Chapter Concept Check Answers

Concept Check 1

• The two types of threshold were given statistical definitions because the experimental data did not permit an absolute definition. There was no none-to-all point of change in the results that were observed when psychophysical researchers attempted to measure the absolute and difference thresholds.
• In switching from a very lax to a very strict decision criterion, a person’s false alarm rate would go down and the miss rate would go up. These changes would be due to the person’s changing from saying “yes” most of the time to saying “no” most of the time.
• A really large constant fraction in Weber’s law would indicate that difference judgments for that type of sensory judgment are not very good; a larger proportion of the standard stimulus is necessary for a difference to be perceived.
• It is adaptive for the exponent to be greater than 1 for dangerous physical energy forms because we would then perceive weak input of that type to be much more intense than it really is. This would possibly allow us to escape before being exposed to more intense energy of that type.

Concept Check 2

• In nearsightedness, we have difficulty viewing distant objects because their images come into focus in front of the retina; in farsightedness, we have difficulty viewing near objects because their images come into focus behind the retina. The focusing problems could be due to defects in the lens or the shape of the eyeball.
• After staring at the flag with alternating yellow and white stripes and a block of green in the middle, the yellow, white, and green parts of the three opponent-process systems would be fatigued and thus unable to oppose the blue, black, and red parts of these systems when you stared at the white sheet of paper. Thus, instead of seeing white, you would see an afterimage of a flag with alternating blue and black stripes and a block of red in the middle. Once the opposing parts of the three systems recovered, the flag afterimage would disappear, and you would see the white surface.
• Longer wavelengths lead to lower frequencies because such wavelengths can only cycle a small number of times per second. Similarly, shorter wavelengths lead to higher frequencies because they can cycle more times per second.
• Neither theory by itself can explain how we hear the entire range of pitches, 20 to 20,000 Hz, because each theory is unable to explain pitch perception for a particular part of this range. Place theory cannot explain how we perceive low pitches (< 500 Hz) because there are no places of maximal firing along the basilar membrane for these frequencies. The firing rate of the entire membrane mimicks these frequencies. Similarly, frequency theory cannot explain how we perceive high pitches, those greater than 5,000 Hz, because there is a physiological limit on the firing rate for cells. Even if the volley principle is employed, this limit is about 5,000 times per second. This means that the hair cells could not generate firing rates to match the high frequencies.

Concept Check 3

• Perceptual processing requires both types of processing because without bottom-up processing you would have nothing to perceive and without top-down processing you would have no knowledge to use to interpret the bottom-up input.
• The similarity is that, in both cases, the brain uses top-down processing to complete the perception. In context effects, the brain uses the present context to complete the perception by determining what would be meaningful in that particular context. In closure, the brain uses the incomplete part of an object to determine what the remaining part should be in order for it to be a meaningful object.
• It is more difficult to recognize your professor because she is in a very different context. Your brain is thrown for a “perceptual loop” because she doesn’t fit in this context (outside the classroom). This is why it takes your brain longer to find the relevant top-down knowledge.
• The retinal image sizes of the two objects in each case are equal because the two objects are actually equal in size and are equidistant from us. Available distance cues, however, lead the brain to believe mistakenly that one of the two objects in each case is farther away. Therefore, the brain enlarges the size of the object that it thinks is more distant because this would have to be the case in order for the geometric relationship between retinal image size and distance from us to hold. For example, in the Ponzo illusion, the linear perspective distance cue leads the brain to think that the top horizontal bar is farther away. Because the two horizontal bars have identical retinal image sizes, the brain then uses the geometric relationship and mistakenly creates an illusion by making the top bar larger in our perception.