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Human Eye
Elaborate, counter-intuitive. The peculiar evolutionary history of eyes has produced a highly effective but odly designed structure.
modified forward surface of sclera, transparent, curved, admits and focuses light
clear, layered tissue behind pupil, convex structure, focuses light on retina
Visual receptors and other neural cells, covers rear wall of eye
Area in the retina where there are no receptiors. The optic nerve leaves the eye so all the receptors are swept to the blind spot. It is also known as the optic disc. The brain fills in the unseen areas with appropriate background based information from the surrounding area.
They are bits of refuse (ex. degenerated neonatal arteries, sloughed photopigment discs)You don't see the actual floaters but you see the shadows that are cast onto the retina. They are not colored, only grey shadows. The gelatinous vitreous humor are like banana bits in jello. They only move when your eyes move. When you try to focus on them, you are trying to align them with your fovea but since they are suspended in a place not aligned with the fovea, you end up endlessly "chasing" them.
Blood Vessels
The grey, squiggly branching lines are the shadows of transparent blood vessels in the vitreous humor. The shadows appear to hang in "midspace" if light is shone on the left (opposite).
Moving Dot
A moving dot in periphery can be detected before its color is determined. This is because peripheral vision is black and white. Color can only be seen in central visual field.
Dominant in periphery. Cannot detect color. Sensitive to motion. Outer segment rod-like. Discs with embedded visual pigment molecules. Larger size (more visual pigment.) About 120 million in the eye. None in the fovea. Low Acuity. High sensitivity to light. High Convergence (many rods: one ganglion)
Color-sensitive High concentration in the central area, including not limiting to the fovea. Outer segment cone-like. Folded sheet with embedded visual pigment molecules. Smaller size (less visual pigment) About 6.5 million in the eye. Dispersed in periphery. Color coding (proportions of red, green, and blue) Poor motion detection. High Acuity esp. in fovea Requires brigher light Low Convergence (1 or few cones: 1 ganglion)
Why do we generally believe that we see color throughout the visual field (not justin the small central area of cones)?
MEMORY and ASSUMPTION When eyes saccade from place to place, they tend to not fixate on anything. Only when they focus do they find color confirming the assumption that we see color everywhere.
Cone Vision
PHOTOPIC. Detail discrimination is best in bright room. In the dark with dim light, vision improves after 7 minutes in comparison to vision after 1 minute. This is due to the cone photo-pigment that rapidly regenerates. YELLOW and GREEN are easiest to detect in dim light.
Purkinje Shift
The shift from photopic to scotopic vision. Shapes even with no details or colors eventually can be seen in the "dark". This process takes awhile as rods regenerate slower than cones. Blue-green stimuli (versus yellow-red) are more likely to be detected by rods, even if their color is not detected.
Vision and proprioception
LAB2: The correlation is violated when we view the image our our hands in one position even when proprioception sends information that our hand is moving. The similar situation arises with visual and auditory information. The ghost walks!
Small area in the human retina that contains only cone receptiors. It is located on the line of sight, so that when a a person looks at an object, the center of its image falls on the fovea. It is ideal for high detail resolution.
Retina images are...
Inverted and mirror-reversed
Focal Length
Distance between the place where light first reaches eye and focal point.
Far Point
The closest point from which a relaxed eye will produce a clear image
20:20 vision
See as well as 20' as normal sees at 20' (i.e. at normal Far Point)
WORSE. See as well as 20' as normal sees 40'.
BETTER. See as well as 20' as normal see 10'.
When lens changes shape to change focus. It can change (shorten) focal length.
Zonules of Zinn
When contracting muscles makes the lens rounder. Analogous to the balloon in the pantyhose. Ciliary muscles attached to the zonule fibers and the front of the sclera (near cornea). When the ciliary muscles contract they pull on the zonule fibers away from the lens allowing the lens to assume its natural rounded shape. MORE Contraction = ROUNDER Lens = MORE Light Refraction
Near Point
Normal Near point = 10 cm at 20 years old , 14 cm at 30 years, 22 cm at 40 years When accommodation can't help. Even when the lens is at its most rounded state, it cannot focus on incoming light on the retina.
Farsightedness - trouble resolving image of close stimuli - Light from beyond the near point is insufficiently refracted by ccornea + lens - Corrective lenses are CONVEX (rounded) to produce more refraction Laser surgery can reshape the cornea to make it more convex
Old eye. - Late onset farsightedness, from the lens losing its elasticity with age - Lens are less able to radically accommodate, so near point gets shifted farther and farther from the eye.
Nearsightedness. - Light from far away is TOO REFRACTED by even a relaxed eye. So Focal point falls short of retina. - Corrective lenses are CONVCAVE to make incoming parallel rays enter the eye at more oblique angels. - Laser surgery can help reshape the cornea or reduce aqueous humor to make it less convex and so less refractive.
Mis-shapened Cornea. - When some parts of an image are in focus while others are not - Specifically, image usually in focus in horizontal but not vertical plans or vice versa. - Laser surgery can help only early on.
Specialized cell that transmits electro/chemical information to other cells
Chemical reaction in cell body
Neurons that form the rear-most neural layer of Retina. Each contains visual pigment that react to light = Isomerization NT release is GRADED TWO TYPES OF RECEPTORS are Rods & Cones
Change in shape of the retinal part of the visual pigment molecule that occurs when the molecule absorbs a quantum of light. Isomerization triggers the enzyme cascade that results in transduction from light energy to electrical energy in the retinal receptors.
Bipolar Cell
Neurons next in pathway - in reaction to NT released by receptors Produce GRADED release of NT to Ganglion cells
If input from bipolars exceeds AP threshold...they release all vs. none. The ganglions form the optic nerve.
Activity is perpendicular to main pathway.
GRADED release of NT that modify interface of receptors and bipolars.
GRADED release of NT that modify interface of bipolars and ganglions.
When whole environment is bright white (no shadows or color). This is due to all visual pigments becoming isomerized.
Dark Adaption
When fully adapted, eyes are 100000x more sensitive.
Coding in Retina (Light)
Light turns Receptor cells OFF (down)
Coding in Retina (Dark)
Darkness turns them ON (up) Few photons --> more NT release
How can receptors that turn off in bright light code for visual stimuli?
Receptors release inhibitory NT, bipolars spontaneously release excitatory NT so Ganglions, by not firing , in effect report to the brain: NO LIGHT. In bright light, receptors are shut down, do not inhibt bipolars, so bipolars spontaneously release enough excitatory NT to trigger ganglions so ganglions send message: BRIGHT LIGHT!
Receptive Field
The set of receptors whose activity influences the activity of the target cell. Target can be simultaneously excited and inhibited. It all depends on the SUMMATION of these effects.

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