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92408 Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose then the blind man made to see Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cube John Locke 1690 ssay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose then the blind man made to see Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cube John Locke 1690 ssay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish Query whether by his sight before he touche them he could distinguish and tell which is the globe which is the cube John Locke 1690 Essay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose then the blind man made to see Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cube John Locke 1690 Essay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cubequot John Locke 1690 Essay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose then the blind man made to see Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cube John Locke 1690 Essay Concerning Human Understanding Molyneux s question quotSuppose a man born blind and now adult and taught by his touch to distinguish between a cube and a sphere Suppose then the blind man made to see Query whether by his sight before he touched them he could distinguish and tell which is the globe which is the cubequot John Locke 1690 Essay Concerning Human Understanding 4 Blinded by a chemical accident at age 3 Light sensitive between a 3 stem cell Yth 2000 Poor spatial acuity ERG responses ok a 2 E q 7 15 o C a 1 o 8 0 MM 5 months 39 o 5 CONTROL 0 0 O 0 1 2 3 Spatial frequency cdeg i I I Ii llllllllli 92408 SB Gregory To what extent does visual processing rely on visual experience Sacks 1991 Hyvarinen et al 1978 1981 Ackroyd et al 1974 Valvo 1971 Gregory amp Wallace 1963 Cheselden 1768 vfa39 15 a quot Wallace 1963 No recovery over time 2 3 9 0 MM 5 months 15 0 MM 11 months 1 to E 1 0 MM 17 months 0 3 MM 21 months 3 Qquot CONTROL 0 O 1 2 3 Spatial frequency cdeg l l l i ltlllltlll HS Valvo I had no appreciation of depth or distance street lights were luminous stains stuck to window panes and the corridors of the hospital were black holes 39 Depth Wrgil Sacks surfaces of objects would seem to loom when they were still quite a distance away sometimes he would get confused by his own shadow Steps posed a particular hazard All he could see was a I confusion a flat surface of 7 V parallel and crisscrossing lines 92408 Depth occlusion ok hadin weak MM96 BUT long RTs C100100100 transparency amp perspective poor 9 MM0 MMNR MM0 bias C100 100 100 C100 100 100 C26 31 29 MM100 C100100100 92408 lime 9 Objects amp Faces Cheselden 1720 Having often forgot which was the cat and which the dog he was ashamed to ask but catching the cat which he knew from feeling he was observed to look at her steadfastly and then have said So puss I shall know you another time Virgil Sacks He did not really look at our faces though he smiled and laughed and listened intently He reminded me of the man who mistook his wife for a hat Poor object amp face identification MM 25 correct control 100 Gender MM 70 correct control 100 n Expression happysadlneutral c control 100 92408 M otlon SB Ackroyd et al His only signs of appreciation were to moving objects M otlon fl ne particularly the pigeons in Trafalgar square He clearly enjoyed watching the movement of other cars on the road He spotted a speeder coming up very fast behind MM 100 MM1OO US I C100 100 100 C100 100 100 Virgil Sacks when the gorilla nally came into the open he thought that though it moved differently it looked just like a large man MM90 MM 100 0 c95 80 85 C100 100 100 MM 10 LEFT HEMI Control 1 LEFT HEMI Medial LEFT HEMI Control LEFT HEMI Lateral Catman has had his face and body altered to mimic that of a cat Assignments Finish reading Chapter 13 in textbook Start reading Chapter 12in the textbook Make a list of anyterms that you don t understand andor any questions that you may have Optional Fiead chapters 10 and 11 in supplementary Course Pack available on website Writing assignment 3 due by midnight on Friday October 24 Submit to the drop box Exam Zooming up on Monday November 3 This exam will cover olfaction somatosensory system and whatever part of the visual system has been covered by then at is included in the catch all term somatosensmy system What is the somatosensory system Sensory receptors are distributed all over the body rather than concentrated in specific locations These receptors respond to many different kinds of stimuli in at least 4 different modalities A simple example of a spatiotemporal pattern across 4 neurons Odor off Each odor elicits a unique pattern of activity across time in each neuron Neuron 1mgl g Neuron Zmu 0 0 05 C mltggoa 00039 00g meow 9 seem Time ms White dots are baseline firing rate Blue dots represent a decrease in firing inhibition and yellow dots represent an increase in firing excitation Somatosensory systems include Cutaneous senses touch pressure vibration temperature heat and cold Proprioception body position Kinesthesis body movement Pain The cutaneous senses The skin contains a variety of sensory receptor types Markers disk Epidermalv dermal border Meissner39s De39rms cumuscle Hair lullicle recepim Pacinian oomuscle Humm s I my Transduction in mechanoreceptors Cutaneous mechanoreceptors typically consist of the distal process of a sensory neuron s axon and a covering that is more or less elastic rigid resilient malleable subject to shearing etc Covering Structure To Cell Body Nerve Endings Each type of receptor is specialized to convey a specific type of information Mechanoreceptors respond to mechanical energy touch pressure vibration stretch etc Thermoreceptors respond to thermal energy heat or lack thereof Chemoreceptors respond to chemical stimuli irritants substances released by injured tissue Transduction in mechanoreceptors Mechanical changes to the covering of the nerve ending activate mechanicallygated ion channels When these channels open they admit positively charged ions and depolarize the nerve ending fthe depolarization receptor potential is large enough action potentials are generated and transmitted along the proximal branch of the axon to the CNS I Key Concept Transduction in sensory systemsl Pacinian corpuscles Located deep in skin Respond best to high frequency vibration Adapt rapidly 39ll39 eters Have large receptive m fields MEIssner norpuscle Merkel disk Rumni endings Pacmian corpuscte What is rapid adaptation gt There is no response to sustained pressure only to changes in pressure I Key Concept Adaptation in sensory systemsl What is a cutaneous mechanoreceptor s receptive field The receptive field is that part of the body which when stimulated evokes activity in the mechanoreceptor Different mechanoreceptors have different receptive fields but there may be overlap I Key concept Receptive field of a nerve fiber or neuron I Key Concept Receptive field Definition For a given sensory receptorcell or neuron the receptive field is that region of sensory space within which an adequate stimulus will evoke a change in the cell s activity n the som atosensory system the region of sensory spacequot refers to a region on or in the body For other systems the region will be in some otherdimension Pacinian corpuscles have large receptive fields Black dots locations of Pacinian corpuscles Grey areas receptive Fields Large receptive fields are correlated with poor spatial resolution Pacinian corpuscles Meissner s corpuscles aielabmus Skin 4 Paplllary ridges Sllalum comeum pidevmls ermls sl lbpapillary plexus f Bare nerve endln Hall receplm Meissner s corpuscles Located superficially in skin Respond best to low X3 frequency flutter Are rapidly adapting Have small receptive fields Melssner39s corpuscles Black areas receptive elds Merkel s disks 7 Harry skm a Glabmus Sklngp Paplllary ridges Stmfum ccmeum pidelmis Merkel39s 39 receptor Melssnel39s calpuscle ermls slmpapillary Hall receptor plfxus Merkel s disks ocated in superficial part of skin Respond to very low frequency pressure changes 0 Slowly adapting Very small receptive fields Mexel s receplofs Slow adaptation When pressure is applied there is a longlasting response that persists for the duration of the pressure and only very slowly decays AJJJ Receptor parermar Ruffini cylinders Epidermalrdevmal runcrrm 7 Market s receptor corpuscle Bare nerve 7 ending Hair receptor 39 Sirarrrrrr carneum m pmermts errrr39rs simpapiuary plfxus Ruffini cylinders Located deep in skin Respond best to high frequency buzz or stretch of the skin in different directions 0 Slowly adapting Large receptive fields Ruffini s corpuscles Large versus small receptive fields Large receptive fields collect and integrate information over a large area of sensory space Spatial resolution is low Small receptive fields collect information from a restricted area of sensory space Spatial resolution is high Relationship between receptive field size and spatial acuity The more closely spaced the receptors are and the smaller their receptive fields the better the ability to resolve two pressure points on the skin 2 point discrimination varies across different parts of the body Forearm 4H Thumb x Hindex finger Stretch receptors Senso39y ending Capsule l MM mmi r mrtd tllllllllal 1ampamph alllllltllllr Cutaneous mechanoreceptors 39 V I z I Alvmm ixSnl Muscles and tendons mquot have embedded within them receptors that respond to stretch Jolrli receptors Hair follicle receptors Hairy skin contains hair follicle mechanoreceptors that sense movement of the hair Animals whiskers are specialized 1M variations of hair follicle receptors I t t L 3quotquot Hair follicle receptor Free nerve endings Free nerve endings throughout the skin respond to heat cold andor pain Hairy If Stratum corneum J Epidermaldermal Merkel s receptor corpuscle Bare ending Ftu ini39s corpuscle corpuscle Thermoreceptors The skin contains receptors that are activated by warmth and others that are activated by cold Skin temperature 0 Com m E 1 Warm a lllllllllllll Cold 9 receptor g g tz Cl lllllll ll llllllllllll w 4 5 20 30 Skin temperature C 5 sec Paradoxical cold Cold receptors are also activated by high temperatures Paradoxical cold lmpulsessec Arbitrary units Estimate of magnitude Individual cold fibers Pain Nociception The sensation of pain is caused by activation of very small diameter nerve endings When tissue is damaged chemical substances are released that stimulate these fibers Som e stimuli that activate nociceptors Thermal high heat or extreme cold WWquot l K I39M39W jquot lt3 Mechanical ntense mechanical stimuli w Chemical rritants or substances released by injured tissue spam we he Chinese giant salamander is the world s largest amphibian with a maximum length 0 about 6 eet Submitted by Leslie Rondeau Two headed girls and two headed snakes Siamese twin girls contributed by Maria Wray Sunseri Assignments and Reminders Exam 2 next Monday November 3 Read Chapter 2 in your textbook Read Chapter 12 in the Coursepack Work on your group presentations Written Assignment 4 due Wednesday November 19 his is a complete dra t 0 your term paper If you receive a score of 95 or above on the draft of your term paper you do not have to rewrite it It Will also count as Assignment 5 Written Assignment 5 Will be your final term paper The visual system processes patterns of light energy Light is a orm o electromagnetic radiation emitted by the oscillation o electrically charged material he visible spectrum is only a small portion 0 the electromagnetic spectrum 10393 1039210 103 105 101 1015 Ultra Gamma lntrared rays erays iolet 39 Broadcast bands C as rays 39 39 r circuits Visible Light short 400 500 600 700 long xWV Wavelength nanometers nm Some properties of light Reflection Light travels very ast about 300000 kms Light travels in straight lines Absorption Light can be reflected absorbed or re racted Refraction bending Eyes Eyes across the animal kingdom take many di erent orms and are placed in di erent configurations Placement of eyes leads to more or less overlap of the two visual fields Animals with eyes on the sides 0 the head have a wide angle visual field but little binocular vision Seen by both eyes Seen by Left Eye Seen by 7 Left Eye Rabbit Eye movements effectively increase the size of the visual field he eyeball is moved by the extraocular muscles Superior rectus muscle Superior oblique muscle Medial rectus Lateral mUSCle BCtUS muscle Inferior rectus muscle inferior oblique muscle The eye like a camera needs a mechanical optical system for optimizing transmission of light to the receptors Brightness Focus 39 Miami The Versace mansion The main parts of the eye Pu Aqueous Humor Iris Cornea in Anterior Chamber cornea Ciliary sclera Muscle o Iris 0 Pupil 0 Lens 0 Retina 0 Optic Nerve Optic Nerve he cornea is the clear he pupil is the opening Light he regulates the amount Aqueous Light passes through several different structures before it reaches the receptor cells Zonule fibers covering over the ronto the eye Lens Fovea through which light passes Cornea humor Optic nerve 0 light passing through the may muscle i A 39 Vitreous humor Sclera The lens focuses light on the back of the eyeball This process is called accommodation Because di erent objects are at di erent distances rom the eye there must be a mechanism to adjust the lights ocus he muscles attached to the lens contract or relax changing the curvature o the lens quot Cornea 39 j 395 Aqueous lmmor Light entering the eye projects an inverted image onto the retina Up to this point the eye works very much like a camera Camera epithelium Q The image in our eye is upside down so why don t we perceive the world as upside down A We do but our brain adjusts so that our visual image 0 the world corresponds to the spatial coordinates that we experience through other senses The retina is the screen upon which light is projected he retina contains three main layers 0 cells Light 2 Ganglion Cells Bipolar Cells Photo receptors Light passes through all the layers of the retina before reaching the receptor cells Information Flow Once the photoreceptors are activated in ormation is transmitted back through the bipolar cells and ganglion cells Blmm to reach the brain via Cells axons o the optic nerve Ganglion 39115 l I W H m Exxz c 4 NA 7 W BYQquot gg gg ss Zake92 The retina contains two types of photoreceptors rods and cones Rods and cones are specialized or processing di erent types 0 in ormation Outer segment Inner t segment Synaptic 1 terminal Cytoplasmic space Plasma membrane Cllium Mitochondria Nucleus Outer J segment Inner segment a gtSynaptic terminal Cone Folding of outer cell rn rane Folding 0 Outer cell membrane Connecting cilium R d Cone Cones are mainly located in the fovea and rods in the remainder Differences between rods and cones of the retina Structure he ovea contains almost exclusively cones Blind spot ype o lightsensitive chemical photopigment Numbers 120 million rods vs 6 million cones mrgitlljfrrig dhsery Contains Number of rods or canes per mm2 Distribution across retina he site at which the optic nerve exits the eye contains no photo receptors and is called the blind spot Relatively long term changes in sensitivity of the eye and retina Dark adaptation n response to a decrease in average brightness the pupil A terimages dilates Photopigments that were bleached by light regenerate in the dark making the receptors more sensitive Light adaptation Dark adaptation Rods and cones darkadapt at di erent rates Rods and cones dark adapt at different rates Key concept Transduction Cones adapt quickly but remain relatively insensitive ransduction is the process by which a specific orm 0 energy in the ROdS adapt SIOWIV abOUt 2O m39nUteS bUt become Very Sens39t39ve39 environment is trans ormed into a pattern 0 electrochemical energy that can be used by the brain he transduction process preserves in ormation about the physical lt39 tagmadapeasenslm properties 0 the stimulus ransduction is the first step in perceiving a stimulus Rodvcone break Maximum cone sensiliwy Loganlhm of sensitivity Dark adapted lmaximum ma sensitivity 10 Time In dark min Transduction in the visual system h Freeiloating dscs 39 When light strikes the retina it interacts with light sensitive molecules in the rods and cones Folding of outer cell membrane hese molecules are called Foldinsoi outer cell photopigments membrane he photopigments are contained in the rOd and ii i quot Cbnnecling cone outer segments quot m 30 Cone Visual pigment Outer segment Rhodopsm A photopigment a woman u molecule consists of two components Disc interior Opsin A large protein Retinal A small molecule derived from Vitamin A Light causes retinal to change its shape and split away from the opsin molecule When retinal leaves its binding site on opsin the opsin changes shape triggering several subsequent steps that happen inside the cell These steps ultimately affect ion channels in the cell membrane The change in the retinal portion of the photopigment is the only step in vision that depends on light Light ultimately causes the closing of a sodium channel that is normally open The whole process occurs in less than 1 millisecond 11000 5 Na ovga W Light i W Photopigment Light causes photoreceptors to hyperpolarize n the dark sodium channels are open so the rod or cone is depolarized n the dark neurotransmitter glutamate is constantly being released 510 p Light causes the sodium channels to close so the photoreceptor is hyperpolarized Glutamate release decreases or stops This might seem counterintuitive backwards but Because we are hardly ever in complete dark or extremely bright light it is useful to modulate the photoreceptor s membrane potential up and down about some intermediate point Painting by Giorgio de Chirico Key Concept Modulation of neural activity around a set point Many sensory neurons are spontaneously active his activity can be modulated upward or downward depending on how stimulus conditions change he set point can also be modulated upward or downward according to average conditions at a given time On center and off center cells Depending on the kinds of receptors present on the cell membrane bipolar cells may be excited by light oncenter or excited by dark offcenter Offcenter L h Oncenter I Bipolar 39 Bipolar E Cell 39 quot Cell l 5Dark l v i Cone in E 8ip0 r EXC Bd Bipolar inhibited by Glutamate 1 I by Glutamate Offcenter Oncenter 39 I Dark Bipolar quot 01 Bipolar Cell 39 Cell r L V Transmission of information in the retina Rods and cones contact bipolar cells Bipolar cells contact ganglion cells Some bipolar cells are emf excited by glutamate and L m Bipolar Oncenter 1 139 Bipolar I 39 Cell are depolarized by dark Gequot and hyperpolarized by light Some bipolar cells are inhibited by glutamate and l are depolarized by light and Momma hyperpolarized by dark gang cell Action on39cemer potentials 39 quot ganglion cell To optic nerve On center and off center cells Ganglion cells are also oncenter or offcenter depending on the nature of the bipolar cells from which they receive input I Ll m A 1L I 3 I l F 391 Action l Offcenter G Ongcelgllne e J potentials 1 7 l L v39 ganglion 23 9 cell To opllc nerve new A A ll quot Acnon potentials I cJL On cemer gang ion cell L Offcenter gangmr cell To optic nerve nformation is first represented as graded potentials then action potentials Rods and cones produce graded potentials receptor potentials Offcenter 393 o I L Bipolar Sagacity I Bipolar cells also produce Veal Cell A graded potentials Ganglion cells produce a Offcenter quot ganglion action potentials which are sent to the brain via their axons in the optic nerve r gt Action it quot e potentials ganglion cell To optic nerve nformation processing in the retina he retina does not just collect and transduce in ormation about patterns 0 light t also per orms important in ormation processing tasks he rod pathway amplifies the signal under lowlight conditions he cone pathway provides in ormation about color and fine structure 0 visual images Lateral inhibition sharpens contrast and contours Imagination is more important than knowledge A1bert Einstein Assignments Read Chapters 13 and 14 in textbook Make a list 0 any terms that you don t understand andor any questions that you may have Optional Read chapter 8 in supplementary Course Pack available on website Writing assignment 2 due by midnight on Friday October 10 Submit to the drop box Exam 1 coming up on Monday October 13 his exam will cover introductory material psychophysics and chemical senses UNIT 2 SENSORY SYSTEMS AND HOW THEY WORK Collection of nformation Transduction Processing Co ect on of lnformat on All sensory systems have specialized organs to gather information bearing forms of energy from the environment Collection of nformation Collector organs perform manv functions including Energy capture Funneling focusing and directing energy to receptor cells Amplification or attenuation gain control Filtering ntegration Analysis Transduction he process through which in ormation in various nonneural orms is converted to patterns 0 neural activity INFORMATION gt Collector gtReceptor Central light sound etc Structures Cells Nervous system Receptor Cells Sensory receptor cells are the site at which environmental energy is converted to neural electrochemical energy Activation o the receptor cell occurs when environmental energy 0 the correct type acts on ion channels and causes them to open or close Receptor Cells he change in ion permeability that results rom the openingclosing o the ion channels 0 the receptor cell causes a graded change in membrane potential the receptor potential he receptor potential unctions in the same way as an EPSP ts cause is just di erent physical stimulus energy a ecting ion channels rather than neurotransmitter molecules Transduction in the receptor cell Transduction in the receptor cell he size 0 the receptor potential is proportional to the amount 0 energy that reaches and activates the cell he amount 0 neurotransmitter released by the receptor is proportional to the size 0 the receptor potential Note receptor cells usually do not produce action potentials just graded potentials and graded amounts 0 neurotransmitter release but there are exceptions Energy Stimulus Oh Receptor E Potential 9 o gt Ion Channel 3965 Time Nerve Neurotransmitter Terminal Release Transduction in the receptor cell Energy Stimulus Receptor g l lt Potential 65 39 Time a 39 g Neurotransmitter 0 Release 9 m a m Stimulus sensory Magnitude Neuron To Brain Di erent receptor cells may have di erent response vs intensity unctions or the same stimulus Threshold and dynamic range vary across sensory receptors and neurons Dynamic Range The range over which a change in stimulus magnitude elicits a change in response Different receptors andor neurons have different thresholds and dynamic ranges Response Stimulus Magnitude Threshold and dynamic range vary across sensory receptors and neurons A wide range 0 thresholds and dynamic ranges allows the sensory system as a whole to transmit in ormation about a broad range 0 intensity or other stimulus parameter The chemical senses provide a good example of the transduction process he chemical senses that we usually think 0 are SMELL TASTE Other chemical senses Chemoreceptors in arteries measure CO2 and O2 in blood Sensory endings in muscles detect lactic acid Nerve endings in skin and mucous membranes detect irritants Quote of the day M We re not here to train students We re here to educate them Comment overheard in a conversation between two pompous professor types walking on the Ave Assignments Read Chapters 14 and 13in textbook Make a list of anyterms that you don t understand andor any questions that you may have Optional Ftead chapters 8 and 9 in supplementary Course Pack a vatable on website Writing assignment 2 due by midnight on Friday October 10 Submit to the drop box Exam 1 coming up on Monday October 13 This exam will cover introductory material psychophysics and chemical senses Different types of sensory adaptation Shortterm adaptation occurs in response to an ongoing stimulus but is reversed when that stimulus is no longer present Longterm cognitivequot adaptation aka habituation occurs with repeated exposure to a stimulus A common stimulus no longer elicits a response or attracts attention How does sensory adaptation occur There are many different mechanisms for sensory adaptation n the chemical senses shortterm adaptation may be a simple matter of equilibrium for an ion being reached across the cell membrane Shortterm adaptation may also be due to saturation of receptor sites Longterm adaptation may resultfrom neural gating processes that block incoming information from sensory organs Longterm adaptation may also be due to peripheral processes including downregulation of receptor sites Chemical Stimuli 0 Cannot be characterized along any single continuous dimension such as the wavelength of light or the frequency of a sound 0 There is no simple relationship between molecular structure and the perception of a taste or an odor 0 Nearly all naturally occurring tastes and odors are composed of more than one type of molecule for example coffee contains hundreds of different molecules Conditioned taste aversion Conditioned taste aversion A specific taste paired with nausea or other noxious stimulus leaves a lasting aversion for the taste A conditioned taste aversion is most likely to develop if the taste is a novel one How is taste quality represented by neural activity One idea is that taste quality is represented by an acrossfiber pattern or spatial pattern with different neurons tuned to different taste qualities A Sour N Salt S Sugar B Bitter l 39j aw an RitzthwhitMi Z Number of Action Potentials Neron 1 Neuron 2 Neuron 3 Neuron 4 quotAcidbestn quotSaltbestquot quotSweetbestquot quotBitterbestquot This idea is accurate but does not fully explain taste discrimination Another idea is that each taste stimulus produces a characteristic temporal pattern Sodium Chloride Salt i is l M I A t The same neuron typicany I g I responds to different tastes with different temporal Hydrochloric Acid Sour patterns It is possible that the kinetics of activation of receptors Quinine Bitter andor ion channels create l fll l in g A the characteristic patterns 39 l I Sucrose Sweet l sec i C Mistretta These temporal patterns are very similar regardless of which neuron responds Sodium Chloride Salt Hydrochloric Acid Sour Note the time scale v l l ill The taste system quotquot 39 39 operates on a scale of Quinine Bitter seconds There is experimental evidence that temporal patterns can convey information about stimulus quality Electrical stimulation of the chorda tympani nerve in rats evokes orofacial reflexes characteristic of the temporal pattern of stimulation Quinine pattern Covey 1 Electrical stimulation of the NTS in behaving rats evokes conditioned taste aversions to stimuli whose taste quality corresponds to the pattern of electrical stimulation Di LOrenzo ampr Hecht 1993 The neural representation of taste quality most likely takes the form of a spatio temporal pattern of neural activity At any given time some neurons are more active than others The relative amounts of activity across all the neurons vary over time Exam Me of a spauotem pora pattern CHAPTER 9 THE CHEMICAL SENSES NEURAL CODING AND CENTRAL NERVOUS SYSTEM PATHWAYS 91 Transduction and patterns of neural activity The purpose of transduction is to convert a pattern of environmental information distributed over space and time to a pattern of neural activity which is also appropriately distributed across space different neurons and time 911 Spatial patterns of activity 9111 Modalityspecific activity Taste stimuli applied to the tongue elicit activity in the taste nerves that innervate the tongue not in peripheral nerves that innervate other structures such as the eye Thus the identity of the nerves that are activated conveys information about what kind of stimulus causes the activity eg chemicals in the case of the taste nerves light in the case of the optic nerve etc Thought question Usually we perceive an electrical current applied to the body as an electric shock However when the inner ear of a deaf individual is stimulated with an electrical current the perception is usually that of sound This technique is applied clinically in the form of a quotcochlear prosthesisquot to restore the hearing of patients who have lost the ability to perform auditory transduction Why do you think this technique works For what sorts of patients would it NOT work 9112 Transduction sensitivity functions and population coding In the gustatory system each individual receptor cell has an assortment of different ion channels that are specifically activated by a class of molecule eg salt acid or sugar Some receptor cells have more of a given type of ion chanel than do others For example one cell may have a high proportion of ion channels that are sensitive to salt and another a high proportion of ion channels sensitive to sugar This means that each cell will respond to both salty and sweet tasting stimuli but the first cell will respond more strongly to salt and the second cell will respond most strongly to sugar The response pro le of each cell across the entire range of stimuli that could potentially activate it is called its sensitivity function The fact that receptor cells in many sensory systems have different often overlapping sensitivity functions is a consequence of the transduction process It is also the basis for population coding Later on we will talk much more about sensitivity functions and population codes in the visual and auditory systems In its simplest form a population code is a neural representation in which information is conveyed by relative amounts of activity across multiple differentially sensitive elements of an array in our example the taste receptor cells This is in contrast to a quotlabeled linequot in which 49 information is represented unambiguously by the absolute amount of activity in each individual element within the array Neuron 1 quotSweetBestquot A 7 quotSaltBestquot m an 0 E e a 1 a 5 I a m m s a a m 3 a E Sweet Salt Sour Bitter Sweet Salt Sour Bitter gure 91 Examples of sensitivity functions or pro les for two taste neurons The quotsweetbestquot neuron on the left responds to all types of stimuli but the largest response is evoked by sweet substances The quotsaltbest neuron on the right also responds to all types of stimuli but the largest response is evoked by salty substances 912 Transduction and temporal response patterns Any system of interactions between or among molecules whether it be a straightforward chemical reaction passage of an ion through a channel or the binding andor disassociation of a molecule tofrom a receptor site has a characteristic time course or kinetics Some of these events happen quickly others slowly Some happen in a continuous smooth pattern and others in a discontinuous jerky pattern Some happen in a single stage others in multiple stages For a sensory receptor cell the kinetics of receptor activation determine the time course of changes in its membrane potential For a taste cell the pattern of action potentials in the nerve ber innervating the cell is determined by the reeptor sites and associated channels activated and their respective kinetics Sudium Chloride1Salty Ouinine Bitter Dilute Hydrochloric Acid Sour Sucrose Sweet 1 Second gure 92 Temporal patterns of action potentials produced by a taste neuron in response to four different stimuli salty sodium chloride NaCl sour dilute hydrochloric acid HCl biner quinine QHCl and sweet sucrose Note that the patterns of action potentials persist for a second or more and that they are very different for each of the four stimuli Within a population of neurons information is represented in both space and time as a spatiotemporal pattern 92 SENSORY INFORMATION FLOW IN THE CENTRAL NERVOUS SYSTEM In most sensory systems information is transmitted in the same general pattern within the central nervous system The sensory nerve projects in a more or less direct way to a specialized region of the thalamus sometimes via one or more groups of cells in the brainstem From the thalamus information is sent to a specialized area of the cerebral cortex the primary sensory area The primary sensory area of the cortex gives rise to direct and indirect projections to secondary sensory areas polysensory areas and ultimately to motor areas and other brain areas associated with memory emotion etc At some point quotsensoryquot information is transformed into something else for example a motor action emotional feeling or abstract idea 39 Processing Motor or p or ewe or Brainstem systems gure 973 Diagram showing the typical panem of information ow in the central nervous system 921 The central gustatory pathways he taste pathways are typical of the general sensory plan outlined above formation about taste is rst relayed via three separate nerves to the nucleus of the solitary tract NTS a group of cells located in the brainstem Cells in the brainstem taste regions have outputs to motor circuits for tasteevoked facial responses as well as to regions involved in emotional responses and feelings of hungersatiety From the brainstem information is transmitted to a taste area in the thalamus the ventral posterior medial nucleus which in turn projects to a taste area in the cortex located in the parietal lobe mainly within a sulcus The primary taste area in turn projects to a secondary polysensory area that also receives input from the olfactory system Cortical Taste Area Tnaxamic Taste rea VPM Nucleus u the Solitary Tract NTS cerebellum pinal cord gure 94 The central pathway for taste 93 THE OLFACTORY SMELL SYSTEM 931 The olfactory receptor cells We are able to discriminate among an extremely large number of odorants most of which are actually mixtures of different molecules Odors reach the receptor cells in the olfactory epithelium through the nose or through the throat The olfactory epithelium lies at the top of the nasal cavity It contains receptor cells and supporting cells Olfactory Nerve Fiber Epithelium Receptor Cell Supporting Cell 39 I 3 Olfactory Mucus Mucus Figure 975 The roof ofthe nasal cavity is lined with tissue called the olfactory epithelium The olfactory receptor cells are long and thin The exposed end ofthe cell faces downward into the nasal cavity and is covered with long hairlike processes called cilia that extend down into a layer ofmucux Olfactory receptor cells unlike other sensory receptors are true neurons They are also the only neurons that regularly die life span 58 weeks and are replaced The dendrites of olfactory receptors have cilia small hairlike structures that stick out into a layer of mucus Odorants once inhaled are picked up by special protein molecules in the mucus layer and transported to receptor sites on the cilia To Ollactory Bulb To Ollactary Bulb 1 t Recepmr Cell Axons 1 g x mam 39 39 Binding Odaram Molecules mucus Protein Figure 976 Left A more intimate View of olfactory receptor cells and supporting cells showing the cilia and the axons of the receptor cells exiting to terminate in the olfactory bulb Right Odorant molecules in the inhaled air are trapped in the mucus where they bind to special molecules called odorant binding proteins These proteins then transport the odorants to the cilia where transduction takes place 932 Olfactory transduction Like taste receptor cells each olfactory receptor neuron has multiple types of receptor sites for different molecules T e v ious types 0 receptor sites are distributed in different proportions on different olfactory receptor neurons Binding of odorants to receptor sites associated With ion channels causes a receptor potential at the dendrites and action potentials at the cell body and axon The axons of olfactory receptor cells project directly to the brain via the olfactory nerve 94 THE CENTRAL OLFACTORY PATHWAYS The olfactory system is the only major sensory system that deviates from the standard pattern of information ow in that information is transmitted to the cortex before it reaches the thalamus Annaquot Poiemlals Recemw Potential 1 second gure 97 The olfactory transduction process is similar to the taste transduction process for sugars and bitter substances Odorant molecules bind to speci c receptor sites in the cell membrane of the cilia These receptor sites are associated with ion channels that are activated when an odorant molecule is bound Binding of an odorant molecule to the receptor site causes various reactions to occur culminating in depolarization of the cell the receptor potential Ifthis depolarization causes the cell to reach its threshold an action potential is generated at the cell body and transmitted along the olfactory nerve ber to the olfactory bulb The axons of olfactory receptor cells project to the olfactory bulb Where they terminate in glomeruli clusters of cells in the olfactory bulb The olfactory bulb projects directly to primary olfactory cortex also called entorhinal cortex The primary olfactory cortex projects to an olfactory area in the thalamus which in turn projects back to secondary areas of olfactory cortex including an area called orbitofrontal cortex Which also receives input from the taste system Receptor Cell gure 98 The central olfactory pathways Axons of the olfactory receptor neurons project to the olfactory bulb enlarged in inset at right Axons from the olfactory bulb form the olfactory tract which projecm to primary olfactory cortex The olfactory cortex projects to the thalamus which projects back to a secondary olfactorytaste area in the orbitofrontal cortex DHch Bull 39ommry Ream I Auburn Molmulli Figure 99 Schematic diagram of the central olfactory pathway The olfactory cortex has connections with motor systems as well as quotlimbic systemquot structures having to do with feeding memory and emotion 95 ENCODING INFORMATION ABOUT TASTE AND SMELL Most taste receptor cells and central neurons respond to all classes of taste stimuli Similarly olfactory receptor cells and central neurons respond to multiple odorants Since there is so little speci city in terms of which receptor cells or neurons are active information about taste or odor quality must be represented by activity across a population One idea of how taste is coded is through an across fiber pattern or population code as discussed earlier Another idea is that taste quality is coded by the pattern of activity over time temporal pattern It is likely that the neural representation of both taste and odor quality takes the form of an across ber pattern that changes over time that is a dynamic spatiotemporal pattern 96 INTERACTION BETWEEN TASTE AND SMELL What we commonly think of as quottastesquot or quot avorsquot are actually a combination of water borne taste and airborne smell molecules If we had to depend on taste alone we would not be able to identify many common foods and beverages such as strawberries cherries tea or coffee Thought Question Taste and olfactory receptor cells are constantly dying and being replaced Why do you think this occurs If the neural code for taste and odor quality is activity in specific neurons what problems are inherent in a system with a constant turnover of receptor cells How do you think the chemical senses solve these problems Water Mohsses Vinegar Sugar Water 35 88 15 5395 15 13 53 E3 2 32 u gg E U H re 98 Bar graph showing percent of subjecm who correctly identi ed a common food avor based on taste alone white bars or using both taste and olfaction black bars Dill Pickle Juicep 97 INDIVIDUAL DIFFERENCES IN TASTE AND SMELL Some individuals are better at discriminating among tastes and odors than are others There are many factors responsible for differing abilities and these factors are interactive They include Genetic makeup as discussed above Age Taste and odor discrimination improves during childhood plateaus during young adulthood to middle age and declines rapidly With old age Sex In general women39s senses of taste and smell are more sensitive than men39s but uctuate during the menstrual cycle or during pregnancy indicating that hormones modulate the acuity ofthe chemical senses T rainin experience It is possible for humans to develop extremely acute chemosensory abilities through training Examples are professional Wine tasters and perfume testers Prolonged exposure to a speci c odorant o en results in decreased sensitivity to that 0 or Thought Question If you were in the commercial food business and wanted to market a new kind of dessert in the US what kinds of psychophysical tests might you perform Do you think the tests you performed would be equally valid for markets in other countries eg Finland India or Nigeria If you wanted to market the product in one of these countries would you perform additional tests Why or why not Number Correct u u a n 8 5 m m m m cu m m m m u39 397 39 1 7 quot9 3 39T 3 a a o a a o a c c a m n a u u s a m Agerup gure 99 Graph showing average number of odors correctly identi ed out of a standard set of 40 by subjecm in different age groups Ability to discriminate odors improves up to about 15 years of age remains relatively constant up to about 60 years of age after which it deteriorates rapidly Pnrcgnragt mm 0 an an no vmunull amen u gure 9710 A rather dif culttoread bar graph illustrating the percentage of male and female subjecm who correctly identi ed common odors Each bar represents the values for males and females For the white bars the lower values are for males the higher for females For the black bars the lower values are for females the higher for ma es
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