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Vision.docx

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Department
Psychology
Course
PSYC 2390
Professor
Lana Trick
Semester
Fall

Description
Stimulus for Vision 9/8/2012 6:12:00 PM Stimulus for Vision What is light?  Visible light is band of electromagnetic spectrum (electromagnetic radiation is stimulus for vision)  Black absorbs a lot of light, does not reflect many wavelengths o White is opposite o Red absorbs all other colours of light and reflects red wavelengths  Produced by electrical charges and is radiated as waves  Newton o Thought light was a stream of particles coming into our eyes o Light consists of small packets of energy (photons)  James Clerk Maxwell o Thought as light as a wave, a disturbance in the electromagnetic continuum o Peaks (like water wave)  Both are right What aspects of light can we pick up?  Amplitude o ½ of height of wave o Seen as brightness  Big amplitude, bright light  Small amplitude, dim light  Wavelength o Distance between peaks of waves (the electromagnetic spectrum)  Range from extremely short (gamma rays, 10^-12m) to long (radio waves, 10^4m)  Measured in nanometers (1nm = 10^-9m) o Visible light is between (blue) 400-700 (red) nm of wavelength o Different wavelengths are associated with different colours What must the eye do to the Light? 9/8/2012 6:12:00 PM What must the eye do to the light? 1. Collect the light 2. Focus light  light goes everywhere  must focus light (bending the light, make them all converge back to one point on retina because they came from one point to create image on back of the eye) 3. Signal brain that the light is there (light  electricity, transduction)  Starts at eye  Light reflected from objects in environment enters eye through pupil  Light is focused by cornea and lens to form sharp images on retina  Retina contains receptors for vision: o Rods  Outer segment: light acts to create energy  Contain sacks of discs  Each disc contains thousands of visual pigment molecules  Each molecule is a long strand of protein called opsin (loops back and forth across disc membrane)  Each visual pigment contains 1 retinal molecule (sensitive to light)  Peripheral retina (everywhere but fovea) contains both rods and cones but more rods than cones o Cones  Fovea contains only cones (when we look directly at object, image falls on fovea) o Blind spot  No receptors  Where optic nerve exits eye  Blind spot is off to the side of our visual field, not in focus therefore hard to detect  Brain also “fills in” information or perception that matches surrounding pattern which also makes blind spot hard to detect  Retinal absorbs one photon of light, changes shape (isomerization) and sticks out of opsin  Triggers transformation of light entering the eye into electricity  Signals emerge from back of the eye in the optic nerve  Optic nerve conducts signals towards the brain Rods and Cones 9/8/2012 6:12:00 PM Rods and Cones Hecht’s Psychophysical Experiment (1942):  Draw conclusions about physiology of transduction by determining a person’s ability to see dim flashes of light  He knew transduction is triggered by isomerization of visual pigment molecules and it takes just one photon of light  How many visual pigment molecules are needed to be isomerized for a person to see? o Used method of constant stimuli o Could determine the threshold in terms of number of photons needed to see  Findings: o Person could detect flash of light that contained 100 photons o Considered what happens to these 100 photons before reaching visual pigments  Half bounce off cornea or absorbed by lens and vitreous humor  Only 50 reach retina  7 of 50 are absorbed by light sensitive retinal part of visual pigment (rest hit opsin or slip between visual receptors)  Person sees flash of light only when only 7 visual pigment molecules are isomerized  How many visual pigment molecules are needed to activate a single rod receptor? o Light flash saw about 500 rod receptors o What is likelihood that any two of these photons would enter the same receptor? (very small) o Therefore only 1 visual pigment molecule is needed to be isomerized in order for rod to be activated  Later Findings: o Isomerization of single visual pigment molecule triggers thousands of chemical reactions (enzyme cascade) Dark Adaptation:  Causes eye to increase its sensitivity in the dark (eye patch)  Dark adaptation curve: o Plot of how visual sensitivity changes in the dark o Initial rapid stage of adaptation is due to cones (3-4 mins)  To measure, observer looks right at test light (light falls on fovea and only cones in fovea) o Rod-cone break: where rods begin to determine the dark adaptation curve, rods “take over” o Slower stage is due to rods (3 /4rd thminute - 20 or 30 mins after light turned off)  To measure, test rod monochromats (genetic disorder, individual has no cones), light sensitivity is only due to rods  as dark adaptation increases, so does the observer’s sensitivity to light Visual Pigment Regeneration:  Visual pigment bleaching: as pigment isomerizes, retinal and opsin break apart and retina becomes “bleached” as indicated by lighter colour  Visual pigment regeneration: occurs in molecules that have been “bleached”, process in which retinal and opsin become rejoined  Under most normal light levels your eye always contains some bleached visual pigments and some intact visual pigments  Turn off lights: o Bleached would continue to regenerate (regains darker colour) o No more isomerization occurs o Eventually your retina would contain only intact visual pigment molecules  Cone pigment takes 6 minutes to regenerate completely  Rod pigments take 30 minutes to regenerate completely  Findings: o Sensitivity to light depends on concentration of visual pigment (more you have, more sensitive you are to light) o Speed at which sensitivity is adjusted in dark depends on chemical reaction (regeneration of visual pigments) Spectral Sensitivity of Rods and Cones:  Observers sensitivity to light at each wavelength in visible spectrum  Use flashes of monochromatic light (contains only one wavelength) to test  Threshold for seeing light is lowest in middle of spectrum (yellows) o Less light is needed to see wavelengths in middle section of spectrum o Can convert threshold to sensitivity by sensitivity = 1/threshold  Can plot spectral sensitivity curve (relative sensitivity vs. wavelength)  Measure differences in cones same as for dark adaptation o More sensitive to longer wavelengths (yellows/reds)  Measure rods by measuring sensitivity after the eye is dark adapted o More sensitive to short wavelength (blue/green)  Purkinje shift: shift from cone vision to rod vision that causes enhanced perception of short wavelengths during dark adaptation o Blue seems brighter than red after dark adaptation  Differences are caused by differences in absorption spectra of rod and cone visual pigments o Plot of amount of light absorbed by a substance vs. wavelength  Rods absorb best in blue green area  Cone has 3 absorption spectra because of 3 different cone pigments  Best in green and yellow Disorders of Receptors: Macular Degeneration  Common in older people  Destroys fovea and small area surrounding it  Creates blind spot in central vision Retinitis Pigmentosa  Degeneration of retina  Hereditary  Attacks peripheral rod receptors and results in poor vision  Severe cases fovea is also attacked leading to complete blindness Detached Retina  Mainly caused by traumatic injuries  Visual pigment cannot regenerate  Part of retina becomes detached, it has become separated from layer that it rests on (pigment epithelium) which contains enzymes needed for pigment regeneration  As a result, retinal and opsin cannot rejoin and person becomes blind Neural Convergence and Perception:  Signals from receptors travel to bipolar cells (do not have long axons) and then to ganglion cells (have axons like neurons) o Axons transmit signals out of retina in optic nerve  Horizontal cells and amacrine cells o Connect neurons across retina o Horizontal connect bipolar cells o Amacrine connect ganglion cells
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