Information Processing Theory 37 



known in sufficient detail. However, some of their basic features — 

 features which undoubtedly are critical in brain function — are 

 well established and can be described accurately. A lot of talent 

 has gone into speculating on the manner in which neurons inter- 

 act to perform the higher functions. One such theory is that of 

 the psychologist Hebb (6), who attempted to explain the phe- 

 nomenon of memory in terms which were physiologically sound 

 and yet psychologically relevant. Hebb proposed three phases in 

 the formation of memory traces. The first is reverberation, the 

 persistence of nervous activity after the termination of the initiating 

 stimulus. The second mechanism, the cell-assembly, consists of a 

 characteristic pattern of firing associated with a particular stim- 

 ulus configuration and comes into being upon adequate repetition 

 of the stimulus. To account for this, Hebb postulated that if one 

 neuron succeeded in firing a second, the synapse, by some un- 

 specified processes, should change so as to make this triggering 

 more probable in the future. The third mechanism, evolving from 

 the second, amounts to the passing of activity from one cell-assem- 

 bly to another as a result of the repeated temporal sequencing of 

 the corresponding stimuli. This mechanism, the phase-sequence, 

 is the primitive basis of expectancy, an important psychological 

 concept. 



Although the neuronal properties which Hebb assumed are 

 well established, and the "growth" hypothesis is almost certainly 

 correct, it is not an easy task to show that these assumptions are 

 sufficient to cause the reverberation, cell-assembly, phase-sequence 

 organization postulated. Rochester et al. (7), attempted to demon- 

 strate the sufficiency of Hebb's assumptions in a novel way. They 

 instructed a digital computer to behave according to the assump- 

 tions, and then simply observed its behavior. It is interesting to 

 note that they were forced to make some additional minor assump- 

 tions before the theory was specified in sufficient detail to be 

 realized. But more important, they found that, although rever- 

 beration was easily achieved and cell-assemblies formed spon- 

 taneously after some suitable modification of the theory, phase- 

 sequences were not achieved. This work has given some important 

 clues as to what is lacking in the theory, and some specific altera- 

 tions have been proposed. 



