UMass Amherst KIN 460 - 09-Vsiual system- Eye - photoreceptors - visual processing (69 pages)

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09-Vsiual system- Eye - photoreceptors - visual processing



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09-Vsiual system- Eye - photoreceptors - visual processing

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Pages:
69
School:
University of Massachusetts Amherst
Course:
Kin 460 - Motor Control
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Somatosensory system Review What would it be like to lose proprioception but have a normal efferent system Ian Waterman Pride and a Daily Marathon Cole 1995 At age 19 got flu his immune system attacked nerves for touch and proprioception Initially did not move he says he could move but his lack of control made it dangerous and ineffective Taught himself how to walk to drive to hold an office job All his movements done under conscious control and continuous visual inspection Without thought there was no movement https www youtube com watch v pMEROPOK6v8 Visual system Visual perception of biological motion https www biomotionlab ca analysisand synthesis of biological motionpatterns http www biomotionlab ca Anatomy of the human eye Animation Eye diseases Glaucoma Aqueous humor fluid is replaced 12x daily in healthy eye Reduced drainage intraocular pressure blood supply damage to retina Eye diseases Cataracts Lens become opaque caused by aging and UV solar radiation Accounts for half of all blindness cases Monocular deprivation binocular vision Overview How are images formed by the eye optics What is the nature and function of retinal neuronal cells The blind spot in your eye How do we perceive color and contrast How does the brain process vision Cornea and lens refraction of light to form a focused image on the photreceptors of the retina Cornea contributes most of the refraction Lens adjustable refraction accomodation Accommodation dynamic changes in the refractive power of the lens Large distance flat lens least refraction unaccomodated Short distance round lens most refraction accomodated Accommodation Amplitude of accomodation decreases with age Refraction Refraction Opticians describe the power of refractive surfaces by the reciprocal of their focal length in metres and these units are called dioptres D for a convex lens the power is positive left for a concave lens the power is negative right Normal refraction Refractive errors Myopia nearsighted If refractive error is too strong the image of a distant object lies inside the vitreous instead of on the retina left It is corrected by using a negative or concave spherical lens right Hypermetropia farsighted If refractive error is too weak the image of a distant object lies beyond the retina instead of on the retina left It is corrected by using a positive or convex spherical lens right Overview How are images formed by the eye optics What is the nature and function of retinal neuronal cells The blind spot in your eye How do we perceive color and contrast How does the brain process vision Anatomy of the human eye Retina The inner surface of the retina viewed with an ophthalmoscope Light Light energy propagated by electromagnetic waves in discrete packets quanta or photons at 300 km ms Structure of the retina Part 1 Structure of the retina Part 2 Animmation Phototransduction Structure of the retina Part 3 low spatial resolution but very sensitive high spatial resolution less sensitive Rods Cones Night vision Daylight vision High sensitivity to light more photopigment Lower sensitivity less photopigment Low temporal resolution slow response long integraton time High temporal resolution fast response short integration time 100 million retina 6 million retina Absent in fovea Concentrated in fovea Achromatic 1 photopigment rhodopsin Chromatic 3 photopigments colour vision Convergent system many rods synapse on same interneuron bipolar cell Single cone fewer cones converge on each bipolar cell Distribution of photoreceptors in the human retina Part 1 Phototransduction Light Phototransduction Cone rod Activation of visual photopigments outer segment Change in membrane potential Hiperpolarization of photo receptor Retinal interneurons Bipolar horizontal and amacrine cells Retinal ganglion cells Area of the retina monitored by a ganglion cell Light on photoreceptors in this area causes a response in the retinal ganglion cell Ganglion cells respond to contrast in light rather than absolute intensity Best response when light intensities in center and suround of receptive field are different Provides sensitivity to borders and contours differences in illumination Receptive field Eye Diseases Macular degeneration Loss of cones legal blindness Central vision Retinitis pigmentosa Loss of rods night blindness Peripheral vision Retinitis Pigmentosa Overview How are images formed by the eye optics What is the nature and function of retinal neuronal cells The blind spot in your eye How do we perceive color and contrast How does the brain process vision Blind spot Blind spot of right eye Hold paper at 1 foot distance Close left eye Fixate on X in figure Pencil in right hand and starting on far right move WITHOUT breaking fixation on X Mark location were tip 1 disappears and 2 reappears Do same for vertical direction create box measure area of blindspot The blind spot Why are we not aware of the blind spot Indicate in the figure the structure related to the blind spot The Blind Spot Demonstration of the blind spot Close your left eye and view the red cross from a distance of about 30 cm the alien will disappear yet no discontinuity in the background will be apparent Overview How are images formed by the eye optics What is the nature and function of retinal neuronal cells The blind spot in your eye How do we perceive color and contrast How does the brain process vision Three types of cones with photopigment sensitive to light of different wavelengths Color blindness or deficiency Dog s view of the world Brighter less detailed Better peripheral vision Better night vision Better at moving objects Less visual acuity 20 75 Contrast sensitivity Subject is asked to view a sinusoidal grating of a particular spatial frequency above as the contrast is reduced until she he reports that he can no longer see it If we plot this threshold contrast as a function of spatial frequency we typically obtain a curve such as the one shown The Importance of Context in Color Perception Color contrast Color constancy The Perception of Light Intensity Perception of color and contrast Firing rate of retinal ganglion cells depends on the luminance contrast between their receptive field center and surrounding area CNS does not simply reads out the relative rates of ganglion cells Color Contrast perception is affected by its context Possible influence of past experiences Overview How are images formed by the eye optics What is the nature and function of retinal neuronal cells The


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