2.4- Problem 1: The Eye and the Visual System

dark adaptation

The process in which the eyes become more sensitive to light in low illumination. - rods & rhodopsin become activated - sodium channels open - ongoing glutamate release

Cataract

clouding of the lens of the eye

Glaucoma

imbalance of drainage/ creation of aqueous humor; high pressure to the eyes --> Can lead to blindness

Presbyopia

impairment of vision as a result of old age - insufficient accomodation --> · Lens becomes harder and the capsule that encircles the lens (enabling it to change shape) loses its elasticity

macular degeneration

progressive damage to the macula of the retina --> can lead to loss of central vision

Cones

retinal receptor cells that are concentrated near the center of the retina (the fovea) and that function in daylight or in well-lit conditions. The cones detect fine detail and give rise to color sensations. 3 types: (1) Short wavelength (blue) (2) Medium wavelength (green) (3) Long wavelength (red)

Rods

retinal receptors that detect black, white, and gray; necessary for peripheral and dark vision, when cones don't respond - more of these found in the retina (compared to the other photoreceptor)

Summary

- Light is reflected from an object into the eye. This light is focused to form an image of that object on the retina. - Light, in a pattern that illuminates some receptors intensely and some dimly, is absorbed by the visual pigment molecules that pack the rod and cone outer segments. Chemical reactions in the outer segments transduce the light into electrical signals. - As these electrical signals travel through the retina, they interact, excite, and inhibit, eventually reaching the ganglion cells, which because of this processing have center-surround receptive fields on the retina. - After being processed by the retina these electrical signals are sent out the back of the eye in fibers of the optic nerve

the eye

- Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, which contains the receptors for vision - Two kinds of visual receptors, rods and cones, which contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals. - These signals flow through the network of neurons that make up the retina. The signals then emerge from the back of the eye in the optic nerve, which conducts signals toward the brain.

Hermann grid

A geometrical display that results in the illusion of dark areas at the intersection of two white "corridors." This perception can be explained by lateral inhibition. The other neurons inhibit the ganglion cell thats under A and cause a slower action potential (weakens A's ganglion cell firing rate) which causes the visual illusion of grayish blobs.

opsin

A membrane protein bound to a light-absorbing pigment molecule/ rhodopsin

aqueous humor and vitreous body

Extracellular/ watery fluid of the eyes

Ganglion cells

In the retina, the specialized neurons that connect to the bipolar cells; - the bundled axons of these cells form the optic nerve.

Retina

Light sensitive layer of the eye; - Contains 3 layers: (1) outermost layer --> photoreceptors (rods and cones) (2) amacrine cells, bipolar cells and horizontal cells (3) ganglion cells

Mach Bands

Perception of a thin dark band on the dark side of a light-dark border and a thin light band on the light side of the border. These bands are an illusion because they occur even though corresponding intensity changes do not exist.

visual process

Pupil,Lens,Retina, Rods and Cones, Bipolar Cells, Ganglion Cells, Optic Nerve, Optic Chiasm, Thalamus, Visual Cortex

pupillary reflex

The automatic process by which the iris contracts the pupil when it is bright and dilates/ relaxes the pupil when it is dark

The near point

The distance at which the lens can no longer accommodate enough to bring close objects into focus. Objects nearer than the near point can be brought into focus only by corrective lenses. - The distance of the near point increases as a person gets older, a condition called presbyopia

trichromatic theory of color vision

The theory that the sensation of color results because cones in the retina are especially sensitive to red light (long wavelengths), green light (medium wavelengths), or blue light (short wavelengths).

Simultaneous contrast

When two different colors come into direct contact, the contrast intensifies the difference between them. - The receptors under the light area surrounding the square are intensely stimulated, so they send a large amount of inhibition to the receptors under the square. - The receptors under the dark area surrounding the square are less intensely stimulated, so they send less inhibition to the receptors under the square. - Because the cells under the right square receive more inhibition than the cells under the left square, their response is decreased more, they fire less than the cells under the left square, and the right square therefore looks darker.

Hyperopia

farsightedness; difficulty seeing close objects when light rays are focused on a point behind the retina

Astygmatism

irregular curvature of the cornea or lens causes blurred vision

Myopia

nearsightedness; difficulty seeing distant objects when light rays are focused on a point in front of the retina

fovea

the central focal point in the retina, made up of only cones

Rhodopsin

the pigment in rod cells that causes light sensitivity

accommodation

the process by which the eye's lens changes shape to focus near or far objects on the retina - far object --> lens thinner (muscles pull out) - near object --> lens thickens/ becomes rounder (muscles push in)

light adaptation

the recovery of the eye's sensitivity to visual stimuli in light after exposure to darkness - rods become "bleached" (shape straightens out from cis --> trans form --> THEY TURN OFF) - cones become activated (ON) - sodium channels close - very little glutamate released

Cornea

the transparent layer forming/ covering the front of the eye. - Transparent because its made up of a highly ordered arrangement of fibers and because it contains no blood vessels or blood which would absorb the light - Has a rich supply of transparent nerve endings which force the eye to close and produce tears in case it is scratched - Highly curved and thus has a high refractive surface

lens

the transparent structure behind the pupil that changes shape to help focus images on the retina - no blood supply

Lateral inhibition

when an excited neuron reduces the activity of its neighboring neurons --> allows enhanced sensory perception - the horizontal/bipolar cells inhibit some activity from the receptors (to focus on the excited receptors) in order to enhance visual edges

blind spot (optic disc)

where the optic nerve leaves the eye; there are no photoreceptor cells here

Transduction in the eye

• light activates neurons in the retina • there are several layers of cells in the retina: • First layer is directly activated by light. - Cones- cells activated by color; Rods- cells that respond to black and white. • Second layer: bipolar cells • Third layer: ganglion cells. - The axons of the ganglion cells make up the optic nerve that sends these impulses to a specific region in the thalamus called the lateral geniculate nucleus (LGN). From there, the messages are sent to visual cortices in the occipital lobes of the brain. • Area where there are no rods or cones: Blind spot • Impulses from left side of each retina go to the left hemisphere of the brain, Impulses from the right side of each retina go to the right hemisphere of the brain. Spot where nerves cross is Optic Chiasm.