Hockenbury Discovering Psychology 5th txtbk

Vision97receives information from the photoreceptors that are located in its receptive field ina particular area of the retina. In this early stage of visual processing, each ganglioncell combines, analyzes, and encodes the information from the photoreceptors in itsreceptive field before transmitting the information to the brain (Masland, 2001).Signals from rods and signals from cones are processed differently in the ganglioncells. For the most part, a single ganglion cell receives information from only oneor two cones but might well receive information from a hundred or more rods. Themessages from these many different rods are combined in the retina before they aresent to the brain. Thus, the brain receives less specific visual information from therods and messages of much greater visual detail from the cones.As an analogy to how rod information is processed, imagine listening to a hundredpeople trying to talk at once over the same telephone line. You would hear thesound of many people talking, but individual voices would be blurred. Now imaginelistening to the voice of a single individual being transmitted across the sametelephone line. Every syllable and sound would be clear and distinct. In much thesame way, cones use the ganglion cells to provide the brain with more specific visualinformation than is received from rods.Because of this difference in how information is processed, cones are especially importantin visual acuity—the ability to see fine details. Visual acuity is strongest whenimages are focused on the fovea because of the high concentration of cones there.optic nerveThe thick nerve that exits from the back ofthe eye and carries visual information to thevisual cortex in the brain.optic chiasm(KI-az-em) Point in the brain where theoptic nerve fibers from each eye meet andpartly cross over to the opposite side ofthe brain.From Eye to BrainHow is information transmitted from the ganglion cells of the retina to the brain?The 1 million axons of the ganglion cells are bundled together to form the opticnerve, a thick nerve that exits from the back of the eye at the optic disk and extendsto the brain (see Figure 3.5). The optic nerve has about the same diameter as a pencil.After exiting the eyes, the left and right optic nerves meet at the optic chiasm.Then the fibers of the left and right optic nerves split in two. One set of axonscrosses over and projects to the opposite side of the brain. The other set of axonsforms a pathway that continues along the same side of the brain (see Figure 3.5).From the optic chiasm, most of the optic nerve axons project to the brain structurecalled the thalamus. This primary pathway seems to be responsible for processinginformation about form, color, brightness, and depth. A smaller number ofaxons follow a detour to areas in the midbrain before they make their way to theOptic nerveThalamusVisualcortexRight visual fieldLeft visual fieldRetinaRetinaRight eyeLeft eyeOpticnerveOpticchiasmVisual cortexFigure 3.5 Neural Pathways from Eye toBrain The bundled axons of the ganglioncells form the optic nerve, which exits theretina at the optic disk. The optic nervesfrom the left and right eyes meet at theoptic chiasm, then split apart. One set ofnerve fibers crosses over and projects tothe opposite side of the brain, and anotherset of nerve fibers continues alongthe same side of the brain. Most of thenerve fibers travel to the thalamus andthen on to the visual cortex of the occipitallobe.