Visual Sensory System

1. Visual Sensory System

2. Wavelengths of Light Electromagnetic Spectrum Light: 1. Wave 2. Stream ofphotons Photon: one quantum of energy Light can be: 1. absorbed 2. diffracted 3. reflected 4. transmitted 5. refracted

3. Light Can Be: • 1. Absorbed: • light energy is taken up by encountered material • 2. Diffracted: • light energy can be bent or scattered • 3. Reflected: • light energy can be redirected back to its source • 4. Transmitted: • light energy can be transmitted through material • 5. Refracted: • light energy can be altered as it passes through material

4. Sensory System: Vision Cornea protects eye refracts light Iris colored muscle regulates pupil size Pupil regulates light input Lens focuses images on retina Ciliary Muscles controls shape of lens accomodation Fovea point of central focus contains most cones birds of prey/rodent variation Retina contains photoreceptors The Structure of the Human Eye

5. Accommodation

6. Astigmatism

7. The Retina (make up the optic nerve) • Rods • 100-120 million • sensitive to dim light • black/white discrimination • large numbers on the periphery • Cones • 4-6 million • used for color vision • located near the fovea • red, green, and blue cones

8. Rod/Cone Distribution

9. The Retina • Ganglion cells • Horizontal cells • Bipolar cells • Amacrine cells • Photoreceptors • Rods • Cones

10. Visual Pathway Light to rods/cones to bipolar cells to ganglion cells to LGN cells to Striate Cortex

11. Bipolar cell Active Not Active Photoreceptor Action • In the Dark: • rods are depolarized • rods release glutamate • glutamate is inhibitory • bipolar cells are inhibited • In the Light: • rods are hyperpolarized • no glutamate is released • bipolar cells are not • inhibited (disinhibition) • bipolar cells undergo • spontaneous activity Glutamate (-) Rod cell LIGHT DARK

12. Light Hyperpolarizes the Rods

13. Rhodopsin Photopigment Rhodopsin: made up of retinal and opsin spans the disc membrane acts as a G-protein

14. Light Transduction LIGHT DARK • trans-retinal transformed to cis-retinal • cis-retinal and opsin form rhodopsin • rhodopsin activates guanylate cyclase (GC) • GC increases the synthesis of cGMP • cGMP opens Na+ channels • rod cell depolarizes • increases the release of glutamate • (darkness adjustment–waiting for rhodosin) • cis-retinal transformed to trans-retinal • trans-retinal and opsin dissociate • now active opsin activates transducin • transducin activates PDE • PDE breaks down cGMP to 5’-GMP • 5’GMP closes Na+ channels • rod cell hyperpolarizes • reduces the release of glutamate

15. Rhodopsin Cascade Rhodopsin molecule LIGHT Rod cell disc Inside Rod cell Outside 1 photon of light can block the entry of 1,000,000 Na+ ions

16. Lateral Inhibition BrightnessContrast is created in part by the wiring of the visual system. Horizontal cells run perpendicular to the photoreceptors. These lateral connections inhibit neighboring cells. This antagonistic neural interaction between adjacent regions of the retina creates brightness contrast

17. Lateral Discrimination Result of Lateral Inhibition: Each strip has a uniform color, but all look lighter on the left. Brightness Contrast

18. Receptive Fields • Visual Field: • the whole area of the world that you can see at any time • Right/Left Visual Field: • the part of your visual field only to the right or left • Receptive Field: • the part of the visual field that only one neuron responds to

19. Receptive Field of a Photoreceptor

20. Receptive Field of a Ganglion cell

21. Determining Receptive Fields Specific stimulus presentation Specific cell recording

22. On-Center Off-Surround Receptive Field

23. Off-Center On-Surround Receptive Field

24. Lateral Geniculate Nucleus (LGN) 6 5 Parvocellular layers layers 3-6 smaller cells 4 3 2 1 Magnocellular layers layers 1-2 larger cells

25. LGN Mapping Input from the right visual field is mapped on the left LGN. Input from the left visual field is mapped on the right LGN. LGN layers 1-6

26. OrientationSensitive Cells LGN Receptive Field MotionSensitive Cells

27. How Receptive Fields Sum to Images There is convergence of information as you move from retina to the visual cortex

28. Striate Cortex

29. Striate Cortex Mapping

30. Orientation and Ocular Dominance Hubel et al. 1978 Column: a vertical arrangement of neurons Orientation Columns: systematic, progressive change in preferred orientation Ocular Dominance Columns: preferential response to one eye stimulation

31. Orientation and Ocular Dominance

32. Color Vision

33. The Trichromatic Theory(Young-Helmholtz) • Can get any color by mixing just three wavelengths • Blue-sensitive cones • Green-sensitive cones • Red-sensitive cones • Each type of cone would have a direct path to the brain • Discriminate among wavelengths by the ratio of activity • across the three different types of cones • To see purple: • 60% (of maximum) blue-sensitive cone response • 50% red-sensitive cone response • 5% green-sensitive cone response • Many areas of the retina lack the diversity to follow this rule

34. Trichromacy

35. Opponent-Process Theory • We perceive color in terms of paired • opposites: • red versus green • blue versus yellow • white versus black • Blue-Yellow Opponent Bipolar cell • Excited by: • short wave or blue light • Inhibited by either: • long wave or red light • medium wave or green light • but strongest by a mix of two-yellow • When Excited – blue perception • When Inhibited – yellow perception

36. Support for the Opponent-Process Monkey LGN cell 1 Monkey LGN cell 2 inhibited excited excited inhibited

37. Negative Afterimage