ANALYSIS OF SOME EVOKED SYNAPTIC ACTIVITIES 



429 



VVW^ /W\AA 



Fig. 2. Characteristics of long-duration superficial cortical responses (s.c.r.'s) 

 recorded 1-5 mm (1-3) and 4 mm (4) from stimulating electrodes on supra- 

 sylvian gyrus. Stimulus frequency 0-5/sec, from six to twenty superposed 

 responses at different stimulus strengths. Negativity upwards in this and all 

 subsequent figures. 1 and 2 from two near-term fetuses; 3, 4-day-old kitten; 

 and 4, 12-hr-old kitten. Duration of s.c.r.'s independent of stimulus strength; 

 threshold of late components in near (3) or distant (4) responses similar to 

 that for the early. Latency of distant responses is not appreciably altered by 

 five to tenfold increase in stimulus strength. Note augmentation of early com- 

 ponent in near responses and late components of distant responses with strong 

 stimulus. Cal. 100 c/s; 0- 1 mV. 



mm from the site of stimulation are of long-duration (50-80 msec) relative 

 to those ordinarily recorded in the adult animal (Adrian, 1936; Brooks and 

 Enger, 1959; Burns, 1958; Chang, 1951; Clare and Bishop, 1955; Purpura 

 and Grundfest, 1956). The 10-20 msec s.c.r. of mature cortex is, however, 

 detectable in the perinatal animal, but a clear dissociation between initial and 

 late components of the response is observed only when care is taken to 

 record responses "at the site of stimulation". With progressive outward dis- 

 placement of the recording electrode from the site of stimulation, additional 

 10-20 msec components increase in magnitude and fuse with the early (Fig. 

 3). During the second and third weeks, dissociation between the early and 

 late surface-negativities of the s.c.r. is more readily obtained at distances 

 comparable to those at which threshold differences in these components are 

 obtained in adult animals. 



S.c.r.'s evoked by weak surface stimulation are detectable up to 5-7 mm 

 from the site of stimulation during the first postnatal week (Fig. 4), and 8-10 

 mm thereafter (Fig. 5). The mean propagation velocity of the s.c.r. in supra- 



