Experimental Psychology. — “Psychical Inhibition’. By Prof 
E. D. Wiersma. 
(Communicated at the meeting of June 25, 1921). 
From daily experience we know that simultaneous sensations 
have an inhibitory influence on one another. During the day the 
light of the stars is not observed. Soft sounds, distinctly audible in 
the dead of night cannot be heard during the randiness of day 
time. The weak stimuli of the skin are nullified by the stronger. 
The psychical inhibition, under which is to be understood, that 
one psychical complex influences the other, so that intensification of 
the one entails a weakening of the other, was investigated by 
Heymans some twenty years ago. By numerous and careful experi- 
ments he determined the underlying principles. Although it is evident 
that the inhibitory process does not limit itself merely to sensations, 
it is important to determine experimentally that also other psychical 
complexes are subjected to similar laws. It is not easy to express 
this in definite measurements for all other complexes of conscious- 
ness, as ideas and emotions; but for volitional acts it is very well 
possible. 
The investigation was carried out as follows: The individuals 
experimented upon were young, students and assistants. They were 
instructed to write down the numbers | to 25 as quickly as possible. 
The time was accurately registered by means of a 4 second-metre. 
Repeatedly after approximately a quarter of an hour, when fatigue 
was out of the question, these experiments were repeated three 
times, once while a dynamometer was squeezed as tightly as possible, 
a second time while a foot was pressed against a fixed resistance, 
and thirdly while the teeth were firmly clenched. In all these expe- 
riments the dynamometer was held with the left hand to equalize 
the relations as much as possible. 
The inhibitory influence from the movement of the left arm, the 
right leg and from the clenching of the teeth is obvious. The 
movement of the left arm inhibits most strongly, the clenching of 
the teeth less, and the movement of the right leg least of all. (See 
table). 
In writing down successive numbers, an associative activily, which 
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Proceedings Royal Acad. Amsterdam. Vol. XXIV. 
