Bauhaus-Universität Weimar

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
Brown, T. Graham
Graham Brown 
for the assumption that the state of facilitation which follows a stimulus 
is altered by a second stimulus which falls within its period. That this 
may be the case is suggested by the progressive augmentation of the 
early members of a series of reactions. But this phenomenon might 
equally be explained on the assumption that the state of facilitation which 
follows a stimulus does not evenly decline after its establishment, but that 
it waxes to a climax, and only on the attainment of this begins to wane. 
This state of deterioration or depreciation at the end of long series of 
stimulations in some cases lasts for longer than periods of 15 seconds. As 
the state of facilitation in such cases appears to have a shorter period, it 
is probable that the depreciation is not only due to a deterioration of the 
state of facilitation—for a previously supra-liminal stimulus may be 
ineffective within this period of depreciation. The “ fatigue ” of the 
mechanism must depend on other factors as well. But that it is in part 
conditioned by a diminution of the period of the state of facilitation seems 
to be shewn by the fact that a stimulus which follows a reaction early in 
a series after a certain longer interval of time than usually elapses in that 
series may evoke a reaction of larger extent than that which follows, at 
the same interval of time, a reaction of similar extent in the same series 
but evoked much later in it. In other words, in these long series of 
reactions in which the initial progressive augmentation is followed by 
progressive diminution of the extents of the reactions, reactions of equal 
extents occur twice in the series—in the first phase and in the second,-— 
but although the reactions are of equal extent, their succeeding states 
of facilitation appear to be of unequal value and duration. Thus it is 
probable that the state of facilitation which follows a reaction is not in all 
cases proportional to the extent of the reaction. During the first phase of 
a series of reactions—that in which progressive augmentation is occurring— 
the state of facilitation gradually augments in value; while during the 
second phase—where progressive diminution of the reactions is occurring— 
it gradually depreciates. 
In these long series of reactions the state of facilitation appears to 
increase more rapidly at first and then progressively more slowly until the 
maximum is attained. Thus the progressive augmentation of the extent 
of the reactions is relatively greater at the commencement of the series 
than at its climax. This is also shewn by the fact that the progressive 
augmentation of three test reactions is proportionally greater after a longer 
interval in a series the earlier that interval falls in the otherwise regular 
series ; and it is also greater the longer the interval when intervals of 
different duration fall at the same point in the series. 
The experiments here described do not localise the part of the 
mechanism in which^the state of facilitation is conditioned. Thus that 
state might be conditioned in the cortex itself, or it might be conditioned 
in the lower centres through which the cortex acts, or it might be condi¬ 
tioned in part in each of these. Experiments intended to demonstrate the


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