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University of Toronto St. George
Christine Burton

Psy280 chapter 3- introduction to vision  Electromagnetic spectrum: continuum of electromagnetic energy that is produced by electric charges and is radiated as waves.  Wavelength: distance between the peaks of the electromagnetic waves. Wavelengths in the electromagnetic spectrum ranges from extremely short-wavelength gamma rays to long wavelengths radio waves.  Visible light: energy within the electromagnetic spectrum that humans can perceive, has wavelengths ranging from about 400 to 700 nm.  For humans and some other animals, the wavelength of visible light is associated with the different colors of the spectrum. Although we will usually specify light in terms of its wavelength, light can also be described as consisting small packets of energy – photons.  Eye: where vision beings. Light reflected from objects in the environment enters the eye through the pupil and is focused by cornea and lens to form sharp images of the objects on the retina: contains receptors for vision.  rods and cones contains light-senstive chemicals called visual pigments. They react to light and trigger electrical signals.  These signals flow through the network of neurons that make up the retina.  Optic nerve: signals then emerge from the back of the eye, which conducts signals toward the brain.  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 occur at the beginning of the perceptual process.  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 8p percent of the eye’s focusing power, but like the lenses in eye glasses, it is fixed in place, so cant adjust its focus.  Lens supplies remaining of 20 percent of the eye’s focusing power, can change its shape to adjust the eye’s focus for stimuli located at different distances.  If the object is located more than about 20 feet away, the light rays that reach the eye are essentially parallel, and these parallel rays are brought to a focus on the retina at point A.  But when object moves closer to the eye, the light rays reflected from this object enter the eye at more of an angle, which pushes the focus point back to point B. light is stopped by the back of the eye before it reaches point B, so image on the retina is out of focus. Objects would be blurred.  Accommodation: ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that it gets thicker. Increased curvature bends the light rays passing through the lens to pull the focus point back to A to create a sharp image on the retina.  This process occurs unconsciously, we are unaware that the lens is constantly changing its focusing power so we can see clearly at different angles. Unconscious focusing process works efficiently that most people assume that everything, near and far is always in focus. Psy280 chapter 3- introduction to vision  Enables us to bring both near and far objects into focus, although objects at different distances are not in focus at the same time. Accommodation has its limits.  Near point: distance at which your lens can no longer adjust to bring close objects into focus.  Presbyopia: distance of the near point increases as a person gets older. Loss of ability to accommodate occurs because the lens hardens with age, and the ciliary muscles become weaker. Changes make it more difficult for the lens to change its shape for vision at close range.  Gradual decrease in accommodative ability poses little problem for most people before the age of 45, at around that age the ability to accommodate begins to decrease rapidly, and the near point moves beyond a comfortable reading distance.  Myopia/ nearsightedness: an ability to see distant objects clearly.  Can be caused by 2 reasons: 1.refractive myopia: which cornea and lens bends the light too much. 2.axial myopia: which eyeball is too long.  Far point: spot of light becomes focused on the retina.  Laser-assisted in situ keratomileusis (LASIK) surgery: involves sculpting the cornea with a type of laser called an excimer laser, which does not heat tissue. Small 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.  Focusing the image clearly onto the retina is the initial step in the process of vision.  Vision occurs not in the retina, but in the brain, and before the brain can create vision, light on the retina must be transformed into electricity.  Outer segment: light acts to create electricity. Rod outer segments contain stacks of disc. Each disc contains thousands of visual pigments molecules.  The molecule is long strand of protein called opsin. Loops back and forth across the disc membrane seven times.  The molecule retina is attached, each visual pigment molecule contains only one of these tiny retinal molecules. Retinal is crucial for transduction, because it is part of the visual pigment that is sensitive to light.  Isomerization: when a photo of light hits the retinal, it changes shape, so it is sticking out from the opsin.  Hecht’s psychophysical experiment:  Determine how many visual pigment molecules need to be isomerized for a person to see, by using the method of constant stimuli.  He found that a person could detect a flash of light that contained 100 photons. Person sees a flash of light when only 7 visual pigment molecules are isomerized.  Only 7 photons but 500 receptors.  Resutls: person can see a light if 7 rod receptors are activated simultaneously. Rod receptor can be activated by the isomerization of just 1 visual pigment molecule.  Isomerization of one visual pigment molecule activates about a million other molecules.  Enzyme cascade: sequence of reactions triggered by the activated visual pigment molecule.  Pigments and perception:  More rods than cones. However ratio depends on location in the retina.  There is one small area the fovea, contains only cones. When we look directly at an object, its image falls on the fovea. Psy280 chapter 3- introduction to vision  The peripheral retina which includes all of the retina outside of the fovea, contains both rods and cones. Important to note that although fovea is in the place where there are only cones, there are many cones in the peripheral retina. Fovea is small that it contains only about 1 percent of the 6 million cones in the retina.  Around 120 million rods and 6 million cones.  One way to appreciate the fact that the rods and cones are distributed differently in the retina is by considering what happens when functioning receptors are missing from one area of the retina.  Macular degeneration: most common in older people, destroys the cone-rich fovea and a small area that surrounds it. Creates a blind spot in central vision, so when a person looks at something he or she loses sight of it.  Retinitis pigmentosa: degeneration of the retina that is passed from one generation to the next. Condition first attacks the peripheral rod receptors and results in poor vision in the peripheral visual field. Eventually, in severe cases, the fovea cone receptors are also attacked, resulting in complete blindness.  The peripheral retina initially degenerates and causes loss of vision in the periphery. Resulting condition is sometimes called ‘tunnel vision’.  Blind spot is that some mechanism in the brain ‘fills in’ the place where the image disappears.  Dark adaptation: causes the eye to increase its sensitivity in the dark.  Dark adaptation curve: a plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished.  Show that the initial rapid stage is due to adaptation of the cone receptors and the second, slower stage is due to adaptation of the rod receptors.  Light-adapted sensitivity: it is measure while the eyes are adapted to the light.  That as dark adaptation proceeds, the observer becomes more sensitive to the light.  Increases in two phase. It increases rapidly for the first 3 to 4 minutes after the light is extinguished and then levels off, it begins increasing again at about 30 minutes after the light was extinguished.  Dark-adapted sensitivity: sensitivity at the end of dark adaptation. About 100
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