Vision

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Vision

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the visual system transduces electromagnetic stimuli(light) into an electrical signal(nerve impulse)

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For vision, we need

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-stimulus

-receptor to detect stimulus

-a transducer to turn detection into graded electrical signals

-a processor that makes sense of the electrical signal

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Sensory Transduction

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process by which sensory stimuli are transduced into electrical potentials

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Light

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-eyes detect presence of light

-electromagnetic energy

-the photon is the basic unit of light

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More light

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-the color of an object is a result of the wavelength of light it reflects

-wavelengths of between 380 and 760 nm is visible to us

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Visual spectrum

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is not a property of the light, but a property of the reception

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Stimulus

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perceived color of light is determined by 3 dimensions: hue, brightness, saturation

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Hue

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the dominant wavelength

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Brightness

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-intensity

-is influenced by the amplitude of the signal

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Saturation

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-purity

-influenced by quality or purity of signal

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Orbits

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bony front of skull in which eyes are suspended

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Sclera

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-white outer coat of eye

-holds eye in place

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Cornea

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outer layer of the front of the eye

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Pupil

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regulates the amount of light that enters the eye

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Iris

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pigmented ring of muscles situated behind the cornea

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Lens

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-altered by contraction of ciliary muscles

-changes shape and permits eye to focus on images of near or distant objects

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Vitreous humor

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-fills main part of eye

-jelly, gelatinous fluid

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Retina

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-contains photoreceptors

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Fovea

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-central point of the retina

-responsible for most acute vision

-contains only cones

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Optic disk

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-exit point from the retina

-axons of the ganglion cells that form the optic nerve

-responsible for the blind spot

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Anatomy of the Visual System

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-image must be focused on retina

-sensory receptors(photo receptors) located in retina

-retina is actually part of the brain

-retina and optic nerve are in the CNS

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Photoreceptors

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-receptors facing backwards(away from front of eye)

-light must pass through nervous tissue before activation photoreceptors

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More photoreceptors

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-specialized type of neuron found in the retina that is capable of phototransduction

-convert light into neural signals

-photoreceptor proteins in the cell that absorb photons triggering a change in the cell's membrane potential

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Rods

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-photoreceptors

-very sensitive

-easily bleached

-concentrated in the periphery

-poor acuity

-useful in the dark

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Cones

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-photoreceptor

-not very active in dim light

-essential for color vision

-daytime vision

-small features of the environment

-highest acuity/sharpness

-located in fovea

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3 types of cones that differ in the wavelength they absord

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-long=red

-medium=green

-short=blue

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Color blindness

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-inability to see color under normal lighting

-lack of cones that perceive color

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Red-green color blindness

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-protanopia

-dueteranopia

-protanomaly

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Blue-yellow color blindness

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-tritanopia

-tritanomaly

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Phototransduction

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-molecule changes when light strikes it

-changes shape

-causes chain reaction that ultimately affects action potentials

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Photopigments

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-phototransduction

-light causes bleaching of the photopigment contained in the photoreceptor

-photopigments are unstable chemicals that are sensitive to different wavelengths of light

-rhodopsin & iodopsin

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Rhodopsin

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-naturally pinkish and bleaches to a pale yellow

when it is exposed to light it breaks into retinal and opsin

-splitting of the photopigment produces the receptor potential

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Dark adaptation

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-rhodopsin bleaches in response to light

-rods are sensitive to light and take longer to fully adapt to changes in light

-cones take about 10 minutes to adapt to the dark

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Negative afterimage

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-image continues to appear in one's vision after the exposure to the original image has ceased

-caused when eye's photoreceptors adapt to overstimulation and lose sensitivity

-photoreceptors that are constantly exposed to the same stimulus will eventually exhaust their supply of photopigment resulting in a decrease in signal to the brain

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Bipolar cell

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-receive info from the photoreceptors

-sends info to the ganglion cells

-synapse with rods or cones(or horizontal cells)

-unlike most neurons biopolar cells communicate via graded potentials rather than action potentials

-receptive fields are made up of photoreceptor cells

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Ganglion cell

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-receives info from bipolar cells

-axons give rise to the optic nerve

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Horizontal cell

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interconnects adjacent photoreceptors and the outer processes of the bipolar cells

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Amacrine cell

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interconnects adjacent ganglion cells and the inner processes of the bipolar cells

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Visual phototransduction

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-photoreceptors are tonically active(depolarized)

-when light hits them, they hyperpolarize causing a decrease in the amount of glutamate released

-their synapse onto the biopolar cell in inhibitory

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Receptive field

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-point in the visual field that an individual neuron responds to

-location of the receptive field depends on the location of the photoreceptors that provide it with visual info

-receptor to axon ratio

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Periphery of retina

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-many receptors coverage to single ganglion cell

-large area of visual field

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Fovea

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-similar number of ganglion cells and cones

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Center-surround receptive fields

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-off & on

-circular field

-on-center- responds most when light strikes center

-off center- responds most when light strikes out portion of receptive field

-Hubel & Weisel

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2 types of bipolar cells

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-on

-off

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Receptive fields of bipolar cells

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-2 parts= center and surround(they are both mutually antagonistic)

-The surround with respond in opposite direction from the center

-the center is more sensitive but the surround is larger

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ON-CENTER

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+= excitation to inhibition to dark

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OFF-CENTER

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-= excitation to dark inhibition to bright

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Retinal ganglion cells

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-different populations of ganglion cells that send visual info to the brain

-3 types

-90% of axons in the optic nerve go to the lateral geniculate nucleus

-parallel processing form M,P, K ganglion cells in the retina for visual construction and perception

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Receptive field of a ganglion cell

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-center & surround

-response are not graded, but are true action potentials

-strong stimulus will lead to higher frequency of action potentials

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Optic chiasm

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-optic nerve from both eyes meet and cross at the optic chiasm

-at this point, the info coming from both eyes is combined and then splits according to the visual field

-corresponding halves of the visual field are sent to left and right halves of the brain for processing

-right side of primary visual cortex deals with left half of the visual field from both eyes

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Primary visual pathway

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-axons from ganglion cells serve nasal halves of retina cross to the other side of the brain

-each hemisphere receives info from contralateral half of visual scene

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Light hits retina

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-image is inverted from cortex lens onto the retina

-all images processed by opposite side of eye than you think-convex lens

-nasal and temporal fibers

-nasal fibers cross

-temporal fibers do not

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Lateral geniculate nucleus

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-thalamus

-6 layers

-1,4,6 correspond to info from contralateral fibers of the nasal retina

-2,3,5 correspond to ipsilateral fibers to temporal retina

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More, lateral geniculate nucleus

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-layers also respond to magnocellular, parvocellula, koniocellular cells differently.

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Primary visual cortex

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-largest system of human brain

-visual processing

-areas V1-V5

-flow of info through hierarchy

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Visual association ares

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-as info passes through visual hierarchy, complexity of neural representation increases

-neural representation may come at a level of specialization into 2 distinct pathways

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Dorsal stream

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-where/how

-spatial attention

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Ventral stream

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-what

-recognition, identification, categorization

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Color vision

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-different mechanisms at different levels of the system

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Trichromatic theory

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-condition of processing 3 independent channels for conveying color info

-derived from 3 cone types

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Opponent process color theory

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-human visual system interprets info about color by processing signals from cones and rods in an antagonistic manner

-3 types of comes have some overlap in wavelengths of light

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Opponent-process coding

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-neurons respond specifically to pairs of primary colors, red opposing green and blue opposing yellowish

-retina contains 2 kinds of color sensitive ganglion cells: red-green, yellow-blue