SOME GENERAL CONDITIONS OF NEURAL ACTIVITY.
=The Nervous Discharge.=--The word discharge is constantly used, and must
be used in this book, to designate the escape of a current downwards
into muscles or other internal organs. The reader must not understand
the word figuratively. From the point of view of dynamics the passage of
a current out of a motor cell is probably altogether analogous to the
explosion of a gun. The matter of the cell is in a state of internal
tension, which the incoming current resolves, tumbling the molecules
into a more stable equilibrium and liberating an amount of energy which
starts the current of the outgoing fibre. This current is stronger than
that of the incoming fibre. When it reaches the muscle it produces an
analogous disintegration of pent-up molecules and the result is a
stronger effect still. Matteuci found that the work done by a muscle's
contraction was 27,000 times greater than that done by the galvanic
current which stimulated its motor nerve. When a frog's leg-muscle is
made to contract, first directly, by stimulation of its motor nerve, and
second reflexly, by stimulation of a sensory nerve, it is found that the
reflex way requires a stronger current and is more tardy, but that the
contraction is stronger when it does occur. These facts prove that the
cells in the spinal cord through which the reflex takes place offer a
resistance which has first to be overcome, but that a relatively violent
outward current outwards then escapes from them. What is this but an
explosive discharge on a minute scale?
=Reaction-time.=--The measurement of the time required for the discharge
is one of the lines of experimental investigation most diligently
followed of late years. Helmholtz led the way by discovering the
rapidity of the outgoing current in the sciatic nerve of the frog. The
methods he used were soon applied to sensory reactions, and the results
caused much popular admiration when described as measurements of the
'velocity of thought.' The phrase 'quick as thought' had from time
immemorial signified all that was wonderful and elusive of determination
in the line of speed; and the way in which Science laid her doomful hand
upon this mystery reminded people of the day when Franklin first
'_eripuit cœlo fulmen_,' foreshadowing the reign of a newer and
colder race of gods. I may say, however, immediately, that the phrase
'velocity of _thought_' is misleading, for it is by no means clear in
any of the cases what particular act of thought occurs during the time
which is measured. What the times in question really represent is the
total duration of certain _reactions upon stimuli_. Certain of the
conditions of the reaction are prepared beforehand; they consist in the
assumption of those motor and sensory tensions which we name the
expectant state. Just what happens during the actual time occupied by
the reaction (in other words, just what is added to the preëxistent
tensions to produce the actual discharge) is not made out at present,
either from the neural or from the mental point of view.
The method is essentially the same in all these investigations. A signal
of some sort is communicated to the subject, and at the same instant
records itself on a time-registering apparatus. The subject then makes a
muscular movement of some sort, which is the 'reaction,' and which also
records itself automatically. The time found to have elapsed between the
two records is the total time of that reaction. The time-registering
instruments are of various types. One type is that of the revolving drum
covered with smoked paper, on which one electric pen traces a line which
the signal breaks and the 'reaction' draws again; whilst another
electric pen (connected with a rod of metal vibrating at a known rate)
traces alongside of the former line a 'time-line' of which each
undulation or link stands for a certain fraction of a second, and
against which the break in the reaction-line can be measured. Compare
Fig. 49, where the line is broken by the signal at the first arrow, and
continued again by the reaction at the second. The machine most often
used is Hipp's chronoscopic clock. The hands are placed at zero, the
signal starts them (by an electric connection), and the reaction stops
them. The duration of their movement, down to 1000ths of a second, is
then read off from the dial-plates.
[Illustration: FIG. 49.]
=Simple Reactions.=--It is found that the reaction-time differs in the
same person according to the direction of his expectant attention. If he
thinks as little as possible of the movement which he is to make, and
concentrates his mind upon the signal to be received, it is longer; if,
on the contrary, he bends his mind exclusively upon the muscular
response, it is shorter. Lange, who first noticed this fact when working
in Wundt's laboratory, found his own 'muscular' reaction-time to average
0´´.123, whilst his 'sensorial' reaction-time averaged as much as
0´´.230. It is obvious that experiments, to have any _comparative_
value, must always be made according to the 'muscular' method, which
reduces the figure to its minimum and makes it more constant. In general
it lies between one and two tenths of a second. It seems to me that
under these circumstances the reaction is essentially a reflex act. The
preliminary _making-ready_ of the muscles for the movement means the
excitement of the paths of discharge to a point just short of actual
discharge before the signal comes in. In other words, it means the
temporary formation of a real 'reflex-arc' in the centres, through which
the incoming current instantly can pour out again. But when, on the
other hand, the expectant attention is exclusively addressed to the
signal, the excitement of the motor tracts can only begin after this
latter has come in, and under this condition the reaction takes more
time. In the hair-trigger condition in which we stand when making
reactions by the 'muscular' method, we sometimes respond to a wrong
signal, especially if it be of the same _kind_ with the one we expect.
The signal is but the spark which touches off a train already laid.
There is no thought in the matter; the hand jerks by an involuntary
start.
These experiments are thus in no sense measurements of the swiftness of
_thought_. Only when we complicate them is there a chance for anything
like an intellectual operation to occur. They may be complicated in
various ways. The reaction may be withheld until the signal has
consciously awakened a distinct idea (Wundt's discrimination-time,
association-time), and may then be performed. Or there may be a variety
of possible signals, each with a different reaction assigned to it, and
the reacter may be uncertain which one he is about to receive. The
reaction would then hardly seem to occur without a preliminary
recognition and choice. Even here, however, the discrimination and
choice are widely different from the intellectual operations of which we
are ordinarily conscious under those names. Meanwhile the simple
reaction-time remains as the starting point of all these superinduced
complications, and its own variations must be briefly passed in review.
The reaction-time varies with the _individual_ and his _age_. Old and
uncultivated people have it long (nearly a second, in an old pauper
observed by Exner). Children have it long (half a second, according to
Herzen).
_Practice_ shortens it to a quantity which is for each individual a
minimum beyond which no farther reduction can be made. The aforesaid old
pauper's time was, after much practice, reduced to 0.1866 sec.
_Fatigue_ lengthens it, and _concentration of attention_ shortens it.
The _nature of the signal_ makes it vary. I here bring together the
averages which have been obtained by some observers:
Hirsch. Hankel. Exner. Wundt.
Sound 0.149 0.1505 0.1360 0.167
Light 0.200 0.2246 0.1506 0.222
Touch 0.182 0.1546 0.1337 0.213
It will be observed that _sound_ is more promptly reacted on than either
_sight_ or _touch_. _Taste_ and _smell_ are slower than either. The
_intensity of the signal_ makes a difference. The intenser the stimulus
the shorter the time. Herzen compared the reaction from a _corn_ on the
toe with that from the skin of the hand of the same subject. The two
places were stimulated simultaneously, and the subject tried to react
simultaneously with both hand and foot, but the foot always went
quickest. When the sound skin of the foot was touched instead of the
corn, it was the hand which always reacted first. _Intoxicants_ on the
whole lengthen the time, but much depends on the dose.
=Complicated Reactions.=--These occur when some kind of intellectual
operation accompanies the reaction. The rational place in which to
report of them would be under the head of the various intellectual
operations concerned. But certain persons prefer to see all these
measurements bunched together regardless of context; so, to meet their
views, I give the complicated reactions here.
When we have to think before reacting it is obvious that there is no
definite reaction-time of which we can talk--it all depends on how long
we think. The only times we can measure are the _minimum_ times of
certain determinate and very simple intellectual operations. The _time
required for discrimination_ has thus been made a subject of
experimental measurement. Wundt calls it _Unterscheidungszeit_. His
subjects (whose simple reaction-time 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 _excess_ of
time occupied by these reactions _over the simple reaction-time_, in
which only one signal was used and known in advance, 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. When two were used (the signals being the sudden
appearance of a black or of a white object), the average times of three
observers were respectively (in seconds)
0.050 0.047 0.079
When four signals were used, a red and a green light being added to the
others, it became, for the same observers,
0.157 0.073 0.132
Prof. Cattell found he could get no results by this method, 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. Reacting in this way, Prof. Cattell found the
increment of time required for distinguishing a white signal from no
signal to be, in two observers,
0.030 and 0.050;
that for distinguishing one color from another was similarly
0.100 and 0.110;
that for distinguishing a certain color from ten other colors,
0.105 and 0.117;
that for distinguishing the letter A in ordinary print from the letter
Z,
0.142 and 0.137;
that for distinguishing a given letter from all the rest of the alphabet
(not reacting until that letter appeared),
0.119 and 0.116;
that for distinguishing a word from any of twenty-five other words, from
0.118 to 0.158 sec.
--the difference depending on the length of the words and the
familiarity of the language to which they belonged.
Prof. Cattell calls attention to the fact that the time for
distinguishing a word is often but little more than that for
distinguishing a letter: "We do not, therefore," he says, "distinguish
separately the letters of which a word is composed, but the word as a
whole. The application of this in teaching children to read is evident."
He also finds a great difference in the time with which various letters
are distinguished, E being particularly bad.
_The time required for association_ of one idea with another has been
measured. Gallon, using a very simple apparatus, found that the sight of
an unforeseen word would awaken an associated 'idea' in about ⅚ of a
second. Wundt next made determinations in which the 'cue' was given by
single-syllabled words called out by an assistant. The person
experimented on had to press a key as soon as the sound of the word
awakened an associated idea. Both word and reaction were
chronographically registered, and the total time-interval between the
two amounted, in four observers, to 1.009, 0.896, 1.037, and 1.154
seconds respectively. From this the simple reaction-time and the time of
merely identifying the word's sound (the 'apperception-time,' as Wundt
calls it) must be subtracted, to get the exact time required for the
associated idea to arise. These times were separately determined and
subtracted. The difference, called by Wundt _association-time_,
amounted, in the same four persons, to 706, 723, 752, and 874
thousandths of a second respectively. The length of the last figure is
due to the fact that the person reacting was an American, whose
associations with German words would naturally be slower than those of
natives. The shortest association-time noted was when the word 'Sturm'
suggested to Wundt the word 'Wind' in 0.341 second. Prof. Cattell made
some interesting observations upon the association-time between the look
of letters and their names. "I pasted letters," he says, "on a revolving
drum, and determined at what rate they could be read aloud as they
passed by a slit in a screen." He found it to vary according as one, or
more than one, letter was visible at a time through the slit, and gives
half a second as about the time which it takes to see and name a single
letter seen alone. The rapidity of a man's _reading_ is of course a
measure of that of his associations, since each seen word must call up
its name, at least, ere it is read. "I find," says Prof. Cattell, "that
it takes about twice as long to read (aloud, as fast as possible) words
which have no connection, as words which make sentences, and letters
which have no connection, as letters which make words. When the words
make sentences and the letters words, not only do the processes of
seeing and naming overlap, but by one mental effort the subject can
recognize a whole group of words or letters, and by one will-act choose
the motions to be made in naming, so that the rate at which the words
and letters are read is really only limited by the maximum rapidity at
which the speech-organs can be moved.... For example, when reading as
fast as possible the writer's rate was, English 138, French 167, German
250, Italian 327, Latin 434, and Greek 484; the figures giving the
thousandths of a second taken to read each word. Experiments made on
others strikingly confirm these results. The subject does not know that
he is reading the foreign language more slowly than his own; this
explains why foreigners seem to talk so fast....
"The time required to see and name colors and pictures of objects was
determined in the same way. The time was found to be about the same
(over ½ sec.) for colors as for pictures, and about twice as long as for
words and letters. Other experiments I have made show that we can
recognize a single color or picture in a slightly shorter time than a
word or letter, but take longer to name it. This is because, in the case
of words and letters, the association between the idea and the name has
taken place so often that the process has become automatic, whereas in
the case of colors and pictures we must by a voluntary effort choose the
name."
Dr. Romanes has found "astonishing differences in the _maximum_ rate of
reading which is possible to different individuals, all of whom have
been accustomed to extensive reading. That is to say, the difference may
amount to 4 to 1; or, otherwise stated, in a given time one individual
may be able to read four times as much as another. Moreover, it appeared
that there was no relationship between slowness of reading and power of
assimilation; on the contrary, when all the efforts are directed to
assimilating as much as possible in a given time, the rapid readers (as
shown by their written notes) usually give a better account of the
portions of the paragraph which have been compassed by the slow readers
than the latter are able to give; and the most rapid reader I have found
is also the best at assimilating. I should further say," Dr. R.
continues, "that there is no relationship between rapidity of perception
as thus tested and intellectual activity as tested by the general
results of intellectual work; for I have tried the experiment with
several highly distinguished men in science and literature, most of whom
I found to be slow readers."
_The degree of concentration of the attention_ has much to do with
determining the reaction-time. Anything which baffles or distracts us
beforehand, or startles us in the signal, makes the time proportionally
long.
=The Summation of Stimuli.=--Throughout the nerve-centres it is a law that
_a stimulus which would be inadequate by itself to excite a nerve-centre
to effective discharge may, by acting with one or more other stimuli
(equally ineffectual by themselves alone) bring the discharge about_.
The natural way to consider this is as a summation of tensions which at
last overcome a resistance. The first of them produce a 'latent
excitement' or a 'heightened irritability'--the phrase is immaterial so
far as practical consequences go;--the last is the straw which breaks
the camel's back.
This is proved by many physiological experiments which cannot here be
detailed; but outside of the laboratory we constantly apply the law of
summation in our practical appeals. If a car-horse balks, the final way
of starting him is by applying a number of customary incitements at
once. If the driver uses reins and voice, if one bystander pulls at his
head, another lashes his hind-quarters, the conductor rings the bell,
and the dismounted passengers shove the car, all at the same moment, his
obstinacy generally yields, and he goes on his way rejoicing. If we are
striving to remember a lost name or fact, we think of as many 'cues' as
possible, so that by their joint action they may recall what no one of
them can recall alone. The sight of a dead prey will often not stimulate
a beast to pursuit, but if the sight of movement be added to that of
form, pursuit occurs. "Brücke noted that his brainless hen which made no
attempt to peck at the grain under her very eyes, began pecking if the
grain were thrown on the ground with force, so as to produce a rattling
sound." "Dr. Allen Thomson hatched out some chickens on a carpet, where
he kept them for several days. They showed no inclination to scrape, ...
but when Dr. Thomson sprinkled a little gravel on the carpet, ... the
chickens immediately began their scraping movements." A strange person,
and darkness, are both of them stimuli to fear and mistrust in dogs (and
for the matter of that, in men). Neither circumstance alone may awaken
outward manifestations, but together, i.e. when the strange man is met
in the dark, the dog will be excited to violent defiance. Street hawkers
well know the efficacy of summation, for they arrange themselves in a
line on the sidewalk, and the passer often buys from the last one of
them, through the effect of the reiterated solicitation, what he refused
to buy from the first in the row.
[Illustration: FIG. 50.--Sphygmographic pulse-tracing. _A_, during
intellectual repose; _B_, during intellectual activity. (Mosso.)]
=Cerebral Blood-supply.=--All parts of the cortex, when electrically
excited, produce alterations both of respiration and circulation. The
blood-pressure somewhat rises, as a rule, all over the body, no matter
where the cortical irritation is applied, though the motor zone is the
most sensitive region for the purpose. Slowing and quickening of the
heart are also observed. Mosso, using his 'plethysmograph' as an
indicator, discovered that the blood-supply to the arms diminished
during intellectual activity, and found furthermore that the arterial
tension (as shown by the sphygmograph) was increased in these members
(see Fig. 50). So slight an emotion as that produced by the entrance of
Professor Ludwig into the laboratory was instantly followed by a
shrinkage of the arms. The brain itself is an excessively vascular
organ, a sponge full of blood, in fact; and another of Mosso's
inventions showed that when less blood went to the legs, more went to
the head. The subject to be observed lay on a delicately balanced table
which could tip downward either at the head or at the foot if the weight
of either end were increased. The moment emotional or intellectual
activity began in the subject, down went the head-end, in consequence of
the redistribution of blood in his system. But the best proof of the
immediate afflux of blood to the brain during mental activity is due to
Mosso's observations on three persons whose brain had been laid bare by
lesion of the skull. By means of apparatus described in his book, this
physiologist was enabled to let the brain-pulse record itself directly
by a tracing. The intra-cranial blood-pressure rose immediately whenever
the subject was spoken to, or when he began to think actively, as in
solving a problem in mental arithmetic. Mosso gives in his work a large
number of reproductions of tracings which show the instantaneity of the
change of blood-supply, whenever the mental activity was quickened by
any cause whatever, intellectual or emotional. He relates of his female
subject that one day whilst tracing her brain-pulse he observed a sudden
rise with no apparent outer or inner cause. She however confessed to him
afterwards that at that moment she had caught sight of a _skull_ on top
of a piece of furniture in the room, and that this had given her a
slight emotion.
=Cerebral Thermometry.=--_Brain-activity seems accompanied by a local
disengagement of heat._ The earliest careful work in this direction was
by Dr. J. S. Lombard in 1867. He noted the changes in delicate
thermometers and electric piles placed against the scalp in human
beings, and found that any intellectual effort, such as computing,
composing, reciting poetry silently or aloud, and especially that
emotional excitement such as an angry fit, caused a general rise of
temperature, which rarely exceeded a degree Fahrenheit. In 1870 the
indefatigable Schiff took up the subject, experimenting on live dogs and
chickens by plunging thermo-electric needles into the substance of their
brain. After habituation was established, he tested the animals with
various sensations, tactile, optic, olfactory, and auditory. He found
very regularly an abrupt alteration of the intra-cerebral temperature.
When, for instance, he presented an empty roll of paper to the nose of
his dog as it lay motionless, there was a small deflection, but when a
piece of meat was in the paper the deflection was much greater. Schiff
concluded from these and other experiments that sensorial activity heats
the brain-tissue, but he did not try to localize the increment of heat
beyond finding that it was in both hemispheres, whatever might be the
sensation applied. Dr. Amidon in 1880 made a farther step forward, in
localizing the heat produced by voluntary muscular contractions.
Applying a number of delicate surface-thermometers simultaneously
against the scalp, he found that when different muscles of the body were
made to contract vigorously for ten minutes or more, different regions
of the scalp rose in temperature, that the regions were well focalized,
and that the rise of temperature was often considerably over a
Fahrenheit degree. To a large extent these regions correspond to the
centres for the same movements assigned by Ferrier and others on other
grounds; only they cover more of the skull.
=Phosphorus and Thought.=--Considering the large amount of popular
nonsense which passes current on this subject I may be pardoned for a
brief mention of it here. _'Ohne Phosphor, kein Gedanke_,' was a noted
war-cry of the 'materialists' during the excitement on that subject
which filled Germany in the '60s. The brain, like every other organ of
the body, contains phosphorus, and a score of other chemicals besides.
Why the phosphorus should be picked out as its essence, no one knows. It
would be equally true to say, 'Ohne Wasser, kein Gedanke,' or 'Ohne
Kochsalz, kein Gedanke'; for thought would stop as quickly if the brain
should dry up or lose its NaCl as if it lost its phosphorus. In America
the phosphorus-delusion has twined itself round a saying quoted (rightly
or wrongly) from Professor L. Agassiz, to the effect that fishermen are
more intelligent than farmers because they eat so much fish, which
contains so much phosphorus. All the alleged facts may be doubted.
The only straight way to ascertain the importance of phosphorus to
thought would be to find whether more is excreted by the brain during
mental activity than during rest. Unfortunately we cannot do this
directly, but can only gauge the amount of PO_{5} in the urine, and this
procedure has been adopted by a variety of observers, some of whom
found the phosphates in the urine diminished, whilst others found them
increased, by intellectual work. On the whole, it is impossible to trace
any constant relation. In maniacal excitement less phosphorus than usual
seems to be excreted. More is excreted during sleep. The fact that
phosphorus-preparations may do good in nervous exhaustion proves nothing
as to the part played by phosphorus in mental activity. Like iron,
arsenic, and other remedies it is a stimulant or tonic, of whose
intimate workings in the system we know absolutely nothing, and which
moreover does good in an extremely small number of the cases in which it
is prescribed.
The phosphorus-philosophers have often compared thought to a secretion.
"The brain secretes thought, as the kidneys secrete urine, or as the
liver secretes bile," are phrases which one sometimes hears. The lame
analogy need hardly be pointed out. The materials which the brain _pours
into the blood_ (cholesterin, creatin, xanthin, or whatever they may be)
are the analogues of the urine and the bile, being in fact real material
excreta. As far as these matters go, the brain is a ductless gland. But
we know of nothing connected with liver-and kidney-activity which can be
in the remotest degree compared with the stream of thought that
accompanies the brain's material secretions.