Page 255 - Wilhelm Wundt zum siebzigsten Geburtstage

Fluctuations of Attention and After-images. 243

increases to a certain point above the threshold; but that, beyond
this point, they cease. He also suggested tbat fatigue of some sort
exerts an influence; but the special character or location of this
fatigue he did not determine. His results, it would seem, might be
explained on the hypothesis that the retina is the seat of fatigue.
A relatively weak Stimulus will not produce its füll effect upon an
Organ that is more or less exhausted; whereas a stronger Stimulus
will be feit 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 Wiersmai) 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 Marbe's results.
But if the fluctuations are ascribed to retinal fatigue, it is not so
easy to see why a Stimulus differing but slightly from the field, should
remain longer out of perception than one which is relatively stronger.
Wiersma does not indicate the corresponding phases of visibihty,
and consequently does not compare the phases in regard to their
duration. It is to be 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-
rich 2) 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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