ODOR SPECIFICITIES OF THE FROG'S OLFACTORY RECEPTORS 33 



has in fact given the frog such an inferior set of analyzers for odor. How- 

 ever, our measurements are apparently made in such a way that we cannot 

 discern the unique properties for which we are searching. The categories 

 that we might construct do not fall into order on the basis of simple 

 chemical properties, or on any psychological odor groupings. There are 

 no data on cross inhibition either at the behavioral or physiological levels 

 for the frog, and data from other vertebrates do not help in achieving this 

 order either. In our earlier experiments in which we used only a few 

 odors, one from each of Zwaardemaker's major groups, it appeared that 

 many units were uniquely sensitive to only one odor of our set. However, 

 when we expanded the collection of stimuli, the exceptions were much 

 more common than were the ones that responded according to our 

 supposition. 



If we assume (as appears reasonable from our data) that the resting dis- 

 charge rate for the receptors is either small or zero, and therefore that 

 inhibition is not a major part of the code for describing an odor, we can 

 consider several different possible types of receptor mechanisms and see if 

 any are preferred by reason of being consistent with our measurements. 



1. All of the receptors are identical. In this case the number of active 

 cells would indicate the intensity of the stimulus and the pattern of the 

 discharge or the topographical position of the receptor on the mucosa or 

 both would indicate the quality of the odor. Under these assumptions, 

 we should see very similar responses to the same odor from one cell to the 

 next, at least when the electrode stays within a restricted area of the 

 mucosa. In fact, of course, the receptors that we record do not behave in 

 this way at all. Differential selectivity is most obvious when the electrode 

 is recording simultaneously from different units with clearly discriminative 

 amplitudes. 



2. There are several different species of receptor cells, each species of 

 which has particular selectivity properties. If the receptors are like this, 

 and if our recording method does not seriously disturb the properties of 

 the units, we ought to see, at least occasionally, two cells that respond in 

 the same way to our entire collection of stimulants. In fact, there is a 

 difference in the response of any two cells with respect to some odors. It 

 is possible that we have seen some identical cells but have disturbed their 

 identity by our manipulations. More sophisticated experimentation might 

 prove this to be the correct description. 



3. There are a great many different receptor types, possibly one for 

 each odor or combination of odors. This seems unlikely, as we see a strong 

 tendency for the receptors to form at least vague odor groups. 



4. There arc different receptor site types that are distributed over each 

 cell. One cell can differ from the next in having different ratios of the 

 receptor sites. In this case the variability that we see is most reasonable, in 



