CHEMORECEPTION 129 



should say that there is no binaural summation. We can imagine, for 

 this purpose, two monaural listeners, one with only a right ear, the 

 other with only a left ear. The two listeners have no means whatsoever 

 of communicating with each other, but both report to the same experi- 

 menter. To obtain measures of monaural and "binaural" sensitivity, 

 the experimenter tests the two listeners separately (successively), then 

 together, in the latter instance recording a positive response whenever 

 either hstener reports hearing the stimulus tone.' The case of unilateral 

 versus bilateral tarsal stimulation of Phormia is similar. The results 

 show that the bilateral threshold for sucrose is lower than the unilateral 

 threshold by a factor that is never greater than can be satisfactorily 

 accounted for on a simple probabihty basis, that is, the greater the 

 number of available receptors, the greater that chance that the number 

 required for a threshold response will be stimulated (Fig. 76) (Dethier, 

 1953 b). 



Discrimination 



Threshold measurements do not permit evaluation of intensity dis- 

 crimination over the complete effective concentration range of a com- 

 pound. Von Frisch (1935) had demonstrated that honeybees responded 

 differently to 0-125M and 0-156M sucrose and to 0-25M and 0-3125M 



Table 5 

 (From Dethier and Rhoades, 1954) 

 Intensity discrimination of sucrose 



