Bauhaus-Universität Weimar

Fluctuations of Attention and After-images. 
243 
increases to a certain point above the threshold; hut that, beyond 
this point, they cease. He also suggested that fatigue of some sort 
exerts an influence; hut the special character or location of this 
fatigue he did not determine. His results, it would seem, might he 
explained on the hypothesis that the retina is the seat of fatigue. 
A relatively weak stimulus will not produce its full effect upon an 
organ that is more or less exhausted; whereas a stronger stimulus 
will he felt in spite of fatigue, even though the fatigue which it 
causes be more complete than that which is due to weaker stimulation. 
The results obtained by Wiersma1) offer, at first sight, greater 
difficulty. He found that as the difference between stimulus and 
field increased, the average length of the phase of invisibility dimi¬ 
nished, so that, in his own case, it became zero when the proportional 
intensity was 2,5. This would agree, in general, with Marhe’s results. 
But if the fluctuations are ascribed to retinal fatigue, it is not so 
easy to see why a stimulus differing hut slightly from the field, should 
remain longer out of perception than one which is relatively stronger. 
Wiersma does not indicate the corresponding phases of visibility, 
and consequently does not compare the phases in regard to their 
duration. It is to he noted, however, that the length of the invisible 
phase depends upon the proportion which the primary stimulus bears, 
in point of intensity, to the condition which it has produced in the 
retina before it^ disappears. A comparatively faint stimulus may re¬ 
quire a considerable time to produce a perceptible impression upon 
the retina that is fatigued even to a small degree; and a stronger 
stimulus may act more rapidly, though it has to overcome more com¬ 
plete fatigue. 
Finally, those changes in other functions should be taken into 
account which accompany the fluctuating retinal condition. Hein¬ 
rich2) found that accommodation ceases when the attention is directed 
towards other than visual impressions. Hence he concludes, with 
Münsterberg, that the fatigue which brings about the fluctuation 
1) Wiersma, Untersuchungen über die sogenannten Aufmerksamkeitsschwan¬ 
kungen. Zeitschr. f. Psych, u. Phys., XXVI, S. 168. 
2) Heinrich, Die Aufmerksamkeit und die Funktion der Sinnesorgane. 
Zeitschr. f. Psych, u. Phys., Bd. IX, S. 342. 
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