How Can Models from Information Theory be Used in Neurophysiology? 235 



enumerated, the phenomenon of lateral interaction among mem- 

 bers of a neuronal population should not escape attention. This is 

 seen, perhaps most strikingly, in the zone of inhibition that develops 

 around a locus of excitation. The "inhibitory surround" has now 

 been demonstrated for the visual, auditory, and somatic afferent 

 systems and emphasizes complex patterns of interaction rather 

 than conduction over isolated paths. There is also the interference 

 with direct routing of impulses from the receptor to the cortex, 

 mediated by recurrent collaterals and centrifugal feed-back control 

 over afferent pathways. Of two more contributions to knowledge 

 which have added to the neurophysiologist's task, one is the 

 realization that the all-or-nothing discharge is a comparatively rare 

 event in the central nervous system, graded responses (which may 

 or may not lead to cell discharges) being all important. Wliat of 

 these graded changes? How do they affect the code? 



One aspect of the problem has been approached by the analysis 

 of the intervals between nerve discharges. Although the action 

 potential of an axon is all-or-nothing and hence digital, the graded, 

 analog character of the receptor's action can be preserved in the 

 code by the intervals between discharges, for intervals between 

 spikes are continuously variable and therefore can transmit graded 

 input. In fact, a great deal of work in many laboratories is cur- 

 rently being devoted to pulse-interval analysis of the message set up 

 by stimulation of receptors. 



More difficult is the problem of graded delivery of the inessage 

 at the higher cerebral level where its result may be effector cell 

 discharge, passage into association areas, passage into storage 

 neurons changing their cellular function, modulation of other 

 currently incoming messages, or dissipation of a type about which 

 we have, as yet, no knowledge. Graded responses in dendrites do 

 not necessarily induce discharges of their cell bodies; nevertheless, 

 their influence as modulators inay be critical for the "meaning'' of 

 the message. 



Last, but not least in importance, is the evidence for a great 

 deal of endogenous discharge of many neurons of the central 

 nervous system in the absence of overt external stimuli. How is the 

 brain to select those discharges that are evoked by messages initiated 

 in its environment from those that form its background activity? 



