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Chapter 3

Sensation and Perception Psych 367 Chapter 3.docx

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Department
Psychology
Course
PSYCO367
Professor
Douglas Wylie
Semester
Winter

Description
Chapter 3 :introduction to vision Focusing light onto the retina Light: the stimulus for vision - Vision is based on visible light, which is a band of energy within the electromagnetic spectrum - Electromagnetic spectrum: a continuum of electromagnetic energy that is produced by electric charges - Energy in this spectrum can be described by its wavelength o Distance between the peaks of the electromagnetic waves -12 - Extremely short wavelength: gamma rays (10 ) +4 - Long wavelengths: radio waves (10 ) - Visible light: the energy within the electromagnetic spectrum that humans can perceive, o 400 to 700 nm The eye - Light enters through the pupil and is focused by the cornea and the lens to form sharp images of the object on the retina - Rods and cones, which contain light sensitive chemicals called visual pigments that react to light and trigger a electrical signal - Optic nerve: which conducts signals toward the brain Light is focused by the eye - The cornea (transparent covering of the front of the eye, accounts for about 80% of the eyes focusing power - The lens supplies the remaining 20% of the eyes focusing power - If the object is located more than about 20 feet away, the light rays that reach the eye are essentially parallel - Accommodation: ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker o The increased curvature bends the light rays passing through the lens to pull the focus point from behind the eye to the fovea - Accommodation enables you to bring both near and far objects into focus, objects at different distances are not in focus at the same time - Near point: the distance as which your lens can no longer adjust to bring close objects into focus - Presbyopia: the distance of the near point increase as a person gets older o Occurs because the lens hardens with age, and the ciliary muscles become weaker o Either hold things longer to read or get glasses that add to the eyes focusing power - Myopia (nearsightedness): inability to see distant objects clearly o Brings parallel rays of light into focus at a point in front of the retina so that the image reaching the retina is blurred o Caused by two factors  Refractive myopia: the cornea or lens bend the light to much  Axial myopia: the eyeball is too long - Far point: the distance at which the spot of light becomes focused on the retina at the far point - A lens placed in front of the eye causes the light to enter the eye at exactly the same angle as light coming from the far point - LASIK: laser assisted in situ keratomiluesis o Sculpting the cornea with a laser o The flap is folded out of the way, the cornea is sculpted by the laser so that it focuses light onto the retina and flap is folded back - Hyperopia (farsighted): can see distant objects clearly but has trouble seeing nearby objects o The focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short o Constant need to accommodate when looking at nearby objects results in eyestrain and headaches Transforming light into electricity Visual receptors and transduction - Transduction is carried out by receptors, neurons specialized for receiving environmental energy and transforming this energy into electricity - The key part of the rod transduction is the outer segment because it is here that the light acts to create electricity - Rod outer segments contain stacks of discs - Each disc contain thousands of visual pigment molecules o Long strand of proteins called opsin, which loops back and forth across the disc membrane seven times o Main concern where a molecule called retinal is attached o Each visual pigment molecule contains only one of these tiny retinal molecules. o The retinal is crucial for transduction, because it is the part of the pigment that is sensitive to light o Transduction is triggered when the light sensitive retinal absorbs one photon of light - When a photon of light hits the retinal, it changes shape, so it is sticking out from the opsin o This change in shape is called isomerisation o This step triggers the transformation of the light entering the eye into electricity in the receptors How does transduction occur - Isomerisation of the visual pigment molecule is a chemical process Hecht’s psychophysical experiment - Transduction is triggered by the isomerisation of visual pigment molecules and that it takes just one photon of light to isomerise a visual pigment molecule - Determine how many visual pigment molecules need to be isomerized for a person to see - Used method of constant stimuli to determine a persons absolute threshold for seeing a brief flash of light - Hetch used a precisely calibrated light source, so he could determine the threshold in terms of the number of photons needed to see - He found that a person could detect a flash of light that contained 100 photons o Half the photons bounce off the cornea o 50 actually reach the retina o Only about 7 are absorbed by the light sensitive retinal part o A person sees a flash of light when only 7 visual pigment molecules are isomerized o A person can see a light if 7 rods receptors are activated simultaneously o A rod receptor can be activated by the isomerisation of just 1 visual pigment molecule The physiology of transduction - Isomerisation of a single visual pigment molecule triggers thousands of chemical reactions which in turn trigger thousands more - Enzyme: A biological chemical that in small amounts facilitates chemical reaction in this - Enzyme cascade: The sequence of reactions triggered by the activated visual pigment molecules Pigments and perception Distribution of the rods and cones - The ratio of rods and cones depends on the location in the retina - Fovea: contains only cones (about 1% of the cones) - Peripheral retina: includes all the retina outside the fovea o Contains both rods and cones - More rods then cones in the peripheral retina because most of the retina receptions are located there and because there are about 120 million rods and 6 million cones - Macular degeneration: o most common in older people o destroys the cone rich fovea and a small area that surrounds it o creates a blind spot in central vision - retinitis pigmentosa o degeneration of the retina that is passed from one generation to the next o first attack the peripheral rod receptors and results in poor vision in the peripheral visual field o severe cases, the foveal cones are attacked, resulting in complete blindness - blind spot: one area in the eye that contains no receptors - we don’t see the blind spot because some mechanisms in the brain “fill in” the place where the image disappears dark adaptation of rods and cones - dark adaptation: eye increases sensitivity in the dark - two stages of eye increase o initial rapid stage o slower stage - dark adaptation curve: a plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished - you ask the observer to adjust the flashing light test so they can barely see it (method of adjustment) - first experiment: observer looks at a fixation point while test light is off to the side o this stimulates both rods and cones - method (measuring dark adaptation) o light adapted sensitivity: when observer adjusts the light so its barely seen in full light - sensitivity increases rapidly for 3-4 minutes, - then incre
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