246 Itiformation Storage and Neural Control 



In recent reviews, both Morrell (19) and I (6) have summarized 

 the large amount of data obtained in many laboratories from many 

 different species of experimental animals, showing that striking- 

 changes in the amplitude and distribution of labeled potentials 

 take place during the establishment of conditioned responses to 

 intermittent stimuli. Although the appearance of labeled potentials 

 in a structure justifies the conclusion that information about the 

 presentation of a TCS is reaching that structure, one cannot assume 

 that such labeled potentials actually are the neural coding of 

 information about stimulus frequency. Labeled potentials may 

 simply be nonfunctional correlates of the actual processing by 

 nerve cells of otherwise coded information about the TCS. Con- 

 versely, one cannot prove on the basis of present evidence that 

 labeled potentials are not the effective neural code for stimulus 

 frequency. 



ASSIMILATION AND MEMORY TRACE 



Many phenomena observed in earlier work directed my at- 

 tention to this problem because they suggested a functional role 

 for labeled potentials. The first of these phenomena was called 

 "assimilation of the rhythm" by Livanov (14), who first observed it. 

 It has since been described by many workers utilizing various 

 species in diverse experimental situations (6, 19). If one studies 

 the resting electrical activity of various brain regions in an animal 

 learning a conditioned response to an intermittent stimulus, one 

 observes that during the intertrial intervals a marked hypersyn- 

 chrony appears at the stimulus frequency, or at a harmonic thereof. 

 This spontaneous, frequency-specific activity can dominate the 

 resting electrical activity in early stages of learning. In our experi- 

 ence, it tends to diminish and disappear as the conditioned response 

 becomes well established, but will return briefly following per- 

 formance of an erroneous response. Figure 1 illustrates assimilation 

 and is taken from a paper by Killam and me (8). Note that the slow 

 hypersynchrony, in this case at one-half the stimulus fr-equency, 

 appears in the reticular formation, fornix, and septum in close 

 relationship. Assimilated rhythms, in our experience, appear 

 earliest, are most marked, and persist longest in nonspecific 



