CHANGES ASSOCIATED WITH FOREBRAIN EXCITATION PROCESSES 



3'7 



ing apparatus is set so that a negative ECG transient 

 registers as an upward deflection, a correlated nega- 

 tive SP change appears as a series of down-going pips, 

 the amplitude of which determines the \oltage of the 

 change. Vice versa, positive SP changes are recorded 

 as up-going pips. The deflection of the pips then is 

 opposite to the direction in which ECG transients 

 are recorded, although each has the same polarity. 

 The reason is that each interruption of the input 

 returns the pens from a positive or a negatise potential 

 value to the zero baseline of the amplifier (fig. i). 

 The conventional ECG can be recorded upon other 

 channels of the same electroencephalograph, using 

 neighboring pairs of polarizable electrodes tor the 

 pickup. 



The recording system must be flexible enough to 

 detect microvolt changes at the same time that it is 

 prepared to register a change of several milli\olts. To 

 accomplish this two devices are used: o) several 



channels of amplification record from the same lead 

 combination at different sensitivities; A) a balancing 

 potentiometer is placed in series with one of the 

 calomel half-cells to oppose the electrical effect of any 

 sizeable shift through the application of a counter- 

 voltage. The amplitude of an ,SP change can then be 

 read directly from the potentiometer. During swings 

 of seseral millivolts the more sensitive channels are 

 first turned ofl' while the least sensitive one is balanced. 

 Thereafter, the others are balanced in order toward 

 the most sensitive one. That one is used continuously 

 to record SP concomitants of evoked and spontaneous 

 activity, and the d.c. changes it records are the ones 

 which can he correlated most directly with ECG 

 manifestations of neuronal activity. From the calomel 

 half-cells, records of the quicker d.c. changes which 

 are registered upon the most sensitive channel of the 

 electroencephalograph can also be led through a d.c. 

 amplifier to a cathode-ray oscillograph. During 



— Mi 



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I sec 



FIG. I. Steady potential change accompanying cortical recruiting in the rabbit. Light ether anes- 

 thesia. Stimulation in medial thalamus. Recording from frontal cortex. A. Recorded by conven- 

 tional condensor -coupled electroencephalograph the input of which is short-circuited 8 times per 

 sec. Negative polarity recruiting responses are recorded as upward deflections, and black dots indi- 

 cate the first and last responses of a series. The SP change is recorded as a series of downward 

 deflections, each indicating a short circuit of the input. Although opposite in direction from the 

 deflections representing the recruiting transients each represents a d.c. change of negative polarity. 

 Records B and C serve to clarify the situation. For each the base line from which the d.c. change is a 

 departure is indicated by a straight white line. B. The same recruiting response recorded upon an 

 oscilloscope with use of a direct-coupled amplifier. Note negative steady potential change accom- 

 panying recruiting series. Surface of the cortex remained negative with respect to the underlying 

 white matter for approximately 750 msec, following the last recruiting response. C. Another re- 

 cruting response series recorded under similar conditions to that of B. Input is again short-circuited 

 8 times per sec. as in A. Note that each short circuit returns the beam to the base line of the amplifier. 

 Had this series been recorded by means of the same amplifier using condensor coupling, the d.c. 

 shift should have been eliminated and the oscilloscope record would have been the counterpart of .4. 

 The chopper signals on C have been retouched due to printing difficulties. 



