Vision

HistoryExtromissive theory

– Plato--light flows from our eye– Ptolemy– Challenged later through experiments by Huygens, Newton

Intromissive Theory– Aristotle--object creates “material images” that enter our eyes

HistoryPointillist theory of vision

– Abu Ali al-Hassan ibn al-Hasan ibn al-Haytham (Alhazen)

– Optical Scientist of Middle Ages

– We see a collection of points on surfaces of objects (1:1 ratio)

OpticsFour basic behaviors of light– Reflection– Refraction– Diffraction– Absorption

ReflectionLight waves bounce off objects

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DiffractionThe bending of waves around small particles

The spreading out of waves past small openings

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DiffractionHuygensThomas Young “Double Slit” Experiment

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AbsorptionTransfer of energy from light to object

Frequency of light is at or near energy levels of electrons of matter

Electrons absorb energy of light wave and change their energy state

RefractionLight bends as it travels from one medium to another

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Our EyesThe most sensitive and delicate organ we have

We are able to see the world and our brains receive the information from the world

The images we see are made up of light reflected from the objects we look at

Components of EyeRetina Membranous, sensitive nerve tissue in the eye

Converts images from the eye’s optical system into electrical impulse

These impulses are sent along the optic nerve to the brain

More than 125 Million Rods and 6 Million Cones

Parts of the EyeCornea Transparent portion of the outer eye Outward curvature Primary refractor of light to eye Cleaned and nourished from aqueous humor and tears

• Iris Muscular diaphragm Controls the size of the pupil Controls the amount of light that enters Colored portion

Parts of the Eye A hole in the center of the iris

Changes size in response to changes in lighting

Larger in dim lighting conditions

Smaller in brighter lighting conditions

Parts of the EyeMacula-highly sensitive center of retina– Detail

Fovea A tiny spot in the center of the retina

Very center of Macula Contains only cone cells Responsible for our sharpness of vision

Parts of the EyeOptic Disk Small area of the retina where the optic nerve leaves the eye: any image falling here will not be seen

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Parts of the Eye• Choroid Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; also, because of the high melanocytes content, the choroid acts as a light-absorbing layer.

Sclera tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye

Parts of the EyeAqueous humor Clear watery fluid found in the anterior chamber of the eye; maintains pressure and nourishes the cornea and lens

• Vitreous humor Clear, jelly-like fluid found in the back portion of the eye: maintains shape of the eye and attaches to the retina

Parts of the Eye Lens Transparent, biconvex structure Refracts light to be focused on the retina

More spherical when focusing on close objects

Flatter when focusing on faraway objects Along with the cornea, light rays are focused back together on the retina

Forms image of object on the back of retina

How We See Light Passes through

– Cornea– Aqueous Humor– Pupil– Lens– Vitreous Humor– Retina-->Phtoreceptors– Optic Nerve-->Brain

Cornea refracts 70-80%– Change from air to cornea is largest

change in index of refraction Lens--20% Ciliary Muscles/Zonule Fibers

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Focal LengthDistance to bring parallel rays into convergence

Diopters--(1/f) measure of lens power

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How We See1:1Light reflected from different points are converged in the eye

Convex lenses--image forms upside down

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AccommodationLens changes shape to accomade vision from far and close distances

Ciliary/Zonule FibersResting state:

– Lens fatAt a distance--ciliary muslce relaxed, zonule fibers are tensioned->lens is pulled flat

Close Distance--ciliary muscle is constricted, zonule fibers relaxed-->Lens rounds

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CellsFive different cells in retina

PhotoreceptorsBipolar CellsGanglion CellsHorizontal CellsAmacrine cells

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Phototransduction Electrical changes in rods and cones cause electrical responses in other cells in the retina

Lead to production of action potentials in neurons

These neurons form optical nerve The place on the retina where the axons of neurons converge is called the blind spot

PhototransductionTo convert light energy to a change in membrane potential--G Protein coupled receptor

Rhodopsin has molecule bound--photon of light releases – Activates G-protein (Transducin)– 2nd Messenger Cascade (Phosphodiesterase)

Turns cGMP to GMPcGMP gated Na+ channels close

– Ion Channel closedDarkness--Rods are depolarized (release glu)Light hyperpolarizes

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Center/Surround Receptive FieldsCenter is from direct connections from photoreceptors

Surround--mediated by horizontal cell connections

on-center cell--stimulated when the center of receptive field exposed to light– inhibited when the surround is exposed to light

Off-center cell--opposite Emphasize contrast at borders (edges)

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Retinofugal ProjectionVisual Pathway from Optic Nerve to Brain

The optic nerves from the left and right eyes partially decussate in the optic chiasma– Fibers from nasal retina cross over– Leads to binocular vision

Left visual field viewed through right hemisphereOverlap in visual fields-->Depth

Travel through optic tracts to the Lateral Geniculate Nucleus (LGN) of the Thalamus

Some Visual Projections

Hypothalamus– A small number of axons connect here– control of sleep wake cycles

Midbrain– Pretectum

controls pupillary light reflex, certain eye movements

– Superior Colliculusvoluntary and involuntary eye movements

Most go to LGN

Lateral Geniculate NucleusRelays vision to visual cortex

Form optic radiations that terminate in the Primary Visual Cortex (Occiptal Lobe)

Layered– 1,2--magnocellular LGN layers, receive inputs from M-type Ganglion cells (color insensitive, respond transiently, center/surround)

– 3,4,5,6--parvocellular LGN layers, receive inputs from P-type ganglion cells (sustained response, center/surround, color sensitive)

– Intralayers ventral to numbered layers--koniocellular layers, inputs from non M/P-type ganglion cells

Combines inputs from two eyesReceptive fields similar to retinal

LGN-Retinotopic MapVisual field is mapped so two points adjacent in visual field and on retina are connected to two adjacent points on neural surface– Retinotopic map is magnified relative to the fovea b/c more photoreceptors

Other Inputs80% of inputs from primary visual cortex

BrainstemModulate responses of LGN neurons– Ex. Responsiveness can be modulated by feeling

Visual CortexAlso known as Striate CortexArea 17--primary visual cortexLayers

– I-Mostly Fibers– II, III, IVA, IVB,V, VI--pyrimdal cells, outputs from cortex

III IVB--other cortical areasV--Pons and superior colliculusVI--LGN

– IVCa and IVCb--stellate cells, receive inputs from LGNIVCa--magnocellular LGN pathwayIVCb--parvocellular LGN pathwayVertical connections from IVC to III

Ocular Dominance Columns

Zebra stripes in layer IV (~0.5mm wide)

Adjacent areas are right or left eye inputs

Wiesel and Hubel– Microelectrodes & cats

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BlobsLayers II and II

– Cytochrome oxidase staining (mitochondrial activity)

Blobs and ocular dominance columbs overlayed in layers II, III and IV

Blobs and interblobs receive parvocellular LGN input (no color)

Blobs also receive koniocellular input (color)

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PathwaysMagnocellular Pathway (M-Channel)

– LGN to IVCa– IVCa to IVB– IVB cells have

Orientation selectivityDirection selectivity for moving stimuli

Binocular selectivityNo Color sensitivity

Specialized for analysis of object motion

PathwaysParvocellular Interblob Pathway (P-IB)– LGN to IVCb– IVCB to layers II and II to interblobs

– HaveVery selective to orientationNo color sensitivityBinocular sensitivitySmall rf

Specialized for analysis of object shape

PathwaysKoniocellular Pathway--Blob Channel– LGN inputs koniocellular layers and parvocellular layers via layer IVCb

– Have:Monocular sensitivityColorNo orientation selectivity

Analysis of Object color

SynthesisCortical Module--basic unit for processing visual stimuli in visual cortex

Hubel and Wiesel--– 2mm X 2mm– Cortical image of a point in space

Two complete sets of ocular dominance columns (layer IV)

Sixteen blobs (III)Two complete orientation columns(interblob layer III)

Striate cortex has about 1000 that act in parallel

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