1676 



HANDBOOK OF PI IVSIl )I < K; Y 



\l 1 Ri ii-in sii 1I1 n;\ 111 



manner, sagittal transection of the corpus callosum 

 was carried out. These eats were then trained to 

 perform pattern discriminations with a mask covering 

 one or the other eye. When a consistently high level of 

 performance had been attained, the mask was shifted 

 to the other previously unmasked eye, whereupon the 

 performance dropped abruptly to a chance level with 

 7 of the [0 discriminations tested. The three deviant 

 results were attributed In Myers to irrelevant generali- 

 zation effects between the several discriminations 

 taughl through the separate eyes. The findings are in 

 marked contrast with the high-level transfer of visual 

 discriminations obtained with the corpus callosum 

 intact. 



Myers subsequently trained two of the eats with 

 combined sagittal transection of the optic chiasma 

 and of the corpus callosum completely conflicting 

 discriminations with the two eyes. The learning with 

 the second eye occurred with relative ease and resulted 

 in no disturbance in performance with the first eye. 

 This finding also contrasts markedly with the per- 

 formance of cats with intact corpus callosum. 



I In -r results seem clearly to implicate the corpus 

 i allosmn in the integration of the two hemispheres in 

 visual learning and transfer of the memory traces. It 

 is possible that Myers has opened the door to a 

 formidable assault upon the memory trace problem. 

 Through ingenious use of combined surgical and 

 training procedures, he has demonstrated that it is 

 possible to trap specific memory traces in one hemi- 

 sphere. He is now in a position to explore system- 

 atically the relevant details of the structures invoh ed. 

 Is the grain or mass necessary for establishing the 

 initial training of one hemisphere exactly homologous 

 with that of the other hemisphere w hich is receptive in 

 the trace-transfer transaction.' In other words, how 

 much ol each hemisphere can be eliminated surgically 

 or otherwise without disturbing trace transfer. 1 Are 

 the functional islands of tissue different chemically or 

 ultrastructurally in die receptive and unreceptive 

 transfer st.iirs.' It seems likely that these are but some 

 11I tin- answerable questions opened up l>\ Myers' in- 

 \ estigations. 



M \l\ UN \M 1 Hi IHiMI iisl \sls IN (III 

 CENTRA] \l kvi us s\sn \i 



Unlike electrical .uu\ mechanical machines which 

 .in- addressed both as to program and operating 

 energy level, tin- brain is capable "1 spontaneous 

 periodic and aperiodic changes in state Perhaps the 



most dramatic example of this is afforded in the many 

 intermediate states between deep sleep and high wake- 

 fulness emphasized by Kleitman (30 j and Hess (21). 

 Bremer (7) points out that "the cerebral cortex par- 

 ticipates actively in its own arousal and in the mainte- 

 nance of its waking state by the corticofugal impulses 

 which it sends to the brain stem reticular formation." 

 He suggests that the act of falling asleep is triggered 

 by the cumulative deactivation or defacilitation of the 

 encephalic neural networks resulting from synaptic 

 fatigue and favored by a reduction in the exterocep- 

 tive and proprioceptive sensory afflux. Jasper (27) 

 fractionates the reticulothalamocortical projection 

 system into diffuse unspecific pathways, regional tin- 

 specific pathways and localized specific pathways. It 

 is not yet possible to state whether any or all of these 

 is directly involved in the defacilitation called sleep, 

 nor is it yet clear to what extent changes in peripheral 

 tone, with associated afferent feedbacks, is primary or 

 secondary in the state of sleep. Sherrington (51 ), Free- 

 man (11, 12) and others are responsible for the ob- 

 servation that muscle tonus waxes and wanes in 

 parallel with mental efficiency, being lowest at waking, 

 rising rapidly during the early morning, declining 

 again in the afternoon and reaching a new peak 

 during the evening hours. Freeman has found that 

 motor tension increases during the execution of a task 

 and drops when it is completed. He concludes that 

 motor tension must be higher under fatigue or dis- 

 traction in order to maintain a normal level of mental 

 work. In this latter connection Bills (3) and Stroud 

 (54) found that a moderate decree of constant pressure 

 on a dynamometer facilitated the learning process for 

 paired associations and lists of nonsense s\ llables. It is 

 difficult to visualize how tonic influences per se, par- 

 ticularly in unrelated muscle groups, could affect 

 favorably the learning process. However, relatively 

 diffuse or regional nonspecific afferent feedbacks to 

 the neural pool might serve a general facilitating 

 function. Segmental effects of muscle relaxation were 

 explored by Jacobson i.v Carlson (26) who found that 



Sufficient relaxation could abolish tin- knee jerk in 

 man. It is ol interest that Miller (45) found that miIi- 



jects in relaxed states rea< ted to electrical shock with 



reduced amplitude .i\n\ speed of reaction. In contrasl 

 with normal tonic levels, the subjects reported under 

 relaxa lii hi an apparent decreased intensity ol sensation 

 induced by the shock. 



The discovery of the reticular activating svstem by 

 Magoun, Lindsley, Morruzzi, Jasper and others has 

 in a lew short years had a far-reaching influence upon 

 brain research. Physiologists have been soniewli.it 



