Hockenbury Discovering Psychology 5th txtbk

96 CHAPTER 3 Sensation and PerceptionFigure 3.4 Demonstration of the BlindSpot Hold the book a few feet in front ofyou. Close your right eye and stare at theinsect spray can with your left eye. Slowlybring the book toward your face. At somepoint the spider will disappear becauseyou have focused it onto the part of yourretina where the blind spot is located. Notice,however, that you still perceive thespider web. That’s because your brain hasfilled in information from the surroundingarea (Komatsu, 2006).The Blind SpotOne part of the retina lacks rods and cones altogether. This area, called the opticdisk, is the point at which the fibers that make up the optic nerve leave the back ofthe eye and project to the brain. Because there are no photoreceptors in the opticdisk, we have a tiny hole, or blind spot, in our field of vision. To experience theblind spot, try the demonstration in Figure 3.4.Why don’t we notice this hole in our visual field? The most compelling explanationis that the brain actually fills in the missing background information(Ramachandran, 1992a, 1992b). In effect, signals from neighboring neurons fill inthe blind spot with the color and texture of the surrounding visual information(Komatsu, 2006; Spillmann & others, 2006).Processing Visual InformationKey Theme• Signals from the rods and cones undergo preliminary processing in theretina before they are transmitted to the brain.optic diskArea of the retina without rods or cones,where the optic nerve exits the back ofthe eye.blind spotThe point at which the optic nerve leavesthe eye, producing a small gap in the fieldof vision.ganglion cellsIn the retina, the specialized neurons thatconnect to the bipolar cells; the bundledaxons of the ganglion cells form the opticnerve.bipolar cellsIn the retina, the specialized neurons thatconnect the rods and cones with theganglion cells.Key Questions• What are the bipolar and ganglion cells, and how do their functions differ?• How is visual information transmitted from the retina to the brain?• What properties of light correspond to color perceptions, and how is colorvision explained?Visual information is processed primarily in the brain. However, before visual informationis sent to the brain, it undergoes some preliminary processing in the retinaby specialized neurons called ganglion cells. This preliminary processing of visualdata in the cells of the retina is possible because the retina develops from a bit ofbrain tissue that “migrates” to the eye during fetal development (see Hubel, 1995).When the numbers of rods and cones are combined, there are over 130 million receptorcells in each retina. However, there are only about 1 million ganglion cells. How dojust 1 million ganglion cells transmit messages from 130 million visual receptor cells?Visual Processing in the RetinaInformation from the sensory receptors, the rods and cones, is first collected by specializedneurons, called bipolar cells (see the lower part of Figure 3.3). The bipolarcells then funnel the collection of raw data to the ganglion cells. Each ganglion cell