Vision

Perception in the brain

Things are only seen when light alters brain activity.

Electromagnetic spectrum

Made up of waves of electromagnetic energy, the wavelegth determines what types of rays are present.

Visible light

The type of light humans can see; 400 nm to 700 nm.

• Shortest is violet, longer are blue, green, yellow orange then red.

• Travels in straight lines, can provide information about objects location, size, shape and features.

• Light goesd from elecromagnatic, to chemical , to electrical.

Pupil

the opening in the center of the iris where light enters the eye.

Lens

n adjustable curved structure behind the pupil that focuses light rays through the pupil to make an image on the retina.

Cornea

a non adjustable clear outer layer that focuses light and protects the eye.

Retina

Back layer of the eye containing receptors

Order that light goes when it enters the eye

Light passes through the cornea, then pupil, then lens, to hit the back of the eye/ the receptors/retina.

Virtreous Humous

The fluid inside the eye that light passes through.

Normal vision

Light focuses on the plane of the retina

Hyperopia

Light focuses behind the plane of the retina.

Myopia

Light focuses before the plane of the retina.

• Some theories believe that focusing on a single place or certain light (artificial light) can lead to myopia.

Retina

The rear surface of the eye, lined with visual receptors

• Light from the left strikes the right half and vice versa.

• The visual system codes the image with neuronal activity.

Rods

• type of photoreceptor in the retina gathering light for low light levels, abundant in periphery of retina, rods greatly outnumber cones (20:1).

  • Contain disks (Rod Lamellae) embedded in pigment epithelial cells (provide nutrients and remove waste), the cell body + nucleus, the rod spherule and the synaptic terminal.

  • Fall beltween short and medium cones on the visible light spectrum

Transduction in the rod

• In the dark, a protein called Rhodopsin containing retinal (in an inactive 11-cis configuration)

• Phosphodiesterase remains inactive allowing high concentrations of cGMP.

• The cGMP is bound to an ion channel allowing it to open and the rod becomes depolarized.

• In the light, the light activates the retinal that changes it from the 11-cis configeration to the all-trans configuration which changes rhodopsin to metarhodopsin II.

• The G protein leaves and changes the cGMP to GMP which reduces how much cGMP is availible to attach to ion channels.

• cGMP’s detach from the ion channels and cause them to close, the rod becomes hyperpolarize, this also leads to reduced internal levels of Ca and activates guanylate cyclase to replenish cGMP.

Cones

• a type of photoreceptor gathering light at higher light levels, abundant in/near fovea.

  • Short cone (purple/blue), medium cone (greenish) and long cone (yellows and reds).

  • Long and medium cones are more abundant and distributed haphazardly.

Photopigments

Chemicals that release energy when struck by light.

• Consist of 11-cis-retinal which is a derivative of vitamin A.

• 11-cis-retinal are bound to a protein called opsins which modify the photopigments’ sensitivity to different wavelenghts of light.

• Rods have rhodopsin and cones have photopsins

• Light cinverts 11-cis-retinal to all-trans-retinal and releases energy that activates second messangers within the cell.

Bipolar cells

Recieve messages that come from the retina through receptors in the back of the eye, located closer to the centre of the eye.

Horizontal cells

A type of cell that receives input from the receptors and delivers inhibatory input to bipolar cells.

Receptors

located at the back of the eye; light must pass through the ganglion, amacrine, and bipolar cells to get to the receptors.

• The cells are transparent to allow much light to pass through.

• In the fovea, the bipolar, amacrine, horixontal and ganglion cells part so that light has direct access to the photoreceptors.

• The muller cells can serve as optical fibres to guide light right to the photoreceptors.

Ganglion cells

located closer ot the center of the eye, recieve messages from bipolar cells.

• The axons unite and travel back to the brain as the optic nerve.

• Three categories of primate ganglion cells: parvocellular, magnocellular and konicellular

Parvocellular neurons

• Within the geniculate cortex of the thalamus

• small cell bodies and small receptive fields mostly in or near the forvea; specialized for colour and fine details.

  • Connect to cones that detect visual details and colors.

Magnocellular neurons

• Within the geniculate cortex of the thalamus

• larger cell bodies and receptive fields, distributed evenly throughout the retina; specialize for perception of movement and patterns.

  • Respond to movement and patterns

Koniocellular neurons

• small cell bodies occuring throughout the retina.

  • Variety of functions.

Amacrine cells

et information from bipolar cells and send it to bipolar cells (connect the two), ganglion cells and other amacrine cells.

• These also refine the responses of bipolar and ganglion cells to enable that certain ones respond mainly to shapes, directions of mocement, colour etc.

Muller cell

a type of glial cell, surrounding the rod and cone cell body.

Optic nerve

ganglion cell axons that exit through the back of the eye and both from each eye meet at the optic chasm.

• The optic nevre from each eye has two parts; right field and left field

• When dividing at the optic chiasm the left side from both eyes goes to the right side of the brail while the right side from each eye goes to the left side

Blind spot

• the point where the optic nerve exits and where blood vessels enter and leave, has no receptors.

  • The brain will often fill the gaps of the blind spot, or the other eye can fill in the missing visual information.

Fovea

a tiny area of the retina that is specialized for acute, detailed vision, packed with receptors that connect to a single bipolar cell connected to a single ganglion cell; each cone has a direct route to the brain.

• Many birds have 2 fovea-like areas per eye, one ahead and one to the side.

Parasol ganglion cells

they have a large receptive field with sensitivity to motion, fast conductance, but no colour information. More likely connected to rods.

Midget ganglion cells

the ganglion cells within the fovea of humans and other primates, each cell is small with a small responsive field needing light and are responsive to just a single cone; responds to red-green sensitivity, slow responsive axon.

Bistratified cells

similar to midget cells, small receptive fields and connected to cones with blue-yellow colour sensitivity, likely slow responsive axons.

Cones in the fovea versus rods in the periphery:

• In the fovea each bipolar cell recieves excitation from just one cone and inhibition from a few surrounding cones with one midget ganglion cell; much better sensitivity to detail and colour vision.

• In the periphery input from many rods converges onto each bipolar cell to result in higher sensitivity to faint light but poor for bright lights, detailed vision and colour vision.

Thomas Young (late 18 to early 19 century)

recognized colour has a biological explanation; we percieve colour by comparing the responses across a few types of receptors which are sensitive to a different range of wavelengths.

Trichromatic theory/young-helmholtz theory

colour is percieved through the relative rates of respose by three kinds of cones, each one maximally sensitive to a different set of wavelengths.

• Humans discriminate among wavelengths by the ratio of activity across the three types of cones; lights at 550 nm excited medium-wavelength and long-wavelength receptors and short-wavelength almost not at all.

  • If all three are equally active we see white or grey.

Negative colour afterimage

the result of staring at a colored object for a prolonged length of time and then looking at a white surface.

Opponent-process theory

created by Ewald Herring, found that we perceieve color in terms of opposites, the brain has a mechanism that perceives color on a continuum from red to green, yellow to blue, and white to black.

• Receptive fields of bipolar cells and ganglion cells respond in a centre-surround, opponent-process way.

• If you stare at one colour in one location long enough the response is fatigued and the response swings to the opposite.

• The cones responding to a light will decrease theyre response, a decreased response in one cone will indicate that the light must be the opposite colour that that cone usually reads.

Colour constancy

the ability to recognize colors despite changes in lighting.

• Regions emitting different wavelengths of light are percieved as having the same colour.

Colour contrast

Regions emitting the same wavelength of light are percieved as having different colours.

Retinex theory

Edwin Land, the cortex comapres information from various parts of the retina to determine the birghtness and color for each area, visual perception is a process of reasoning and inference as well as retinal stimulation.

Colour vision deficiency

colourblindness, or a decresed ability to percieve differences in colours.

• Poeple with certain genes will fail to develop one type of cone or develop an abnormal cone.

Red-green color deficiency

• most common, people have troubke distinguishing red from green because their long and medium-wavelength cones have the same photopigment instead of different ones.

  • Transferred on the ‘x’ chromosome, more common in males.

  • Recessive so all x chromosomes must have it; for men thats only one versus women 2

Protanopia

one type of red-green color deficiency, red cones contain green opsin, responsive to the wrong wavelenghts, seeing greenish when red or green.

Deuteranopia

one type of red-green color deficiency, green cones containing red opsin, responsive to the wavelengths where red is seen versus green.

Tritanoia

blue-yellow colourblindness, it is very rare it is not on the X chromosome and it occurs in males and females, the retina lacks blue cones and objects are perveived in reds and greens.

Fourth cone

• a condition in women, where they have two versions of cone receptor proteins, one being serine and the other alanine. They have a better distinction between one and another colour.

  • Cannot happen in men as they only have one x chromosone, so they will either have the protein that codes for serine or alanine.

Retinocollicular pathway

• Eye recieves information from both left and right visual field.

  • Right visual field goes to left side of eyeball and left visual field to right.

  • Optic nerve to optic disc exiting through blindspot which has no photoreceptors.

  • Crossover at optic chiasm where the info from the right visual field goes to left hemisphere and vice versa.

  • Goes to superior colliculus and pulvinar.

Geniculostriate pathway

• Information crosses the same way at the optic chiasm but the information goes to the lateral geniculate nucleus at the thalamus.

• After this information goes to the visual cortex in the occipital lobe.

• There are parallel pathways in the visual system carrying different visual information.

  • There are pathways for each of the threee retinal ganglion cell types:

    • Parasol cells forming the magnocellular layers, midget cells creating the parvocellular layers and the bistratified creating the koniocellular layers.

Lateral geniculate nucleus

Most of the optic nerve goes here; the thalamic nucleus that recieves incoming visual information.

Superior colliculus

some axons also go here, part of midbrain/tectum, important for eye movements.

Cortical magnification

information from the fovea is allocated disproportionatley more tissue in the primary visual cortex than any peripheral information.

Lateral inhibition

the retina’s way of sharpening contrasts to emphasize the borders of objects; the reduction of activity in one neuron by activity in neighboring neurons.

• The receptors send messages to excite nearby bipolar cells, and they also send messages to horizontal cells that slightly inhibit those bipolar cells and the neighbors to their sides.

• The result is a heightened contrast between an illuminated area and its darker surround.

• The output on receptors is exitation of bipolar cells.

Receptive field

an area in visual space from where light excites or inhibits a visual neuron.

• The receptive field of a rod or cone is the point in space where light strikes the cell.

• Receptive field gets bigger past the receptors, bipolar and up to ganglion cell.

Center-surround receptive fields

• some of the surrounding photoreceptors may have inderect effects/connections through the horizontal cells.

  • The center of the receptive field has direct relations to the bipolar cell while the surround is responsive to the surrounding photoreceptors.

On-center/off-surround ganglion cell:

• In darkness ganglion cell fires at a moderate rate.

• When light is shined onto the surround, the firing rate is reduced.

• If light is shined directly in the center of the receptive field, the firing rate of the ganglion cell increases.

Primary visual cortex

area in the occipital cortex also known as area V1 or sriate cortex, sends information to the frontal cortex which can send information back to the V1.

• Cells in this area respond to patterns of light and dark.

• Damage to this area often results in lack of vision, even in dreams.

Hubel and Wiesel expiriment

cat viewed slides being projected, found that some cells are responsive to bars of light and darkness in different orientation.

Secondary visual cortex

V2 area, processes the information and transmits it to additional areas; could include V3, V4, V5 and inferotemporal cortex.

• In V2 and V3, some cells are responsive to colour and some to disparity between left and right eyes.

• V4 has clusters of cells responsive to speciifc colors like green, orange and purple; responds most strongly to colour imagery.

• Receptive field V2 larger than V1

Ventral stream

visual paths in the temporal cortex that are specialized for identifying and recognizing objects; also known as the “what” path.

• Ganglion cells for colour and definition mostly in ventral pathways

Dorsal stream

visual path in the parietal cortex that helps the motor system locate objects; the “where” or “how” path.

• Ganglion cells for movement and location mostly in dorsal pathways.

Inferior temporal cortex

a portion of the cortex where neurons are highly sensitive to complex aspects of the shape of visual stimuli within a very large receptive field.

• Prefrontal cortex forms hypotheses about an object and feeds it back to the temporal cortex to confirm or deny.

Visual agnosia

damage to the ventral pathway that causes an inability to recognize objects despite otherwise satisfactory vision. 2 types; apperceptive and associative.

Apperceptive agnosia

inability to put features together to recognize a larger object.

Associative agnosia

able to copy an object but are not too sure what it is/unable to recogize what the objects are.

Fusiform gyrus

a brain area of the inferior temporal cortex that recognizes faces (especially right hemisphere).

Parrahipocapal place area

cells in the temporal lobe parrahipocampal region mostly responding to faces.

Propagnosia

impaired ability to recognize faces, often due to damage to the right fusiform gyrus, failure of the gyrus to develop fully, or fewer connections between it and the occipital cortex.

Optic ataxia

a specific inability to guide body and hand to grasp objects in space, specifically when objects are in periphery.

Cerebral chromatopsia

cortical colourblindness; colourblindness stemming from injuries in or around area V4.

MT (middle temporal cortex)

or area V5, an area in the middle temporal lobe that is important for perception of visual motion, colour insensitive.

• Small receptive field.

• Retinotopic organization

• Notice acceleration, deacceleration and absolute speed.

• Also notice photos that imply movement.

Saccades

oluntary eye movements; area MT and parts of the parietal cortex decrease their cactivity during coluntary eye movements.

MST (medial superior temporal cortex)

temporal cortex area that responds to the expansion, contraction or rotation of a visual display, colour insensitive.

• Dorsal stream

• Large receptive field

• Coarses retinotopic map.

• Occurs when someone moves forward, bakcwards, or tilts their head.

Motion blindness

Also known as Akinetopsia; being able to see objects but unable to see whether they are moving, or which direction and how fast.

Visual imagery

the recreation of an area in the mind, starts in language and memory areas then gradually activates V1.

Aphantasia

little or no auditory imagery, tend ot be attraced towards math and science.

Hyperaphantasia

can imagine a visual scene almost as vividly as actually seeing it.

Blindsight

damage to area V1 , the ability to respond to visual information without percieving it consciously.

• Small areas of healthy tissue surviving in otherwised damaged area, or input from eyes may go to other areas than V1.

• Patient DB, loss of right primarl visual cortex meaning lost awareness of information presented to his left visual field.

• The pulvinar sends output to the cortical areas that process object location and motion, bypassing the primary visual cortex.

Left primary visual cortex damage

loss of awareness of information presented to the right visual field.

Simple cell

A type of visual cortex cell that has a receptive field with fixed excitatory and inhibitory zones, the receptive field is a vertical bar and if the bar moves angled or location wise it reduces the response.

Complex cells

in areas V1 and V2, respond to bars and edges equally anywhere within a large receptive field, if the bar is oriented/angled differently it has less activation.

• Respond mostly to stimuli moving

End-stopped/hypercomplex cell

resembles a complex cell and in areas V1 and V2 but it has strong inhibatory areas at one end of its bar-shaped receptive field, the light must be contained within the receptive field so this cell must detect edges.

Feature detectors

neurons whose responses indicate the presence of a particular feature (ex. downward motion).

Cells in the cortical layers

cells in a certain column were responsible for a specific orientation of a bar of light, while the other columns had other orientations.

Ocular dominance column

a column responsive for information from one of the eyes (right or left)

Orientation column

the columns that represent specific orientations of light.

Retinal disparity

the discrepancy between what the left and right eyes see.

Strabismus

• aka. lazy eye, a condition where the eyes dont point in the same direction, leads to most children attending to one eye and not the other.

  • Treatment is to cover one eye to encourage attention to the ignored eye, or encourage activities that must use both eyes.

astigmatism

a decreased responsiveness to one kind of line over another, caused by an asymmetric curvature of the eye

Cataracts

cloudy spots on the lenses that prevent perception of anything other than bright versus dark. Must be surgically removed.