5 Steps to a 5 AP Psychology, 2010-2011 Edition - Laura Lincoln Maitland [50]
Vision
While psychologists study all sensory processes, a major focus is visual perception because most of us depend so much on sight. Initial visual sensation and perception take place in three areas: in the cones and rods of the retina located at the back inner surface of your eye; in the pathways through your brain; and in your occipital lobes, also called the visual cortex. The image formed on your retina is upside down and incomplete. Your brain fills in information and straightens out the upside down image almost immediately.
Visual Pathway
Millions of rods and cones are the photoreceptors that convert light energy to electrochemical neural impulses. Your eyeball is protected by an outer membrane composed of the sclera, tough, white, connective tissue that contains the opaque white of the eye, and the cornea, the transparent tissue in the front of your eye.
Rays of light entering your eye are bent first by the curved transparent cornea, pass through the liquid aqueous humor and the hole through your muscular iris called the pupil, are further bent by the lens, and pass through your transparent vitreous humor before focusing on the rods and cones in the back of your eye (see Figure 8.1).
You are said to be near-sighted if too much curvature of the cornea and/or lens focuses an image in front of the retina so nearby objects are seen more clearly than distant objects. You are said to be far-sighted if too little curvature of the cornea and/or lens focuses the image behind the retina so distant objects are seen more clearly than nearby ones. Astigmatism is caused by an irregularity in the shape of the cornea and/or the lens. This distorts and blurs the image at the retina.
The more abundant rods have a lower threshold than cones and are sensitive to light and dark, as well as movement. Three different kinds of cones are each most sensitive to a different range of wavelengths of light, which provides the basis for color vision. When it suddenly becomes dark, your gradual increase in sensitivity to the low level of light, called dark adaptation, results from a shift from predominantly cone vision to predominantly rod vision. Rods and cones both synapse with a second layer of neurons in front of them in your retina, called bipolar cells. More rods synapse with one bipolar cell than do cones. Small amounts of stimulation from each rod to a bipolar cell can enable it to fire in low light. In bright light, just one cone can stimulate a bipolar cell sufficiently to fire, providing greater visual acuity or resolution. Bipolar cells transmit impulses to another layer of neurons in front of them in your retina, the ganglion cells. Axons of these cells converge to form the optic nerve of each eye. Where the optic nerve exits the retina, there aren’t any rods or cones, so the part of an image that falls on your retina in that area is missing—the blind spot. At the optic chiasm on the underside of your brain, half the axons of the optic nerve from each eye criss-cross, sending impulses from the left half of each retina to the left side of your brain and from the right half of each retina to the right side of your brain. The thalamus then routes information to the primary visual cortex of your brain, where specific neurons called feature detectors respond only to specific features of visual stimuli, for example a line in a particular orientation. Many different feature detectors can process the different elements of visual information, such as color, contours, orientation, etc., simultaneously. Simultaneous processing of stimulus elements is called