Visual Sensory System Wavelengths of Light Electromagnetic Spectrum

Visual Sensory System

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

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

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

Accommodation

Astigmatism

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

Rod/Cone Distribution

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

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

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

Light Hyperpolarizes the Rods

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

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

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

Lateral Inhibition Brightness Contrast 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

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

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

Receptive Field of a Photoreceptor

Receptive Field of a Ganglion cell

Determining Receptive Fields Specific stimulus presentation Specific cell recording

On-Center Off-Surround Receptive Field

Off-Center On-Surround Receptive Field

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

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 layers 1 -6

LGN Receptive Field Orientation Sensitive Cells Motion Sensitive Cells

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

Striate Cortex

Striate Cortex Mapping

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

Orientation and Ocular Dominance

Color Vision

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

Trichromacy

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

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

Negative Afterimage