QUANTITATIVE ESTIMATES OF SENSORY EVENTS 285 



depressed and enhanced sensitivity, obtained by previously stimulating the 

 ear to be tested, or the other, with tones of the same frequency as the test 

 tone and about 15 to 20 times its intensity. 



Gage (16) investigated the variation of the uniaural difference threshold 

 with simultaneous stimulation of the other ear by tones of the same fre- 

 quency. Gage's detailed results are not relevant to the present purpose, 

 but his general results suggest the occurrence of high values of the threshold 

 at low intensities to a very marked degree. 



An unusual approach to the continuity of the value of the difference 

 threshold is seen in the work of Kenneth and Thouless (27), who claimed 

 to show that this continuity extended to the absolute threshold. By start- 

 ing at zero intensity and taking several intermediate values of the stimulus 

 between this and the absolute threshold of hearing, they demonstrated 

 that the just noticeable difference varied continuously until a point was 

 reached at which it so happened that the requirements of Weber's law were 

 fulfilled over a certain range. 



The work of Churcher, King and Davies (10) is interesting (a) for its 

 stress on the continuity of variation of the difference threshold, and (b) for 

 the fact that the just noticeable increments and decrements of intensity 

 found were not such as would occur if Weber's law held good. This 

 suggests the importance of ' set,' a point more fully dealt with by 

 Montgomery (38, see below). Differential sensitivity to cyclical changes 

 of intensity was also studied, though the discrepancy noted under (b) above 

 made this method, in the authors' opinion, not strictly valid. If this 

 objection is a real one, it would seem to apply also to all the ' alternation ' 

 methods used in other investigations. 



In addition to a direct investigation of differential sensitivity at four 

 intensity levels, Montgomery (38) made a systematic survey of the effect 

 of ' set ' as determined by variations in experimental conditions. Table II 

 shows thresholds, expressed in both db and fractional form, for the thermal 

 noise from a high-gain amplifier at an intensity level of 40 db. These 

 figures stress the difficulty, noted also in connection with loudness estima- 

 tion, of comparing results obtained by different experimenters using different 

 methods. 



Table II. (Montgomery.) 



A further point was made by Upton and Holway (58), whose chief 

 interest was to demonstrate that differential sensitivity to sound intensity 

 was a specific reproducible function of exposure-time. Interpreted from 

 the point of view of Weber's law, the results indicate that while relative 

 constancy of the threshold was obtained with long exposure-times, with 



