132 SENSORY RECEPTORS 



Exercise XXVI 



or that two lights are equal in brightness. What 

 would it mean, however, for me to say that one 

 light is 2.3 times as bright as another? 



Weber (1834), experimenting initially with 

 weights, discovered that the difference threshold 

 varies with the intensity of the stimulus in a 

 peculiar way that became known as Weber's 

 law: the difference threshold (A/) is a constant 

 fraction of the intensity of stimulus (/), that is, 

 A/// = constant. The ratio A/// is called the 

 Weber fraction. It is an inverse measure of the 

 capacity to discriminate intensities; the larger 

 this ratio, the poorer is the capacity for making 

 such discriminations. Weber's law holds only 

 approximately and over a limited range. The 

 ratio A/// remains approximately constant for 

 many senses over the middle ranges of intensity, 

 but rises at both the low and high extremes of 

 intensity. 



Estimation of weight 



A weight held in the hand is supported by 

 muscle tensions in the hand and arm. These 

 stimulate tension receptors in the muscles and 

 tendons, the reports of which help to guide the 

 limb and also excite sensations. If two weights 

 are successively lifted, there exists a minimal 

 difference in weight such that one is judged 

 just heavier than the other, the difference thresh- 

 old. This may be determined for various weights, 

 and the constancy of the Weber ratio tested. 



Working in pairs, perform the following ex- 

 periment. You have two 125-ml Erlenmeyer 

 flasks. Mark one of them with a crayon, to be 

 the test flask. Add water to both so as to bring 

 them to equal weight at about 50 grams. The 

 experimenter now hands the flasks to the sub- 

 ject, whose eyes are closed. The subject holds 

 the flasks either cupped in his palms or with his 

 fingers by the necks, but whichever way he 

 chooses should be maintained throughout the 

 experiment. For this first experiment the flasks 

 should be held steady, and the subject says 

 whether they feel equal or unequal in weight. 

 Presumably they feel equal. Now the experi- 

 menter takes the flasks again, and adds water 



to the test flask in 2-ml portions, each time 

 handing the flasks back to the subject, randomly 

 mixing right and left, each time giving the sub- 

 ject all the time he needs to decide the relative 

 weights. When the test flask feels just per- 

 ceptibly heavier than the other, record the vol- 

 ume of water that was added to it. This is also 

 the added weight, since 1 ml of water weighs 

 1 gram. The difference in weight is then the 

 difference threshold for a weight of 50 grams. 



Now repeat this procedure with the flasks 

 initially made equal in weight at about 100, 

 200, and 500 grams (the latter two in 500-ml 

 flasks) and tabulate the results. 



Prepare a graph plotting the Weber ratio 

 (A weight/lower weight) on the vertical axis 

 against the lower weight on the horizontal axis. 



Repeat this experiment for at least one 

 weight, or all the way through if you have time, 

 with the subject wagging the flasks up and down 

 as he estimates their weights. Do you find a 

 difference in the Weber ratio? Motion in gen- 

 eral produces a much stronger and more per- 

 sistent excitation than a stationary stimulus. 

 Why? (Recall the effectiveness of flickered as 

 compared with steady light, in Exercise XX, 

 p. 108.) 



Has the Weber ratio remained approximately 

 constant in your experiments? What do you 

 conclude of the accuracy with which weights 

 can be estimated? How do the Weber ratios of 

 other subjects compare with yours? This last 

 question illustrates one example of the "per- 

 sonal equation" that is involved in every type 

 of sensory judgment. 



VISION 



The blind spot 



The point at which the optic nerve leaves the 

 retina is blind, since this area contains no visual 

 receptors. Lay a sheet of blank white paper on 

 the desk, and draw a small cross to the left of 

 center. The subject, holding his left eye closed, 

 should stare fixedly at the cross with his right 

 eye 30 cm from it. (Staring fixedly at anything 

 means holding its image within the central fovea 



