122 BRAIN MECHANISMS AND LEARNING 



Stimulus (again well known from the CR experiments) and also for the 

 recent memory traces of this cessation. And so while neurones of group 2 

 account for comiiioii features of a stimulus and its traces and provide a 

 basis for their mutual generalization, neurones of group 3 account for 

 diversity of the stimulus and its trace and form a basis for their possible 

 differentiation. Various moments of the trace of the stimulus differ among 

 themselves in that with a lapse of time fewer and fewer elements of 

 groups 2 and 3 arc activated. And so the more remote the trace of the 

 stimulus, the less its refiexogenic strength, a fact which is again supported 

 by much experimental evidence. 



We have a strong inclination to believe that the 'sense of time' of men 

 and other animals, i.e. the sense of the varying durations of time which 

 have elapsed since a defmite event, is based on nothing else than the 

 strength of traces left by this event at various moments after its cessation. 

 The weaker these traces the more remote in time the given event seems 

 to be. 



5. THE PROBLEM OF LOCALIZATION OF REVERBERATING CIRCUITS 



If the above hypothesis is correct then the problem arises as to where the 

 reverberating circuits responsible for recent memory are situated. The 

 simplest assumption is that they are localized in the associative areas of the 

 cerebral cortex — as decorticated animals possess hardly any recent 

 memory — in close vicinity to the respective sensory projection areas. In 

 consequence we may expect that the destruction of these associative areas 

 should lead to the abolition of recent memory for stimuli in the given 

 sensory modality. 



Let us analyse from this point of view the functions of the prefrontal 

 area, i.e. that associative area which was first recognized as having to do 

 with recent memory in the classical experiments by Jacobsen (1936). As is 

 well known this author demonstrated that after bilateral ablations of the 

 prefrontal area in monkeys delayed responses are abolished and he 

 attributed this defect to the loss of 'immediate niemory', as contrasted 

 with the full preservation of 'stable memory'. The results of Jacobsen were 

 afterwards confirmed by many authors, but his interpretation was sub- 

 jected to much criticism. It was argued that the impairment of delayed 

 responses is due not to the lack of 'immediate memory' but to the enhanced 

 distractability of prefrontal animals (Malmo, 1942; Wade, 1947; Harlow 

 et al., 1952), to their hypermotility (Wade, 1947) or to the impairment of 

 associative functions (Nissen et al, 1938; Finan, 1942). 



