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PSY280H1 (40)
Chapter 3

chap 3 textbook notes

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University of Toronto St. George
Kristie Dukewich

CH3 – INTRODUCTION TO VISION  MYOPIA (nearsightedness) – inability to see distant objects clearly Focusing Light onto the Retina o Parallel light rays entering eyes focus in front of retina, reasons:  Light: The stimulus for vision  REFRACTIVE MYOPIA – cornea and/or lens bends light too much o ELECTROMAGNETIC SPECTRUM – continuum of electromagnetic  AXIAL MYOPIA – eyeball too long energy, waves, produced by electric charges, o Solutions:  Vision based on visible light  bring stimulus closer, push focus point back  WAVELENGTH – distance btwn peaks of EM wave  corrective lens, bend incoming light as if were at far point -12  Ranges: ex. gamma (10 m) to radio (10km)  Laser-Assisted In Situ Keratomileusis (LASIK) surgery o VISIBLE LIGHT – energy within EM spec human can perceive  Sculpt cornea w/ laser  Wavelength 400 to 700 nm (1 nm = 10 m) -19  HYPEROPIA (farsightedness) –inability to see near objects clearly  Different wavelength = different colors o Parallel light rays entering eyes focus behind eyeball o Light also has particle like properties o Far objects can accommodate  PHOTONS – small packets of energy, makes up light  near objects req. too much accommodations  headaches  1 photon = smallest possible packet of light energy o Solution: also corrective lenses  ASTIGMATISM – cornea not perfectly round,  EYE – where vision begins o IRIS – control amount of light entering o PUPIL – here light reflect from objects in environment enter eye o CORNEA – entering light focused by the cornea o LENS – form sharp images of the objects on the retina o RETINA – contains receptors for vision,  Two kinds of visual receptors: Rods, Cones  Visual Pigments – light-sensitive chemicals within both receptors  Reacts to light, trigger electrical signal  flow through Transforming Light into Electricity  Focusing light = initial step, but seen images = in brain not on retina network of neurons making up the retina o Need transform light on retina into electricity o OPTIC NERVE – conducts signals towards the brain o VITREOUS HUMOR – holds retina in place, unreplaceable, gelatinous  The visual receptors and transduction o Front eye: cornea, lens; rest = back o RECEPTORS – neurons specialized in receiving environmental energy, transform to electrical energy  Carries out transduction o OUTER SEGMENT – of rod, key to transduction; light  electricity  Contains stacks of discs, each disc contains thousands of visual pigment molecules RHODOPSIN = Retinal + Opsin  VISUAL PIGMENT MOLECULE – long strand of OPSIN protein, loops back/forth across the disc membrane seven times  RETINAL – light sensitive molecule, only one attached to each visual pigment molecule  Light is focused by the eye o Cornea = transparent covering of front of the eye  Does 80% of eye’s focusing power  Fix in place, can’t adjust focus o Lens = 20% eye’s focusing power  Not fixed, can shape & adjust for diff. distance stimuli  Objects >20 ft. away, light reach eye = parallel  Brought to focus on retina naturally  Objects closer to eye, light rays enter eye at angel  Naturally focus point behind eyes  ACCOMMODATION – done by lens, focus light on retina  ciliary muscles at front eye tightens & increase curvature of lens  thickens, increase curvature  result = bend light rays, focuses on retina  NEAR POINT – distance where lens can no longer adjust to accommodate o 10cm (age 20)  14cm (age 30)  22cm (age 40)  100cm (age 60)  Photoreceptor transduction process o When receptor at resting stage: o PRESBYOPIA – condition, near point distance increase with age  DARK CURRENT – inward Na+ ions flow through cGMP via outer  b/c lens harden, ciliary muscles weaken  less malleable segments and outward K+ flow via inner segment ion channels  FAR POINT – distance which spot of light becomes focused on the retina  Sodium Potassium pump counter DK  3 Na+ out for 2 K+ in o Transduction triggers when light- sensitive retinal within Rhodopsin absorbs one photon, cause:  1. ISOMERIZATION – Retinal change shape when absorb photon, becomes erect from opsin protein  Rhodopsin Isomerization = chemical process o Physiology of Transduction  From Hecht’s experiment: isomerization of 1 out of millions of visual pigment molecules in rod activates receptor, reason:  ENZYME CASCADE – Isomerization of single visual pigment molecule triggers thousands of chemical reaction  triggers thousands more, uses enzyme (facilitates chemical reaction)  2. Rhodopsin Activated  Activates G-protein  Activates enzyme  Convert cGMP to GMP  Na+ channels close  Hyperpolarize membrane (-40 mV  -70 mV)  3. Glutamate (neurotransmitter) released by photoreceptors  When photo pigment activates, less Glutamate released  Less glutamate  depolarize on bipolar cells, carry signal Pigments and Perception  Visual pigment molecules determine detection/not and shape specific aspects of perception o Some properties help determine sensitivity to light o rod receptors & cone receptors has diff. perception o HECHT’S PSYCHOPHYSICAL EXPERIMENT – explore physiological process of transduction through psychophysical experiments, measure PP  to determine how many visual pigment molecules needed isomerized for person to see  Used method of constant stimuli, determine absolute threshold for seeing brief flash of light  Controlled number of photons released  Determined 100 photons needed for detection  Only 50 photons reach retina, 50 reflected  7/50 photos absorbed by light-sensitive retinal parts, rest  Distribution of the rods and cones, ratio depends on location in retina o 120 mill. rods 6 million cones in total in retina hit opsin or slip btwn visual receptors o FOVEA – contains only cones, only 1% (500k) of all cones in retina  Conclude, 7 isomerization = abs threshold o PERIPHERAL RETINA – retina outside fovea, both rods and cones  Conclude, rod receptors activated by isomerization of just 1 visual pigment molecule  More rods here than cones  7 photons hit area cover 500 receptors o MUSCULAR DEGENERATION – destroy fovea & surrounding area  Effects: creates blind spot in central vision o RETINITIS PIGMENTOSA – degeneration of retina, genetic  Condition first attacks peripheral rod receptors,  poor vision in peripheral visual area  eventually fovea receptors attacked  complete blindness o BLIND SPOT – where optic nerve leaves eye, no receptors to allow ganglion cells fibers to flow to optic nerve  Normally unaware to humans b/c:  blind spot located side of visual field  not in sharp focus  normally people unaware, hard to detect  brain fills in the blind spot w/ surrounding patterns  Cones sensitivity levels off after, rods continue increase  Minute 7 rods sensitivity same as cones, then becomes increasing sensitivity  control person’s vision  ROD-CONE-BREAK – place where rods begin to determine dark adaptation curve o VISUAL PIGMENT BLEACHING – after light hits light-sensitive retinal parts of visual pigment molecule, isomerizes and triggers transduction of process; retinal then separates from opsin  cause retina to become light in color (red  white) o VISUAL PIGMENT REGENERATION – retinal and opsin rejoin  Retinal combines w/ opsin in the dark  Pigment regains its darker red color  Cone pigments 6 minutes to regenerate completely  Rod pigments take >40 minutes to generate completely  DARK ADAPTION – process of the eye, increase sensitivity in the dark  Rate of cone/rod dark adaptation match the rate of cone/rod o Increase sensitivity when change brightness pigment regeneration o DARK ADAPTATION CURVE – plot how visual sensitivity change in dark, shows 2 stages of dark adaptation  PIGMENT EPITHELIUM – rests below part of the retina, contains enzymes necessary for pigment regeneration  Initial change b/c adaption of cone receptors (faster)  cones control vision during early stages of dark adaption  Second change b/c adaption of rod receptors (slower) o Experiment: ask observer adjust intensity of small flashing test light so it’s barely seen with adapting light (LIGHT-ADAPTED SENSITIVITY)  Then, adapting light extinguished  Observer looks at small fixation point, also pay attention to flashing test light in periphery  test light simulates both rods and cones  Dark adaptation curve: observe sensiti
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