Volltext: Studies in the physiology of the nervous system, XXII: On the phenomenon of facilitation, 1: Its occurence in reactions induced by stimulation of the "motor" cortex of the cerebrum in monkeys (9)

Studies in the Physiology of the Nervous System 
“motor” area of the cerebral cortex. These are described here for the 
movement of flexion at the elbow-joint as studied in a pair of isolated 
antagonists in monkeys and the anthropoid ape. 
When a second cortical stimulation is made to follow upon a first the 
reaction evoked is greater than that evoked at the first stimulus if the 
interval of time between the two is not too great. For just liminal 
strengths of stimulation the interval of time during which the mechanism 
is facilitated appears to be about ten seconds in duration. The value of the 
state of facilitation—as measured by the extent of the second reaction— 
appears to be greater the stronger the stimulus. It appears also to be 
greater the longer the duration of the first period of stimulation. The 
state of facilitation seems also rapidly to deteriorate as time elapses after 
the first stimulus, for the extent of the second reaction is less the longer 
is the interval of time between the two stimuli. Increasing depth of 
chemical narcosis seems to diminish the value of the exciting stimulus. 
A second stimulus which falls within the period of a state of facilita¬ 
tion alters that state (as will be shewn in a following note). This altera¬ 
tion may be looked upon as equivalent, to a certain extent, to a summation 
of the state of facilitation produced by the first stimulus with that 
produced by the second stimulus which falls within it. Or perhaps it 
may be looked upon as due to the state of facilitation which follows a 
reaction being to a certain extent proportional to the value of that 
reaction. Thus a second reaction is greater than the first if it falls within 
the state of facilitation conditioned by that first reaction, and the state of 
facilitation conditioned by the second may possibly be greater than that 
conditioned by the first—so that a third reaction again would be greater 
than the second (as it actually is). In any case, when stimuli are repeated 
in series, the reactions evoked shew progressive augmentation until a 
maximum is attained. Thereafter they diminish progressively in extent 
until they either disappear or become of a constant value. Were the state 
of facilitation conditioned by a period of stimulation one which gradually 
diminishes in value after its establishment, and if it were unaltered by a 
second stimulus which falls within it, then it might be expected that the 
reaction evoked by a third stimulus which also falls within the period of 
facilitation conditioned by the first stimulus would be smaller in extent 
than the second reaction—and so on. That this is not the case is shewn 
by the progressive augmentation of the reactions conditioned by a series 
of stimuli the intervals between which are not too great. An explanation 
of this is that the state of facilitation of the mechanism appears gradually 
to be augmented with repeated stimulation. This would then be the case 
even where the strength of stimulation used is at first subliminal. The 
diminution of the extents of the reactions at the end of long series may 
best be looked upon as conditioned by a state of “ fatigue ” wherein there 
occurs a deterioration of the state of facilitation. 
It must, however, clearly be noted that there is no positive evidence 
VOL. IX., NO. 1.—1915. 7


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