When the things to be attended to are small sensations, and when the effort is to be exact in noting them, it is found that attention to one interferes a good deal with the perception of the other. A good deal of fine work has been done in this field, of which I must give some account.
It has long been noticed, when expectant attention is concentrated upon one of two sensations, that the other one is apt to be displaced from consciousness for a moment and to appear subsequent; although in reality the two may have been contemporaneous events. Thus, to use the stock example of the books, the surgeon would sometimes see the blood flow from the arm of the patient whom he was bleeding, before he saw the instrument penetrate the skin. Similarly the smith may see the sparks fly before he sees the hammer smite the iron, etc. There is thus a certain difficulty in perceiving the exact date of two impressions when they do not interest our attention equally, and when they are of a disparate sort.
Professor Exner, whose experiments on the minimal perceptible succession in time of two sensations we shall have to quote in another chapter, makes some noteworthy remarks about the way in which the attention must be set to catch the interval and the right order of the sensations, when the time is exceeding small. The point was to tell whether two signals were simultaneous or successive; and, if successive, which one of them came first.
The first way of attending which he found himself to fall into, was when the signals did not differ greatly—when, e.g., they were similar sounds heard each by a different ear. Here he lay in wait for the first signal, whichever it might be, and identified it the next moment in memory. The second, which could then always be known by default, was often not clearly distinguished in itself. When the time was too short, the first could not be isolated from the second at all.
The second way was to accommodate the attention for a certain sort of signal, and the next moment to become aware in memory of whether it came before or after its mate.
"This way brings great uncertainty with it. The impression not prepared for comes to us in the memory more weak than the other, obscure as it were, badly fixed in time. We tend to take the subjectively stronger stimulus, that which we were intent upon, for the first, just as we are apt to take an objectively stronger stimulus to be the first. Still, it may happen otherwise. In the experiments from touch to sight it often seemed to me as if the impression for which the attention was not prepared were there already when the other came."
Exner found himself employing this method oftenest when the impressions differed strongly.[331]
In such observations (which must not be confounded with those where the two signals were identical and their successiveness known as mere doubleness, without distinction of which came first), it is obvious that each signal must combine stably in our perception with a different instant of time. It is the simplest possible case of two discrepant concepts simultaneously occupying the mind. Now the case of the signals being simultaneous seems of a different sort. We must turn to Wundt for observations fit to cast a nearer light thereon.
The reader will remember the reaction-time experiments of which we treated in [Chapter III]. It happened occasionally in Wundt's experiments that the reaction-time was reduced to zero or even assumed a negative value, which, being translated into common speech, means that the observer was sometimes so intent upon the signal that his reaction actually coincided in time with it, or even preceded it, instead of coming a fraction of a second after it, as in the nature of things it should. More will be said of these results anon. Meanwhile Wundt, in explaining them, says this: