The time required for discrimination has been made a subject of experimental measurement. Wundt calls it Unterscheidungszeit. His subjects (whose simple reaction-time—see [p. 85] ff.—had previously been determined) were required to make a movement, always the same, the instant they discerned which of two or more signals they received. The exact time of the signal and that of the movement were automatically registered by a galvanic chronoscope. The particular signal to be received was unknown in advance, and the excess of time occupied by those reactions in which its character had first to be discerned, over the simple reaction-time, measured, according to Wundt, the time required for the act of discrimination. It was found longer when four different signals were irregularly used than when only two were used. In the former case it averaged, for three observers respectively (the signals being the sudden appearance of a black or of a white object),
0.050 sec;
0.047 sec.
0.079 sec.
In the latter case, a red and a green signal being added to the former ones, it became, for the same observers,
0.157;
0.073;
0.132.[432]
Later, in Wundt's Laboratory, Herr Tischer made many careful experiments after the same method, where the facts to be discriminated were the different degrees of loudness in the sound which served as a signal. I subjoin Herr Tischer's table of results, explaining that each vertical column after the first gives the average results obtained from a distinct individual, and that the figure in the first column stands for the number of possible loudnesses that might be expected in the particular series of reactions made. The times are expressed in thousandths of a second.
| 2 | 6 | 8.5 | 10.75 | 10.7 | 33 | 53 |
| 3 | 10 | 14.4 | 19.9 | 22.7 | 58.5 | 57.8 |
| 4 | 16.7 | 20.8 | 29 | 29.1 | 75 | 84 |
| 5 | 25.6 | 31 | ... | 40.1 | 95.5 | 138[433] |
The interesting points here are the great individual variations, and the rapid way in which the time for discrimination increases with the number of possible terms to discriminate. The individual variations are largely due to want of practice in the particular task set, but partly also to discrepancies in the psychic process. One gentleman said, for example, that in the experiments with three sounds, he kept the image of the middle one ready in his mind, and compared what he heard as either louder, lower, or the same. His discrimination among three possibilities became thus very similar to a discrimination between two.[434]
Mr. J. M. Cattell found he could get no results by this method,[435] and reverted to one used by observers previous to Wundt and which Wundt had rejected. This is the einfache Wahlmethode, as Wundt calls it. The reacter awaits the signal and reacts if it is of one sort, but omits to act if it is of another sort. The reaction thus occurs after discrimination; the motor impulse cannot be sent to the hand until the subject knows what the signal is. The nervous impulse, as Mr. Cattell says, must probably travel to the cortex and excite changes there, causing in consciousness the perception of the signal. These changes occupy the time of discrimination (or perception-time, as it is called by Mr. C.) But then a nervous impulse must descend from the cortex to the lower motor centre which stands primed and ready to discharge; and this, as Mr. C. says, gives a will-time as well. The total reaction-time thus includes both 'will-time' and 'discrimination-time.' But as the centrifugal and centripetal processes occupying these two times respectively are probably about the same, and the time used in the cortex is about equally divided between the perception of the signal and the preparation of the motor discharge, if we divide it equally between perception (discrimination) and volition, the error cannot be great.[436] We can moreover change the nature of the perception without altering the will-time, and thus investigate with considerable thoroughness the length of the perception-time.
Guided by these principles, Prof. Cattell found the time required for distinguishing a white signal from no signal to be, in two observers: