40 BRAIN MECHANISMS AND LEARNING 



association areas, and also to other rhinencephalic and basal-gangliar regions 

 which seem to be involved in higher mental processes (cf. the recent 

 review by Rosvold, 1959). Whether a given synaptic junction will be 

 activated in such regions of divergent conduction cannot be determined 

 by control of the stimulating environment, the experimental situation in 

 which the learning is to be established. It is fully dependent also on 

 whether the units concerned are ready to be fired, and on the concurrent 

 activity of other units, not controlled by the present sensory stimulation, 

 which impinge on the synapse in question and which may produce either 

 summation or inhibition. Cumulative learning, in which the second trial 

 adds to a synaptic modification begun by the first, thus depends on what 

 activity is already going on in these regions. 



Here is a point at which we tmd a full convergence of the physiological 

 with the psychological evidence. Psychologically, the problem is that of 

 'set', 'attention', 'motivation', 'attitude', or the like: all terms developed 

 in an earlier day to refer to (i) the puzzling but unmistakable deviation of 

 behaviour from control by environmental stimulation, and (2) deviations 

 of learning from the simple S-R and CR conceptions, implying a direct, 

 through connection which, as we have seen, cannot be expected to occur 

 in regions of divergent conduction, though it was taken for granted by 

 earlier physiologists as well as psychologists. 



Even today, the problems involved here have not always been faced, 

 perhaps because of the complications which they entail. The histological 

 structure of the tissue in question appears to mean that transmission is via a 

 series of the closed pathways described by Lorente dc No (1943), or 

 groups of them functioning as systems capable of self-maintained activity. 

 Such systems are what I have called 'cell-assemblies' (Hebb, 1949), and 

 Lashley (1958) 'trace systems'; the most adequate discussion of their pos- 

 sible mode of development and internal function is that of Milner (1957). 

 Such conceptions certainly introduce complexities into behavioural 

 theory, but on the other hand the behaviour itself is even more complex, 

 and means of experimental analysis arc becoming available. Burns (1958) 

 for example has provided us with much information about the conditions 

 in which self-maintained activity is possible in a slab ot isolated cortex, 

 from a physiological approach ; and such studies of perception as those of 

 Broadbcnt (1956) with respect to hearing, and Heron (1957), Kimura 

 (1959) and Bryden (1958) with respect to vision, have begun to transform 

 a very speculative class of theory into something more solidly grounded 

 in fact and susceptible of direct experimental attack. The following experi- 

 mental work attempts to carry this process further. 



