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Chapter 2

PSY 280 CH. 2 Summary.docx

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University of Toronto St. George
Kristie Dukewich

PSY 280 CH.2 The Beginnings of Perception Starting at the Beginning • Looking at vision, though many of the processes also describe means of perception used in other senses as well. • Step 1: Environmental stimulus, e.g. a tree. Step 2: Light is reflected and transformed to create an image of the tree on the retina. Step 3: Receptor processes: Receptors transform light into electricity. Step 4: Neural processing: Signals travel in a network of neurons. Light and Focusing • The ability to see a tree, or any other object, depends on information contained in light reflected from that object into the eye. Light: The Stimuli for Vision • Electromagnetic spectrum: Continuum of electromagnetic energy that extends from very- short-wavelength gamma rays to long-wavelength radio waves. Visible light is a narrow band within this spectrum. • Wavelength: For light energy, the distance between one peak of a light wave and the next peak. • Visible light: The band of electromagnetic energy that activates the visual system and that, therefore, can be perceived. For humans, visible light has wavelength between 400 and 700 nanometres. • Different wavelengths are associated with different colours. 400 being more violet, 700 being more red. • Light can be described as consisting of small packets of energy called photons, with one photon being the smallest possible packet of light energy. The Eye • Eye: The eyeball and its contents, which include focusing elements, the retina, and supporting structures. • First eyes could distinguish light from dark, but couldn’t detect features. • Pupil: The opening through which light reflected from objects in the environment enters the eye. • Cornea: The transparent focusing element of the eye that is the first structure through which light passes as it enters the eye. They cornea is the eye’s major focusing element. • Lens: The transparent focusing element of the eye through which light passes after passing through the cornea and the aqueous humour. The lens’change in shape to focus at different distances is called accommodation. • Retina: A complex network of cells that covers the inside back of the eye. These cells include the receptors, which generate an electrical signal in response to light, as well as the horizontal, bipolar, amacrine, and ganglion cells. • Light enters the eye through the pupil, is focused by the cornea and lens to form sharp images on the back of the retina. • Rods: A cylinder-shaped receptor in the retina that is responsible for vision at low levels of illumination. • Cones: Cone-shaped receptors in the retina that are primarily responsible for vision in high levels of illumination and for colour vision and detail vision. • Visual pigment: A light-sensitive molecule contained in the rod and cone outer segments. The reaction of this molecule to light results in the generation of an electrical response in the receptors. • Optic nerve: Bundle of nerve fibres that carry impulses from the retina to the lateral geniculate nucleus and other structures. Each optic nerve contains about 1 million ganglion cell fibres. • Once on the retina, the visual data goes to the visual receptors which contain the visual pigments. The pigments trigger an electrical signal to be sent down the optic nerve. • Two transformations are involved in this process: 1. The transformation from light reflected from an object into an image of the object. 2. The transformation from the image of the object into electrical signals. Light is Focused by the Eye • Light reflected from an object into the eye is focused onto the retina by a 2-element optical system: the cornea and the lens. • Cornea accounts for about 80% of the focusing power, but it is in a fixed position, so it can’t adjust its focus. • The lens accounts for about 20% of the focusing power. It can change shape to adjust focus for nearer of farther objects. • Change in lens shape done by the ciliary muscles. They increase the focusing power of the lens by increasing its curvature. • Athicker lens will bring a nearer object into focus. Without it, the focus point will be farther back than usual, and the image will be blurry. • Accommodation: In vision, bringing objects at different distances into focus by changing the shape of the lens. • Near point: The distance at which the lens can no longer accommodate enough to bring close objects into focus. Objects nearer than the near point can be brought into focus only by corrective lenses. Loss ofAccommodation With IncreasingAge • Presbyopia: The inability of the eye to accommodate due to a hardening of the lens and a weakening of the ciliary muscles. It occurs as people get older. • The lens hardens with age and the ciliary muscles become weaker. • At around 45 the ability to accommodate decreases rapidly. Myopia • Myopia AKA nearsightedness: An inability to see distant objects clearly. • Myopia happens when rays of light are brought into focus in front of the retina. Caused by 2 factors: Refractive myopia or axial myopia. • Refractive myopia: Myopia (nearsightedness) in which the cornea and/or lens bends the light too much. • Axial myopia: Myopia (nearsightedness) in which the eyeball is too long. • Aconcave corrective lens can be used. • Far point: As a light is moved towards the eye, the distance at which the light becomes focused on the retina. • Surgical procedures to change the shape of the cornea have been sued to fix myopia. • Laser-assisted in situ keratomileusis (LASIK): A process in which the cornea is sculpted with a laser in order to achieve clear vision by adjusting the focusing power of the cornea so it focuses light onto the retina. Hyperopia • Hyperopia AKA farsightedness: A condition causing poor vision in which people can see objects that are far away but do not see near objects clearly. • Corrective lenses (convex) can be used. Receptors and Perception • Light entering visual receptors triggers electrical signals when the light is absorbed by light-sensitive visual pigment molecules in the receptors. Transforming Light Energy Into Electrical Energy • Transduction: In the senses, the transformation of environmental energy into electrical energy. For example, the retinal receptors transduce light energy into electrical energy. • Transduction occurs in the rods and cones in the eyes. • Outer segments: Part of the rod and cone visual receptors that contain the light-sensitive visual pigment molecules. • Visual pigments are located in the outer segments of the rods and cones. • Visual pigments have 2 parts: a long protein called opsin and a much smaller light sensitive component called retinal. • Retinal changes shape when it absorbs light, called isomerization. It triggers a sequence of reactions that culminates in generation of an electrical response in the receptor. • Isomerization: Change in shape of the retinal part of the visual pigment molecule that occurs when the molecule absorbs a quantum of light. Isomerization triggers the enzyme cascade that results in transduction from light energy to electrical energy in the retinal receptors. • The chain reaction from isomerization amplifies its effects. Adapting to the Dark • Increasing the sensitivity to light allows us to see in the dark. • Dark adaptation: Visual adaptation that occurs in the dark, during which the sensitivity to light increases. This increase in sensitivity is associated with regeneration of the rod and cone visual pigments. • The receptors for rods and cones adapt to the dark at different rates, depending on their visual pigment. Distribution of the Rods and Cones • At the fovea, there are no rods, only cones. Cone density diminishes greatly as you move away from the fovea. • Fovea: A small area in the human retina that contains only con receptors. The fovea is located on the line of sight, so that when a person looks at an object, the center of its image falls on the fovea. • Peripheral retina: The area of retina outside the fovea. • The peripheral retina contains many more rods than cones. • Macular degeneration: A clinical condition that causes degeneration of the macula. An area of the retina that includes the fovea and a small surrounding. • Macular degeneration creates a blind spot right in the middle of your visions. Retinitis pigmentosa creates a blind spot in your peripheral vision. • Retinitis pigmentosa: A retinal disease that causes a gradual loss of vision, beginning in the peripheral retina. • Retinitis pigmentosa is passed through families. It can end up affecting the cones in the fovea in severe cases. • Blind spot: The small area where the optic nerve leaves the back of the eye. There are no visual receptors in this area, so small images falling directly on the blind spot cannot be seen. • The blind spot is located off to the side of our visual field, where objects are not in sharp focus. This is one of the reasons why we do not notice a blind spot. The main reason though is that the brain ‘fills-in’the place where the image disappears. Measuring the DarkAdaptation Curve • Dark adaptation curve: The function that traces the time course of the increase in visual sensitivity that occurs during dark adaptation. • First step in measuring a dark adaptation curve is to have the observer look at a small fixation point while paying attention to a flashing test light that is off to the side. The fixation point falls on the fovea while the light falls in the peripheral vision. The observer measures their threshold for seeing the light by adjusting its intensity. The amount of energy required to barely see the light is converted into sensitivity (sensitivity=1/threshold). At the beginning of this experiment, threshold is high and sensitivity is low. The adapting light is extinguished; the observer is now in the dark. The observer then adjusts the flashing light so that it can barely be seen.As the observer adapts, the light intensity needed will go down. • Light-adapted sensitivity: The sensitivity of the eye when in the light-adapted state. Usually taken as a starting point for the dark adaptation curve because it is the sensitivity of the eye just before the lights are turned off. • Rods adapt slowly over a longer period and to a higher sensitivity than cones. Cones adapt quickly over a short period, but have a lower level of sensitivity. • Dark-adapted sensitivity: The sensitivity of the eye after it has completed adapted to the dark. Measuring ConeAdaptation • Observer looks directly at the test light so that it is solely on cones in the fovea. Measuring Rod Adaptation • Need to measures dark adaptation in a person who has no cones. • Rod monochromats: A person who has a retina in which the only functioning receptors are rods. • The light-adapted sensitivity measured just before the lights are turned off is determined by the rods only. • As soon as a light is extinguished, the sensitivity of rods and cones begins to increase. • Rod-cone break: The point on the dark adaptation curve at which vision shifts from cone vision to rod vision. Visual Pigment Regeneration • Visual pigment bleaching: The change in the colour of a visual pigment that occurs when visual pigment molecules are isomerized by exposure to light. • Light causing retinal to change shape and separate from opsin. This causes visual pigment bleaching. • When pigments are in their bleached state, they are no longer useful for vision. • Visual pigment regeneration: Occurs after the visual pigment’s 2 components – opsin and retinal – have become separated due to the action of light. Regeneration, which occurs in the dark, involves a rejoining of these 2 components to reform the visual pigment molecule. This process depends on enzymes located in the pigment epithelium. • At normal light levels, your eyes contain some bleached and some non-bleached photo pigments. • The increase in regeneration when we turn off a light is responsible for the increased sensitivity as time goes by. • Rods take about 30 minutes to regenerate, whereas cones take about 6 minutes. • Our sensitivity to light depends on the concentration of a chemical – the visual pigment. • The speed at which our sensitivity increases in the dark depends on a chemical reaction – the regeneration of the visual pigment. • Detached retina: A condition in which the retina is detached from the back of the eye. • The pigment epithelium contains the enzymes responsible for the regeneration. • Spectral sensitivity: The sensitivity of visual receptors to different parts of the visible spectrum. • Spectral sensitivity is the eye’s sensitivity to light as a function of the light’s wavelength. Spectral Sensitivity • Spectral sensitivity curve: The function relating a subject’s sensitivity to light to the wavelength of the light. The spectral sensitivity curves for rod and cone vision indicates that the rods and cones are maximally sensitive at 500nm and 560nm, respectively
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