1、 Sensory SystemsnThe brain detects events in the external environment and directs the contractions of the muscleslAfferent neurons carry sensory messages to brainlEfferent neurons carry motor messages to the musclesnStimulus:any energy capable of exciting a receptorlMechanicallChemicallThermallPhoto
2、nicnSensory energies are measurable(unlike ESP)6.1 Sensory ReceptorsnReceptors are specialized nerve cells that transduce energy into neural signalslReceptors lack axons,form synapses with dendrites of other sensory neuronsnReceptors are“mode”specificl“Law of Specific Nerve Energies”:sensory message
3、s are carried on separate channels to different areas of the brain nReceptors detect a small range of energy levelslEye:400-700 nMlEar:20-20,000 HzlTaste buds:specific chemicals6.2 Visual SystemsnThe function of a visual system is to detect electromagnetic radiation(EMR)emitted by objectsnHumans can
4、 detect light with a wavelength between 400-700 nMlPerceived color(hue)is related to the wavelength of lightlBrightness is related to the intensity of the radiationnFunctions of visionlDiscriminate figure from background(food or rock?)lDetect movement(predator/prey?)lDetect color(adaptive value of c
5、olor vision)6.3 Eye DetailsnAn eye consists of:lAperture(pin hole,pit,or pupil)to admit lightlLens that focuses lightlPhotoreceptive elements(retina)that transduce the light stimulusSource:nei.nih.gov/nei/vision/vision2.htm6.4 RetinanLight passes through the pupil and is focused by the lens onto the
6、 retina at the back of the eyenThe retina consists of three layers of cellslGanglion cell layerlBipolar layerlPhotoreceptor layer:receptors in this layer transduce lightnThe ganglion cell layer is the outermost layer and the photoreceptor layer is the innermost layerlIn order to reach the photorecep
7、tor layer,light actually passes through the outer two layers of the retina6.5 Rods and ConesnTwo types of photoreceptors are located within the retinalRods:120 millionuLight sensitive(not color)uFound in periphery of retinauLow activation thresholdlCones:6 millionuAre color sensitiveuFound mostly in
8、 fovea nThe outer segments(O.S.)of a rod or a cone contain different photopigments that react to lightSource:insight.med.utah.edu/Webvision/imageswv/rodcoEM.jpeg6.6 Retinal Circuitry6.7Adapted from Dowling,J.E.,and Boycott,B.B.Proceedings of the RoyalSociety of London,B.,1966,166,80-111.Primary Visu
9、al PathwaynInformation from each visual field crosses over at the optic chiasm and projects to the opposite side of the primary visual cortex6.8 Visual Pathways:LGNnSignals from the ganglion cells of the retina are sent to the thalamus via the optic nerve/tractnThe dorsal lateral thalamic nucleus(LG
10、N)has 6 layerslEach layer receives input from only one eyelThe inner 2 layers contain large cells(magnocellular)lThe outer 4 layers contain small cells(parvocellular)lKoniocellular sublayers are ventral to each of the 6 layersnNeurons of the LGN project through the optic radiations to a region of oc
11、cipital cortex termed primary visual cortex(striate)6.9 Visual Pathways:LGN6.10 Overview of the Visual CortexnHuman visual areasV1V2V3V3AV4PosteriorAnteriorcm6.11Visual areasoccipital lobes,posterior viewV1V2V3V3aV4vMT+leftrightcm Organization of V180Upper VMLower VMHM6.13 Visual TransductionnPhotop
12、igments are located in the membrane of the outer segment of rods and conesnEach pigment consists of an opsin(a protein)and retinal(a lipid)lIn the dark,membrane NA+channels are open-glutamate is released which depolarizes the membranelLight splits the opsin and retinal apart-uActivates transducin(G
13、protein)-uActivates phosphodiesterase-uReduces cGMP-closes NA+channelsnThe net effect of light is to hyperpolarize the retinal receptor and reduce the release of glutamate6.14 Retinal Responses to Light6.15 Receptive FieldsnMicroelectrodes can be used to record the firing activity of a single sensor
14、y neuronlSensory neurons have a background rate of firing(impulses/sec)lThis rate of firing can increase or decrease in response to a stimulus nReceptive Field(RF):Those attributes of a stimulus that will alter the firing rate of sensory celllThe general pattern of the RF can be recorded at each lev
15、el of a sensory system(e.g.from a peripheral sensory receptor,the thalamus,or the cortex)lRF analyses can indicate the manner in which sensory information converges from level to level 6.16 Ganglion Cell Receptive FieldsnGanglion cells in the retinal periphery receive input from many photoreceptors
16、while ganglion cells in the fovea receive input from one photoreceptorlThe receptive fields of ganglion cells are circular with a center field and a surround fieldl“ON-Cell”:uCell exhibits a low baseline firing rateuLight placed in center ring increases firing rateuLight placed on surround decreases
17、 firing ratel“OFF-Cell”uLight placed in center ring reduces firing rateuLight placed on surround increases firing rateON OFF 6.17 Color Vision TheoriesnTrichromatic theory(Thomas Young 1802)argued there are 3 different receptors in the eye,with each sensitive to a single huelAny color could be accou
18、nted for by mixing 3 lights in various proportionsnOpponent theory(Ewald Hering 1905/65)notes that people perceive opponent colors:yellow/blue and red/greenlYellow is a primary color rather than a mixture of red and blue-green lightlNegative color afterimages suggest that red and green are complemen
19、tary colors as are blue and yellow6.18 Color Vision Systems6.19Tritanopia deuteranopia protanopia Color Vision SystemsnPrimate retina contains 3 types of photoreceptorslEach cone uses a different opsin which is sensitive to a particular wavelength(blue,red,green),supporting trichromatic theorynAt th
20、e ganglion cell level,the system responds in an opponent-process fashionlA given cell might increase its firing rate to yellow light in the center,but decrease it to blue light6.20 Center-Surround:LGN/Retina6.21 Visual Pathways:Striate CortexnStriate cortex is organized into 6 layerslLayer 4c receiv
21、es information from the parvocellular and magnocellular layers of the LGNlThe visual information is the sent to layers above and below layer 4c for analysisnMicroelectrode receptive field studies have sought to identify the features of the external world that activate cells in striate cortexlOrienta
22、tion sensitivity:some cells fire best to a stimulus of a particular orientation and fire less when orientation is shiftedlSpatial frequency:cells vary firing rate according to the sine wave frequency of the stimulus6.22 Orientation Sensitivity6.23Best orientation Spatial FrequencynVisual neurons res
23、pond to a sine wave grating:lAlternating patches of light and darkuLow frequency:large areas of light and darkuHigh frequency:fine details6.24 Modular Organization of Striate CortexnStriate cortex is organized into modules(2500)lStains for cytochrome oxidase(CO)reveal two”CO blobs”in each moduleuCel
24、ls within each CO blob are sensitive to color and to low frequency informationuOutside each blob,neurons respond to orientation,movement,spatial frequency and texture,but not to color informationnStriate modules showlOcular dominance:cells in each half of the module respond to only one eyelOrientati
25、on columns:uCells respond to same orientation,adjacent cells are shifted by 10 degreesuAre organized at right angles to the ocular dominance columns6.25 Modules in Visual Cortex6.26Cytochrome Oxidase blobs in V1 Visual System DivisionsnMagnocellular systemuCells from retina terminate in LGN layers 1
26、,2 and then project to layer 4C of striate cortexCarry info on contrast and movement(color insensitive)System is found in all mammalsnParvocellular system uCells from retina terminate in LGN layers 3-6 and then project to layer 4C of striate cortexCarry info on fine detail,and color(red,green)System
27、 is found in primatesnKoniocellular systemuSystem projects from LGN to blobs in striate cortexSystem carries color information(blue)System is found only in primates6.28 Primary Visual Pathway6.29GIRKIN AND MILLERSurv Ophthalmol 45(5)MarchApril 2019Visual Areas in Lateral and Medial Views of Occipita
28、l Cortex Visual Association CortexnVisual information is transmitted to extrastriate cortex(termed visual association cortex)via two streamslDorsal stream:“where”an object isuReceives mostly magnocellular inputuProjects to post.parietal association cortexlVentral stream:“what”an object is(analysis o
29、f form)uReceives an equal mix of magnocellular and parvocellular inputuProjects to extrastriate cortex(V2,V3,V4,V5)and to inferior temporal cortex(TEO,TE,STS)6.31 Visual Cortex:what/where6.32 PET study of where/what dichotomy6.33 Face cells in STS of macaque monkey6.34 nAgnosia refers to a failure t
30、o perceive or identify a stimulus by means of a sensory modalitylApperceptive visual agnosia is a failure in higher level perceptionuPerson has normal visual acuity,but cannot recognize objects based on their shapeuProsopagnosia is a form of apperceptive visual agnosia in which the person cannot rec
31、ognize a face visually,but can do when hearing their voicelAssociative visual agnosia refers to a disconnection between perceptions and verbal systemsuPerson cannot name what they seelBalints syndrome relates to failures in spatial/location perceptionuOptic ataxia,ocular apraxia,simultanagnosiaAgnos
32、ia6.35 Agnosia different from memory loss6.36 Apperceptive/Associative Visual Agnosia6.37 Summary of Visual CortexnV4:responds to color(and form perception)uLesions of V4 impair color perceptionnV5:responds to movementnTEO:involved in color discrimination,2-d pattern discriminationuTEO projects to area TEnTE:neurons here respond to 3-d objects(a face or a hand)6.38docin/sanshengshiyuandoc88/sanshenglu 更多精品资源请访问更多精品资源请访问
