38 D. OTTOSON 



Stimulus intensity it has further been possible to evaluate the stimulus- 

 response relationship of the olfactory organ (Fig. 2). 



It is a common experience that the duration of the olfactory sensation 

 varies for different substances. Some produce a short-lasting effect, while 

 others give rise to a more persistent sensation. A comparison of the 

 responses evoked by different substances of equal stimulus intensities shows 

 that they may differ considerably in duration (Fig. 3). No systematic 



Fig. 3. Responses to different types of stimuli. A, amy! acetate ; B, butanol ; 

 C, oil of cloves. Vertical line 1 mV. Time bar 1 sec. (From Ottoson, 1956.) 



Study has as yet been made about the relation between the time-course of 

 excitation in the olfactory membrane and the properties of the stimulating 

 substance. It seems most likely, however, that the solubility properties 

 and the vapour pressure of the stimulus are of decisive importance with 

 respect to the onset and duration of the response. It is of particular in- 

 terest to note that corresponding differences have also been demonstrated 

 in the time-course of the discharge in the secondary pathways (Adrian, 

 1951). These observations show that the temporal characteristics of the 

 response are encoded in the pattern of the impulse discharge carried to the 

 olfactory cortex and may serve an important function in the discrimination 

 of different substances. 



The notion seems to be widely held that the olfactory receptors adapt 

 rapidly. However, this view is not supported by the results obtained in 

 electrophysiological studies. In recording the olfactory discharge of the 

 secondary neurons in the bulb of the rabbit, Adrian (1950) found that each 

 inspiration of odorous air was followed by a distinct burst of impulses 

 with no appreciable decrease of the activity for a period of 1 hr. or 

 more. This observation clearly showed that the receptors are able to 

 maintain their activity during prolonged periods of stimulation. Further 

 evidence on this point has been obtained in recordings of the response of 

 the olfactory membrane. As shown in Fig. 4, a potential with a sustained 



