234 Information Storage and Neural Control 



peripheral loci could fire an individual cell in the brain. Many 

 examples drawn from the somatic and other sensory systems have 

 been published, but Miiller would have been even more dismayed 

 had he been shown that there are cells within the brain that are 

 no respecters of sense modalities. Proof has been given of con- 

 vergence of sensory modalities onto individual neurons of the 

 midbrain and thalamic brain stem and onto units in the limbic 

 system. Even cortical neurons are not simon-pure. 



The complexities do not cease there, for such coding as can be 

 established for individual presynaptic fibers is found to be trans- 

 formed at the synapse and to send on a different pattern of discharge 

 in the postsynaptic output. Some of the exquisite response patterns 

 that investigators have been able to identify in the primary neurons 

 from the receptors are therefore only one link in a chain of se- 

 quential codes. Moreover, the evidence that recoding in post- 

 synaptic fibers varies in different neuronal aggregates is over- 

 whelming, for there is great variation in the degree of convergence 

 and divergence. 



The biologist has long known how rare is a one-to-one relation- 

 ship between input and output of a synapse but is now beginning 

 to realize that the relationships may not even be linear. He may 

 well have to wait for the mathematicians to progress further with 

 their analyses of nonlinear systems before he himself can master the 

 transformation characteristics of the code as it passes seriatim 

 through a chain of synaptic relays. 



As an example of changing code, the findings of Whitfield (13) in 

 the auditory system may be quoted. Whitfield found that the rate 

 of firing becomes progressively less as the impulses proceed through 

 the serial synaptic stations on their way to the cortex. Moreover, 

 the rate of unit firing becomes less and less dependent on the 

 strength of the stimulus at each successive relay station. In other 

 words, the intensity of the stimulus is no longer being signalled 

 simply by frequency of discharge. The coding has changed and 

 some clues to its nature are already known. These point to the 

 distribution of excitation and inhibition among the fibers of the 

 pathways as being a crucial factor. 



Although in this outline which poses the problem, all the facets 

 that must enter into any consideration of neural coding cannot be 



