Somatosensory system - ReviewWhat 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=pMEROPOK6v8Visual system• Visual perception of biological motion• https://www.biomotionlab.ca/analysis-and-synthesis-of-biological-motion-patterns/• http://www.biomotionlab.ca/Anatomy of the human eyeAnimationEye diseasesGlaucoma  Aqueous humor: fluid is replaced 12x daily in healthy eye Reduced drainage  ↑intraocular pressure  ↓ blood supply damage to retinaEye diseasesCataracts Lens become opaque caused by aging and UV solar radiation Accounts for half of all blindness cases Monocular deprivation – binocular visionOverview① How are images formed by the eye (optics)?② What is the nature and function of retinal neuronal cells?③ The blind spot in your eye….④ How do we perceive color and contrast?⑤ 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 = accomodationAccommodation: dynamic changes in the refractive power of the lensLarge distance – flat lens: least refraction = unaccomodatedShort distance – round lens: most refraction = accomodatedAccommodationAmplitude of accomodation decreases with ageRefractionRefractionOpticians 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 refractionRefractive errorsMyopia = 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).Overview① How are images formed by the eye (optics)?② What is the nature and function of retinal neuronal cells?③ The blind spot in your eye….④ How do we perceive color and contrast?⑤ How does the brain process vision?Anatomy of the human eye: Retina The inner surface of the retina, viewed with an ophthalmoscopeLightLight: 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: PhototransductionStructure of the retina (Part 3)low spatial resolution but very sensitivehigh spatial resolution less sensitiveRods• Night vision• High sensitivity to light (more photopigment)• Low temporal resolution: slow response, long integraton time• 100 million/retina• Absent in fovea• Achromatic (1 photopigment: rhodopsin)• Convergent system: many rods synapse on same interneuron (bipolar cell)Cones• Daylight vision• Lower sensitivity: less photopigment• High temporal resolution: fast response, short integration time• 6 million/retina• Concentrated in fovea• Chromatic (3 photopigments: colour vision)• Single cone/fewer cones converge on each bipolar cellDistribution of photoreceptors in the human retina (Part 1)Cone/rodPhototransductionLightActivation of visual photopigments (outer segment) Change in membrane potentialHiperpolarization of photo-receptorRetinal ganglion cellsRetinal interneurons (Bipolar, horizontal and amacrine cells)PhototransductionReceptive field• 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)Eye DiseasesMacular degeneration• Loss of cones: ‘legal blindness’• Central visionRetinitis pigmentosa• Loss of rods: night blindness• Peripheral visionRetinitis PigmentosaOverview① How are images formed by the eye (optics)?② What is the nature and function of retinal neuronal cells?③ The blind spot in your eye….④ How do we perceive color and contrast?⑤ How does the brain process vision?Blind spotBlind 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 blindspotThe blind spotWhy are we not aware of the blind spot?Indicate in the figure the structure related to the blind spotThe Blind SpotDemonstration 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.Overview① How are images formed by the eye (optics)?② What is the nature and function of retinal neuronal cells?③ The blind spot in your eye….④ How do we perceive color and contrast?⑤ How does the brain process vision?Three types of cones with photopigment sensitive to light of different wavelengthsColor blindness or deficiencyDog’s view of the worldBrighter, less detailedBetter peripheral visionBetter night visionBetter at moving objectsLess visual acuity (20/75)Contrast sensitivitySubject 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 PerceptionColor contrast Color constancyThe Perception of Light IntensityPerception of color and contrast• Firing rate of retinal ganglion cells depends