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Chapter 2

Psy 280 - chapter 2.odt

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Chapter 2: The first steps in vision From light to neural signals Alittle light Physics • light is a form of electromagtic radiation • two ways to conceptualize light: as a we or as a stream of photo, tiny particples that each consist ofone quantum of energy • visible light waves have wavelengths between 400 and 700 nanometers • wavelengths carrie sin the visible spectrum the color we observe changes from violet at about 400nm thorugh the whole spectrum of the rainbow up to red at about 650nm • light wave themselves are not colors, it is only after our visual system interprets an incoming wave that we perceive the light as a specific color • example about star to eye. (first page Eyes that see light • Image: a picture or likeness • figure of the eye p, 33 • first tissue that light from the star will encounter is the cornea • cornea is transparent – most light photons are transmitted through it, rather then being reflected or absorbed • it is transparent because it is made of a highly ordered arrangement of fibers and it contains no blood vessels or blood, which would absorb light. • Cornea has a transparent sensory nerve endings which are there to foce the eyes to close and produce tears if the cornea is scratched to preseve it transparency • cornea regenerate very quickly within 24, even when its scratched • e.g. When its scratched or worn contacts too long • Aqueous humor ( the watery fluid in the anterior chamber of the eye) : a fluid derived from blood, fills the space behind the cornea, supplying oxygen and nutrients to and removing waste from the cornea and crystalline lens • Crystalline lens: the lens inside the eye that enables the changing of focus • the shape of the lens is controlled by the ciliary muscle • to get the light from the star to the lens, it must pass through the pupil which is a hole in the muscular structure called : iris • iris give the eye its distinctive color • pupil controls the amount of light that reaches the retina, via the pupillary light reflex • when the level of light increases or decresase, the risi automatically expands and contracts to allow more or less light into the eye When you emerge from a dark room into bright light, not only does your pupil constrict but there a good chance that you will sneeze. • “photic sneeze reflex”- may be due formation of tears or crossed wires in the brain • after passing throuh the lens, our starlight enters the vitreous chamber (the space between the lens and the retina) where it will be refracted for the fourth and final time by vitreous humor • Vitreous humor: the transparent fluid that fillst he vitreous chamber in the posterior part of the eye - the longest journey through the eyeball 80% of the internal volume of the eye - vitreous is gel like and viscous (egg white) and generally transparent • After through the vitreous chamber, the light is brough into focus at the retina • only some of the light will reach the rest is lost in space due to absorption • half the light is lost in the eyeball only half reaches the cornea and arrive at the retina • the role of the retina is to detect light and tell the rain about the aspect of light that are related to object in the world Focusing light onto the retina • cornea has a higher refrative index than air – the most powerful refractive surface in the eye • aqueous and vitreous help refract light – but the structures are fixed • Accommodation: the process by which the eye changes its focus( lens gets fatter as gaze is directed toward nearer objects) • accommodation is accomplished through contraction of the ciliary muscles • lens is attached to the ciliary muscles • when tthe ciliary muscles is relaxed the zonule are stretched and the lens is flat ( focus on distant objects) • to focus something closer: contraction is reduces the tension on the zonules and enables the lens to bulge – fatter the lens is the more powerful • accommodation enables the power of the lens to vary as much as 15 diopters • if your eyes corrected for distant vision, 15 diopters will enable you to read your watch at adistace of about 6.7 centimeters or 0.067 meters • our ability to accomodate declines with age, starting from about 8 years old and we lost about diopter of accommodation every 5 years up to age 30 • by the time people are between 40 to 50 years old they find that their arms are too short beacue tey can no longer easily accomodate the 2.5 diopters or to see at 40 cm • “” Presbyopia: old sight, the loss of near vision because of insufficient accommodation - its is inevitable Why do we all have presbyopia? • The lens bcomes harder and the capsule that encircles the lens loses it elasticity • Benjamin franklin invested bifocals – lenses that hav eon power that the top (to see distant objects ) and a different power at the bottom ( focus at a comfortable reading distance • lens are typically transparent because the crystallins are packed together very densely • anything that interfers with the regualrity of the crystallins will result in loss of transparency • Opacities are known as catarcts • Catarcts: an opacity of the crystalline lens • cataract occur at different ages and take many forms • congential catarcts are rare and can affect vision if it is not properly treated • Cataracts are discovered mostly after 50 and increases with age by 70 most people have some loss of transparency • Cataracts can interfere with vision because they absorb and scatter more light than the normal lens do To focus on a distant star on the retina, the refractive power of the four optical components of the eye must matched to the length of the eyeball continue Communicating to the Brain via Ganglion cell • by the time it has reached the ganglion cell there is already information processing • ganglion cell receive their input from bipolar and amarine cell, process this input further and send messages off to the brain through their axon which gather from the back of the eyeball and emerge together as the optic nerve • P Ganglion cell : a small ganglion cell that receive excitatory input from single midget bipolar cells it nhe central retina and feeds the parvocellular layer of the lateral geniculate nucleus - 70 p cell in human retina • M ganglion cell: a ganglion cell resembling a little umbrella that receive excitat
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