Watson, Sr., R.I. (1978). The great psychologists. (4th
edition). New York: J.B. Lippincott Co.
CHAPTER 11
HELMHOLTZ:
NEURAL PHYSIOLOGY
One of the sources from which experimental psychology was to
emerge was experimental physiology, the study of the functioning
of the organ systems of the body. It is customary to say that
general physiology became a separate discipline with the appointment
of Johannes Muller to the first professorship of physiology in
1833 at the University of Berlin. Here between 1834 and 1840 Muller
compiled his Handbuch der Physiologie des Menchen,1 a systematic
organization of comparative anatomy, chemistry, and physics, as
related to physiology. More pertinent to psychology at this time,
however, was the study by physiologists of the nervous system.
In this field the giant among pioneer neural physiologists was
Hermann von Helmholtz. But, as always, the way was prepared by
lesser men.
PHYSIOLOGY BEFORE HELMHOLTZ
In the eighteenth century, during the time of Hartley and Hume,
some research on reflex action was being carried on. According
to Whytt (17161766), the "fundamental" experiment on
the nature and existence of the reflex had been performed about
1730 by Stephen Hales, who had found that a decapitated frog which
had responded to pinching by withdrawing the legs, failed to continue
to do so when the spinal cord was destroyed 2 Whytt, himself,
repeated and extended this experiment, in the course of which
he introduced the terms "stimulus," and "response."
Unzer (17271799) in 1771 utilized the word, "reflex"
to distinguish between this kind of action and that carried on
volitionally.
The first half of the nineteenth century yielded a variety of
developments firmly establishing physiology as a science. Simultaneous
with these was the rise of phrenology, which, pseudoscientific
though it may have been, served to bring to the attention of the
general public the mind and its relation to the brain.
The GREAT PSYCHOLOGISTS
The Bell-Magendie Law
During the years 1811-1822 Charles Bell (1774-1842 ) in Great
Britain and Francois Magendie (1783-1855 ) in France, working
independently, performed the research which distinguished between
the sensory and motor nerves. In a general way this distinction
had been known to Galen, and others had from time to time rediscovered
it. Prior to the work in question, however, the distinction had
been lost sight of and the nerves were conceived as if all of
them more or less indiscriminately carried on both motor and sensory
functions. A controversy developed over the priority of the work
of Bell and Magendie.3, 4. s Bell published first, but it is not
entirely clear that his original paper reported what he later
claimed for it; while Magendie, when he published eleven years
after Bell, stated the matter with unequivocal clarity.6 The important
thing, after all, is that through their work a clearcut research
distinction had been made.
Magendie established the distinction between motor and sensory
nerves by cutting the nerve roots at the spinal cord and studying
the functions that were lost. He found that it is through the
posterior (dorsal) roots of the spinal cord that sensory fibers
enter, while the anterior ( ventral ) roots are the route through
which the motor fibers leave the spinal cord. For instance, when
Magendie cut an anterior root, he found a complete paralysis,
which was not present when the posterior root was cut. Since movement
was prevented by this cutting, it must have been the anterior
root that had to do the movement.
One effect of this work was to separate clearly neural physiology
into the study of sensory and of motor functions or, to use more
obviously psychological terminology, to establish a distinction
between that of sensation and of movement. This distinction between
sensory and motor nerves, according to what in time came to be
called the Bell-Magendie law, is still useful today in bringing
order into the study of physiology and psychology.
Gall and Phrenology
At about the same time as Bell and Magendie made their important
contributions, the long-lived craze of phrenology was rampant.
Franz Joseph Gall (1758-1828 ), first in Vienna and then in Paris,
had been working for many years on the localization of the functions
of the brain. Some of his early work was straightforward and competent;
he attempted to relate functioning of specific areas of the brain
to particular sensory and motor activities. However, he goes down
in history as the originator of the pseudoscientific theory of
phrenology.
While still a boy, Gall had been impressed by what he thought
was a relationship between prominent mental characteristics of
his schoolmates and the shape of their skulls.' As an anatomist,
years later, when he embarked on mapping out the location of the
centers for various functions on the surface of the cerebrum,
the cerebral cortex, he tried to verify this impression. He reasoned
that if a region of the cortex was well developed there would
be a characteristic bump or profusion of the skull at this point,
or a comparable hollow if underdeveloped. Gall collected instances
of what appeared to be over- or underdevelopment of areas of the
brain as reflected in the shape of the skull and formed an impression
of their possessors' striking characteristics. He did this by
seeking out instances where he thought he might find them. Among
pick pockets from the prisons he found the bump of acquisitiveness.
Later he extended his search among friends and public figures!
He also sought out persons who seemed to show a same trait to
a striking degree and received permission to search their skulls
for bumps and hollows. Once he found some sort of correspondence
between one, two or a few individuals in the possession of the
certain characteristic and their bumps or hallows, he named it,
and assumed that when the bump was found thereafter the characteristic
would be prominent.
From a modern perspective one is struck by the glaring inadequacy
of the method that Gall used.8 Today, any college sophomore in
psychology would be able to tell him that his research called
for taking an "unselected" sample of the population,
measuring all of their bumps, and without knowledge of these measurements,
estimating their standing on the entire list of psychological
characteristics and then comparing the two streams of data for
the degree and nature of the relationship that they showed.
It was not long, however, before Gall's teaching excited popular
attention, and soon he had disciples and practitioners, including
one Spurzheim, who did much thereafter to extend the list of characteristics
and to spread the phrenological doctrine. When the mapping was
completed, each characteristic had a definite place of localization
in an area of the cerebral cortex. Thus, with the aid of a chart,
the phrenologist could then examine the subject's skull and from
what he found plot the individual's strength and weaknesses in
abilities and personality.
Phrenology was a faculty psychology to end all faculty psychologies
with an eventual formulation by Spurzheim9 of thirty-seven characteristics
or faculties. He divided them into affective and intellectual
faculties and further subdivided these two classes into propensities
and sentiments, and perceptive and reflective faculties. Some
of the affective faculties were destructiveness, amativeness,
self-esteem and benevolence, while the intellectual capacities
included calculation, order, and causality. So far as the particular
faculties were concerned, Spurzheim merely borrowed the lists
of Thomas Reid and Dugald Stewart of the Scottish School (page
192) . As in all faculty psychologies, the major task was naming
the function which, once done, made further analysis superfluous.
Thus, it served as a block to further advancements.
Phrenology was never generally accepted by scientists even while
it was still possible to hold this implausible hypothesis. It
was opposed by such familiar figures as Charles Bell and Thomas
Brown in Great Britain and Flourens in France.10
Spurzheim, Gall's erstwhile collaborator, was most active in
spreading phrenological views in England and in the United States.
His visit to the United States in 1832 produced considerable interest
among physicians practicing in the mental hospitals." Rather
than necessarily accepting his views en toto psychiatrists found
that phrenology offered them general guidance in their thinking
about the abnormal functioning of the mind as it depends upon
the condition of specific areas of the brain. To this extent phrenology
was not entirely without value, and its appeal died hard for acceptance
continued, despite the scientific opposition, and lasted even
into the present century. A phrenological journal first appearing
in 1823 did not expire until 1911.12 The display of phrenological
charts with their neat squares superimposed on an outline of the
head still occurs at the booths of fortune tellers at amusement
parks and fairs.
Phrenology, wrong as it was, at least made the man in the street
aware he had a brain. Even more important, it stimulated scientists
to take up research on the brain and established, once and for
all, that the brain was the organ of the mind.
Flourens and Localization o f Function in the Cerebrum
One of Gall's sharpest critics was Pierre Flourens (1794-1867),
who was also engaged in investigating the function of various
parts of the brain. His research led him to the conclusion that
removal of the cerebrum, while leaving the reflexes intact, abolishes
thought and volition.13 Moreover, he concluded that the cerebrum,
as a whole, not limited portions of it, is responsible for all
thought and volition. Other functions he localized in the other
major parts of the brain, the cerebellum, and the medulla oblongata,
for example, but each part functioned as a whole. This theory
was, of course, directed against phrenology since the phrenologists
localized each of their thirty-seven faculties in definite parts
of the cerebrum. For years thereafter, Flourens' view of the cerebrum
functioning as a whole prevailed, if anything, strengthened by
being in opposition to the phrenologists.
It was not until 1861 that Broca14 showed that the loss of speech
was due to a lesion "of the posterior part of the third frontal
convolution of the left hemisphere." This was the first successful
challenge to Flourens' doctrine of the unity of the cerebrum,
since Broca had found a localization of function within a specific
area of the cerebrum. That speech was too complicated a mechanism
to be confined to one specific region, as later research showed,
does not detract from this first demonstration of specific localization.
Evidence concerning the reality of localizations of function within
the cerebrum in a very complicated interlocking fashion is well
established today. Needless to say, the functions isolated, bear
no resem blance to those of the phrenologist.
Muller and the Specific Energy o f Nerves
In 1833 Johannes Muller (1801-1858 ) was named to the just-created
Chair of Physiology at Berlin. Previously it had been combined
with that of anatomy. His appointment signalized the recognition
of physiology as an independent sphere of science. His major work,
Handbuch der Physaologie des Menchen,15 which appeared between
1834-1840, systematized and exhaustively summarized the knowledge
of physiology of his time in a primarily inductive fashion of
which Bacon would have approved. This work served to place general
physiology upon a scientific footing. In it, he brought comparative
chemistry and physics to bear upon physiological problems. Miller
may be said to have done for general physiology what Helmholtz
in the next generation was to do for neural physiology.
The most direct contribution of Muller to psychology was his
doctrine of the specific energy of nerves.16 He arrived at the
conclusion that each sensory nerve, however stimulated, gives
rise to only one type of sensory process, and no other. l' The
sensation that is experienced is due, not to the nature of the
stimulus, but to the sense organ, nerve or brain center that is
stimulated. For example, the optic nerve always responds by the
sensation of light whether it is stimulated in the usual fashion
or it is pinched, heated, irritated by acid or shocked by electric
current. (An easily verified every-day illustration is that of
pressing with the thumb against the closed eye, and by so doing,
producing a flood of light). Miller was saying that every sensory
nerve responds in its own characteristic way, no matter how stimulated.
To paraphrase a statement of James,18 if we were to interchange
surgically the optic and auditory nerves, we would hear the lightning
and see the thunder. Muller a Kantian, saw his doctrine as unequivocal
support for nativism since what is more innate than the nervous
system itself? This, however, should not deter us, for in a moment
we shall find Helmholtz using the same doctrine to support empiricism.
Where does the specificity arise? Miller considered seriously
two possibilities, either that it was in the nerves themselves
or in the centers in the brain where the nerves terminate. Both
views seemed to him to be defensible, but he decided in favor
of the specific energies being in the nerves themselves. In view
of Flourens' researches bringing discredit upon the doctrine of
cortical localization, this conclusion is hardly surprising. Miller's
version of specific energies was consequently not brought into
question by the arguments concerning brain localization. Nowadays,
with increased knowledge, we would place the specificity in the
brain, not in the nerves.
Despite some adverse evidence,19 it is still argued today that
"specific energy" is the basic principle governing the
physiology of the special senses. To place Muller's doctrine in
its broadest perspective is to say that we are not aware of objects
directly but only through some form of intermediary, in this case,
the sense organs and nerves. This principle of knowing only through
an intermediary was understood since Herophilus and the British
associationists had, in their various interpretations, made much
of the fact. Muller, however, placed it upon a scientifically
solid footing in a way that still prevails.
Donders and Reaction Time
The speed of reaction had been a research problem ever since
the end of the eighteenth century when the Astronomer Royal at
Greenwich dismissed his assistant who was apparently making errors
in observing the time at which stars crossed the meridian.20 A
moment's reflection can show that this is a very serious matter
because the astronomical findings were calibrated by this observation.
This astronomer not only dismissed his assistant but wrote and
published a short account of the incident.
A few years later Bessel, an astronomer at Helmholtz's own University
of Konigsberg, came upon the account of the incident, and realized
that the unlucky assistant might not be unique and that astronomers
in general might differ in accuracy of their observation of the
times of stellar events. When structured this way, other astronomers
agreed with Bessel and the "personal equation," as it
came to be called, became a matter of intense and widespread interest
among them. A variety of methods of comparison of the personal
equation of one astronomer to another came into vogue. That is,
relative personal equations were established-astronomer A was
shown to be one-tenth of a second slower than B, but twotenths
of a second faster than C in carrying out his stellar observations.
A constant correction, the personal equation of B - A - .10 second
could then be used in collating their astronomical observations.
After the discovery of electric currents and electromagnets these
relative personal equations became absolute measures through the
invention and perfection of the chronograph and chronoscope. The
chronograph was an instrument which in pressing a key a mark is
placed upon a drum which also has constant marks placed upon it
by a tuning fork vibrating 500 times a second. It measured the
time accurately to less than a tenth of a second. Technical advances
continued rapidly. By 1862 an instrument, the Hipp Chronoscope,
was being used to measure the personal equation to a thousandth
of a second. The measurements these instruments supplied could
be now stated in absolute terms-A took 300 milliseconds while
B took 200 milliseconds to respond.
The absolute personal equation of the astronomer, when adapted
to physiological tasks, came to be called reaction time. Donders,
a Dutch physiologist, a contemporary of Helmholtz, became interested,
and in 1868 extended its study by going beyond simple reaction
time or the reaction to a predetermined stimulus by a given predetermined
response. For example, a reaction already agreed on, pressing
a key, would be made when a light went on. The elapsed time was
simple reaction time. Donders asked whether, if this reaction
was made more complicated, could not the increased time taken
to react be attributed to that which was added to complicate the
reaction? He performed experiments to study "choice"
and "discrimination" times.
Although he studied them in the reverse order, it is clearer
to start with discrimination. Simple reaction time had called
for subjects to respond to stimulus A with response a. Donders
presented several stimuli A, B, C, D in irregular order but his
subjects were instructed to respond only to A, not to the others.
Consequently, they had to discriminate A from all of the other
stimuli before responding with a. Suppose simple reaction time
had been measured by the subject knowing in advance that when
a red light flashed on A he was to respond a. No other light than
the red was used in this phase of the study. In the new discrimination
reaction his reaction, a was to be made to a red light, but he
was to withhold his reaction if a green light, B, or a yellow
light, C, was flashed. As he predicted, his subjects took longer
to make the discrimination reaction than they did to make the
simple reaction. Subtracting simple reaction time from the discrimination
reaction time gave him the discrimination time. This was called
the subtractive procedure. Choice was even more complicated: subjects
responded to A with a, B with b, C with c, and so on. This gave
a still longer reaction time; the choice time was the total time
minus the discrimination and simple reaction times. From the measures
Donders obtained three reaction times-simple, discrimination,
and choice. The very time taken by mental events had been brought
to heel and measured. Thus, the temporal study of various mental
functions=`mental chronometry," as it was called-was launched
to be followed up by Wundt and others later.
The existence of compartmentalized knowledge is a limitation
that most scientists, including psychologists, suffer gladly,
human capabilities being what they are. They find one thing quite
enough to do. The great abilities of Hermann von Helmholtz permitted
him to disregard the convenient but artificial boundaries that
had been set up between the sciences.
By what resembled a process of natural growth, Helmholtz was
led by his research interests through physics, as a co-formulator
of law of the conservation of energy; neural physiology, including
the measurement of the rate of the neural impulse; optics, extending
from the invention of the ophthalmoscope to the theory of color
vision associated with his name; acoustics, extending from Oriental
music to the theory of resonance as the basis for hearing; and
other important, but less relevant work, in hydrodynamics, electrodynamics,
and meteorological physics. In all, he wrote more than 200 papers
and books, a high proportion of which made definite important
contributions to science.
Helmholtz conceived of psychology as a separate discipline, but
as one allied to metaphysics insofar as it ascertained the laws
and nature of the products of the mind 21 He made an exception
of the psychology of the senses because of its close alliance
with physiology. Consequently, he had no hesitation in discussing
psychological issues pertinent to his interests.
Helmholtz believed that an attempt to deduce objective reality
from subjective experience was utterly wrong. Rather, the task
of science is to find and to express in its own way the objective
reality toward which the subjective experience pointed 22 He made
specific his objections to vitalism. Years later, one of his popular
lectures was directed against the delusion of vitalism, particularly
as it was present in medicine. He claimed it was his microscope
that saved him from this source of error.23 Prophetic of a modern
view toward philosophy, Helmholtz would limit it to its critical
function in specifying the nature and conditions of knowledge,
denying its claim to deal with deeper problems, such as the nature
and meaning of reality and the universe 24 Nevertheless, he held
that philosophy does have an important function in supplying a
theory of knowledge.
Helmholtz's own views were more in the tradition of Locke and
Hartley than o£ Leibniz, Kant, or Herbart. He was an empiricist.
With admirable brevity he states his major argument against nativism.
It is not that it is disprovable; it is simply non-parsimonious
25 In spite of his strong empiristic tendencies, he was also influenced
to some extent by Kant. For example, he accepted the law of causality
as a priori and transcendental and not demonstrable from anything
else 2g Causality was not a law of nature but a regulative principle
which guides the scientist in comprehending phenomena.
Life and General Scientific Endeavors o f Helmholtz
Herman Ludwig Ferdinand von Helmholtz27, 28 was born in Potsdam,
Germany, in 1821. His father taught in the Gymnasium. of that
city, and, because of his son's delicate health, at first tutored
him at home; at the age of nine the boy entered the Gymnasium,
proceeding so rapidly as to graduate at seventeen. Lacking the
financial means to study physics, an already formed major interest,
Helmholtz continued his studies at the MedicoChirurgical Friedrich-Wilhelm
Institute in Berlin where no tuition was charged those promising
to serve as surgeons in the army upon graduating. Although Helmholtz
was never a student at the University of Berlin, Johannes Muller,
its Professor of Physiology, was the teacher that had the most
profound effect upon him during these student days. Students under
Muller, who became his friends, included DuBois Reymond, Virchow,
and Brucke. They all admired Muller immensely, but he was of an
older generation and, although he had helped to win physiology
away from the philosophy of nature, still held to the prevailing
vitalistic theory of biological activity. This, his students,
could not accept. The spirit of their attitude is caught in a
solemn oath that Brucke and DuBois Reymond imposed upon themselves
during their student days, pledging to prove and expound the principle
that "no other forces than common physical chemical ones
are active within the organism."29 Such was the temper of
these young scientists, all under thirty, which Helmholtz shared.
Shortly after graduation in 1'842 Helmholtz became an army surgeon
at Potsdam. While carrying on his duties with the military, he
continued his studies in physics and mathematics, and wrote and
published several papers. Already his tremendous energy, which
never left him, was becoming evident.
In 1847, less than five years after his graduation, Helmholtz,
a twentysix-year-old army surgeon, read before the Physical Society
of Berlin his classic paper on the indestructibility of energy,
giving mathematical formulation to the law of conservation. A
few years earlier in 1842, Julius Mayer (1814-1878) had published
a theoretical paper on the topic, along with the method for calculating
the dynamical equivalent of heat 39 J. P. Joule, almost immediately
after Mayer, published the experiments of several years which
he had conducted and which led to a theory substantially similar
to Mayer's. Controversy about priority sprang up. Helmholtz freely
acknowledged their priority. All of these men deserve credit,
Mayer more for the theoretical formulation, Helmholtz more for
its mathematical statement, and joule more for its research verification.
It is worthy of remark that this paper by Helmholtz was in the
spirit of that oath taken by his friends a few years before, since
the theory of the conservation of energy is simultaneously a denial
of the existence of a biological vital force and the substitution
for it of the physical and chemical analysis of energy transformation.
After serving in the army for five years and after a short stint
as instructor in anatomy at a Berlin art school, Helmholtz was
called to Konigsberg as Associate Professor of Physiology. Since
he had a reasonably secure position, he married. He then turned
to what proved to be his second major contribution, the measurement
of the speed of the neural impulse, a topic to which we shall
return presently.
During these years he also worked in physiological optics. In
1851 he invented the ophthalmoscope, which was a concave mirror
with a small hole in the middle through which the observer looked
as he reflected light into the eye of the patient. Once thought
of, this relatively simple device to look into the eye practice.
In 1856 the first volume of the Handbuch der physiologischen
Optik31 appeared, with the last of its three volumes to appear
ten years later. Nearly sixty years later in 1924-25 it32 was
translated into English, not as a classic, but as an indispensable
tool for the serious student of vision.
In 1855 Helmholtz went to Bonn as Professor of Anatomy and Physiology.
It was during this period that the Ministry of Education became
somewhat displeased with his teaching. 33 In his lectures on anatomy
he had the temerity to introduce mathematics! It might be mentioned
in passing that his academic, as distinguished from his popular
lectures, were not always too well prepared. Relatively little
of his time went into their preparation and gaps and errors in
his presentations were not infrequent. 34
Such things are trivial as compared to the magnitude of his scientific
achievements, the next one itself was of enormous value in research
and medical of which centered on acoustics. While still at Konigsberg
he had become interested in the field. During his three years
at Bonn, his first major research on hearing was carried out.
In 1858 during the period in which he centered his attention
on acoustical problems, he moved to an even more important position
in the German university system, the Professorship of Physiology
at Heidelberg, a chair he occupied until 1871. If anything, his
already tremendous productivity increased during this period.
In addition to the research on audition relevant to this particular
account, he showed a remarkable ability to come to grips with
important and crucial problems in hydrodynamics and electrodynamics.
He ranged over many areas. The titles of some of his papers between
1858 and 1871 were on such topics as after-images, color blindness,
the ArabianPersian musical scale, relation between the natural
sciences and the totality of sciences, the form of the horopter,
the movements of the human eye, the regulation of ice, the axioms
of geometry, and hay fever. His acoustical researches culminated
in the appearance of On. the Sensations o f Tone in 1863. just
as did his book on vision, this new book summarized not only his
investigations but sifted, summarized, and systematized the entire
available literature. During this period, honors were showered
on him, invitations to lecture, calls from foreign countries,
and the Prorektorship of the University of Heidelberg among them.
In 1870, when Helmholtz was fifty years of age, the Chair in
Physics at the University of Berlin became vacant and he was asked
to set his own conditions of acceptance. He asked for and received
a salary of 4,000 thalers (a huge sum for that day), a promise
of a new institute of physics, its directorship and living quarters
in that institute.35 Until arriving at Berlin in 1871, Helmholtz
had been somewhat cramped for space and limited as to apparatus.
Now all was changed for he had what, according to Hall.36 were
perhaps the best research facilities in the world, about which
Hehnholtz could speak, without the humorous overtones such an
expression would produce today, as a "temple of physics."
In 1887 he was made the first director of the new Physics-Technical
Institute at Charlottenberg, near Berlin, while still retaining
his professorship.
Such were his interests in espousing empiricism that during the
period extending from 1866-1894, he published five papers on geometrical
axioms with the intention of showing, contrary to Kant, that they,
too, are products of experience.37 He did his job so well that
his discussion of nonEuclidian space made those who came after
him cite his work as evidence for a fourth dimension. This turned
the tables with a vengeance since, of course, this dimension had
never been experienced.
A bizarre consequence of this work was the support some contemporaries
thought it gave to spiritualism. The American medium, Slade, had
been performing such feats as seeming to remove objects from sealed
boxes. From this a fuzzy explanation in terms of the fourth dimension
became popular. It was argued that Slade's feats were capable
of performance because he worked in the "fourth dimension"
where it would be as easy to remove an object from a sealed box
as it was in the world of three dimensions to lift an object out
of a square. Such ran the argument which various scholarly figures
in Germany hotly and acrimoniously debated. In the course of this
Helmholtz was blamed for creating a scientific scandal. Such was
the passion aroused that one professor was dismissed from Berlin
because of his accusations against Helmholtz.
Although he continued his interests in physiological problems,
revised his books on hearing and vision, and made that spirited
defense of empiricism mentioned earlier, his researches during
these years tended to center on problems of physics less close
to present interests. For example, he helped to direct his pupil,
Hertz, to the problems which made a crucial contribution to the
founding of wireless telegraphy and radio. He developed popular
and elaborate lecture demonstrations and continued to give scientific
lectures designed to be of popular nature of which a total of
four different collections appeared in print. In 1877 academic
freedom was the basis of one of his popular lectures in which
the "admirable" state of German universities, where,
having thrown off the yoke of the Church, its professors taught
unhindered,38 was contrasted with the lowly backwardness of British
universities which had not made these advances. His first wife
having died, he married again in 1861. His second wife made their
home in Berlin something of a salon for scientific, artistic,
and literary figures where, unlike his usual taciturn laboratory
self, Helmholtz managed to unbend quite charmingly. In society
he was such a success as to become a favorite with the Imperial
family. In 1893 Helmholtz came to the United States for the first
and only time to attend the Chicago World's Fair as a delegate
from the government of Germany. While returning, he had a severe
fall down the ship's stairs from which he never fully recovered.
He died from a cerebral hemorrhage in September 1894.
The Speed of the Neural Impulse
The scientific opinions prevailing until the research of Helmholtz
was either that the speed of the neural impulse was instantaneous
or at least so fast as to be incapable of measurement. The latter
view was one in which even the great Muller concurred. Estimates
of speeds many times the velocity of light had been obtained by
assuming that the rate of flow of "animal spirits" would
be similar for vessels of the same size and that speed varied
inversely with the diameter of the conductor. Since the diameters
of nerve fibers were very, very minute, indications of tremendous
speeds were obtained. The method used by Helmholtz, which would
have been entirely inadequate had the previous estimates been
true, was to take a motor nerve and attached muscle from a frog's
leg, the so-called "nerve-muscle preparation," and arrange
it so that both the moment of stimulation and the resultant movement
would be recorded on a drum revolving at a known speed. Helmholtz
measured the time between stimulation and the muscle twitch for
different lengths of nerve. The difference in time interval between
stimulation of the nerve near the muscle and its recorded reaction
and that for stimulation far from the muscle gave him the time
taken for passage. Since he knew the distance between the points
of stimulation, he could now calculate its speed per second. Helmholtz
found the speed to be the very modest one of about ninety feet
per second.
The Bell-Magendie law, which distinguished between sensory and
motor nerves, made it unsafe for him to leave the problem measured
with motor nerves alone, so he now turned to sensory-motor nerves
to find out whether their speed was similar or different. He also
turned to human subjects. When stimulated on the toe and on the
thigh, a man can respond with his hand. The difference in the
time between stimulation and reaction over differing lengths traversed
in the two instances gave Helmholtz his measure of speed of reaction-something
between fifty and 100 feet per second, but variability from trial
to trial and subject to subject was so great that he did not follow
up the original study. The individual differences, later to become
so important in themselves, were, for Helmholtz, nothing more
than an indication of inadequate control,39 as indeed they turned
out to be. Nevertheless these two phases of the study gave him
rough measures of the speed of sensory and motor neural impulses.
In general, later, more accurate research has demonstrated that
the neural impulse varies enormously. The speeds, Helmholtz found,
proved to be too slow. Nevertheless it was Helmholtz who carried
out the pioneer study.
The fact that the nervous impulse is not instantaneous but takes
appreciable amounts of time signified that mental events were
definitely limited by the properties of the body and that an analysis
of bodily motion was relevant to psychological phenomena. Mental
events that seem instantaneous may actually be temporal events.
As Boring so aptly phrases it, ". . . it brought the soul
to time."40
This was the first "reaction-time" study as it came
to be called. However, Helmholtz was interested only in the sheer
speed of the neural impulse, and it was the work of Donders published
some fifteen years later (already described) which showed a grasp
of the psychological significance of the problem.
Vision
In the field of visual physiology Helmholtz did an enormous amount
of original and important work which is presented in his Handbuch
der physiologischen Optik,41 the three volumes of which have been
characterized respectively as physiological, sensory, and perceptual
accounts of vision.42 Some of his specific research studies show
a similar close amity to psychological problems.
He measured the optical constants of the eye, demonstrated how
the eye accommodates for different distances, and developed and
supported by research a theory of visual space. Most important
of all was his theory of color vision. In 1802 Thomas Young (1773-1829
) had published a theory of color vision in which he postulated
that the retina is equipped with three kinds of color sensitive
points.43 These three primary colors, working cooperatively, were
said to furnish the range of experienced colors. The derivation
of all colors from three colors was not new having been implied
by Newton among others. Young's contribution was the suggestion
of a physiological basis of three kinds of "particles on
the retina" ( receptors ) each kind acting independently.
This suggestion laid fallow until Helmholtz espoused the theory,
to be known thereafter as the Young-Helmholtz theory of color
vision. In Helmholtz's theory three sets of fibers in the retina
are said to give rise respectively to sensations of red, green,
and violet.44 There are supposed to be three kinds of photo-chemically
decomposible substances in the end organs, each with different
degrees of sensitiveness to different parts of the visible spectrum.
With disintegration of these substances, neural excitation occurs.
The three kinds of excitation act differently on the brain only
because, while playing the part of connecting wires, they are
united to different functioning parts of the brain. Using these
physiological particulars, Helmholtz then proceeded to suggest
how they could be used to explain various psychological visual
experiences. On stimulation of the red and green fibers together,
yellow results. Other combinations produce other colors. When
all three kinds of organs are stimulated in the right proportion,
white results. Negative after-images on looking at a white surface,
after colored stimulation, arise because after one of the organs
have been thoroughly fatigued by use, we see with the unfatigued
organs alone. Color blindness ( red-green blindness) could result
from lack of either red or green organs, or both. An objection
to the theory that arose from critics was that yellow, postulated
as arising from stimulation of red and green organs, should not
be seen by color-blind individuals, whereas actually they can
see yellow. In spite of criticism, modification, elaboration,
and correction, the Young-Helmholtz theory is still taken very
seriously to-day. Helmholtz45 stated that this color theory is
a particular application of Muller general law of the specific
energy of nerves, with "three nerve systems:" Muller
himself had applied the principle of the specific energy of nerves
to the differention of one sense from another, but not to the
separate qualities within a single sense, such as sweet and sour
or red and green. Helmholtz was applying the rule within a single
sense modality.If we speak of Muller's theory as one of specific
nerve energies, then Helmholtz was advancing a theory of specific
fiber energies .46 Although Helmholtz did much to advance this
theory of fiber energies, he had been anticipated by several other
investigators, including Thomas Young himself who, by advancing
his theory of specificity for different color qualities, anticipated
Miller's more general doctrine. Despite his citing Miiller, the
theory of Helmholtz turned out to be more a doctrine of the specific
energies of cortical areas than it did of nerves 4' As we know,
the cortical area alternative had been raised, but not accepted
by Mfiller. In the location of specificity, later research has
proven Helmholtz right and Muller wrong.
Space Perception
In interpreting space perception, Helmholtz was an empiricist.48
In this he was partially dependent for support upon Muller's doctrine
of specific energies. Helmholtz held that the various sense organs
and nerves each have characteristic qualities, which are not,
in themselves, meaningful, being bare sense impression. Here experience
enters, for recurring association gives them meaning. The doctrine
of specific nerve energies interpreted by Muller as a defense
of Kantian nativism was now being interpreted by Helmholtz as
a defense of empiricism.
Helmholtz stressed the importance of the process of unconscious
inference in space perception 49 Perception of space, he said,
is not inherent; instead we "infer" space from past
experience, but without awareness that this process of inference
is going on. Certain small cues constitute a sign that the object
is to be found at a certain distance. Without noticing the sign
itself, we infer how far away the object is. To use only one example
-in gazing at an object twenty feet away there are intervening
objects at various closer distances that are actually seen as
double, but without our noticing it. (This can be verified by
anyone who takes the trouble. Look across the room while holding
a pen first, a few inches from the nose, and then at arm's length,
while still looking across the room. The doubleness of the pen
will be evident in both instances but greatest when close up.)
Closer objects are seen, while looking at the far object, with
varying degrees of doubleness. A specific degree of doubleness
is unconsciously interpreted, without noticing the doubleness
at a certain distance to the object. This is what Helmholtz meant
by "unconscious inference."
Audition
In the field of hearing Helmholtz made many contributions. Among
them were his clarification of the role of timbre to round out
the major harmonic components and his theory of how hearing is
mediated by the ear and the relation of this theory to the doctrine
of specific energy of nerves. Each of these must be examined in
more detail.
That pitch depends primarily on frequency of sound waves and
intensity upon the amplitude of the waves had been understood
and explained in Muller's Handbuch; timbre had not. The fact of
timbre, that is to say, that there are qualitative differences
between tones other than pitch was, of course, well known. Anyone
hearing the same note being played by different instruments can
not escape realizing the existence of something which make them
sound different despite their identity of pitch. Helmholtz explained
this experience of timbre by the presence, in addition to the
fundamental pitch, of so-called overtones or vibration rates more
rapid than the fundamental tone which fixes the pitch.50 Most
vibrating bodies, vibrate not only as wholes, but simultaneously
as parts. It is these partial vibrations which give rise to timbre.
The shape of waves making up a tone from a musical instrument,
if visualized, would show the main wave being the same as that
for all other instruments and giving the fundamental pitch when
playing that note, but with the part waves unique to this particular
instrument, giving rise to the timbre. The more similar the tone
of two instruments, the more similar the characterizing overtones
were found to be. Here again, Helmholtz was not content merely
to advance a theory; he conclusively demonstrated his contention
by building a series of tuning forks and resonators which permitted
systematically varying the intensity of the overtones accompanying
a fundamental tone. From these he produced synthetically the characteristic
timbre of various instruments.
His theory of hearing evolved over the years, and this is not
the place to trace these modifications. His theory was essentially
a theory of pitch; he assumed that intensity was more or less
explained by varying degrees of excitation of the fibers.
Helmholtz marshalled evidence that a portion of the inner ear
responded to an auditory wave stimulus by resonance, vibrating
in tune with the frequency of the sound wave. The ear behaved
similarly in the same way as striking a note on one tuning fork
will set in vibration an unstruck tuning fork. This is the phenomena
called resonance.
After discarding the organs of Alfonso Corti as the basis of
differential tuning, he finally reached the conclusion that the
basilar membrane with its many hair cells in the cochlea of the
inner ear is the resolving organ of hearing.-" This basilar
membrane is trapezoidal in shape (narrow at one end and increasing
gradually in width). The hairs on its narrower end he thought
to be tuned to high pitches; the hairs on the wider portion to
the low pitches. Pitch, then, transmitted to the brain after analysis
by resonance on the basilar membrane, is dependent upon place
of stimulation. The differential action of a portion of the basilar
membrane is the means whereby the pitch of the heard sound is
determined.
The resonance theory gave rise to many alternative theories.52
The most formidable competition to the resonance theory has been
from the frequency theory in which the basilar membrane is said
to vibrate as a whole, thus, giving rise to nerve impulses which
preserve, unchanged, the frequency of the stimulus so that the
brain, itself, not the basilar membrane, becomes the place where
auditory analysis into different pitches takes place.53 Today
each theory is seen as explaining some, but not all, of the evidence.
It is clear that the old theories are too simple. Resolution of
the present discrepancies is a task for future research.
The doctrine of specific nerve energies was regarded by Helmholtz
as an important established fact. It was natural for him to apply
it to hearing. Differences of quality ipso facto thus meant that
there are differences in the conducting sensory fibers. Since
research of Helmholtz's day by Seebeck demonstrated the existence
of some 5,000 distinguishable pitches, this meant that Helmholtz54
was arguing that there must be some 5,000 specific energies within
hearing! This bold step could hardly avoid being noticed, and
the issue of the specific energies in hearing became a prominent
one in all of the senses.
Helmholtz as a Great Scientist
Helmholtz was deeply devoted to an empirical approach to science
and yet had a broad philosophical outlook and possessed curiosity
and originality. He had the skill to carry a problem through from
its conception, the construction of the necessary apparatus, and
the execution of the research design. He was cautious in the interpretation
of the results, yet he possessed great synthesizing ability in
coordination of his findings with discovery in other related fields.
In many ways Helmholtz was one of the great scientists of modern
times.
