Overview of the Visual System Dr. Patrick Degenaar Lecturer in Neurobionics (Institute of Biomedical Engineering) (Division of Neuroscience, Faculty of Medicine)
Dr. Patrick Degenaar Biological Vision VS Hawk Oscar “He/she has eyes like a Hawk!” Hawk’s eyes are smaller! Hawks have fewer neurons to perform visual processing! Why???
Dr. Patrick Degenaar What is the purpose of vision? Scene selection Light intensity Colour Night vision Image processing (head and eye movements) (retina and pupil) (Retina cones) (Retina rods) (post-receptoral pathway) The Eye is not a camera!!!!!! -- The purpose of vision is to understand the world around us!
Dr. Patrick Degenaar Lateral Geniculate Nucleus (LGN) Visual Cortex Optic Nerve The visual system
Dr. Patrick Degenaar Glaucoma Myopia Optic Neuritis Optic Neuropathy Papilledema Diplopia Retinitis Pigmentosa Macular Degeneration Cancer Cataphracts Orbital cellulitis Diabetic retinopathy Things that can go wrong……..
Dr. Patrick Degenaar The eye: Physiological Optics
Dr. Patrick Degenaar The eye as an imaging system Principal refraction occurs at the cornea Focal power of the cornea is ~ 60 dioptres The lens provides variable focus (accommodation) Lens power varies with age ~ 6 dioptres in adults For small pupil sizes resolution is limited by diffraction For large pupil sizes aberration limits resolution
Dr. Patrick Degenaar Anterior chamber Formation Secretion / diffusion / ultrafiltration from ciliary body ‘Plasma-like’ Replaced every 100 minutes Drains through trabecular meshwork Function Maintenance of IOP (16mm Hg) Contributes to transparency Metabolic support for lens, cornea & vitreous Lens Anterior chamber Cornea Trabecular meshwork Iris Schlemm’s canal
Dr. Patrick Degenaar Glaucoma Lens Anterior chamber Cornea Trabecular meshwork Iris Schlemm’s canal
Dr. Patrick Degenaar Inner Nucleus Layer Ganglion Cell Layer Outer Nucleus Layer Inner Plexiform Layer Outer Plexiform Layer Pigment Epithelium Layer Photoreceptor Layer Structure of the Retina Ramon y Cajal (1900) Retina section Cajal’s drawing
Dr. Patrick Degenaar Photoreceptor cells Horizontal Cells Bipolar Cells Amacrine Cells Ganglion cells Retina Function Retinal Epithelium
Dr. Patrick Degenaar Photoreceptors Rods Scotopic Monochromatic Cones Photopic Colour vision
Dr. Patrick Degenaar Why do we need colour?
Dr. Patrick Degenaar Colour sensitivity L - cone M - cone S - cone Rod
Dr. Patrick Degenaar Colour blindness Congenital colour deficiencies - affecting 8% population Protanopia: (protanomally): missing (abnormal) L cone Deuteranopia : missing (abnormal) M cone Tritanopia : missing (abnormal) S cone Monochromacy
Dr. Patrick Degenaar Ishihara tests for colour blindness
Dr. Patrick Degenaar Photoreceptors and photopigments
Dr. Patrick Degenaar Light closes the cGMP channels in the outer segment Physiology of phototransduction In darkness photoreceptors are relatively depolarised. A constant depolarising current flows through cGMP gated cation channels in the outer segment Light hyperpolarises photoreceptors
Dr. Patrick Degenaar A biochemical cascade links photo-isomerisation of the visual pigment to the closure of the cGMP cation channel GAIN…….. hyperpolarisation Phototransduction
Dr. Patrick Degenaar Photoreceptor distribution
Dr. Patrick Degenaar Equal visibility with eccentricity Arises from: arrangement of the photoreceptor mosiac post receptoral processing of the image
Dr. Patrick Degenaar Fovea – avascular zone Photoreceptors Fovea Maximum density of photoreceptors Maximum density of photoreceptors High spatial resolution High spatial resolution
Dr. Patrick Degenaar Electroretinogram ERG- retinal activity ERP: phototransduction A-wave (LRP): photoreceptor (16-25ms) B-wave: ON-bipolar (+HC’s) Ops: amacrine cell circuits C-wave: K+ flux PR-RPE space ( ms)
Dr. Patrick Degenaar Visual Processing begins in the retina Red Channel Green Channel Red-green edges via subtraction
Dr. Patrick Degenaar Colour opponent channels Only two colour opponent channels are necessary Red and Green cone sensitivitiesare very close together
Dr. Patrick Degenaar Initiated at in the outerplexiform layer of the retina Bipolar cells receive direct photoreceptor input at the receptive field centre together with surround input fed via horizontal cells Visual Processing begins in the retina
Dr. Patrick Degenaar B on / Y off R on / G off R off / G on G on / R off G off / R on R = L cone G = M cone Y = L + M B = S cone Basic Currency of Parvocellular Pathway Colour opponent primate ganglion cells
Dr. Patrick Degenaar Cones: All hyperpolarize Bipolar Cells: hyperpolarize –OFF depolarize – ON Ganglion Cells: ON and OFF The retinal cone pathway On - centre Off - centre
Dr. Patrick Degenaar ON – centre ganglion cell OFF – centre ganglion cell Centre surround physiology
Dr. Patrick Degenaar Retina Summary Layered structure Photoreceptors are the last layer in the light path More rods than cones Non-uniform and different spacing of rods and cones Cones mediate high spatial resolution at high light levels Rods operate at low light levels
Dr. Patrick Degenaar Retinal physiology - summary Wavelength encoding is achieved by a subtractive process - different spectral classes of cone are combined in opponency. Signals in the retina are all analogue apart from the ganglion cells which produce action potentials for communication with the CNS Ganglion cells are of two broad types; P-type, which are colour opponent and have sustained responses, and M type, which are achromatic and transient. Centre-surround opponency provide the first spatial processing of visual information. Surround receptive fields result from horizontal and amacrine cells.
Dr. Patrick Degenaar Central projections of retinal Ganglion cells Major Pathway: Primary Visual Pathway retina - lateral geniculate nucleus (LGN) - cortex Other Pathways: Pretectum - pupillary light reflex Hypothalamus - circadian axis input Superior colliculus - eye movements orientation
Dr. Patrick Degenaar What happens here ? Primary visual pathway Retina optic nerve Chiasm LGN optic radiation cortex
Dr. Patrick Degenaar Anatomy LGN shows precise segregation of Left/Right eye. Together with functional segregation of P and M retinal inputs Physiology The receptive fields are generally of the simple concentric centre-surround of the types. LGN – structural segregation
Dr. Patrick Degenaar What do we know about the visual cortex? Single unit recording Primate Cortex Organisation: Hubel and Wiesel
Dr. Patrick Degenaar V1 – characterised by the emergence of edge detection Generation of simple receptive fields
Dr. Patrick Degenaar What is the functional significance of bringing Left and Right together ? Ocular dominance stripes in V1
Dr. Patrick Degenaar Random Dot Stereograms All dots are exactly the same in each slide Except the dots in the square The dots in Slide one are moved over one column in slide two (shown by the yellow section)
Dr. Patrick Degenaar Random Dot Stereograms Moving the dots creates disparate points for that square region when seen through the stereoscope. Even without the aid of any monocular cues, you still see a 3D pop- out square.
Dr. Patrick Degenaar Very complex Understanding of visual system decreases rapidly after v1 Visual Cortex Map
Dr. Patrick Degenaar Primary visual cortex - summary Layered structure, with specific cells types in different layers Information from each eye is incorporated to generate neurones sensitive to depth Cells are sensitive to edges – orientation columns Many cells are motion sensitive. Colour cells are segregated from non colour cells
Dr. Patrick Degenaar Biological Vision VS Hawk Oscar “He/she has eyes like a Hawk!” Hawk’s eyes are smaller! Hawks have fewer neurons to perform visual processing! Why???
Dr. Patrick Degenaar Reading and Learning Web resource on the retina: Visual Cortex “A Vision of the Brain” – S. Zeki [Blackwell]
Dr. Patrick Degenaar Supplementary information Ref:
Dr. Patrick Degenaar Supplementary information Intensity Centre surround response x x Cones Horizontal cells Bipolar cell The original 2D image that hits the retina If we take a 1D intensity profile… The centre surround processing will output an image which effectively transmits features which can be interpreted as edges later in the visual cortex