320 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



tion during repetitive stimulation. The after-effects 

 of singl\- evoked responses may not be evident at the 

 beginning of an experiment but appear as the experi- 

 ment proceeds. They may be positise, negative or 

 diphasic when evoked by stimuli applied to the op- 

 posite optic nerve or the corresponding lateral genicu- 

 late nucleus. The positive SP component follows the 

 evoked response transient immediately and may 

 reach a voltage of 0.4 mv and persist for 1.5 sec. (fig. 

 4). In diphasic after-effects the positive component 

 is usually of briefer duration (0.5 sec.) and is followed 

 by a negative one of similar voltage lasting for 1.5 

 sec. When a negative after-effect alone is encountered 

 its start is delayed for 0.3 to 0.5 sec. after the evoked 

 respon.se is over. 



When repetitive stimulation of the optic nerve 

 (submaximal for cortical evoked responses^ with fre- 

 quencies of 20 to 30 per sec. continued for 2 to 10 

 sec. is used, negative after-effects are oljserved to sum, 

 and positive after-effect is minimal (12, fig. 5). How- 

 ever, a positive change may replace the negative one 

 if cortical excitabilitx' had been significantly changed 

 by a preceding major paroxysm (fig. 5F, G). Near- 

 threshold stimulation of the lateral geniculate nucleus 

 also gives chiefly summation of negative after-effect 

 (12). At one half maximum to maximum stimulus 

 strength, positivity usually develops early in the 

 course of stimulation, to he replaced later by a nega- 

 tive SP change which persists past the end of stimula- 

 tion. From this and other evidence we have concluded 

 that in the response to repetitive stimulation, positive 

 and negative after-effects sum algeijraically, different 

 effects predominating at different strengths of stimu- 

 lation. This indication of the existence of processes of 

 opposite electrical sign, the electrical manifestations 

 of which can cancel completeh', may relate to the 



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FIG. 4. .^fter-effects which follow primary visual response in 

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 negative) after-effect. Black dots indicate position of first short- 

 circuiting signal to occur after each response. [From Goldring 

 & O'Leary (11).] 





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FIG. 5. SP change resulting from repetitive stimulation of 

 the optic nerve at different strengths. A. Diphasic after-effect 

 of a single evoked visual response for comparison. Stimulus 

 frequency for subsequent records of figure was 25 per sec. 

 For each strip the initial evoked response in ECG marks the 

 beginning of stimulation and the arrow its cessation. B. Nega- 

 tive SP change during repetitive stimulation at threshold. 

 C. Stimulus strength one-third maximal; here negativity is 

 most prominent. I). Stimulus strength half ma.ximal; the 

 negativity develops somewhat later. E. .Stimulus strength 

 maximal; the negativity appears later and is not as maiked as 

 in C and D. Initial positivity in SP is seen to occur at the start 

 of stimulation. F. Similar repetitive stimulation 5 min. following 

 a major lasting shift in SP; stimulus strength as in C. The SP 

 change is now surface-positive. G. Same as F except that stimu- 

 lus strength is that o{ E. [From Goldring S" O'Leary (12).] 



ditiiculty of demonstrating SP after-effects early in the 

 course of some experiments. 



SP Changes Associated zvith Recruiting Responses 



With repetitive stimulation (6 to 20 per sec, 30 v, 

 o. I msec, duration) in the midline thalamus of the 

 rabbit under light ether anesthesia, a negative cortical 

 SP change amounting to 0.2 to 0.6 inv ordinarily 

 develops during the rise in amplitude of the conven- 

 tional ECG transients of negative polarity as shown 



