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Chapter 3

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PSYC 2390
Lana Trick

PSYC 2390: SENSATION AND PERCEPTION Chapter 3 – pg. 61 Focusing Light Onto The Retina • Vision begins when visible light is reflected from objects into the eye Light: The Stimulus for Vision • Vision is based on visible light, which is a band of energy within the electromagnetic spectrum- a continuum of electromagnetic energy that is produced by electrical charges and is radiated as waves • The energy in this spectrum can be described by its wavelength – the distance between the peaks of the electromagnetic waves • Visible light, the energy within the electromagnetic spectrum that humans can perceive, has wavelengths ranging from about 400 to 700 nanometers The Eye • The eye is where vision begins. Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to forms sharp images of the objects on the retina, which contains the receptors for vision • There are two kinds of visual receptors, rods and cones, which contain light- sensitive chemicals called visual pigments that react to light and trigger electrical signals • These signals flow through the network of neurons that make up the retina. The signals then emerge from the back of the eye in the optic nerve, which conducts signals toward the brain • The cornea and lens at the front of the eye and the receptors and neurons in the retina lining the back of the eye shape what we see by creating the transformations that occurs at the beginning of the perceptual process Light is focused by the Eye • Once light is reflected from an object into the eye, it needs to be focused onto the retina • The cornea, the transparent covering of the front of the eye, accounts for about 80 percent of the eye’s focusing power, but like the lenses in the eyeglasses, it is fixed in place, so cant adjust its focus. • The lens, which supplies the remaining 20 percent of the eye’s focusing power, can change its shape to adjust the eye’s focus for stimuli located at different distances • We can understand how the lens adjust its focus by first considering what happens when the eye is relaxed and a person views a small object that is far away → If the object is located more than about 20 feet away, the light rats that reach the eye are essentially parallel, and these parallel rays are brought to a focus on the retina at pointA → But if the object moves closer to the eye, the light rays rejected from this object enter the eye at more of an angle, which pushes the focus point back to point B (further back thanA) PSYC 2390: SENSATION AND PERCEPTION → However, the light is stopped by the back of the eye before it reaches point B, so the image on the retina is out of focus. If things remained in this state, the person would see the object as blurred → Aprocess called accommodation keeps this from happening. The ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker. This increased curvature bends the light rays passing through the lens to pull the focus back toAto create a sharp image on the retina • Accommodation enables you to bring both near and far objects into focus, although objects at difference distances are not in focus at the same time • But accommodating as its limits. When the pencil was too close, you couldn’t see it clearly, even though you were straining to accommodate. The distance at which your lens can no longer adjust to bring close objects into focus is called the near point • The distance of the near point increases as a person gets older, a condition called presbyopia. The loss of ability to accommodate occurs because the lens hardens with age, and the ciliary muscles become weaker. These changes make it more difficult for the lens to change its shape for vision at close range • Myopia or nearsightedness, an inability to see distant objects clearly → The myopic eye brings parallel rays of light into focus at a point in front of the retina so that the image reaching the retina is blurred → This problem can be caused by either of two factors (1) refractive myopia, in which the cornea and/or the lens bends the light too much, or (2) axial myopia, in which the eyeball is too long → Can solve by either moving the object closer which pushes the focus point farther back and if object moved close enough can push focus point onto the retina. The distance at which the spot of light becomes focused on the retina is called the far point; when the spot of light is at the far point, a myope can see it clearly → Another solution is corrective glasses or lenses → Surgical procedures in which lasers are used to change the shape of the cornea have been introduced that enable people to experience good vision without corrective lenses. More than 1 millionAmericans a year have laser-assisted in situ keratomileusis (LASIK) surgery ⇒ LASIK involves sculpting the cornea with a type of laser called an excimer laser, which does not hear tissue.Asmall flap, less than the thickness of a human hair is cut into the surface of the cornea. The flap is folded out of the way, the cornea is sculpted by the laser so that it focuses light onto the retina, and the flap is then folded back into place. • Aperson with hyperopia, or farsightedness can see distant objects clearly but has trouble seeing nearby objects → In the hyopertic eye, the focus point for parallel rays of light is located behind the retina, usually because the eyeball is too short PSYC 2390: SENSATION AND PERCEPTION → By accommodating to bring the focus point back to the retina, people with hyperopia are able to see distant objects clearly → Nearby objects are more difficult for a person with hyperopia to deal with because a great deal of accommodating is required to return the focus point to the retina → The constant need to accommodate when looking at nearby objects results in eyestrain and, in older people, headaches • Focusing the image clearly onto the retina is the initial step in the process of vision. But it is important to realize that although a sharp image on the retina is essential for clear vision, we do not see the image on the retina. Vision occurs not in the retina, but in the brain and before the brain can create vision, he light on the retina must be transformed into electricity Transforming Light Into Electricity • The process of light into electricity is the process of transduction The Visual Receptors and Transduction • Transduction is carried out by receptors, neurons specialized for receiving environmental energy and transforming this energy into electricity • The receptors for vision are the rods and the cons. • Rods: rod-shaped receptor in the retina primarily responsible for vision at low levels of illumination. The rod system is extremely sensitive in the dark but cannot resolve fine details • Cones: cone-shaped receptors in the retina that are primarily responsible for vision in high levels of illumination and for color vision and detail vision • The key part of the rod for 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 contains thousands of visual pigment molecules • Zooming in on an individuals molecule, we can see that the molecule is a long strand of protein called opsin, which loops back and forth across the disc membrane seven times • Our main concern is one particular place where a molecule called retinal is attached. Each visual pigment molecule contains only one of these tiny retinal molecules. The retinal is crucial for transduction, because it is the part of the visual pigment that is sensitive to light • Transduction is triggered when the light-sensitive retinal absorbs one photon to light (remember that a photon is the smallest possible packet of light energy). Before the light is absorbed, the retinal is next to the opsin. When the photon of light hits the retinal, it changes shape, so it is sticking out from the opsin. This change in shape is called isomerization, and it is this step that triggers the transformation of the light entering the eye into electricity in the receptors How Does Transduction Occur? • Psychologist Selig Hecht was able to draw conclusions about the physiology of transduction by determining a person’s ability to see dim flashes of light • Hecht’s Psychophysical Experiment PSYC 2390: SENSATION AND PERCEPTION → The starting point for his experiment was his knowledge that transduction is triggered by the isomerization of visual pigment molecules and that is takes just on photon of light to isomerize a visual pigment molecule → He then went on to determine how many visual pigment molecules need to be isomerized for a person to see. He accomplished this by using the method of constant stimuli to determine a person’s absolute threshold for seeing a brief flash of light. → He concluded that only 1 visual pigment molecule per receptor was isomerized when his observers reported seeing the light; therefore, a rod receptor can be activated by the isomerization of only 1 visual pigment molecule → Hecht’s conclusions: 1. Aperson can see a light if 7 rod receptors are activated simultaneously 2. Arod receptor can be activated by the isomerization of just 1 visual pigment molecule • The Physiology of Transduction → Physiological and chemical search determined that isomerization of a single visual pigment molecule triggers thousands of chemical reactions, which in turn trigger thousands more. → Abiological chemical that in small amounts facilitates chemical reactions in this way is called an enzyme; therefore, the sequence of reactions triggered by the activated visual pigment molecule is called the enzyme cascade – sequence of reactions triggered by an activated visual pigment molecule that results in transduction Pigments and Perception • Vision can occur only if the rod and cone visual pigments transform the light entering the eye into electricity Distributions of the Rods and Cones • From the cross section of the retina you can see that the rods and cones are interspersed in the retina. The ratio of rods and cones depends on the location in the retina • The retina 1. Has one small area, the fovea that contains only cones. When we look directly at an object its image falls on the fovea 2. The peripheral retina, which includes the entire retina outside of the fovea, contains both rods and cones. It is important to note that although the fovea is the place where there are only cones, there are many cones in the peripheral retina, the fovea is so small that it contains only about 1 percent of the 6 million cones in the retina 3. There are many more rods than cones in the peripheral retina because most of the retina’s receptors are located there and because there are about 120 million rods and 6 million cones PSYC 2390: SENSATION AND PERCEPTION • One way to appreciate the fact that the rod and cones are distributed differently in t
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