ACTA OPHTHALMOLOGICA 1957

Uttiversity of Manitoba, Canada

T H E MEASUREMENTS OF VISUAL ACTION-TIME OF MCDOUGALL

BY

Frank Allen'>)

Action-time, a term introduced by McDougall, is the least time during which a light of given intensity must act upon the retina in order to produce its full effect in sensation; or, in other words, to elicit the sensation of the highest intensity it is a t all capable of exciting. In his paper McDougall ( 5 ) criticizes the methods of Exner (2) and Kunkel(3) whose measurements had been accepted as standards. The measurements of Martius ( 4 ) were also criticized, since those of all three observers differed widely from one another.

McDougall especially objected to the measurements of Swan (6) and Char- pentier ( 1 ) since these investigators had concluded that the visual action-times were the same for all intensities of light, the former giving a value of one-tenth and the latter one-eighth of a second; whereas the measurements of Exner and of Kunkel showed generally shorter action-times for high intensities of light than those for dim lights. Exner had concluded that his measurements followed a law; that whereas the intensities of light increased in geometrical progression, the times that intervene between the beginning of stimulation and the highest intensity of the resulting sensation diminish in arithmetical progression. This ),law<< apparently was based on four measurements:

Stimulus of value 1 requires 0.1508 sec. )) )) )) '12 )) 0.2000 sec. )) y )> N 0.2460 sec. )> >> N l/s P 0.2873 sec.

The measurements of Martius were in terms of an initial intensity of light, j, which was reduced by progressive fractions, as shown in Table 1.

When these values are plotted with fractions of J as abscissae and action- times, A, as ordinates the curve is an exponential one; but if values of loga- rithms of the fractions of J (taking J = 1 ) are plotted as abscissae and action-

") Received September 20th 1956.

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Intensity Log I

- 0.5 1.699 0.25 1.398 0.125 1.097 0.063 2.799 JI16

5/32 0.051 2.491

- Jl2 514 J/S

- - -

Action-time (u)

39 53 66 78 93

times, A, as ordinates as before, the graph obtained consists of two intersecting straight lines as shown in Fig. 1 (right), which conform to the Fechner law of response: A = - k log I + C, where I represents values of the fractions of J, the intensity, and k and C are constants having different numerical values for the two separate lines.

With the greatest care and very precise experimental arrangements, McDou- gall made two lengthy series of measurements of action-times for twelve in- tensities of stimulation in one case and nine in the other; which are given in Table 2 and Table 3, respectively. The values in Table 2 are plotted in Fig. 1, left, with logarithms of intensities of stimulation, log I, as abscissae and cor- responding measurements of the action-times, A, as ordinates. The values in Table 3 are similarly plotted 'in Fig. 2. It will be noted that in Table 2 and Fig. 1 the action-times are given absolutely in thousandths (u) of a second; whereas in Table 3 and Fig. 2, they are given relatively as the duration of

90

180 7 0

140 5 0

100 3 0

60 I0

30 Z O l o 0

Fig. 1. Action-time. A. Thousandths of a second. A = - k log I + C.

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46

35

2 1

u

1.0 i. 5 0

Fig. 4. Action-time. Degrees. A. McDougall. A = - k log I + C.

open sectors of a disc rotating at constant speed, to which the action-times are strictly proportional.

In both cases the graphs consist of three intersecting straight lines all of which conform to the Fechner logarithmic law of response,

A = - k l o g I + C

where, as before, I represents the intensity of stimulation and k and C are constants which differ numerically for each of the three straight lines of the graphs. The intensity, I, in the equations represents fractions of L in the Tables, which is given an arbitrary value of unity for logarithmic purposes.

With the exception of two measurements in Fig. 1 and one measurement in Fig. 2, all observations of McDougall lie accurately on the graphs, which attest the remarkable accuracy of measurement of that distinguished observer in so difficult an investigation as the observation of action-times.

Since the graph of measurements of Martius in Fig. 1, is also a straight line, though the numerical values differ from those of McDougall, it may be con- cluded that his method of observation differed somewhat from that of the latter investigator.

The different slopes of the straight lines in each graph may be thus ex- plained. The responses of every sense organ are controlled by the reflex neural processes of inhibition and facilitation; the former process, by its depressing influence, preserves the sensations from over-intensity which might be harm- ful, and the latter process, by its enhancing power, from under-intensity which might render them ineffective. The different slopes also indicate that the balance of the controlling processes differs for the various ranges of intensities of stimulations.

Mc Dougall did not graphically plot his measurements in any way. From the general trend of the numerical values he, however, concluded that the saction-time of white light varies inversely with its intensity* within the limits

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Table 2. Measurements of McDougall.

I

Iatensity I. I

Action-time A (4 Log I

L 1 0 49

L/2 L/4 L/8 L/l6 LlS2 L/64 LJ128 L/256 L/512 L/1024 L/2048

0.5 0.25 0.125 0.063 0.031 0.016 0.008 0.004 0.002 0.001 0.0005

- 1.699 1.398 1.097 2.799 2.491 2.204 3.903 3.602 3.301 3.0 4.699

- - - - - - - - - -

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61 66 78 89 100 127 142 150 183 200

~ ______ ________ ~~ ______

Note: In both tables L is given an arbitrary value of unity.

Table 3. Measurements of McDougall.

Intensity I Log I

Action-time Degrees of

open sectors

L 1 0 18'

L/2 L13 L/4 L/5 L/6 L:8 L/14 L/16

0.5 0.33 n.25 0.20 0.1 7 0.125 0.071 0.063

- 1.699 1.519 1.398 1.301 1.230 1.097 2.851 2.799

- - - - - - -

22 25 29 33 36 38 42 44

of 0.2 to 0.03 second; the former value being the action-time of the minimum perceptible light. From his measurements he also concluded that red, green and blue lights of equal intensities have action-times of equal or nearly equal

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duration. In other words only the intensity oi light, and not its colour, deter- mines the action-time. This implies that all three primary colour sensations, red, green and blue, are of the same responsiveness. Since, according to the tri- chromatic theory, the sensation of white arises from equal stimulation of the three primary sensations, they must be of the same responsiveness, or else flashes of white light would be initially perceived tinged with colour.

From the linear character of the graphs it may be concluded that the action- time varies inversely with the logarithm of the intensity of stimulation, within the same limits, and with different numerical values of the proportionality constant for different ranges of intensity. This principle may be termed .McDougall’s law of action-times<<.

Exner’s four measurements on page one may also be represented logarith- mically as in Table 4, where the initial intensity, I, is taken as unity.

Intensity I

Action-time Second Log I

112 I/4 I/8

- 0.5 1.699 0.2000

1.398 0.2460 0.25 0.125 1.097 0.2873

- -

They are also plotted in Fig. 3, where the graph conforms to the Fechner equation:

Exner’s measurements are of perfect accuracy. A = -k log I + C.

Fig. 3. Action times. Exner. A = -log I + C.

REFERENCES

I . Charpentier, A.: Compt. Rend. de la SOC. de Biol. 1887. 2. Exner, S.: Uber die zu einer Gesichtswahrnehmung nothige Zeit. Ber. d. Wiener

3. KunkeZ, A.: Uber die Abhangigkeit der Farbenempfindungen von der Zeit. Pfliiger’s

4 . Martius, G.: Uber die Dauer der Lichtempfindungen. Beitrage zur Psychol. u.

5. McDougaZZ, W.: The variation of the intensity of visual sensation with the duration

6. Swan, W.: On the gradual production of luminous impressions on the eye and other

Akad. 1868, Abth. 11, Bd. 58; S. 601.

Arch. f . Physiol. Bd. IX, 1874, S. 197.

Philos. Leipzig. 1902, Bd. 1, Heft. 3.

of the stimulus. Brit. Jour. Psychol. 1, 1904, 151.

phenomena of vision. Trans. Roy. SOC. Edin. 1849.

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