OLFACTORY IDENTIFICATION OF CHEMICAL UNITS 339 



For animals, I shall give below an example in which a flavour was dis- 

 criminated by a rat when it was added to the famihar synthetic food. 



In all such discrepancies between the olfactory and the chemical identity 

 of molecular units and mixtures, it is very important to point out that dis- 

 criminatory performance, in man, depends to a great extent on learning 

 and previous experience and the subject's knowledge of the meaning 

 of the stimuli concerned. The same is true in animals, where discriminatory 

 performance also depends on learning, on experimental or physiological 

 conditioning of the individual. A given chemical mixture, such as a food, 

 which is not olfactively analysed by a novice, can be by an expert. Two 

 molecules whose odour is confused by a novice will be distinguished 

 olfactively by a chemist " knowing " them. 



These commonplace facts indicate the origin of distortions of olfactory 

 information, and of the variable and apparently capricious limits of olfac- 

 tory discriminatory performance. The factors determining these distor- 

 tions and limits are chiefly .central in nature. The functioning by the 

 olfactory apparatus is integrated, more than other systems are, in the 

 regulatory schemes of the central nervous system. Modern neurophysio- 

 logy has emphasized the very important role of centrifugal pathways acting 

 at different levels of sensory systems to regulate and modulate their 

 quantitative and qualitative responses. It is highly probable that these 

 centrally regulated responses, depending on inherited connections and 

 on the physiological role of olfactory stimuli in alimentary, sexual and 

 other behaviour, play a fundamental role in olfaction and especially in 

 qualitative discrimination of diff'erent stimuli. The peripheral mech- 

 anism for molecular diff'erentiation, of course, forms the background 

 for this and determines the ultimate limits of discriminatory capacity. But 

 upon these messages coming up from the periphery — possibly even before 

 arrival in the central nervous system— inhibitory and facilitatory processes 

 obviously act, operating selectively within separate or mixed patterns, and 

 analogous to those which have recently been shown to act at the retinal 

 level in the visual discrimination of forms. 



This analogy between the peripheral mechanisms for olfactory and for 

 pastial visual discrimination — first suggested by Adrian in 1942 — has also 

 allowed an idea to be formed of the relation between peripheral and central 

 mechanisms in discriminative performance as it appears in behaviour. 



According to Adrian's well-known electrophysiological data, confirmed 

 at the peripheral level by Ottoson, the specific message more or less charac- 

 teristic of a given stimulating molecule is elicited by means of a pattern of 

 differential activation by this given molecule of all or of specific numbers of 

 functionally independent units. Thus, the action of a molecule on the 

 olfactory surface would be the equivalent of the action of the image of an 

 object on the retina, with its spatial and perhaps also its chromatic pattern. 



