Psychology: Vision
Terms
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Describe the Photopic System
of Dark Adaptation. - Acts rapidly reaching a plateau at 5-10 mins. Cones serve this system.
- Photoreceptors
- receptors of the retina (rods& cones) which transduce photic energy into electrical potentials.
- Accomodation (monocular cue)
- Changes in shape of the lens to focus on near or more distant objects.
- Fovea
- Region of the retina that mediates the most acute vision.
- Texture gradient
- Retinal images of texture elements get smaller & closer together w/ increased distance.
- Rods
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150 million in the eye spread out over entire surface of the retina except for fovea
much more sensitive to light than cones-allow us to see in dim light.
Provide low acuity vision:
a)info from many rods combined on a single ganglion cell.
b)rods are not dense enough anywhere in the retina to provide high acuity.
-they provide no info about colour.
-like cones they are more sensitive to yellow & green than red light. - Describe the Scotopic System of Dark Adaptation.
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More potent, slower acting, accounting for extreamly high sensitivity after 20 mins. Rods serve this system.
When we are dark adapted & sensitive to dim lights we are said to be in a Scotopic State.
- What parts exists in the Frontal Chamber of the eye?
- Lens, Iris, Pupil, Cornea, Aqueous Humor
- Dark Adaptation
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Gradual lowering of visual threshold w/ time spent in darkness.
Involves 2 mechanisms:
1)Photopic System
2)Scoptic System
SS is more sensitive than PS for all wavelengths except very long. When we are fully adapted we detect weakest lights (except red) by the SS. - Pupil
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Hole in the centre of the iris thru which the light passes
In young adults, the diameter of the pupil ranges from 2 to 8 mm. The range is somewhat smaller in older adults. Pupil diameter decreases with the level of illumination, and therefore plays a (minor) role in regulating visual sensitivity in response to changes in light level. Pupil size influences the quality of the retinal image: smaller pupils tend to produce clearer images and increase the depth of field. However, if the pupil is too small (roughly, less than 2-2.5mm), retinal image quality is degraded due to diffraction. Finally, it is interesting to note that the size of the pupil is controlled by the autonomica nervous system, and therefore pupil size is affected by your emotional state and level of arousal.
- Amacrine cells in the retina.
- Neurons in the retina that interconnect ganglion cells and bipolar cells.
- Retina
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Thin membrane of cells that lines the rear interior of the eyeball.
Contains receptor cells for vision (rods & cones). - Cells in the retina?
- Horizontal, Bipolar, Amacrine, Ganglion
- Lens
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Transparent structure behind the iris that helps focus the light that has passed through the pupil.
Can change shape (adapt) by contracting ciliary muscles. - Retinal Disparity
- Retinal disparity is a cue to depth, based on fact that each retina recieves a slightly different image of an object.
- Iris
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Coloured (usually brown or blue) dohnut shaped muscular stucture of the eye.
-located behind the chornea & infront of the lens that controls the size of the pupil & in that way controls the amount of light that can enter the eye's interior. - Describe the path of light.
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A)Outside Eyeball
-chornea->aqeous humor->pupil(thru iris)->lens
B)Inside Eyeball
-viterous humor (fluid filling centre of eye)->retina(photoreceptors)->horizontal cells->bipolar cells-> amacrine cells->ganglion cells - What parts of the eye exist in the rear chamber?
- Retina, Fovea, Photoreceptors
- Cones
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6 mill in retina.
Most tightly packed in the fovea. Outside the fovea, cone density drops.There are few cones at the periphery of the retina.
-responsible for vision in normal light. Are relatively insensitive to light so they may not work at night.
Cones provide most info about visual detail-high acuity vision b/c each cone goes to a seperate ganglion cell. Provide info the brain uses to percieve colour: a)3 types of cones: red, blue, green. b)each type of cone is maximally sensitive to wavelengths in a particular range of spectrum. - Chornea
- Curved, tansparent tissue infront of the eyeball which helps to focus light rays as they 1st enter the eye.
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cataract
- An opacity of the lens is called a cataract. A cataract blocks the transmission of light and can lead to severe visual impairment. Fortunately, cataracts can be treated by removing the lens and implanting and artificial one. A cataract is shown in the figure on the right.
- Aqueous Humour
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Fluid in the anterior chamber of the eye.
Aqueous humor is used to transport nutrients and waste products to and from cells in the cornea. In a normal eye, the amount of aqueous flowing into and out of the anterior chamber is controlled carefully, so that the pressure within the eye remains at a tolerable level. When not enough aqueous is drained from the eye, intraocular pressure increases and can lead to the death of light-sensitive cells in the retina. This condition is known as glaucoma, and it is a leading cause of blindness.
- Convergence
- A binocular cue-the inward & outward movements of eyes to focus on near or more distant objects
- Horizontal cells in the retina
- neurons in the retina that interconnect adajecent photoreceptors & outer aborizations of bipolar cells.
- Where does the info from the outer 1/2 of each retina go?
- To the ipsilateral visual cortex.
- Superposition
- If one object partly hides another, than the hidden object must be further way.
- Ganglion cells in the retina.
- Sensory neurons for vision. Cell bodies located in the retina & their axons run by way of the optic nerve to the brain.
- Ariel perspective
- Images of outdoor objects get hazier & bluer w/ increasing distance.
- Blind spot
- Place in the retina where axons of visual sensory neurons come together to form the optic nerve. B/c the blind spot lacks receptor cells, light that strikes it is not seen.
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crystalline lens
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In adults, the lens is approximately 9mm in diameter and 4mm thick. It continues to grow throughout life, quadrupling in size from birth to 90 years of age. The lens hardens with age, a condition known as sclerosis, and also becomes less transparent, especially to short wavelength light.
Muscles are attached to the lens capsule, and variations in muscle tension alter the shape of the lens, which in turn changes the lens' optical power. By varying its optical power, the lens provides a mechanism that allows us to focus on objects at different viewing distances, a process known as accommodation. - Optic Nerve
- Cranial nerve that contains the sensory neurons for vision which runs from the retina to the brain.
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Relative size
- For familar or mulitple objects, smaller retinal images=greater distance from us.
- Motion Parallax (monocular cue)
- As we move past objects those that are close to us move by faster than those that are farther away.
- Linear perspective.
- Images of parellel lines appear to converge the further they are away from us.
- Monocular cues
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Relative size
Superposition
Texture gradient
Ariel perspective
Linear perspective. - Bipolar cells in the retina.
- Constitute the middle layer of the retina, conveying info from the receptor cells to the ganglion cells.
- Path of light from ganglion cells to visual cortex?
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Ganglion cell axons form the optic nerve, and travel to Lateral Geniculate Nucleus (LGN) of the thalamus and form the optic chiasm. Axons from LGN go to primary visual cortex.
Axons from the outer 1/2 of each retina go to ipsilateral visual cortex. Axons from the inner 1/2 of each retina travel to the contralateral visual cortex. - Where does info from the inner 1/2 of each retina travel?
- To the contralateral visual cortex.