Textbook Notes (367,993)
Canada (161,547)
Psychology (527)
PSYCO367 (36)
Chapter 3

Sensation and Perception Psych 367 Chapter 3.docx

10 Pages
Unlock Document

Douglas Wylie

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
More Less

Related notes for PSYCO367

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.