370 BRAIN MECHANISMS AND LEARNING 



SUMMARY 



1. According to the generally accepted view, the basic process under- 

 lying any kind of learning behaviour at the neuronal level seems to be the 

 persistence of synaptic facilitatory (or inhibitory) effects as a result of the 

 repeated presentation of the same stimulus or of that ot the same set of 

 associated stimuli. 



2. The only post-activation facilitatory (or inhibitory) residue that 

 has so far been disclosed and is now recognized as a general pre-synaptic 

 process is the so-called 'post-tetanic potentiation' (PTP). Actually it would 

 be better termed a 'protracted tetanic potentiation'. 



3. Classical studies of PTP represent it as a process strictly localized in 

 the presynaptic terminals of the fibres that have been involved in a 

 rather strong hypcractivation (stimulation at high frequency). It the 

 benefit of the PTP cannot be transferred from one kind of affercnts to 

 another, if it remains monosynaptic while learning processes actually 

 need the participation of polyneuronic structures, how can we here give 

 a role to this phenomenon? 



4. The key seems to be in the now well established fact that most 

 neurones in the brain of an animal in the waking state are continuously 

 delivering autogenic repetitive discharges. These maintain in the axon 

 terminals a certain level of transmissive efficacy we propose to call 

 'tetanic potentiation' (PT). Any transient increase in frequency AF will 

 result in an increase APT that will outlast its cause by seconds or minutes. 

 Consequently, the post-synaptic potentials of all neurones receiving 

 afferents from those in which some A? has occurred will be enhanced in 

 amplitude as long as the potentiation increment will persist. This effect 

 in its turn will increase the firing frequency of the neurones involved, and, 

 among those previously silent, will induce some of them to start firing. 



5. It is easy to understand how the same sequence of events operating 

 several times along a chain-like polyneuronic structure can result in a 

 very important lengthening of the final trace left by a brief input hyper- 

 activity. Negative AF due to a drop in activity or to the intervention of 

 inhibitory processes can similarly give rise to long-lasting traces of 

 inactivation. 



6. Network-like characteristics are found in all polyneuronic structures 

 of the brain. There many neurones are connected to heterogeneous afferent 

 pathways and can thus be activated by signals of different sensory quali- 

 ties. A multivalent neurone of that kind can obviously transfer to any of 

 its group of afferences the benefit of a residual APT caused by the 



