STIMULI.
IV. THE PROBLEMS AND POSSIBILITIES OF COMPARATIVE REACTION-TIME
STUDIES.
Animal reaction time is at present a new field of research of evident
importance and full of promise. A great deal of time and energy has
been devoted to the investigation of various aspects of the time
relations of human neural processes; a multitude of interesting facts
have been discovered and a few laws established, but the results seem
disproportionate to the amount of patient labor expended.
Physiologists have determined the rate of transmission of the neural
impulse for a few animals, and rough estimates of the time required
for certain changes in the nervous system have been made, but this is
all we have to represent comparative study. Just the path of approach
which would seem most direct, in case of the time of neural changes,
has been avoided. Something is known of the ontogenetic aspect of the
subject, practically nothing of the phylogenetic; yet, in the study of
function the comparative point of view is certainly as important as it
is in the study of structure. In calling attention to the importance
of the study of animal reaction time I would not detract from or
minimize the significance of human investigations. They are all of
value, but they need to be supplemented by comparative studies.
It is almost impossible to take up a discussion of the time relations
of neural processes without having to read of physiological and
psychological time. The time of nerve transmission, we are told, is
pure physiological time and has nothing whatever to do with psychic
processes; the time occupied by the changes in brain centers is, on
the contrary, psychological time. At the very beginning of my
discussion of this subject I wish to have it clearly understood that I
make no such distinction. If one phase of the neural process be called
physiological time, with as good reason may all be so named. I prefer,
therefore, to speak of the time relations of the neural process.
Of the value of reaction-time studies, one may well believe that it
lies chiefly in the way of approach which they open to the
understanding of the biological significance of the nervous system.
Certainly they are not important as giving us knowledge of the time of
perception, cognition, or association, except in so far as we discover
the relations of these various processes and the conditions under
which they occur most satisfactorily. To determine how this or that
factor in the environment influences the activities of the nervous
system, and in what way system may be adjusted to system or
part-process to whole, is the task of the reaction-time investigator.
The problems of reaction time naturally fall within three classes:
Those which deal with (1) nerve transmission rates; (2) the time
relations of the spinal center activities, and (3) brain processes.
Within each of these groups there are innumerable special problems for
the comparative physiologist or psychologist. Under class 1, for
instance, there is the determining of the rates of impulse
transmission in the sensory and the motor nerves, (_a_) for a variety
of stimuli, (_b_) for different strengths of each stimulus, (_c_) for
different conditions of temperature, moisture, nourishment, fatigue,
etc., in case of each stimulus, (_d_) and all this for hundreds of
representative animals. From this it is clear that lines of work are
not lacking.
Closely related to these problems of rate of transmission are certain
fundamental problems concerning the nature of the nerve impulse or
wave. Whether there is a nerve wave, the reaction-time worker has as
favorable an opportunity to determine as anyone, and we have a right
to expect him to do something along this line. The relations of the
form of the nerve impulse to the rhythm of vital action, to fatigue
and to inhibition are awaiting investigation. Some of the most
important unsettled points of psychology depend upon those aspects of
neural activities which we ordinarily refer to as phenomena of
inhibition, and which the psychologist is helpless to explain so long
as the physiological basis and conditions are not known.
Then, too, in the study of animals the relation of reaction time to
instincts, habits, and the surroundings of the subject are to be
noted. Variability and adaptability offer chances for extended
biological inquiries; and it is from just such investigations as
these that biology has reason to expect much. The development of
activity, the relation of reflex action to instinctive, of impulsive
to volitional, and the value of all to the organism, should be made
clear by reaction-time study. Such are a few of the broad lines of
inquiry which are before the comparative student of animal reaction
time. It is useless to dwell upon the possibilities and difficulties
of the work, they will be recognized by all who are familiar with the
results of human studies.
In the study of the time relations of neural processes Helmholtz was
the pioneer. By him, in 1850, the rate of transmission of the nerve
impulse in the sciatic nerve of the frog was found to be about 27
meters per second[4]. Later Exner[5] studied the time occupied by
various processes in the nervous system of the frog by stimulating the
exposed brain in different regions and noting the time which
intervened before a contraction of the gastrocnemius in each case.
Further investigation of the frog's reflex reaction time has been made
by Wundt[6], Krawzoff and Langendorff[7], Wilson[8] and others, but in
no case has the method of study been that of the psychologist. Most of
the work has been done by physiologists who relied upon vivisectional
methods. The general physiology of the nervous system of the frog has
been very thoroughly worked up and the papers of Sanders-Ezn[9],
Goltz[10] Steiner[11] Schrader[12] and Merzbacher[13],[14] furnish an
excellent basis for the interpretation of the results of the
reaction-time studies.
[4] Helmholtz, H.: 'Vorläufiger Bericht über die
Portpflanzungsgeschwindigkeit der Nervenreizung.' _Arch. f.
Anal. u. Physiol._, 1850, S. 71-73.
[5] Exner, S.: 'Experimentelle Untersuchung der einfachsten
psychischen Processe.' _Pflüger's Arch._, Bd. 8. 1874, S.
526-537.
[6] Wundt, W.: 'Untersuchungen zur Mechanik der Nerven und
Nervencentren.' Stuttgart, 1876.
[7] Krawzoff, L., und Langendorff, O.: 'Zur elektrischen
Reizung des Froschgehirns.' _Arch. f. Anal. u. Physiol._,
Physiol. Abth., 1879, S. 90-94.
[8] Wilson, W.H.: 'Note on the Time Relations of Stimulation of
the Optic Lobes of the Frog.'_Jour. of Physiol._, Vol. XI.,
1890, pp. 504-508.
[9] Sanders-Ezn: 'Vorarbeit für die Erforschung des
Reflexmechanismus in Lendentmark des Frosches.' _Berichte über
die Verhandlungen der Kgl. sächs. Gesellsch. d. Wissensch. zu
Leipzig_, 1867, S. 3.
[10] Goltz, F.: 'Beiträge zur Lehre von den Functionen der
Nervencentren des Frosches.' Berlin, 1869, 130 S.
[11] Steiner, J.: 'Untersuchungen über die Physiologie des
Froschhirns.' Braunschweig, 1885, 127 S.
[12] Schrader, M.G.: 'Zur Physiologie des Froschgehirns.'
_Pflüger's Arch._, Bd. 41, 1887, S. 75-90.
[13] Merzbacher, L.: 'Ueber die Beziebungen der Sinnesorgane zu
den Reflexbewegungen des Frosches.' _Pflüger's Arch._, Bd. 81,
1900, S. 223-262.
[14] Merzbacher, L.: 'Untersuchungen über die Regulation der
Bewegungen der Wirbelthiere. I. Beobachtungen an Fröschen.'
_Pflüger's Arch._, Bd. 88, 1901, S. 453-474, 11 Text-figuren.
In the present investigation it has been my purpose to study the
reactions of the normal frog by the reaction-time methods of the
psychologist. Hitherto the amount of work done, the extent of
movements or some other change has been taken as a measure of the
influence of a stimulus. My problem is, What are the time relations of
all these reactions? With this problem in mind I enter upon the
following program: (1) Determination of reaction time to electrical
stimuli: (_a_) qualitative, (_b_) quantitative, (_c_) for different
strengths of current; (2) Determination of reaction time to tactual
stimuli (with the same variations); (3) Auditory: (_a_) qualitative,
(_b_) quantitative, with studies on the sense of hearing; (4) Visual:
(_a_) qualitative, (_b_) quantitative, with observations concerning
the importance of this sense in the life of the frog, and (5)
Olfactory: (_a_) qualitative, (_b_) quantitative.
The present paper presents in rather bare form the results thus far
obtained on electrical, tactual, and auditory reaction time;
discussion of them will be deferred until a comparison of the results
for the five kinds of stimuli can be given.
V. METHOD OF STUDY.
The measurements of reaction time herein considered were made with the
Hipp Chronoscope. Cattell's 'Falling Screen' or 'Gravity Chronoscope'
was used as a control for the Hipp. The Gravity Chronoscope consists
of a heavy metal plate which slides easily between two vertical posts,
with electrical connections so arranged that the plate, when released
from the magnet at the top of the apparatus, in its fall, at a certain
point breaks an electric circuit and at another point further down
makes the same circuit. The rate of fall of the plate is so nearly
constant that this instrument furnishes an accurate standard time with
which Hipp readings may be compared, and in accordance with which the
Hipp may be regulated. For, since the rate of a chronoscope varies
with the strength of the current in use, with the variations in
temperature and with the positions of the springs on the magnetic bar,
it is always necessary to have some standard for corrections. In these
experiments the time of fall of the gravity chronoscope plate, as
determined by the graphic method with a 500 S.V. electric tuning fork,
was 125[sigma] (_i.e._, thousandths of a second).
This period, 125[sigma], was taken as a standard, and each hour,
before the beginning of reaction-time experiments, the time of the
plate's fall was measured ten times with the Hipp, and for any
variation of the average thus obtained from 125[sigma], the standard,
the necessary corrections were made by changing the position of the
chronoscope springs or the strength of the current.
The standard of comparison, 125[sigma], is shorter than most of the
reaction times recorded, but since the time measured was always that
from the breaking to the making of the circuit passing through the
chronoscope it cannot be urged that there were errors resulting from
the difference of magnetization which was caused by variations in the
reaction time. But it is evident that the danger from differences in
magnetization, if such exists, is not avoided in this way; instead, it
is transferred from the reaction time proper to the period of
preparation immediately preceding the reaction; for, from the moment
the chronoscope is started until the stimulus is given a current is
necessarily passing through the instrument. At a verbal signal from
the operator the assistant started the chronoscope; the stimulus was
then given by the operator, and the instrument recorded the time from
the breaking of the circuit, effected by the stimulating apparatus, to
the making of the circuit by the reaction of the animal. Despite
precautions to prevent it, the period from the starting of the
chronoscope to the giving of the stimulus was variable, and errors
were anticipated, but a number of the tests proved that variations of
even a second did not cause any considerable error.
A fairly constant current for the chronoscope was supplied by a
six-cell 'gravity battery' in connection with two storage cells, _GB_
(Fig. 6). This current could be used for two hours at a time without
any objectionable diminution in its strength. The introduction of
resistance by means of the rheostat, _R_, was frequently a convenient
method of correcting the chronoscope.
[Illustration: FIG. 6. General Plan of Apparatus in Diagram. _H_, Hipp
Chronoscope; _R_, rheostat; _C_, commutator; _SC_, storage cells;
_GB_, 'Excello' gravity battery; _F_, Cattell's falling screen; _T_,
reaction table; _RK_, reaction key; _SK_, Stimulating apparatus; _K_,
key in chronoscope circuit; _S_, stimulus circuit.]
Fig. 6 represents the general plan of the apparatus used in these
experiments.
The general method of experimentation is in outline as follows:
1. At a 'ready' signal from the operator the assistant makes the
chronoscope circuit by closing a key, _K_ (Fig. 6), and then
immediately starts the chronoscope.
2. Stimulus is given by the operator as soon as the chronoscope is
started, and by this act the chronoscope circuit is broken and the
record begun.
3. Animal reacts and by its movements turns a key, _RK_ (Fig. 6), thus
making the chronoscope circuit and stopping the record.
4. Assistant stops chronoscope and takes reading.
[Illustration: FIG. 7. Reaction Key. _l_, lever swung on pivot; _p,
p_, posts for contacts with platinum plates on base; _b_, upright bar
for string; _s_, spring for clamping string; _w_, wheel to carry
string; _c, c_, chronoscope circuit; 1 and 2, points which are brought
into contact by animal's reaction.]
The steps of this process and the parts of the apparatus concerned in
each may be clearly conceived by reference to the diagram given in
Fig. 6. The various forms of stimulating apparatus used and the
modification of the method will be described in the sections dealing
with results. The same reaction key was used throughout (see Fig. 7).
Its essential features are a lever _l_, pivoted in the middle and
bearing a post at either end, _p, p_. From the middle of this lever
there projected upward a small metal bar, _b_, through the upper part
of which a string to the animal ran freely except when it was clamped
by the spring, _s_. This string, which was attached to the subject's
leg by means of a light elastic band, after passing through the bar
ran over a wheel, _w_, and hung tense by reason of a five-gram weight
attached to the end. Until everything was in readiness for an
experiment the string was left free to move through the bar so that
movement of the animal was not hindered, but the instant before the
ready-signal was given it was clamped by pressure on _s_. The diagram
shows the apparatus arranged for a reaction. The current is broken,
since 1 and 2 are not in contact, but a slight movement of the animal
turns the lever enough to bring 1 against 2, thus making the circuit
and stopping the chronoscope. When the motor reaction of the subject
was violent the string pulled out of the clamp so that the animal was
free from resistance, except such as the string and weight offered.
The five-gram weight served to give a constant tension and thus
avoided the danger of error from this source. Between experiments the
weight was placed on the table in order that there might be no strain
upon the subject.
That the subject might be brought into a favorable position for an
experiment without being touched by the operator a special reaction
box was devised.
The animals used in these studies were specimens of _Rana clamitans_
which were kept in a tank in the laboratory throughout the year.
VI. ELECTRIC REACTION TIME.
The reaction time to electrical stimuli was determined first because
it seemed probable that this form of the pain reaction would be most
useful for comparison with the auditory, visual, olfactory and tactual
reactions. In this paper only the electrical and the tactual reaction
times will be considered. The former will be divided into two groups:
(1) Those resulting from a stimulus given by touching electrodes to
the leg of the frog, and (2) those gotten by having the frog resting
upon wires through which a current could be passed at any time.
_Group 1 of the electrical reactions_ were taken under the following
conditions. A reaction box about 40 cm. in diameter was used. The mean
temperature of the experimenting room was about 20° C. In all cases
the string was attached to the left hind leg of the frog, and the
stimulus applied to the middle of the gastrocnemius muscle of the
right hind leg. Reaction times were taken in series of ten, excluding
those which were imperfect. As the moistness of the skin affects the
strength of the electric stimulus received, it was necessary to
moisten the animal occasionally, but as it did not seem advisable to
disturb it after each experiment this was done at intervals of five
minutes throughout the series. Were it not for this precaution it
might be said that lengthening of the reaction times toward the end of
a series simply indicated the weakening of the stimulus which resulted
from the gradual drying of the skin. The stimulus in this group was
applied by means of the stimulating apparatus of Fig. 6. It is merely
two wire electrodes which could be placed upon the animal, with the
additional device of a key for the breaking of the chronoscope circuit
the instant the stimulus was given. The most serious objection to this
method of stimulating is that there is a tactual as well as an
electrical stimulus.
Before presenting averages, two representative series of reactions may
be considered.
SERIES I. FROG B. APRIL 9, 1900. 10 A.M.
Temperature 19° C. String to left hind leg. Stimulus to right hind
leg.
Strength of stimulating current 1.0 volt, .0001 ampère.
Number of
Experiment. Hour. Reaction Time. Remarks.
1 10.25 No reaction.
2 10.27 No reaction.
3 10.30 139[sigma]
4 10.34 164
5 10.35 102
6 10.37 169
7 10.39 151
8 10.40 152
9 10.42 144
10 10.43 152
11 10.45 122
12 10.51 179
13 10.54 No reaction.
Average of 10, 147.4[sigma]
SERIES 2. FROG F. ELECTRICAL STIMULUS.
No. Hour. Reaction Time. Remarks. Deviation from Mean.
1 10.19 35[sigma] Probable reaction
to visual stim.
2 10.22 173 4.7
3 10.24 161 - 7.3
4 10.25 133 -35.3
5 10.26 199 30.7
6 10.28 130 -38.3
7 10.32 179 10.7
8 10.34 187 18.7
9 10.35 60 Probable reflex.
10 10.37 183 14.7
11 10.38 166 - 2.3
12 10.39 172 3.7
Average of 10, 168.3[sigma] Average of first 5, 159.2[sigma]
Average Variation, 16.64[sigma] Average of second 5, 177.4[sigma]
Both are fairly representative series. They show the extremely large
variations, in the case of series 1, from 102 to 179[sigma]. In all
these experiments such variation is unavoidable because it is
impossible to have the conditions uniform. A very slight difference in
the frog's position, which could not be detected by the operator,
might cause considerable difference in the time recorded. Efforts were
made to get uniform conditions, but the results seem to show that
there is still much to be desired in this direction.
Tables VII. contains the results of four series of ten reactions each
for frog _A_. It will be noticed that the time for the first five in
each series is much shorter than that for the last five; this is
probably indicative of fatigue.
TABLE VII.
REACTION TIME OF FROG _A_ TO ELECTRICAL STIMULI.
Series of Averages Averages of Averages of
ten reactions. of series. first five. second five.
1 163.1[sigma] 134.6[sigma] 191.6[sigma]
2 186.2 176.2 196.2
3 161.1 125.2 197.0
4 158.3 101.6 215.0
General averages 167.2[sigma] 134.4[sigma] 199.9[sigma]
TABLE VIII.
REACTION TIME OF FROG _B_ TO ELECTRICAL STIMULI.
1 132.7[sigma] 118.2[sigma] 147.4[sigma]
2 196.6 167.8 225.4
3 147.4 145.5 149.8
4 157.5 152.0 163.0
General averages 158.6[sigma] 145.9[sigma] 171.4[sigma]
TABLE IX.
NORMAL AND REFLEX REACTION TIME OF SIX ANIMALS TO ELECTRICAL STIMULUS.
Normal. Reflex.
Average for 20 Average for 20
Frog. reactions. Mean Var. reactions. Mean Var.
_A_ 149.5[sigma] 24.0[sigma]
_B_ 158.3 16.0 51.5[sigma] 8.0[sigma]
_C_ 191.0 24.3
_D_ 167.0 10.1
_E_ 182.4 28.0 45.1 5.5
_F_ 176.3 10.2 46.0 4.5
General
Average. 167.9[sigma] 18.8[sigma] 47.5[sigma] 6.0[sigma]
For _D_ the average is for ten reactions.
_B_ and _E_ were males, _F_ a female; the sex of the others was
not determined by dissection and is uncertain.
Early in the experiments it became evident that there were three
clearly defined types of reactions: there were a number of reactions
whose time was shorter than that of the ordinary quick voluntary pain
reaction, and there were also many whose time was considerably longer.
The first type it was thought might represent the spinal reflex
reaction time. For the purpose of determining whether the supposition
was true, at the end of the series of experiments three of the frogs
were killed and their reflex reaction time noted. This was done by
cutting the spinal cord just back of the medulla, placing the animal
on an experimenting board close to the reaction key with the thread
from the key fastened to the left leg as in case of the previous work
and stimulating the gastrocnemius with an induced current by the
application of wire electrodes.
In Table IX. the reflex reaction times for the three animals are
given.
The following results obtained with frog _E_ show that the time of
reaction increases with the increase in the time after death. The
average of 20 reactions by _E_ taken an hour after the cord had been
cut was 45.5[sigma]; the average of 20 taken twenty hours later was
55.85[sigma].
As a rule the reflex reactions were but slightly variable in time as
is indicated by the accompanying series.
SERIES OF REFLEX REACTIONS OF FROG _F_.
Taken at rate of one per minute.
1 50[sigma]
2 58
3 55
4 59
5 48
6 46
7 45
8 51
9 42
10 44
Throughout these experiments it was noticed that any stimulus might
cause (1) a twitch in the limb stimulated, or (2) a twitch followed by
a jump, or (3) a sudden jump previous to which no twitch could be
detected. And it soon appeared that these types of reaction, as it
seems proper to call them, would have to be considered in any
determination of the mean reaction time. As proof of the type theory
there is given (Fig. 8) a graphic representation of 277 reactions to
the electrical stimulus.
[Illustration: FIG 8: Distribution of 277 reactions.]
The column of figures at the left indicates the number of reactions at
any point. Below the base line are the classes. For convenience of
plotting the reactions have been grouped into classes which are
separated by 25[sigma]. Class 1 includes all reactions between
1[sigma] and 25[sigma], class 2 all from 25[sigma] to 50[sigma], and
so on to 400[sigma], thereafter the classes are separated by
100[sigma]. It is noticeable that there is one well-marked mode at
75[sigma]. A second mode occurs at 175[sigma]. This is the primary and
in our present work the chiefly significant mode, since it is that of
the quick instinctive reaction to a stimulus. At 500[sigma] there is a
third mode; but as such this has little meaning, since the reactions
are usually pretty evenly distributed from 300[sigma] on to
2000[sigma]; if there is any grouping, however, it appears to be about
500[sigma] and 800[sigma].
The first mode has already been called the reflex mode. The short
reactions referred to usually lie between 40[sigma] and 80[sigma], and
since experiment has shown conclusively that the spinal reflex
occupies about 50[sigma], there can be little doubt that the first
mode is that of the reflex reaction time.
The second mode represents those reactions which are the result of
central activity and control. I should be inclined to argue that they
are what we usually call the instinctive and impulsive actions. And
the remaining reactions represent such as are either purely voluntary,
if any frog action can be so described, or, in other words, depend
upon such a balancing of forces in the brain as leads to delay and
gives the appearance of deliberate choice.
Everything points to some such classification of the types as follows:
(1) Stimuli strong enough to be injurious cause the shortest possible
reaction by calling the spinal centers into action, or if not spinal
centers some other reflex centers; (2) slightly weaker stimuli are not
sufficient to affect the reflex mechanism, but their impulse passes on
to the brain and quickly discharges the primary center. There is no
hesitation, but an immediate and only slightly variable reaction; just
the kind that is described as instinctive. As would be expected, the
majority of the frog's responses are either of the reflex or of this
instinctive type. (3) There is that strength of stimulus which is not
sufficient to discharge the primary center, but may pass to centers of
higher tension and thus cause a response. This increase in the
complexity of the process means a slower reaction, and it is such we
call a deliberate response. Precisely this kind of change in neural
action and in reaction time is at the basis of voluntary action. And
(4) finally, the stimulus may be so weak that it will not induce a
reaction except by repetition. Just above this point lies the
threshold of sensibility, the determination of which is of
considerable interest and importance.
_Group 2 of the electrical reactions_ consists of three series taken
to determine the relation of strength of stimulus to reaction time.
The conditions of experimentation differed from those for group 1 in
the following points: (1) The stimulus was applied directly by the
making of a circuit through wires upon which the subject rested (Fig.
9); (2) the thread was attached to the right hind leg; (3) the thread,
instead of being kept at the tension given by the 5-gram weight as in
the former reactions, was slackened by pushing the upright lever of
the reaction key one eighth of an inch toward the animal. This was
done in order to avoid the records given by the slight twitches of the
legs which precede the motor reaction proper. For this reason the
reactions of group 2 are not directly comparable with those of group
1. Fig. 9 is the plan of the bottom of a reaction box 15 cm. at one
end, 30 cm. at the other, 60 cm. long and 45 cm. deep. On the bottom
of this, at one end, a series of interrupted circuits were arranged as
shown in the figure. The wires were 1.2 cm. apart, and an animal
sitting anywhere on the series necessarily touched two or more, so
that when the stimulus key, X, was closed the circuit was completed by
the animal's body; hence, a stimulus resulted. The stimulus key, X,
was a simple device by which the chronoscope circuit, _c_, _c_, was
broken at the instant the stimulus circuit, _s_, _c_, was made.
Cells of 'The 1900 Dry Battery' furnished the current used as a
stimulus. Three different strengths of stimulus whose relative values
were 1, 2 and 4, were employed in the series 1, 2 and 3. Careful
measurement by means of one of Weston's direct-reading voltmeters gave
the following values: 1 cell, 0.2 to 0.5 volt, 0.00001 to 0.00003
ampère. This was used as the stimulus for series 1. 2 cells, 0.5 to
1.0 volt, 0.00003 to 0.00006 ampère. This was used for series 2. 4
cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampère. This was used for
series 3.
[Illustration: Fig. 9. Ground Plan of Reaction Box for Electrical
Stimuli (Group 2). _IC_, interrupted circuits; _CC_, chronoscope
circuit; _X_, key for making stimulus circuit and breaking chronoscope
circuit; _B_, stimulus battery; _S_, string from reaction key to
animal. Scale 1/2.]
The reactions now under consideration were taken in sets of 24 in
order to furnish evidence on the problem of fatigue. The stimulus was
given at intervals of one minute, and the subject was moistened at
intervals of ten minutes. To obtain 24 satisfactory reactions it was
usually necessary to give from thirty to forty stimulations. Five
animals, numbers 1, 2, 4, 5, and 6, served as subjects. They were
green frogs whose size and sex were as follows:
Length. Weight. Sex.
Number 1 7.5 cm. 35 grams. Male.
Number 2 7.3 " 37 " Male.
Number 4 8.2 " 50.4 " Female?
Number 5 7.1 " 25 " Female.
Number 6 7.8 " 42 " Male.
For most of these frogs a one-cell stimulus was near the threshold,
and consequently the reaction time is extremely variable. In Table X.
an analysis of the reactions according to the number of repetitions of
the stimulus requisite for a motor reaction has been made. Numbers 1
and 5 it will be noticed reacted most frequently to the first
stimulus, and for them 48 satisfactory records were obtained; but in
case of the others there were fewer responses to the first stimulus,
and in the tabulation of series 1 (Table XI.) averages are given for
less than the regular sets of 24 reactions each.
TABLE X.
ANALYSIS OF REACTIONS TO ONE-CELL STIMULUS.
Frog. Reactions to To 2d. To 3d. To 4th. To 5th. More. Total No.
first Stimulus. of Reactions.
1 53 2 1 0 0 1 57
2 20 12 5 5 4 12 58
4 31 15 1 0 2 8 57
5 51 11 1 2 0 1 66
6 45 15 6 3 1 5 75
Totals, 200 55 14 10 7 27 313
Table XI. is self-explanatory. In addition to the usual averages,
there is given the average for each half of the sets, in order that
the effect of fatigue may be noted. In general, for this series, the
second half is in its average about one third longer than the first
half. There is, therefore, marked evidence of tiring. The mean
reaction time for this strength of stimulus is difficult to determine
because of the extremely great variations. At one time a subject may
react immediately, with a time of not over a fifth of a second, and at
another it may hesitate for as much as a second or two before
reacting, thus giving a time of unusual length. Just how many and
which of these delayed responses should be included in a series for
the obtaining of the mean reaction time to this particular stimulus is
an extremely troublesome question. It is evident that the mode should
be considered in this case rather than the mean, or at least that the
mean should be gotten by reference to the mode. For example, although
the reaction times for the one-cell stimulus vary all the way from
150[sigma] to 1000[sigma] or more, the great majority of them lie
between 200[sigma] and 400[sigma]. The question is, how much deviation
from the mode should be allowed? Frequently the inclusion of a single
long reaction will lengthen the mean by 10[sigma] or even 20[sigma].
What is meant by the modal condition and the deviation therefrom is
illustrated by the accompanying curve of a series of reaction times
for the electric stimulus of group I.
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_3_|____________|___________________|____|____|___________________________
_2_|_______|____|____|_________|____|____|____|____|______________________
_1_|__|____|____|____|_________|____|____|____|____|____|____|____|____|__
100 110 120 130 140 150 160 170 180 190 200 210 220 230
The column of figures at the left indicates the number of reactions;
that below the base line gives the reaction times in classes separated
by 10[sigma]. Of thirty-one reactions, seven are here in the class
170[sigma]. This is the model class, and the mean gotten by taking the
average of 31 reactions is 162[sigma]. If the mode had been taken to
represent the usual reaction time in this case, there would have been
no considerable error. But suppose now that in the series there had
occurred a reaction of 800[sigma]. Should it have been used in the
determination of the mean? If so, it would have made it almost
30[sigma] greater, thus removing it considerably from the mode. If
not, on what grounds should it be discarded? The fact that widely
varying results are gotten in any series of reactions, points, it
would seem, not so much to the normal variability as to accidental
differences in conditions; and the best explanation for isolated
reactions available is that they are due to such disturbing factors as
would decrease the strength of the stimulus or temporarily inhibit the
response. During experimentation it was possible to detect many
reactions which were unsatisfactory because of some defect in the
method, but occasionally when everything appeared to be all right an
exceptional result was gotten. There is the possibility of any or all
such results being due to internal factors whose influence it should
be one of the objects of reaction-time work to determine; but in view
of the fact that there were very few of these questionable cases, and
that in series I, for instance, the inclusion of two or three
reactions which stood isolated by several tenths of a second from the
mode would have given a mean so far from the modal condition that the
results would not have been in any wise comparable with those of other
series, those reactions which were entirely isolated from the mode and
removed therefrom by 200[sigma] have been omitted. In series I alone
was this needful, for in the other series there was comparatively
little irregularity.
The results of studies of the reaction time for the one-cell electric
stimulus appear in Table XI. The first column of this table contains
the average reaction time or mean for each subject. Nos. 2 and 4
appeared to be much less sensitive to the current than the others, and
few responses to the first stimulus could be obtained. Their time is
longer than that of the others, and their variability on the whole
greater. Individual differences are very prominent in the studies thus
far made on the frog. The one-cell stimulus is so near the threshold
that it is no easy matter to get a mean which is significant. Could
the conditions be as fully controlled as in human reaction time it
would not be difficult, but in animal work that is impossible. No
attempt has thus far been made to get the reaction time in case of
summation effects except in occasional instances, and in so far as
those are available they indicate no great difference between the
normal threshold reaction and the summation reaction, but on this
problem more work is planned.
There are large mean variations in Table XI., as would be anticipated.
Since the reactions were taken in sets of 24, the means of each set as
well as that of the total are given, and also, in columns 4 and 5, the
means of the first half and the last half of each set.
A comparison of Tables XI., XII. and XIII. makes clear the differences
in reaction time correlated with differences in the strength of an
electric stimulus. For Table XI., series I, the relative value of the
stimulus was I; for Table XII., series 2, it was 2, and for Table
XIII., series 3, it was 4. Throughout the series from I to 3 there is
a rapid decrease in the reaction time and in the variability of the
same. The reaction time for stimulus I, the so-called threshold, is
given as 300.9[sigma]; but of the three it is probably the least
valuable, for reasons already mentioned. The mean of the second
series, stimulus 2, is 231.5[sigma] while that of the third, stimulus
4, is only 103.1[sigma]. This great reduction in reaction time for the
four-cell stimulus apparently shows the gradual transition from the
deliberate motor reaction, which occurs only after complex and varied
central neural activities, and the purely reflex reaction, which takes
place as soon as the efferent impulse can cause changes in the spinal
centers and be transmitted as an afferent impulse to the muscular
system.
TABLE XI.
ELECTRICAL STIMULUS REACTION TIME. SERIES 1.
Average Average of Average Average Mean Var
Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.
1 238.5* 33.3* 216.0* 205.6* 226.7* 33.2*
261.0 248.0 274.1 33.3
2 458.0 219.0 458.0 270.4 643.8 219.0
4 273.4 59.9 273.4 245.7 301.1 59.9
5 263.9 50.5 268.6 244.7 292.5 44.9
259.2 236.0 282.4 56.1
6 271.1 65.1 322.6 273.2 372.0 87.9
219.6 208.5 230.6 42.3
Gen Av. 300.9 85.5 300.9 244.8 356.8 85.5
Totals.
For No. 1 the averages are for 2 sets of 24 reactions each, 48
" 2 " " one set of 12 " " 12
" 4 " " one set of 24 " " 24
" 5 " " two sets of 24 " " 48
" 6 " " two sets of 24 and 12 reactions,
respectively, 36
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
TABLE XII.
ELECTRICAL STIMULUS REACTION TIME. SERIES 2.
Average Average of Average Average Mean Var
Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.
1 227.3* 33.7* 229.4* 209.1* 249.6* 25.5*
225.2 207.3 243.0 42.1
2 240.1 30.9 239.0 222.3 255.1 29.0
241.3 220.2 262.4 32.8
4 270.3 56.5 298.5 285.3 311.4 62.8
242.2 206.0 278.4 50.2
198.5 26.2 195.0 197.5 193.0 33.5
202.0 195.2 209.0 18.8
6 224.4 24.4 221.6 209.7 233.7 23.6
227.2 213.5 241.0 25.1
Gen. Av. 231.5 34.3 231.0 216.6 246.6 34.3
For No. 5 the averages are for two sets of 18 each; for all the
others there are 24 in each set.
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
TABLE XIII.
ELECTRICAL STIMULUS REACTION TIME. SERIES 3.
Average Average Average Average Mean Var.
Frog. of all. Mean Var. of all. of 1st h. of 2d h. of Sets.
1 93.6* 13.5* 91.8* 93.2* 90.4* 13.5*
95.4 91.8 99.0 13.5
2 99.9 12.8 92.2 89.4 95.0 17.4
107.5 105.9 109.0 8.2
4 125.2 16.3 113.5 106.5 120.5 13.6
136.0 135.7 138.2 19.0
5 94.4 8.0 88.6 90.5 88.6 8.2
100.2 97.8 102.7 7.9
6 102.5 12.2 104.2 98.6 109.9 12.8
100.9 101.0 108.3 11.6
Gen. Avs. 103.1 12.5 103.1 101.0 105.9 12.5
For each animal there are two sets of 24 reactions each.
*Transcriber's Note: All values in [sigma], 1/1000ths of a second.
The spinal reflex for a decapitated frog, as results previously
discussed show, is approximately 50[sigma]; and every time the
four-cell stimulus is given this kind of a reaction results. There is
a slight twitch of the legs, immediately after which the animal jumps.
Now for all these series the thread was slackened by one eighth of an
inch, but the reflex time was determined without this slack.
Calculation of the lengthening of the reaction time due to the slack
indicated it to be between 20 and 30[sigma], so if allowance be made
in case of the reactions to the four-cell stimulus, the mean becomes
about 70[sigma], or, in other words, nearly the same as the spinal
reflex. The conclusion seems forced, therefore, that when a stimulus
reaches a certain intensity it produces the cord response, while until
that particular point is reached it calls forth central activities
which result in much longer and more variable reaction times. It was
said above that the series under consideration gave evidence of the
gradual transition from the reflex to the volitional in reaction time.
Is this true, or do we find that there are well-marked types, between
which reactions are comparatively rare? Examination of the tables
VII., VIII., IX., XI., XII. and XIII. will show that between 70[sigma]
and 150[sigma] there is a break. (In tables XI., XII. and XIII.,
allowance must always be made for the slack in the thread, by
subtracting 30[sigma].) All the evidence furnished on this problem by
the electrical reaction-time studies is in favor of the type theory,
and it appears fairly clear that there is a jump in the reaction time
from the reflex time of 50-80[sigma], to 140 or 150[sigma], which may
perhaps be taken as the typical instinctive reaction time. From
150[sigma] up there appears to be a gradual lengthening of the time as
the strength of the stimulus is decreased, until finally the threshold
is reached, and only by summation effect can a response be obtained.
The most important averages for the three series have been arranged in
Table XIV. for the comparison of the different subjects. Usually the
reaction time for series 3 is about one half as long as that for
series 2, and its variability is also not more than half as large. In
the small variability of series 3 we have additional reason for
thinking that it represents reflexes, for Table IX. gives the mean
variation of the reflex as not more than 8[sigma], and the fact that
the means of this series are in certain cases much larger is fully
explained by the greater opportunity for variation afforded by the
slack in the thread.
TABLE XIV.
MEANS, ETC., FOR EACH SUBJECT FOR THE THREE SERIES. (TIME IN [sigma])
Mean First Second Mean Frog.
Half. Half. Variation.
Series 1 238.5 226.8 259.4 33.3
Series 2 227.3 208.2 246.3 33.7 No. 1
Series 3 93.6 92.5 94.7 13.5
Series 1 458.0 270.4 643.8 219.0
Series 2 240.1 221.2 258.8 30.9 No. 2
Series 3 99.9 97.6 102.0 12.8
Series 1 273.4 245.7 301.1 59.9
Series 2 270.3 245.6 294.9 56.5 No. 4
Series 3 125.2 121.1 129.3 16.3
Series 1 263.9 240.4 287.4 50.5
Series 2 198.5 196.4 201.0 26.2 No. 5
Series 3 94.4 94.2 94.7 8.0
Series 1 271.1 240.8 301.3 65.1
Series 2 224.4 211.6 237.3 24.4 No. 6
Series 3 102.5 99.8 109.1 12.2
A striking fact is that the averages for the first and last half of
sets of reactions differ more for the weak than for the strong
stimulus. One would naturally expect, if the increase were a fatigue
phenomenon purely, that it would be greatest for the strongest
stimulus; but the results force us to look for some other conditions
than fatigue. A stimulus that is sufficiently strong to be painful and
injurious to an animal forces an immediate response so long as the
muscular system is not exhausted; but where, as in series 1 and 2 of
the electrical stimulus, the stimulus is not harmful, the reason for a
sudden reaction is lacking unless fear enters as an additional cause.
Just as long as an animal is fresh and unfamiliar with the stimulus
there is a quick reaction to any stimulus above the threshold, and as
soon as a few experiences have destroyed this freshness and taught the
subject that there is no immediate danger the response becomes
deliberate. In other words, there is a gradual transition from the
flash-like instinctive reaction, which is of vast importance in the
life of such an animal as the frog, to the volitional and summation
responses. The threshold electrical stimulus does not force reactions;
it is a request for action rather than a demand, and the subject,
although startled at first, soon becomes accustomed to the experience
and responds, if at all, in a very leisurely fashion. The reaction
time to tactual stimuli, soon to be considered, was determined by
giving a subject only three or four stimulations a day; if more were
given the responses failed except on repetition or pressure; for this
reason the data on fatigue, or lengthening of reaction time toward the
end of a series, are wanting in touch. A few tests for the purpose of
discovering whether the time would lengthen in a series were made with
results very similar to those of the threshold electrical stimulus;
the chief difference lies in the fact that the responses to touch fail
altogether much sooner than do those to the electrical stimulus. This,
however, is explicable on the ground that the latter is a stimulus to
which the animal would not be likely to become accustomed so soon as
to the tactual.
First Half. Second Half. Second % Greater.
Series 1 244.8[sigma] 356.8[sigma] 46 per cent
Series 2 216.6 246.6 14 "
Series 3 101.0 105.9 5 "
If pure fatigue, that is, the exhaustion of the nervous or muscular
system, appears anywhere in this work, it is doubtless in series 3,
for there we have a stimulus which is so strong as to force response
on penalty of death; the reaction is necessarily the shortest
possible, and, as a matter of fact, the motor reaction (jump forward)
here occupies little more time than the leg-jerk of a decapitated
frog. This probably indicates that the reaction is a reflex, and that
the slight increase in its length over that of the spinal reflex is
due to occasional cerebellar origin; but of this there can be no
certainly from the evidence herewith presented. At any rate, there is
no possibility of a voluntary reaction to the strong current, and any
changes in the general character of the reaction time in a series will
have to be attributed to fatigue of the nervous or muscular systems.
The second halves of the sets of series 3 are 5 per cent. longer than
the first, and unless this is due to the partial exhaustion of the
nervous system it is hard to find an explanation of the fact. Fatigue
of the muscles concerned seems out of the question because the
reactions occur at the rate of only one per minute, and during the
rest interval any healthy and well-nourished muscle would so far
recover from the effect of contraction that it would be able to
continue the rhythmic action for long periods.
To the inquiry, Does fatigue in the experiments mean tiring by the
exhaustion of nerve energy, or is the lengthening in reaction time
which would naturally be attributed to tiring due to the fact that
experience has shown quick reaction to be unnecessary? we shall have
to reply that there is evidence in favor of both as factors. There can
be little doubt that in case of the strong stimuli there is genuine
fatigue which makes quick reaction impossible; but at the same time it
is certain that the 40 to 50 per cent. increase of the second half of
sets in series 1 over the first half can not be due to fatigue, for
the strain is here evidently much less than for series 3. Rather, it
would seem that habituation instead of exhaustion is the all-important
cause of the difference in series 1 and 2. It becomes clear from these
considerations that the repetition of a stimulus can never mean the
repetition of an effect.
VII. TACTUAL REACTION TIME.
In the following work on the reactions to tactual stimulation the
subject was placed in a large reaction box with a thread attached to
one of its legs and passing to a reaction key, as in the experiments
already described. The box in which the subject was confined was
surrounded by movable cloth curtains to prevent the animal's escape
and at the same time permit the experimenter to work without being
seen by the frog.
Tactual stimulation was given by means of a hand key[15] similar to
that used for electrical stimulation which is represented in Fig. 6.
The touch key ended in a hard-rubber knob which could be brought in
contact with the skin of the subject. This key was fixed to a handle
of sufficient length to enable the operator to reach the animal
wherever it chanced to be sitting in the reaction box. Stimulation was
given by allowing the rubber point of the touch key to come in contact
with the skin in the middle region of the subject's back. As soon as
the point touched the animal the chronoscope circuit was broken by the
raising of the upper arm of the key.
[15] This apparatus was essentially the same as Scripture's
device for the giving of tactual stimulation.
As a precaution against reactions to visual stimuli, which it might
well be supposed would appear since the subject could not in every
case be prevented from seeing the approaching apparatus, the frog was
always placed with its head away from the experimenter so that the
eyes could not readily be directed toward the touch apparatus.
Notwithstanding care in this matter, a reaction occasionally appeared
which was evidently due to some disturbance preceding the tactual
stimulus which served as a warning or preparation for the latter. All
such responses were at once marked as questionable visual reactions
and were not included in the series of touch reactions proper.
As has been mentioned in connection with the discussion of fatigue, it
was found absolutely necessary to have the subjects perfectly fresh
and active, and for this purpose it was advisable to give not more
than three or four stimulations at any one time. The subject was
usually kept in the reaction box from 30 to 45 minutes, dependent upon
the success of the experiments. As the work progressed it became
evident that the responses to the stimulus were becoming less and less
certain and slower, that the subjects were becoming accustomed to the
novel experience and no longer suffered the surprise which had been
the cause of the prompt reactions at first. It seemed best for this
reason not to continue the work longer than two weeks, and as a
consequence it was impossible to base the averages on more than twenty
reactions for each subject.
So far as the tension of the thread is concerned, the condition for
the tactual reaction time was the same as that for the first group of
electrical reaction-time experiments. In comparing the tactual with
the electrical of series 1, 2 and 3, allowance must be made for the
slack in the latter cases.
Selection of the tactual reaction times upon which the mean is based,
has been made with reference to the mode for each set of experiments.
Inspection of the curves given by the reactions of each subject
indicated that the great majority of the responses lay between 100 and
300[sigma], and that those which were beyond these limits were
isolated and, in all probability, exceptional reactions due to some
undetected variation in conditions which should throw them out of the
regular series. On this account it was thought best to use only
reactions between 100 and 300[sigma].
For convenience of comparison, again, the averages for the electrical
reaction time of subjects _A_, _B_, _C_, _D_, _E_ and _F_, and the
same for the tactual reaction time of subjects 1, 2, 3, 4, 5 and 6 are
herewith given together. All averages are for twenty reactions, except
for _D_ and 5, for which there are ten.
Besides the usual determination for the tactual reaction-time work on
the six subjects named, there is given in Table XVI. the electrical
reaction time of these animals to a two-cell current. Comparison of
the electrical and tactual results are of interest in this case
because the mean variation for each is about 34[sigma], being
34.3[sigma], for the electrical and 33.8[sigma], for the tactual.
TABLE XV.
Average of 20 Electrical Average of 20 Tactual
Frog. Reactions. Frog. Reactions.
_A_ 149.5[sigma] 1 188.3[sigma]
_B_ 158.3 2 199.1
_C_ 191.0 3 212.1
_D_ 167.0 4 213.0
_E_ 182.4 5 199.8
_F_ 176.3 6 221.9
Gen. Avs. 167.9 205.7
TABLE XVI.
REACTION TIME FOR TACTUAL AND ELECTRICAL STIMULI.
Tactual Reaction Time. Electrical Reaction Time.
Frog. Average. Mean Variation. Average. Mean Variation.
1 188.3[sigma] 167.3[sigma]
2 199.1 180.1
3 212.1
4 213.0 210.3
5¹ 199.8 138.5
6 221.9 164.4
Gen. Avs. 205.7 33.8 172.1 34.3
¹For 5 the average of ten instead of twenty is given.
VIII. EQUAL VARIABILITY AS A CRITERION OF COMPARABILITY OF REACTION
TIME FOR DIFFERENT KINDS OF STIMULI.
Since variability as indicated in the study of the influence of
different strengths of electrical stimulus becomes less as the
stimulus increases, parity in variability for different stimuli offers
a basis for the comparison of reaction times. Certain it is that there
is no use in comparing the reaction times for different senses or
different qualities of stimuli unless the relative values of the
stimuli are taken into consideration; but how are these values to be
determined unless some such index as variability is available? If the
reaction time to tactual stimuli as here presented is to be studied in
its relation to the electrical reaction time, it will mean little
simply to say that the former is longer than the latter, because the
electrical reaction time for a one-cell stimulus happens to be
somewhat less than that for the particular tactual stimulus used. For
it is clear that this tactual reaction time is really shorter than the
reaction time to a weak current. In making variability a basis of
comparison it must be assumed that the strength of stimulus is the
important factor, and that all other variable conditions are, so far
as possible, excluded. If, now, on the basis of parity in variability
we compare the tactual and electrical reaction times, it is apparent
that the tactual is considerably longer. The tactual average of Table
XV. is 205.7[sigma], while the electrical reaction time which has
approximately the same variability is 172.1[sigma]. It may well be
objected that I have no right to make variability the basis of my
comparison in these experiments, because the work for the various
kinds of stimuli was done under different conditions. Admitting the
force of this objection, and at the same time calling attention to the
fact that I do not wish to lay any stress on the results of the
comparisons here made, I take this opportunity to call attention to
the possibility of this criterion.
The use of variability as a basis of comparison would involve the
assumptions (1) that a certain intensity of every stimulus which is to
be considered is capable of producing the shortest possible, or reflex
reaction, and that this reaction is at the same time the least
variable; (2) that as the strength of a stimulus decreases the
variability increases until the threshold is reached.
Suppose, now, it is our desire to compare the results of reactions to
different intensities of electrical and tactual stimuli; let the
figures be as follows:
Reaction Time. Variability.
Stimulus Strength. Elect. Touch. Elect. Touch.
8 50[sigma] 50[sigma] 10[sigma] 10[sigma].
4 130 155 25 30
2 175 220 40 40
1 300 320 50 60
In the double columns the results for electrical stimuli are given
first, and in the second column are the tactual. Stimulus 8 is assumed
to be of sufficient strength to induce what may be designated as
forced movement, and whatever the quality of the stimulus this
reaction time is constant. I make this statement theoretically,
although all the evidence which this work furnishes is in support of
it. So, likewise, is the variability of this type of reaction time
small and nearly constant. At the other extreme, stimulus 1 is so weak
as to be just sufficient to call forth a response; it is the so-called
threshold stimulus. Whether all qualities of stimulus will give the
same result here is a question to be settled by experimentation. Wundt
contends that such is the case, but the observations I have made on
the electrical and tactual reactions of the frog cause me to doubt
this assumption. It seems probable that the 'just perceptible stimulus
reaction time' is by no means the same thing for different qualities
of stimulus. Those modifications of the vital processes which alone
enable organisms to survive, make their appearance even in the
response to the minimal stimulus. In one case the just perceptible
stimulus may cause nothing more than slight local changes in
circulation, excretion, muscular action; in another it may produce,
just because of the particular significance of the stimulus to the
life of the organism, a violent and sudden motor reaction. But grant,
if you will, that the threshold reaction time is the same for all
kinds of stimuli, and suppose that the variability is fairly constant,
then, between the two extremes of stimuli, there are gradations in
strength which give reaction times of widely differing variabilities.
If, now, at some point in the series, as, for instance, to stimulus 2,
the variability for different kinds of stimuli is the same either with
reference to the reaction time (ratio) or absolutely, what
interpretation is to be put upon the fact? Is it to be regarded as
merely a matter of chance, and unworthy of any special attention, or
should it be studied with a view to finding out precisely what
variability itself signifies? It is obvious that any discussion of
this subject, even of the possible or probable value of variability as
a criterion for the comparative study of stimuli, can be of little
value so long as we do not know what are the determining factors of
variations of this sort. The only suggestion as to the meaning of such
a condition (_i.e._, equal variability at some point)--and our studies
seem to show it for touch and electrical stimulation--which I feel
justified in offering at present, is that parity in variability
indicates equality in strength of stimuli, that is, the electrical
stimulus which has a reaction time of the same variability as a
tactual stimulus has the same effect upon the peripheral nervous
system as the tactual, it produces the same amplitude and perhaps the
same form of wave, but the reaction times for the two stimuli differ
because of the biological significance of the stimuli. The chances are
that this is wholly dependent upon the central nervous system.
IX. SUMMARY.
1. This paper gives the results of some experiments on the frog to
determine its electrical and tactual reaction time. It is the
beginning of comparative reaction-time studies by which it is hoped
important information may be gained concerning the significance and
modes of action of the nervous system. Comparative physiology has
already made clear that the time relations of neural processes deserve
careful study.
2. According to the strength of the stimulus, electric stimulation of
the frog causes three types of reaction: (1) A very weak or threshold
stimulus results in a deliberate or delayed reaction, the time of
which may be anywhere from 300[sigma] (thousandths of a second) to
2,000[sigma]. (2) A very strong stimulus causes a spinal reflex, whose
time is from 50 to 80[sigma]; and (3) a stimulus of intermediate
strength causes a quick instinctive reaction of from 150 to 170[sigma]
in duration.
3. The reaction time for electric stimuli whose relative values were
1, 2 and 4 were found to be 300.9[sigma], 231.5[sigma] and
103.1[sigma].
4. The reaction time of the frog to a tactual stimulus (contact of a
rubber point) is about 200[sigma].
5. The variability of reaction times of the frog is great, and
increases as the strength of the stimulus decreases.
6. When two kinds of stimuli (_e.g._, electrical and tactual) give
reaction times of equal variability, I consider them directly
comparable.
7. According to this criterion of comparability the reaction time to
electric stimulation which is comparable with that to tactual is
172.1[sigma]; and it is to be compared with 205.7[sigma]. Both of
these have a variability of approximately 34[sigma]. On this basis one
may say that the tactual reaction time is considerably longer than the
electrical.