Hockenbury Discovering Psychology 5th txtbk

92 CHAPTER 3 Sensation and PerceptionThe other important threshold involves detecting the difference between twostimuli. The difference threshold is the smallest possible difference between twostimuli that can be detected half the time. Another term for the difference thresholdis just noticeable difference, which is abbreviated jnd.The just noticeable difference will vary depending on its relation to the originalstimulus. This principle of sensation is called Weber’s law, after the Germanphysiologist Ernst Weber (1795–1878). Weber’s law holds that for each sense,the size of a just noticeable difference is a constant proportion of the size of theinitial stimulus. So, whether we can detect a change in the strength of a stimulusdepends on the intensity of the original stimulus. For example, if you are holdinga pebble (the original stimulus), you will notice an increase in weight if a secondpebble is placed in your hand. But if you start off holding a very heavy rock (theoriginal stimulus), you probably won’t detect an increase in weight when the samepebble is balanced on it.What Weber’s law underscores is that our psychological experience of sensationis relative. That is, there is no simple, one-to-one correspondence between the objectivecharacteristics of a physical stimulus, such as the weight of a pebble, and ourpsychological experience of it.Sensory AdaptationSuppose your best friend has invited you over for a spaghetti dinner. As you walk inthe front door, you’re almost overwhelmed by the odor of onions and garlic cookingon the stove. However, after just a few moments, you no longer notice the smell.Why? Because your sensory receptor cells become less responsive to a constant stimulus.This gradual decline in sensitivity to a constant stimulus is called sensoryadaptation. Once again, we see that our experience of sensation is relative—in thiscase, relative to the duration of exposure.Because of sensory adaptation, we become accustomed to constant stimuli,which allows us to quickly notice new or changing stimuli. This makes sense. If wewere continually aware of all incoming stimuli, we’d be so overwhelmed with sensoryinformation that we wouldn’t be able to focus our attention. So, for example,once you manage to land your posterior on the sofa, you don’t need to be constantlyreminded that the sofa is beneath you.VisionFrom Light to SightKey Theme• The receptor cells for vision respond to the physical energy of light wavesand are located in the retina of the eye.difference thresholdThe smallest possible difference be tweentwo stimuli that can be detected half thetime; also called just noticeable difference.Weber’s law(VAY-berz) A principle of sensation that holdsthat the size of the just noticeable differencewill vary depending on its relation to thestrength of the original stimulus.sensory adaptationThe decline in sensitivity to a constantstimulus.Key Questions• What is the visible spectrum?• What are the key structures of the eye and their functions?• What are rods and cones, and how do their functions differ?A lone caterpillar on the screen door, the pile of dirty laundry in the corner of the closet,a spectacular autumn sunset, the intricate play of color, light, and texture in a paintingby Monet. The sense organ for vision is the eye, which contains receptor cells that aresensitive to the physical energy of light. Before we can talk about how the eye functions,we need to briefly discuss some characteristics of light as the visual stimulus.