent on the balance between input from large nerve fibers (touch) and that from small nerve fibers (pain) o If there is more large than small, there should be little or no pain o If there is more small than large, then one will sense pain o By activated large fibers through rubbing, one can alleviate pain Labeled lines 2  For the brain to recognize that a stimulus is a vibration that is coming from the surface of the skin, the brain must label the afferent type that has been activated as an RA1 afferent  Each type of touch sensor has its own private line – a labeled line  Labeled line = a label attached to each afferent fiber as to what sub-modality it is o Because of this, there is no reason for encoding and decoding each packed of info o Cortex learns from experience what travels along the fiber o Similar coding in the auditory system  We sense the frequency of a sound from where the fiber is originating on the basilar membrane  Information here is encoded in two ways: 1. The firing frequency of a particular neuron 2. Which neuron this is Pathway to the primary sensory cortex  Pathway for transmission of touch is the dorsal column medial lemniscal system  Path for labeled line to the cortex  In the spinal cord the dorsal column is the first stage in the development of somatotopic organization o In the lower segments, only afferents from the leg are found o As one moves up the spinal cord, new afferents enter laterally o Thus in high segments of the spinal cord from medial to lateral: leg, trunk, arm  The signal crosses over to the other side of the brain and synapses the ventral posterior nucleus of the thalamus 3 o From there the signal goes to the arm area of the primary somatosensory cortex (area S1)  Pathway for transmission of pain and temperature info to the primary sensory cortex is the anterolateral system o First makes a synapse in the spinal cord then crosses at the same level of the cord to the opposite send o Ends in the same region of cortex as touch The three functions of the dorsal column nuclei (DCN)  There is no need of relay nucleus because AP do not need to be boosted by a synapse  A synapse on its own simply adds an unnecessary delay  Neurons from synapses in a nucleus to transform or change the incoming signal  There are three distinct transformations in the DCN 1. Convergence a. The skin on your back has a low afferent density i. Many afferents converge onto a single DCN neuron ii. Because of these two factors only a few DCN neurons are required to represent a given area of skin on the back iii. Therefore there is a large RF and a low tactile discrimination (like the peripheral retina) b. The skin on your fingertips has a high afferent density i. Few afferents converge onto a single DCN neuron ii. Therefore many DCN neurons are required to represent a given area of skin iii. Therefore there is small RF and a high tactile discrimination (like the fovea) iv. This is why you can use your finger trip to read braille c. Because of these differences in convergence the representation of the body in the DCN begins to become distorted 2. Inhibitory surround a. As with retinal ganglion cells, a stimulus in the center will activate a DCN neuron while in the surround it inhibits the same DCN neuron b. This first occurs in the DCN, not the skin c. In contrast to the skin, the retina, being part of the brain, can be organized into a complex neural network while the skin cannot d. Function of this is the same as in vision: it accentuates the activity associated with the edge of an object and enhances two-point discrimination 4 3. Cortical Gating a. Why can’t you tickle yourself? b. When you try to tickle yourself or make any movement, a copy of the command (corollary discharge) inhibits the touch signals ascending through the DCN c. The purpose of this inhibition is to block some of the touch signals from the skin arising from the movement itself d. This inhibition is presynaptic i. Because post synaptic inhibition directly on the DCN would turn off all three afferents, this can be directed to just one Features of somatosensory cortex 1. Somatotopic organization a. Primary somatosensory cortex, S1, is somatotopically organized wit the body surface laid down sequentially on the postcentral gyrus b. This body map is distorted with the lips, tongue and fingertips having a large representation – this reflects that of the DCN c. The skin of the back has a small representation because of the high convergence and large RF in DCN neurons  Phantom limbs o Patient X has an arm amputated up the shoulder o About a year later he complains of a phantom sensation of his hand coming from his cheek o The face area is adjacent to the arm area in the somatosensory cortex, because the arm area no longer receives input, it is gradually taken over by the face area o It sometimes surrounds the arm area, temporarily leaving an island o Demonstrates that the somatotopic organization of this area retains a great deal of plasticity even in adulthood 2. Multiple Maps a. S1 is subdivided into 4 parallel strips: areas 3a, 3b, 1 and 2 b. Thus the homunculus is repeated 4 times c. Areas 3b, 1 and 2 code touch d. Area 3a codes the signals from muscles and joints, afferents that signal limb position and movement e. Note that the hand representation is mirrored in areas 3b and 1 as is the rest of the body i. Similar to the retinotopic representation in V1 and V2 f. Each area extracts different features 5 i. Area 1 – receives input from RA afferents from the skin surface, important in texture recognition ii. Area 2 – receives input from SA afferents deep in the skin, estimates joint position (important in recognizing the size and shape of objects) g. Moving to areas 1 and 2, the cell’s RF become more complex: i. Area 3b have small circular surround RF ii. Area 1 cells have large RF that can encompass more than one finger and are orientation and movement direction selective h. Somatosensory info is then sent to area 5 in the parietal association cortex where stereognosis takes place: the 3D identification of an object through tough i. Eg. When you reach into a bag and identify objects by touching them 3. Columns a. In area 3b there are modality specific columns b. Each column receives input from one afferent type c. Separation of afferent types into columns is what produces labeled lines Taste – the five basic tastes  Taste and smell (chemical senses) are important in distinguishing between foods that are nutritious and those that are harmful  The tongue is sensitive to touch, temperature and pain  In addition the tongue performs the chemical analysis of substances dissolved in saliva  Five basic tastes: bitter, sour, salty, sweet and umami  Each taste can be sensed everywhere on the tongue but different areas have different preferences o Middle = relatively few taste cells o As we sip wine – it first activates sweetness on the front of the tongue, then sour in the middle and bitter at the back o Thus the tongue sends a spatial temporal pattern to the brain that allows us to differentiate between a variety of wines or juices  Sourness (H+) and saltiness (Na+) act on cell ion channels directly  Bitterness, sweet and umami taste are amplified by specific GPCRs which activate second messenger cascades to depolarize the cell as in the retina’s receptors  Umami are activated by monosodium glutamate and other proteins  Each receptor site is specific for a particular molecule like a lock  Goal of artificial sweetener is to make a molecule that looks like sugar but has no nutritional value The taste bud  On the tongue there are taste buds, a cluster of about 100 taste cells  Each taste cell is most sensitive to one of the 5 tastes  Because the tongue is exposed to hazards these cells are constantly being replaced 6 o Over a 2 week life span, basal cells become supporting cells which become taste cells  Need innervation to survive, if the afferent fibers are damaged, taste cells degenerate  Axonal transport along fibres provides some important trophic factor The taste pathway  Taste afferents project via cranial nerves 7,9,10 to the nucleus of the solitary tract  From there the signal goes to the ventral posterior medial nucleus of the thalamus then to areas of the cortex, including the hypothalamus, which regulates hunger and the insular taste cortex which has a rough topographic representation of different tastes  The taste signal is projected to the cortex using a labeled line system similar to touch Inborn hunger  We have an inborn ability to compensate for diet deficiencies by selecting foods that will compensate  We crave salts because they are essential to maintaining our electrolyte balance  Activation of umami and sweet afferents produces pleasurable sensation which promotes eating proteins and sugars Generic Taste Deficiencies  Taste deficiencies can be genetic  Genes code development of particular receptor sites  Some individuals cannot detect different forms of bitterness because of the absence of a particular receptor on the tongue o Cannot detect a form of bitterness found in cabbage Learnt Taste Aversion  Animals have an innate aversion to bitter or sour substances because this signifies food that is spoiled or poisoned  If a rat is given a food with a tasteless poison that results in nausea, the rat will be conditioned to avoid that food  If you go to a restaurant and get food poisoning you may develop an aversion to that particular food o This can last a lifetime o Not the same as classical conditioning because you would not become aversive to the music playing or the people you were with Smell  Oldest of all senses  Combination of smell and taste give food their flavor  Becomes less sensitive with age, this can lead to a loss of appetite and sometimes weight loss  Most odors are composed of a complex mixture of odorants 7 o Each odorant has a distinctive molecular shape o These molecules enter the roof of the nasal cavity, dissolve in the moist mucosal protective layer and are recognized by receptors in the dendrites of olfactory cells o These cells project directly through the skull to mitral cells of the olfactory bulb, part of the cortex o Mitral cells project to the pyriform cortex which codes mixtures of odorants present in a particular smell o Sends info to the amygdala and hippocampus through the medial dorsal thalamus to the orbital frontal cortex  After a sudden head impact, these olfactory afferents can be sheared off, but regrow in a bout a month  Olfactory cell axons are unmyelianted because the distance to the bulb is short so speed isn’t important o It takes time for an odor molecule to diffuse across the mucus film and attach to a receptor  When we sniff, different odor molecules diffuse across the mucus film at different rates o Because of this, the response at any particular time provides little info as to the odor  Remember the temporal pattern of the whole sniff  For this reason memory is an important element in recognizing odors  Odors can elicit strong memories  The amygdala is activated by the pleasant or unpleasant aspects of odors and the hippocampus facilitates the storage of these memories  The orbital frontal cortex combines our sensation to odors with those of taste, texture (somatosensory), spiciness (pain) and vision which results in the perception of flavor  Cells have multimodal input and can respond for example to the smell, sight or taste of a banana  Patients with lesions of the orbitofrontal cortex are unable to discriminate odors Are there basic smell qualities?  No – the sense of smell does not have a small number of basic receptor types  Instead humans have over 300 receptor types, other specie have more  Receptors are randomly distributed in the nasal cavity  Each odor molecule is composed of odorant molecules  Each odorant molecule can activate several receptors because its molecular shape fits more than one receptor  To identify a particular odor, the cortex examines the pattern of afferents activated  A molecular cascade amplifies the taste receptor’s sensitivity (as in the retina’s receptors)  Genetic defects such as those that produce color blindness can also result in the absence of particular subtypes of olfactory cells that can produce anosmia for specific odors The mapping of smell in the olfactory bulb  Each mitral cell receives input from many olfactory cells that expresses the same subtype  As in the receptors, each odor activates a particular combination of mitral cells 8  A given mitral cell is activated by more than one odor  Similar odors stimulate adjacent mitral cells  The nasal cavity is not mapped somatotopically on the olfactory bulb, rather I is arranged in a topographical map of smells  When olfactory cells are damaged, they are replaced within about a month from basal cells which regrow to the same region that was most sensitive to that particular odor o Thus the same representation of odors in the bulb is maintained as their memory Summary  Both olfactory cells and taste cells in the taste bud are constantly replaced  Both taste and smell project to the newer cerebral cortex, the neocortex for perception and to the older cortex in the limbic system for automatic responses of hunger, pleasure etc.  Smell is the only sensory system which projects from the periphery directly to the cortex (hippocampus and amygdala) Lecture 8 – Muscle Sense Receptors signal the position and movement of your limbs  Joint afferents are nerve fibers located in the joints which are most sensitive to positions at extreme joint angles  Muscle spindles are located in the muscle and are very important for sensing position and movement (velocity)  Golgi tendon organs are located in the muscle tendon and detect tension  Tactile receptors in the overlying skin (recall that the slowly adapting Ruffini afferents are important for sensing the position of your fingers) What else might contribute to one’s sense of position?  When you command your hand to move, you estimate its position by internal sense of effort  The signal is derived from corollary discharge Note 1: all receptors contribute proprioception  Different receptor types are better at providing different info o Eg. Spindles – position and velocity; GTO – force 9  While all receptors can provide some info on position, the brain relies more heavily on particular receptors in different body parts Note 2: If one receptor is lost  If one is lost, the others can provide missing info  Ex. o Patients with artificial joints lack joint receptors but their position sense is almost normal o Position sense in the hand is reasonable after the skin is anaesthetized because of position signal from muscle spindle o If one has no afferent input at all, one has a sense of position from motor commands one sends out (corollary discharge)  Eg. If one can still drive a car that one is used too Anatomy of muscle spindles and GTO  Muscle spindles are located in parallel with the regular muscle fibers o They undergo the same length changes as the rest of the muscle  GTO (1b) are located in the tendon of the muscle o In series with muscle fiber o They sense the force the muscle exerts  Within the spindles are two types of fibers: nuclear bag and nuclear chain o Large, primary afferents (1a) originate from both bag and chain fibers o Smaller secondary afferents (2) originate only from chain fibers  Numbers 1a, 1b, 2 refer to the size of the fiber o 1a = largest; most rapidly conducting o 1b (GTO) = slower o 2 = slowest  Afferents from the bag fibers signal velocity o Like the Pacinian corpuscles, they adapt quickly when stretched o Give a phasic response during stretch o Adapt quickly to a constant position  Nuclear chain fibers signal position and contribute both to the 1a and to the 2 afferents Comparing passive muscle stretch to active muscle contractions  Passive stretch (someone else stretches your muscle) 10 o 1a afferents come from both bag and chain fibers and therefore sensitive to velocity, rate of change in length (phasic response) and to position (tonic response) o Also sensitive to vibration o 2 afferents come from chain fibers so they’re sensitive to position o 1b afferents activity changes a little, primarily because the force acting on the tendon during passive stretch is small  active contraction (You contract your muscle) o Contraction causes the tendon to stretch and the muscles to shorten o Spindles become silent o 1b afferents are very sensitive to active contraction because the tendon is stretched Gamma drive controls sensitivity of muscle spindle  Arises from gamma neurons in the spinal cord causes the contraction of the ends of bag and chain fibers  This stretches the central region where the afferents are located, increasing their sensitivity  Gamma static drive contracts chain fibers  Gamma dynamic drive activates bag fibers During passive stretch  Each row shows the activity with: o No gamma drive o Gamma static drive o Gamma dynamic activity  Left shows the activity of 1a afferent o When gamma static is activated the 1as position sensitivity is increased (from chain fiber) o When gamma dynamic fiber is activated the 1as velocity sensitivity is increased (from bag fiber)  Right shows activity of 2 afferent fiber o Gamma static activated = 2 position activity is increased (from chain fiber) 11 o Gamma dynamic is activated = no change in 2 afferent because gamma static only contracts chain During active contraction  Activation of alpha motoneurons but not gamma motoneurons, the muscle shortens and silences the spindle  Activation of alpha and gamma = fibers ends in spindles contract and maintain spindle sensitivity  Called alpha gamma co-activation Three spinal reflexes  Monosynaptic stretch reflex activated by 1a afferents o When muscle lengthens, the spindle is stretched = 1a activity increases o Increases alpha motoneuron activity o Muscle contracts, length decreases o This reflex regulates length, tries to maintain constant length o Monosynaptic and carried by large fibers so it has shortest latency of all reflexes  Reflex mediated by GTO o Too much force generated by the muscle = GTO activated o Through an inhibitory interneuron in the spinal cord this activation reduces firing rate of motoneurons o Decreases muscle contraction and muscle force o Regulates force ie. Maintains constant force (used when maintaining a constant grip on a cup)  Reflex mediated by pain and cutaneous receptors o Produces 2 responses:  Contraction of flexors on same side, results in withdraw from painful stimulus  Contraction of extensors on opposite side, allows one to maintain posture and balance Tremor  Why not keep gamma activity high all the time?  Too much is a bad thing, it can cause tremor  Normally the 1a reflex stops the return movement exactly in the correct position o When a limb position changes the 1a reflex is activated o Stretch of the agonist causes a reflex activation of the agonist o Initiates return movement o Return movement stretches antagonist spindle = reflex activation of antagonist muscle; if the response is the right size, it will correctly stop the return movement 12  If spindle sensitivity is too high, stretch reflex in the agonist will be too large, causing a fast return movement that overshoots the target o This fast return and high spindle sensitivity will produce a large reflex response in the antagonist o Will reverse the movement rather than stop it o Process is repeated, causing tremor Cortical response in stretch reflex  Muscle stretch activates two EMG responses: o Early spinal monosynaptic stretch reflex o Later cortical long loop response  Through motor cortex, under voluntary control  Responses are set or context dependent and controlled by the cerebellum o Adds learnt motor skills to our responses Experiment involving muscle vibration  Muscle spindle afferents contribute to a conscious sensation of muscle length  Vibrate the tendon of the biceps, vibrates the whole muscle and activates 1a afferents  Subject with their eyes closed indicates felt limb position with the other arm  Perceived position is longer than actual position of vibrated arm  Because vibration activates 1a afferents  This additional activity is interpreted as longer length How is actual muscle length sensed  Contracted arm has a lot of alpha gamma co-activation, other has little  Presumably in the contracted arm spindles will be more active  You can sense correctly when both are at the same length  Can do so because of corollary discharge  Helmholtz proposed that motor commands go both to muscles and to areas of brain that sense position  Discharge modifies how the afferent signal is interpreted  Relaxed muscle: o Low alpha with low gamma = low spindle activity  Maintains same position while actively contracting the muscle = high alpha and high gamma = high spindle activity  Brain compares command to the arm to the afferent spindle feedback  Sensed position is the same because during active contraction, high spindle activity is cancelled by high corollary discharge Coordinate transformation  Afferents discussed project to area 3a of somatosensory cortex on bottom of central sulcus  Spindles in the biceps muscle code length of that muscle, length is proportional to elbow angle 13  In a similar way the angle of the finger relative to the forearm is coded and the angles of all body parts relative to each other  Sense of where the finger is relative to your body is thought to be computed in association areas within dorsal stream along intra-parietal areas (IPS)  The relative proprioceptive info passed from area 3a, could be combined into an egocentric finger position  Position is represented in a nested reference frame Lecture 9 – hearing The importance of hearing  People who lose hearing feel isolated from the world  Children hard of hearing are often misdiagnosed as cognitively impaired What are sound waves  Sound is produced when something vibrates  Speaker pushes on the air and compresses it  The vibrating speaker produces a series of pressure waves  Waves travel to the ear causing the ear drum to vibrate  Only when the sound waves move your ear drum and activity reaches your cortex that you perceive sound  Amplitude is perceived as loudness  Normal range of frequencies audible to humans is 20 to 20,000 Hz  We are most sensitive to frequencies between 2000 to 4000 Hz How does the sound wave reach the inner ear?  Sound waves passes through your outer ear down the air filled ear canal  Sound vibrates the ear drum and vibration is conveyed across the air filled middle ear by the ossicles o The ossicles pass vibrations to the oval window, a membrane which seals the opening to the cochlea  Vibrations reach the fluid filled inner ear, inside the cochlea has neurons which are activated Why such a complicated chain of transmission?  Middle ear has two functions: 14 a. Impedance matching – fluid in the cochlea is much harder to vibrate than air. If sound waves in air struck the oval window directly, they would bounce off. The ear drum picks up weak vibrations over a large area, ossicles act like a lever system, concentrate these movements to forceful vibrations over the smaller area of the oval window, these displace the oval window against cochlear fluid b. Gating – muscles in the middle ear are able to reduce the transmission efficiency of the ossicles to protect the inner ear from loud noises. These muscles are activated before you speak (preprogrammed response) or after a sustained loud noise such as a rock concert (reflexive response) What is the function of the round window?  Final goal of the system is to displace the basilar membrane  This membrane stretches the length of the cochlea and is embedded with hair cells whose motion is converted to electrical activity  This membrane is surrounded by fluid which can’t be compresses o So when the oval window is pressed in some portion of the basilar membrane also bulges, this causes a bulge in the round window  So pressure waves are transmitted across the basilar membrane to the round window which acts as a pressure outlet How does deformation of the basilar membrane activate auditory afferents?  Hair cells are located on the basilar membrane  When the BM bends, the hairs are also bent  A filament between adjacent hairs opens ion channels allowing K+ to enter the hair cells causing it to depolarize  This mechanical opening of the channel is very fast – precision that is useful for sensing high frequencies or locating the source of sound th  NT is released, increasing firing rate of 8 nerve neurons Describing the traveling wave theory  Von Bekesy made the direct observation that a travelling wave sweeps down the BM starting near the oval window  You can mimic this by taking a heavy cord and attaching one end to something at the end of the room and then flick one end  A wave travels the length of the cord  The cord has the same properties throughout so the size of the wave remains fairly constant  BM properties change How is the frequency of a sound coded?  Helmholtz noted that the BM is narrow and stiff near the oval window and wide and floppy near the other end  So each portion of the BM vibrates maximally for a particular frequency of sound o High f maximally displaces hair cells near the oval 15 window o Low f maximally displaces hair cells at the other end  Thus sound frequency is topographically represented on the BM – place coding  Frequency is coded by which about 16,000 hair cells are activated, not by it’s firing rate  This is like labeled lines in the sense of touch How is loudness coded  Loud sounds produce a larger amplitude vibration of the BM than soft sounds  The large vibration produces more displacement of the hair cells and a larger change in potential inside these cells  Thus loudness is encoded by the frequency of AP that travel down a particular afferent fiber  Most every day sounds are complex, they contain many frequencies  The hair cells decompose these sounds into different frequencies like a synthesizer  Each hair cell encoding the loudness of a particular frequency Four Major Causes of Hearing Loss 1. Loud sounds break parts in the ear a. Extremely loud sounds from explosions and gunfire can rupture the ear drum, break the ossicles or tear the BM b. Hair cells are fragile and once damaged do not recover c. Loud sounds can shear the hairs off the hair cells or break the filaments that open the ion channels 2. Infections a. Middle ear infections in rare cases rupture the ear drum b. Inner ear infections can damage the hair cells c. The Middle ear fills with fluid 3. Toxic drugs a. Toxins and some antibiotics can enter hair cells through the open channels and poison them b. The hair cells are not replaced and must last a life time 4. Old age a. The parts simply wear out with time b. Blockage in the blood supply kills cells What are the cues to sound direction  The auditory system also computes the direction from which sound originates  There are two methods for detecting this direction: a. Sound intensity differences – the sound striking the ear facing away from the source will be muffled by the head i. This works best for high frequencies over 3000 Hz ii. Low frequency sounds wrap around the head b. Timing difference – The peak of the sound wave strikes the ear facing the source before it strikes the other ear i. is least for sounds coming in from the front and most when coming from one side 16 ii. Used to localize low frequency sounds  Humans can resolve direction with an accuracy of about 1 deg, this is timing difference of 1/100,000 sec  The shape of the earlobe amplifies sound and helps distinguish sounds coming from in front vs behind (perhaps above and below). The earlobe acts like a directional antenna  Can also localize sound by turning ones head to where the sound is loudest  Other sensory modalities, especially vision work with audition to localize the sound source  Sometimes vision can mislead us The role of the superior olive in sound localization  The superior olive in the brainstem, is the first place where signals from two ears come together to be compared  Cells in the lateral superior olive encode differences in intensity  Cells in the medial encode particular timing differences The central auditory pathway  Auditory information is then sent: o To the inferior colliculus which encodes the location of a sound and signals the SP to generate orientating movements of the eye and head towards a sound o To the MGN of the thalamus and then to the primary auditory cortex which is the first relay involved in the conscious perception of sound The primary auditory cortex has columnar organization  The BM is mapped topographically in a strip of columns (low to high frequency)  The columns have tonotopic representation What happens beyond the primary auditory cortex?  Sounds processed first by the primary auditory cortex (A1) are further processed by higher order areas (A2)  A2 is best activated by word-like sounds  There are building blocks of words, phonemes  A1 is the core are and A2 is the belt  Together these two areas have been found to contain mirror image tonotopic representations similar to visual areas V1, V2 and V3  Then the sounds that are words are processed by Wernicke’s area (W) o Responsible for word comprehension  Newborns initially have the ability to distinguish a common set of phonemes  Peter Eimas found that babies habituated to the repetition of one sound and that they started sucking much more rapidly when the sound changed o She found that after 6 months, the auditory system starts to filter phonemes o These filters act as magnets which: 17  Attract phonemes that are slightly different to make them sound like familiar phonemes  Produce a clear boundary between different familiar phonemes The sequence of activity when reading out loud  Werniche-Geshwind Model  Primary and higher order visual cortex detects simple features such as the line elements of a letter  In the Visual Word Form Area (VWFA) the left and right side of the word is put together  The VWFA is located posterior to where one would expect to find the FFA but on the left side  A lesion in the FFA results on proposagnosia, similarly a lesion in the VWFA produces dyslexia, a reading disability  In the PTO association area there is convergence of visual, auditory and tactile info = object is recognized o The object apple can be recognized by the written word or picture of the apple, sound of the spoken word, feel of an apple through touch and by taste  Wernicke’s area is involved in verbal understanding and associating the objects wit the sound of a word o This is a true association area which is activated by hearing words, reading words or touching braille  Recall that biological motion produced by lip movements is analyzed in the STS o This can transform what we hear o For example hearing ba while seeing the lips form ga leads to the perception of ga o Visual predominates  Broca’s area is responsible for language production o Involved in the grammatical rules and verbal expression o This is part of frontal working memory, used to order words in a sentence o Used for writing language, it activates the arm and hand area of the motor cortex  The facial area of the motor cortex contracts the right muscles to produce the required sound Lesion Patient cannot Patient can Wernicke’s aphasiaunderstand language say words(often nonsense) Broca's aphasia say the right word or grammarunderstand language  The deaf use ASL to communicate o Lesions in Broca’s area produce deficits in expression with hand gestures o Lesions in Wernicke’s produces a deficit in the comprehension of these gestures o Thus these areas are not limited to the understanding or production of language through sounds Auditory what and where streams  Auditory what stream flows into the anterior temporal lobe and then to the prefrontal cortex o Used to identify the sound (is it grandma’s voice?)  Auditory where stream flows in the posterior direction through Wernicke’s area to the parietal lobe then to the prefrontal cortex o Used to identify the sound’s location and its temporal properties 18  Important for speech perception o Infants begin babbling at 6 months, this is critical for adjusting both production and perception of language Lecture 10 – Balance The sense of balance originates in the labyrinth  The bony labyrinth is a convoluted system of tunnels in the skull that contains the sensory for the auditory and vestibular systems  The vestibular system is responsible for one’s sense of balance  Inside of these tunnels is lined with a membrane  Perilymph (similar to extracellular fluid) is found between the bone and the membrane  Endolymph (similar to intracellular fluid, high in K and low in Na) fills the inside of the membrane and surrounds the hair cell receptors The auditory and vestibular systems that have a common origin  The common origin is the lateral line organ in early aquatic animals  Organ consists of a system of tubes lined with sensory cells with hair like projections that are in communication with the surrounding water  Sensory cells are activated by fluid movement in the tubes caused by: o Waves produced by some external disturbance in the water (precursor of the auditory system) o The fish’s own motion (precursor of th
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