D. O. HEBB 45 



readily occur where they begin to enlarge to form knobs — that is, short 

 of the synapses proper. When there is already some depolarization of cell 

 body or dendrites, the flow of current, it is argued, will make the bottle- 

 neck traversable, and the subsequent depolarization will keep it open for 

 an appreciable period of time. This will be particularly true if there is any 

 considerable activity of the dendrites. 



Such a mechanism would clearly help to account for the short-term 

 memories of the present experiment, where we are dealing with associative 

 connections which are already highly practised (i.e. whatever growth 

 process there may be at the synapse is near its maximum), and where a 

 momentary experience is able to make one set of well-developed synaptic 

 connections temporarily dominant over others. The conditions of the 

 experiment demand, of course, that this dominance must be very brief, 

 lasting only for the period in which one set of digits is being held and 

 giving way when the next is presented. 



The explanation is of course speculative, but it accounts in principle for 

 phenomena which cannot be plausibly dealt with solely in terms of (i) 

 a reverberatory trace, and (2) growth at the synapse. What I am saying, in 

 short, is that there may be three mechanisms of the memory trace and not 

 two, as suggested earlier. It should be clear, of course, that these are not 

 alternative mechanisms in the actual phenomena of brain function; they 

 occur together, and reinforce one another's actions. 



WIDER IMPLICATIONS 



It has been urged above that we have no hope of understanding learning 

 in the adult mammal until we know much more about the organized 

 activity (in cell-assembly or trace system) of the regions of divergent 

 conduction in the cerebrum. Lack of such knowledge is the main reason 

 for the great gap between the theory of learning and the practical advice 

 one can give to the student who wants to know how to study more 

 efficiently. The great question always is how to 'concentrate', and how to 

 'motivate oneself — that is, to keep on concentrating — and this, clearly, 

 is the theoretical problem of the control of the excess activity, to prevent 

 its interference with the task in hand. If such experiments as the one des- 

 cribed can help us codify the ideas involved, and if further they provide 

 some information about the interaction of cell-assembly groups in 

 learning, we can also see them as a slight contribution to the ultimate 

 understanding of the problem of serial order which, as Lashley (195 1) has 

 shown, is the crux of the problem of behaviour in the higher animal. 



