ODOR SPECIFICITIES OF THE FROG'S OLFACTORY RECEPTORS 31 



and cyclo-hexanol. Group four responds to benzaldehyde, nitrobenzene, 

 benzonitrite, musk, and amyl alcohol. Group five responds to pyridine, 

 musk, cinnamaldehyde, and /?-butanol. Musk is the strongest stimulus for 

 a sixth type that does not show much response to benzaldehyde or nitro- 

 benzene. A seventh type responds to pyridine more strongly than to most 

 of our other stimuli. The eighth group, which is very common, responds to 

 butanol, ethyl butyrate, amyl alcohol, and geraniol. There seem to be 

 other types but we have not seen them often enough to be able to charac- 

 terize them at all. Furthermore, it is possible and likely that these types 

 may be condensed into fewer groups or expanded into more. If we have 

 not used stimuli that are especially effective and, instead, are seeing res- 

 ponses to some of the large number of odors that weakly affect a type, it 

 would account for much of our uncertainty and for the fact that no two 

 units seem to be completely alike. 



Fig. 14. A unit that shows a strong response to butyric acid, and weak response 



to pyridine and //-butanol. It could also be interpreted as showing inhibition 



for //-butanol. Sweep lengths, 10 sec. 



To give some indication of the complexity that we face, we have com- 

 piled the table shown in Fig. 1 5. It is a list of all of the stimuli that we have 

 used in many experiments. The second column lists the number of cells 

 that we attempted to stimulate with each odor. The third column is the 

 number of cells that showed a repeatable response measured as a transient 

 increase in discharge rate. Most units respond to many things. Each cell 

 shows such individuality in its weaker responses that, in spite of a rather 

 large number of attempts, we have not been able to discover a unique set 



