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Chapter 3: Vision, Part 1

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Western University
Psychology 2115A/B
Christine Tsang

Chapter 3 Review: Vision, Part 1 By the end of this section, you should know about: - Slight and the eye - Visual transduction - Pigments and perception Light  The quantum theory of light o Quantum = particle of light o Photon = 1 quantum  Intensity of light measured in photons  Smallest amount of light possible = a single photon of light  We measure light in terms of its intensity – how bright it is, the number of photons present The Electromagnetic Spectrum  Wavelength: the physical distance between the two highest peaks of a photon wave  Can be very short or very long  Very short and very long wavelengths of light are not visible to the human eye – we are restricted to a very small spectrum of light  Visible wavelengths of light correspond to a small portion of the entire electromagnetic spectrum  The longer the wavelength, the more red they appear to be  The shorter they are, the more blue/purple they appear to be  Greens and yellows in the middle The Structure of the Eye o Eyeball contained in the eye socket: a protective bony socket o The cornea: Primary visual element that does almost all of the bending of light  Approximately 13mm in diameter, very transparent, contains no blood vessels, contains many nerve endings  Functions to serve as a fixed lens  Responsible for gathering light and concentrating it so that it gets into the eye  Bulges forward form the eyeball – football shaped  This bulging allows reception of light from slightly behind your eyeball – allows you to see slightly behind where your eye is o Aqueous humor: transparent liquid that fills the chamber behind the cornea o Pupil: a whole bounded by two membranes (iris)  Iris is a coloured membrane that surrounded the pupil  Can contract and expand depending on how big the whole needs to be  Iris membrane forms a ring around the hole (pupil) itself  The muscle in the iris is a dilator muscle  Controlling the amount of light that can enter the eye o Lens: contain no blood vessels or nerves  Slightly yellow and gets more discoloured as we get older  Observed about 10% of the light that is allowed to enter the eye  Held in place by cilliary muscles that allow the lens to move in particular ways – changes the shape of the lens o All of these elements of the eye are all trying to focus the light onto the back of the eye where the retina is o Vitreous humor: liquid that keeps the eyeball relatively round The Optical Elements of the Eye  Cornea refracts light the most o Hits the cornea, goes through the pupil, hits the lens, accommodates the signal onto the retina  The changing shape of the lens provides fine focus, allowing us to focus from infinity to as close as 10 cm  Slightly yellow to correct for chromatic aberration o Everything looks a little yellow in order to correct for the slight aberration in the environment o Chromatic aberration makes you not able to see colours as well because the eye is yellowing o Visible spectrum runs from 300-1600 nanometers in wavelength The Pupil  Limits the amount of light that enters the eye  Diameter ranges from 2-8 mm  Small pupil size reduces image degradation, by increasing depth of field  Optimal pupil size is 2-3 mm  The pupillary reflex: automatic reflexive response that makes the two pupils equivalent in size at all times The Lens  Located directly behind the pupil and iris  Critical for bringing an image into focus on the retina o Without the lens, vision is difficult and focused vision is impossible  The lens is relatively relaxed when the object is far  When the object is closer, the lens becomes much tighter Accommodation  The process by which the lens varies its focus  When lens is spherical (increased curvature), near objects are in focus  When lens is flattened (decreased curvature), far objects are in focus  Near point: distance at which the lens can no longer adjust to bring close objects into focus o Presbyopia: are related condition where there are changes in the near point where reading is not at a comfortable distance o Occurs because the muscles that are controlling the flatness and roundness of the lens are getting stiff  Far point: distance at which a spot of light becomes focused on the retina Optical Defects  Emmetropic eye o Normal eye shape o Image focused on the retina  Hyperopic eye: ‘far sighted’ o Eyeball too short o Near objects do not focus on the retina  Myopic eye: ‘near sighted’ o Eyeball too long o Far objects do not focus on the retina  Blind spot: point in the optic nerve where all the nerves bundle together to leave the eye, and therefore you cannot see from this part of the eye The Retina  Retina is a transparent sheet of tissue that covers most of the interior surface of the globe that makes up the back of the eye itself  Composed of 5 different types of cells: rod and cone receptors, bipolar cells, horizontal cells (stretch laterally across retina rather than horizontal), amacrine cells (lateral), ganglion cells  More concerned with rod and cone receptors and the ganglion cells  Retina positioned in unusual way o Would assume that the rod and cone receptors would be the first set of cells that the light passes through but light has to pass through the other four cell types in order to be transduced into electrical energy o The photosensitive cells are actually at the back of the eye – means that all the cells that precede the photoreceptor layer have to be transparent so the light can pass through as strongly as possible o The reason that they are at the end of all the neurons is because they need a lot of energy (rich oxygen supply) to work and the only way they get this is through a dense layer of blood vessels at the back of the eye o Body of photoreceptors positioned the most energy consuming cells at the back of the eye so the blood doesn’t have to travel farther though the eye  The ganglion cell axons all move in a particular direction o The last cells to receive energy before leaving the eye o Bundle together to create the optic nerve – what transmits the electrical visual signal to the brain o Bind spot created from all the ganglion cells bundled together Cones  The photoreceptors: the outer segment of the photo receptor is what actually does transduction – turning light from a physical energy into a neural form  Specialized for photopic vision – fine detail and colour and relatively high light levels  Chromatic: sensitive to colour Rods  Specialized for scotopic vision: vision in very dim light  The difference between the two types of vision is that one is for day and the other is for night  Achromatic: not sensitive to colour, only to differences in brightness – whether something is more or less bright than something else, but not the different hues Phototransduction (Rods) – Part 1  Outer segment contains disks of visual pigment molecules o The outer segment does all the phototransduction o The rod itself contains little disks in the segment – lined with visual pigment disks/molecules  In rods, the visual pigment molecules are called rhodopsin (a protein)  Rhodopsin has 2 components: o Opsin o Retinal: light absorbing part of the visual pigment  Regenerated by Vitamin A – critical component that is a main player in the phototransduction process Phototransduction – Part 2  When retinal absorbs a photon: o It isomerizes: changes in shape  Occurs when a photon of light needs to be caught by the eye  When retina absorbs light, it changes shape o Isomerization triggers a cascade of biochemical events  The cells become more negative – hyperpo
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