Principles of Stimulus Coding 17 



similar relationships to the strength of the applied stimulus, and it 

 should therefore be evident that quantitative comparisons between 

 different receptors, or the different responses of a single cell to 

 different stimulus intensities, should include a statement of the 

 period during which the measurements were made. 



Disparities in the time-course of the sensory response, as 

 described above, contribute to the difficulties involved in arriving 

 at a quantitative analysis of the effects of stimulus magnitude, and, 

 therefore, in investigating excitatory mechanisms at the molecular 

 level. Some of these difficulties are amenable to an eventual 

 electrophysiological solution, such as deciding what factors are 

 responsible for the phasic and tonic properties of impulse generat- 

 ing regions of neuronal membranes. But at the present time it is 

 often difficult to pose proper questions about the nature of the 

 changes, induced by the stimulus, at the level of the membrane 

 architecture. It is difficult even to give an explanation for the 

 fact that certain types of sensory structure are associated with 

 the detection of particular types of stimuli. Probably other bio- 

 physical approaches will be necessary to examine the changes in 

 molecular configuration of sensory membranes which result from 

 a stimulus and trigger the electrical events that have been studied 

 so successfully in recent years. 



