Chapter 9 The Eye

Introduction Significance of vision –Relationship between human eye & camera –Retina Photoreceptors: Converts light energy into neural activity Detects differences in intensity of light –Lateral geniculate nucleus (LGN) First synaptic relay in the primary visual pathway Visual information ascends to cortex  interpreted and remembered

Properties of Light Light –Electromagnetic radiation –Wavelength, frequency, amplitude

Properties of Light Light –Energy is proportional to frequency –e.g., gamma radiation and cool colors - high energy –e.g., radio waves and hot colors - low energy

Properties of Light Optics –Study of light rays and their interactions Reflection –Bouncing of light rays off a surface Absorption –Transfer of light energy to a particle or surface Refraction –Bending of light rays from one medium to another

The Structure of the Eye Gross Anatomy of the Eye –Pupil: Opening where light enters the eye –Sclera: White of the eye –Iris: Gives color to eyes –Cornea: Glassy transparent external surface of the eye –Optic nerve: Bundle of axons from the retina

The Structure of the Eye Ophthalmoscopic Appearance of the Eye

Cross-Sectional Anatomy of the Eye

George Wald 1906 – 1997 received the Nobel Prize in 1967 for discoveries concerning the primary physiological and chemical visual processes in the eye

Image Formation by the Eye Refraction of light by the cornea –Eye collects light, focuses on retina, forms images

Image Formation by the Eye Accommodation by the Lens –Changing shape of lens allows extra focusing power

Image Formation by the Eye The Pupillary Light Reflex –Connections between retina and brain stem neurons that control muscle around pupil –Continuously adjusting to different ambient light levels –Consensual –Pupil similar to the aperture of a camera

Image Formation by the Eye The Visual Field –Amount of space viewed by the retina when the eye is fixated straight ahead

Visual Acuity –Ability to distinguish two nearby points –Visual Angle: Distances across the retina described in degrees

Microscopic Anatomy of the Retina Direct (vertical) pathway: –Ganglion cells  –Bipolar cells  –Photoreceptors

Microscopic Anatomy of the Retina Retinal processing also influenced lateral connections: –Horizontal cells Receive input from photoreceptor s and project to other photoreceptor s and bipolar cells –Amacrine cells Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

Microscopic Anatomy of the Retina The Laminar Organization –Inside-out –Light passes through ganglion and bipolar cells before reaching photoreceptors

Photoreceptor Structure –Converts electromagnetic radiation to neural signals –Four main regions Outer segment Inner segment Cell body Synaptic terminal –Types of photoreceptors Rods and cones

Regional Differences in Retinal Structure –Varies from fovea to retinal periphery –Peripheral retina Higher ratio of rods to cones Higher ratio of photoreceptors to ganglion cells More sensitive to light

Regional Differences in Retinal Structure (Cont’d) –Cross-section of fovea: Pit in retina where outer layers are pushed aside Maximizes visual acuity –Central fovea: All cones (no rods) 1:1 ratio with ganglion cells Area of highest visual acuity

Phototransduction Phototransduction in Rods –Light energy interacts with photopigment to produce a change in membrane potential –Analogous to activity at G- protein coupled neurotransmitter receptor - but causes a decrease in second messenger

Phototransduction in Rods –Dark current: Rod outer segments are depolarized in the dark because of steady influx of Na + –Photoreceptors hyperpolarize in response to light

Phototransduction in Rods –Light activated biochemical cascade in a photoreceptor –The consequence of this biochemical cascade is signal amplification

Color Blindness - a genetic disorder where individuals lose the ability to distinguish some or all variations in color vision

Phototransduction in Cones –Similar to rod phototransduction –Different opsins Red, green, blue Color detection –Contributions of blue, green, and red cones to retinal signal –Spectral sensitivity –Young-Helmholtz trichromacy theory of color vision

Phototransduction Dark and Light Adaptation –Dark adaptation—factors Dilation of pupils Regeneration of unbleached rhodopsin Adjustment of functional circuitry All-cone daytime vision All-rod nighttime vision 20–25 minutes

Deuteranopia & Deuteranomaly (forms of GREEN deficiency) Deuteranopia and deuteranomaly are the most common forms of color-blindness. People with these conditions have cones that are insensitive to medium wavelengths (greens), but the end result is similar to protonopia, with the exception that reds do not look as dark. Deuteranomaly is the less severe of the two conditions. Individuals with deuteranomaly cannot see reds and greens in the same way that people with full color vision can, they are able to distinguish between shades of reds and greens relatively accurately. full color vision green deficiencies

Protanopia & Protanomaly (forms of RED deficiency) Color receptors in the eyes of people with protanopia are not sensitive to long wavelengths (the reds). Reds look more like beiges and appear to be somewhat darker than they actually are. The greens tend to look similar to the reds. Protanomaly is milder than protanopia, but the end result is similar. Although many people with protanomaly can distinguish some reds and greens, they cannot do so as easily as someone with color-normal vision, and, as with protanopia, reds tend to look darker as well. full color vision red deficiencies

Tritanopia (a form of BLUE deficiency) Note… Tritanopia is much less common than the other categories mentioned above. Tritanopia is insensitivity to short wavelengths (the blues). Blues and greens can be confused, but yellows are also affected in that they can blend with blues or appear as lighter shades of red. full color vision blue deficiencies

Rod Monochromacy or Achromacy (NO color vision) This group constitutes an extremely small minority among people who are color- blind. All cones of the eye are non-functional, so the rods (receptors which can only differentiate between light and dark) are the only available source of visual information. Individuals with achromacy see no color at all. People with achromacy often have poor visual acuity and have an aversion to bright light. full color vision complete cone deficiency

Phototransduction Dark and Light Adaptation –Calcium’s Role in Light Adaptation Calcium concentration changes in photoreceptors Indirectly regulates levels of cGMP  channels

Retinal Processing Research in ganglion cell output by –Keffer Hartline, Stephen Kuffler, and Horace Barlow –Only ganglion cells produce action potentials Research in how ganglion cell properties are generated by synaptic interactions in the retina –John Dowling and Frank Werblin –Other retinal neurons produce graded changes in membrane potential

Retinal Processing Transformations in the Outer Plexiform Layer –Photoreceptors release less neurotransmitter when stimulated by light –Influence horizontal cells and bipolar cells

Retinal Processing Receptive Field: “On” and “Off” Bipolar Cells –Receptive field: Stimulation in a small part of the visual field changes a cell’s membrane potential –Antagonistic center-surround receptive fields

Retinal Processing On-center Bipolar Cell –Light on (less glutamate); Light off -> more glutamate

Ganglion Cell Receptive Fields –On-Center and Off-Center ganglion cells –Responsive to differences in illumination

Types of Ganglion Cells –Appearance, connectivity, and electrophysiological properties M-type (Magno) and P-type (Parvo)ganglion cells in monkey and human retina

Retinal Output Color-Opponent Ganglion Cells

Retinal Output Parallel Processing –Simultaneous input from two eyes Information from compared in cortex –Depth and the distance of object –Information about light and dark: ON-center and OFF-center ganglion cells –Different receptive fields and response properties of retinal ganglion cells: M- and P- cells, and nonM-nonP cells

Concluding Remarks Light emitted by or reflected off objects in space  imaged onto the retina Transduction –Light energy converted into membrane potentials –Phototransduction parallels olfactory transduction Electrical-to-chemical-electrical signal Mapping of visual space onto retina cells not uniform