J7oumnal ofNeurology, Neurosurgery, and Psychiatry 1 993;56:329-333
oumrnal of
NEUROLOGY
NEUROSURGERY
& PSYCHIATRY
Editorial
Penfield's homunculus: a note on cerebral cartography
In 1937, Penfield and Boldrey published a paper of
major importance.' They described their work on the
effects of stimulation of the cerebral cortex in man, the
procedures being carried out as exploratory manoeuvres
to delineate the appropriate area for subsequent surgical
intervention. They confirmed the precise topography of
cortical localisation, and were able to relate stimulation
of a discrete part of the brain with motor and sensory
phenomena affecting a particular part of the body. Whilst
these now classical studies confirmed and greatly extended
what had been known from earlier observations in
awake humans and from experiments in animals, the
manner of presentation of their findings was remarkable.
It was the first time pictorial means of illustrating cortical
representation had been attempted; it was thus an entirely
new concept, but it was also one which has proved a
curious method of illustration and one which gives rise to
a number of unforseen problems.
Penfield and Boldrey set out to illustrate "the order
and comparative extent" occupied in the sensorimotor
strip.' To represent the topography of their observations,
the authors departed from the rigorous textual description
of the effects of stimulation of the brain and
achieved an extraordinary conceptual leap: an artist, Mrs
H P Cantlie, was employed to draw a sensory and motor
homunculus-a term discussed below. This first
homunculus to be created (fig 1) was described as giving
"a visual image of the size and the sequence of the cortical
areas". It was symmetrical in shape and illustrated
both motor and sensory features together; what actually
was represented, however, was ambiguous and confusing.
The authors state that the size of the parts was determined
"not so much by the number of responses...but by
the apparent perpendicular extent of representation of
each part when these responses were multiple for the
same part". What is meant by "perpendicular" is not
stated, and it becomes even less clear from the subsequent
comment: "that the large size of the thumb and
the lips indicates the vertical extent of Rolandic cortex
devoted to those parts in individual cases is very large". It
is unclear whether "vertical" is the same as "perpendicular",
and whether these terms mean longitudinal over the
cortical surface, or deep-since the authors also stimulated
deeper layers of the brain after having resected areas
for extirpation of abnormal brain. The referral to
responses which were multiple for the same part is also
difficult to interpret, since it could imply multiple sites
were effective in eliciting responses, or that only in certain
areas were multiple sites actually stimulated. Whilst
the homunculus appears to relate to length rather than
width, the parts illustrated are in fact enlarged in both
dimensions. Also noteworthy is that responses have been
transposed to the same side of the brain for the purposes
of representation, and it is not possible from the illustration
to distinguish unilateral from bilateral effects of cortical
stimulation.
Thirteen years later, in their monograph entitled The
cerebral cortex of man, Penfield and Rasmussen made
another attempt to illustrate cortical representation.2 This
was preceded by a diagram of a cross-section of the cere-
'.9t
Figure 1 Thefirst homunculus, Penfield and Boldrey, 1937. Reprinted
from Brain 1937;60:389-443, with permission of The Macmillan Press
Ltd.
329
Editonial
Figure 2 The motor and
sensory homunculus: thefirst
map. Penfield and
Rasmussen, 1950. Reprinted
with permission of
Macmillan Publishing
Companyfrom The cerebral
cortex ofman by Wilder
Penfield and Theodore
Rasmussen. Copyright 1950
Macmillan Publishing
Company; copyright renewed
1978 Theodore Rasmussen.
rlip
bral hemispheres in which were drawn solid bars at the
periphery, the length of the bars giving an indication of
the relative cortical areas from which the corresponding
responses were elicited. This implies somewhat different
information was being illustrated compared with the
1937 illustration. A few pages later, overlying the bars is
superimposed the homunculus, which is again "brought
forth" by the same artist, but in a different and even
more familiar form, draped along the cortical surface and
interhemispheric fissure. In this homunculus, however,
the motor and sensory representations have been separated,
slightly modified and corrected (fig 2). These drawings
are so memorable and have been reproduced so
often that it is difficult to appreciate that here, even more
so than with the first homunculus, a new concept in representation
and imagery had been created. Moreover, for
the first time the homunculus can be considered as some
form of "map" of human cortical representation, being
more or less precisely in relation to actual brain areas
identified at surgery. This compares with the first
homunculus, which was not drawn in relation to the
hemispheres at all. It is unclear whether the authors
appreciated the visual significance of the homunculi, but
these figurines created a precedent which has had a major
influence on subsequent forms of related graphic illustra-
tion.
Degradation of the homunculus subsequently ensued,
with the appearance in 1954 of multiple homunculi.
Penfield and Jasper now illustrated three sets of homunculi
(fig 3), the first set (motor and sensory) in relation to
the Rolandic fissure, a second apparently purely sensory
homunculus in the secondary sensory area near the
Sylvian fissure, and a third homunculus (virtually only
motor) in the supplementary motor cortex.3 The homunculi
change considerably in this process; for instance the
fingers and toes are made to seem important in the secondary
sensory figurine, which is crouched, and is to
some extent bilateral, whereas the supplementary
homunculus is straight and extremely vaguely represented.
The precision intended in the earlier homunculi has
been almost completely lost; indeed Penfield and Jasper
stated that "the exact position of the parts must not be
considered topographically accurate. They are aids to
memory, no more". And again, "the figurines...have the
defects, and the virtues, of cartoons in that they are inaccurate
anatomically...". It suggests that the homunculus
has taken over from its authors, and illustration has outstripped
the scientific evidence available. Indeed, the
bilateral leg movements that can be elicited by stimulation
of medial structures led Bates to denigrate the concept
of the homunculus thus: "May it not be, in other
words, that in depicting the 'average' representation the
motor homunculus should have two back legs?"4
Homunculi now appeared in other areas of the brain,
such as the thalamus, and Penfield and Jasper3 were the
first to place a homunculus in this subcortical region (fig
4). Whilst they stated a precise topographical organisation
is present in the somatic relay nuclei of the thalamus,
the homunculus drawn there "makes no pretence to
SECOND > ' Ib A s
SENSORY
Figure 3 Mulltiple sets ofhomunculi. Penfield andJasper, 1954.
Reprintedfrom Epilepsy and thefunctional anatomy of the human brain,
with permission ofChurchill Livingstone.
330
Editorial
Figure 4 The homunculus in the thalamus. Penfield and Jasper, 1954.
Reprintedfron: Epilepsy and the functional anatomy ofthe human brain,
with pernission of Churchill Livingstone.
detailed accuracy, though the relationships between body
parts are probably roughly correct, judging from animal
experiments". The homunculus appears now to have
degenerated into a purely artistic device vaguely based on
animal experimental work, without any scientific basis to
justify the human form portrayed. Final relegation to
mere amusement must be the riot of homunculi with
numerous images in the thalamic region depicted by
Hassler et al5 and further annotated by Creutzfeldt.6
Whilst recognising the professional medical illustrator's
role as an interpreter,7 any scientific significance of these
displays is difficult to discern.
An interesting correspondence is referred to between
Penfield and an un-named "distinguished neurologist",
the latter pointing out that Hughlings Jackson had suggested
the index finger would have a larger representation
than the others; the discussion concludes by Penfield
stating "the purpose of this monograph is to analyse the
data, however imperfect they may be, without preconception".)
With this stated aim, however, it seems curiously
contradictory to represent meticulous scientific observations
by means of an homunculus. As implied by the preceding
comments on Penfield and Jasper's multiple
homunculi, even more than in the earlier homunculi,
there is uncertainty as to what actually was being represented.
Was it the surface area of the cortex, or the
length or depth, or in addition some measure of the
threshold of stimulation that produced motor or sensory
effects?
Homunculi perhaps represent different things in different
circumstances. The motor homunculus is some form
of representation of areas from which involuntary movements
can be produced by artificial and passive means; is
this the same homunculus that would be derived following
a different form of cortical excitation such as magnetic
stimulation, or when voluntary movements are
generated? This contrasts with the sensory homunculus,
which in Penfield's observations denotes areas from
which subjectively reported sensations in the periphery
are produced by antidromic rather than orthodromic
stimulation. The sensations reported were of various
types, and included tinglings, numbness, sensations of
movement, and rarely pain, feelings of thickness, and
other sensations.
This is of considerable importance, since the
homunculus as represented by Penfield gives the impression
of single motor and sensory maps. From experiments
using electrical stimulation techniques in monkeys,
however, it seems probable that the concept of simple
somatotopic organisation is inadequate.8 For the sensory
homunculus, Merzenich et aO provide evidence for multiple
maps representing different functions, and there is
evidence in non-human primates that there are as many
as seven or more somatosensory areas, "most of which
appear to constitute complete, orderly representations of
the body surface or deep body tissues".10 Apart from differences
that might result from different stimulation techniques,
it may also be maps of cortical potentials evoked
peripherally are not identical to maps of perceived sensations
induced by direct cortical stimulation. Thus
Penfield's homunculus is not only ambiguous in what
exactly it does represent, but also gives an unjustified
impression of cortical representation which is both single
and established.
Moreover, not only were Penfield's data obtained
using comparatively large-diameter stimulating electrodes
in patients with brain disease, but different individuals,
both normal and those with diseases of the nervous system,
have very different cortical areas from which sensory
and motor phenomena can be elicited. Penfield was
aware not only of this individual variation but also that
the effects of cortical stimulation could change after
surgery.' Whilst the immense plasticity of cortical maps
in animals is established (for review, see Wall"), there are
also several examples of plasticity in humans. Confirming
observations in animals, patients with damage affecting
somatosensory systems show evidence of marked somatotopic
reorganisation at thalamic level.'2 At cortical level,
plasticity of maps of outputs of the human motor system
has been demonstrated after amputation, paraplegia and
hemispherectomy, and changes in sensorimotor representation
have been demonstrated in blind braille readers
(for review, see Pascual-Leone et at"). What of the
homunculus in these patients?
Results of cortical stimulation in animals had of course
been illustrated before, for instance by superimposing
numbers and then words for the relevant part on drawings
of the brain.'4 Nevertheless, the first figurine was
human, with Penfield's homunculus the precedent for
the profusion of illustrations of experimental work on
animals. Fundamental studies on cortical localisation
using stimulation and evoked potential techniques, particularly
by Bard, Woolsey et al, were illustrated in analogous
fashion; the simiusculus, the cortical representation
of the monkey, soon appeared, as did cortical representation
of the rabbit, rat, cat and doubtless other animals
(see Woolsey'5). The pig and its large snout representation
was another example illustrated in a different form
by Adrian.'6 Interesting, since it showed another form of
331
Editorial
physiological process is the figurine placed in the cerebellum
by Fulton,'7 and which he called a "primunculus"
rather than homunculus. It illustrated the approximate
distribution of excitatory and inhibitory foci in the anterior
cerebellum. All these animal representations are more
figurative than accurate, contrasting with the extremely
accurate mapping of localised evoked potentials illustrated
diagrammatically rather than by means of figurines.'5
Even as a form of straightforward representation, the
homunculus is curious. Just as the head has been the site
of numerous fantastic yet symbolic compositions since
Arcimboldo in the sixteenth century,'8 so with the
twentieth century homunculus there is artistic licence
and embellishment. The border between fact and imagination
has been blurred: the homunculus has been
imbued with character, an attribute which many a viewer
will experience, and which is perhaps unique in terms of
scientific illustration. Thus the first homunculus drawn in
1937 is described by Penfield and Boldrey as
"grotesque".' The later ones are "brought forth", and the
drawings clearly have expressions and a personality. In
1937, the face is smiling broadly and is missing half its
teeth; the 1950 versions show a wide-open mouth on the
motor half of the homunculus, and a closed mouth and
rather sad expression on the sensory half.2
It is tempting to dismiss this anthropomorphic development
as an artistic whim or amusement. Whilst later
relegating homunculi to the status of cartoons and aidesmemoires
for students,23 at least initially the drawings
were a serious endeavour, confirmed by comments such
as that "certain inaccuracies in this figurine are corrected
in the text"'-comments which preclude a frivolous
intent. Not only was at least the first homunculus therefore
a carefully thought out device, but as a consequence
it could provoke strong reactions. Thus the concept of
the homunculus was roundly castigated by Walshe:
"even today cortical cartography is eagerly pursued as
new modes of electrical stimulation uncover fragments of
electrical excitability in new cortical territories. Nor are
the moderns content with maps, for homunculi and simiusculi
have now made their horrid appearances, lineal
descendants of Lewis Carroll's Jabberwock, purporting to
depict the fair face of nature, but in fact achieving something
quite unnatural".'9 It is ironic that Walshe, too,
imbues the homunculi he so dislikes with personality, in
this instance, horrid.
Noteworthy is Walshe's reference to cartography, since
this alludes to one of the functions of such drawings: to
represent the link between points on the surface of the
body with areas within. The essence of all such drawings
is that they are forms of maps, which represent how one
collection of points is linked to another.'0 Maps, however,
are designed to be accurate as well as informative, and
apart from the paucity of information available particularly
in humans, there are formidable cartographic difficulties
in using a device such as an homunculus as a type
of map. These difficulties include two-dimensional representation
of diverse, three-dimensional information both
in the periphery and in the brain, the representation of
multiple and sometimes overlapping images, and simultaneous
representation of bilateral and unilateral data. The
particular problems and potential misuse of mapping the
electrical activity of the brain have been admirably
reviewed by Binnie and MacGillivray."
It is clear that both scientifically and graphically there
is considerable ambiguity in what Penfield was intending
to illustrate. This produces unease-even the preceding
example of the map fails. Perhaps this is because the
homunculus is a form of representation, and any representation
of cerebral function is also ambiguous. The
definition and nature of representation in science have
never been satisfactorily defined. As Gooddy points out,
"representation is at the bottom of so many schemes of
neurological thinking, whether they be clinical or experimental"."
This is of considerable importance, not least
since inappropriate representation could actually impede
scientific advance. It might even be the homunculus is an
instance of this. Gooddy's conclusion that "A function
does not originate in a specific region of brain tissue,
where its 'representation' has been traditionally located"
is still valid today, but it points to the problem of how
brain function can ever be illustrated. Amongst other
aspects discussed above, it is this unresolvable difficulty
which makes the homunculus an unsatisfactory and perhaps
even misleading image.
The periphery of the homunculus is an additional
aspect that requires comment. Penfield's homunculus is
shown with an outline which encompasses the area of
cortical representation. What exactly does this outline
represent, for we do not feel the outline or periphery of
our bodies? As Schilder comments, "...the outline of the
skin is not felt as a smooth and straight surface. This outline
is blurred. There are no sharp borderlines between
the outside world and the body".23 The homunculus,
however, is outlined by an imaginary 'envelope' which
does not exist.
Another phenomenon where the border is blurred
between fact (pathophysiology) and imagination (subjectivity)
concerns phantom sensations. Some patients
report that the phantom that follows amputation often
fades, but it does so in a non-sequential manner. For a
phantom limb, it has been stated that: "The sequence of
disappearing parts follows, with the possible exception of
the joints, the Penfield-Boldrey homunculus. Those parts
which have large areas of representation on the
homunculus...are the very same parts which have the
longest phantom life. By contrast, those which have minimal
representation on the homunculus are relatively
short-lived as phantoms".'4 Whilst this concept has been
rejected for a number of reasons,'5 it has to be accepted
that newer theories are as tenuous as those that invoke
the homunculus. It requires considerable ingenuity to
attempt to describe a phantom phenomenon in terms of
phantom representation.
It can scarcely have escaped Penfield's notice that both
the term and the concept of the "homunculus" are old,
dating at least since medieval times; they are also strange.
The term merely means "little man" or "manikin", and
again it is curious that Penfield and his collaborators
should have used this term. A special use for the term
seems implied, since, for example, a man portrayed on a
street sign would not be designated a homunculus.
Derived from the medieval idea of creating gold from
base metals, the concept of the homunculus in the past
embodied the idea of being able to create a diminutive
human, a concept familiar to Paracelsus and later
alchemists. Modern psychology has also embraced the
homunculus-the concept of a little person within oneself,
having a personal, internal role, and perhaps the
inner person with whom one converses in internal
speech: the function of this inner person is to provide an
explanation or interpretation of the outside world-for
example, an inner person that is responsive to one's pain,
or to visual images, or other experiences. The inner
person, however, explains nothing, and the usefulness of
this concept is dubious.'6
Even beyond the human and animal brain, the
homunculus has acquired status, for it has actually been
endowed with teleological attributes. Thus "The gross
distortion of the imaginative 'little man' shows the high
332
Editorial
P-AW
F'giure 5 Photomiicrograph shozving the relationship between sonmatotopic subareas and architectural organisation of the rat sensory cortex, with parts of
the bodv drawn around the cortical areas to which they project. Welker, 1976. Reprintedfronm Journal ofConmparative Neurology 1976;166: 173-190,
with penmission of The Wistar Inistitute Press.
priority that nature has placed on dexterity in the fingers 5 Hassler R, Mundinger F, Riechert T. Stereotaxis in Parkinson syndrome.prioritythat nature has placed on dexterity in the fingers Berlin: Springer Verlag, 1979: (front cover).
and in the control of speech muscles. These two func- 6 Creutzfeldt OD. Cortex cerebri. Berlin: Springer Verlag, 1983: 216.
tional areas account for almost half the total area of the 7 Cull P. Medical Illustration. In: Jennings S, ed. The new guide to professionial
illustration and design. London: Headline, 1987: 40-43.
motor strip! A similar analysis in other mammals shows 8 Gould HJ, Cusick CG, Pons TP, Kaas JH. The relationship of corpus calquite
a different allocation of these areas, reflecting a losum connections to electrical stimulation maps of motor, supplementary
motor, and frontal eye fields in owl monkeys. J Comparative
different set of behavioural priorities".' For the Neurology 1986;247:297-325.
homunculus, science has now yielded to philosophy. 9 Merzenich MM, Kaas JH, Sur M, Lin C-S. Double representation of thehomunculus,
~~~~~~~~~~~~~~~~~bodysurface within cytoarchitectonic areas 3b and 1 in "51I" in the owl
Penfield's homunculus was a deceptively simple and monkey (Aotus trivi'rgatus). J7 Comparative Neurology 1978;181:41-74.
yet naive concept. This type of illustration, a form of 1o Merzenich MM, Kaas JH. Principles of organization of sensory-perceptualyetnafvecncept.Thi type of ilustration a form
ofsystems in mammals. Pr~ogr Psychobiol Physiol Psychol 1980;9: 1-42.
map, was a highly original attempt to portray graphically 1I1 Wall JT. Variable organization in cortical maps of the skin as an indication
the observations of brilliant and painstaking research and of the lifelong adaptive capacities of circuits in the mammalian brain.
Trends Neurosci 1988;1 1:549-57.
one which has had a lasting influence as a mode of repre- 12 Lenz FA. The thalamus and central pain syndromes: human and animal
sentation. It is memorable and useful. It has, however, studies. In: Casey KL, ed. Pain and central nervous system disease: the central
pain syndromes. New York: Raven Press, 1991:171-182.
been of limited and even doubtful scientific value, since 13 Pascual-Leone A, Cohen LG, Hallett M. Cortical map plasticity in
fact and fancy have been confused. Illustration of brain humans. Trends Neurosci 1992;15:13-14.
function by projected drawings may best be reserved for 14 Leyton ASF, Sherrington CS. Observations on the excitable cortex of the
function by projected drawings may best be reserved for chimpanzee, orang-utan and gorilla. Q JExper Physiol 1917;1 1:135-222.
those rare instances where true images can be derived 15 Woolsey CN. Organization of somatic sensory and motor areas of the
cerebral cortex. In: Harlow HF, Woolsey CN, eds. Biological and bioand
recorded. An example is the different cytoarchitec- chemincal bases of behavior. Madison: The University of Wisconsin Press,
tural subareas that can be discerned on photomicro- 1958:63-81.16 Adrian ED. Motor and sensory areas of the brain. The physical backgrouind
graphs of the rat sensory cortex. These disjunctive ofperception. Oxford: Oxford University Press, 1946:31-47.
somatotopic areas, correlating with the projected periph- 17 Fulton JF. Functional localization in the frontal lobes and cerebellutml.
London: Oxford University Press, 1949:129-130.
eral body part, allow the different parts to be identified 18 Kaufmann TD. The allegories and their meaning. In: Hulten P, ed. The
and drawn, and a sensory "rattunculus" can be derived Arcim"boldo effect. Milan: Bompiani, 1987:89-109.
2^1 Representation of everything else may best be
19 Walshe FMR. Some reflections upon the opening phase of the physiology
(fig 5).2 Representation of everything else may best be of the cerebral cortex, 1850-1900. In: Poynter FNL, ed. The history,
served by an unambiguous diagram or words. anzd philosophy of knowledge of the brain and its fuinctions. Amsterdam: B.
M. Israel, 1973:223-4.
I am indebted to Dr P W Nathan and Dr D Fish for their most helpful com- 20 Dykes RW, Ruest A. What makes a map in somatosensory cortex? In:
ments, and Dr E Whitcombe for drawing my attention to the paper bv Dvkes Jones, EG, Peters A, eds. Cerebral cortex, V7ol 5, Sensory-motor areas and
and Ruest aspects ofcortical conzniectivity. New York: Plenum, 1986:1-29.
G D SCHOTT 21 Binnie CD, MacGillivray BB. Brain mapping-a useful tool or dangerous
Thlc .National Hospital for Neurology and Neurosurge, Pschiatr 199255:527-9.
Quecn Squarc, London W'CIN 3BG, UK 22 Gooddy W. Cerebral representation. Brail 1956;79: 167-87.
23 Schilder P. The imtage and appearance of the hum"ian body. London: Kegan
Paul, Trench, Trubner, 1935:85.
24 Simmel ML. On phantom limbs. Arch Neuirol Psychiatry, (Chicago)
1 Penfield W, Boldrev E. Somatic motor and sensorv representation in the 1956;75,637-47.
cerebral cortex of man as studied by electrical stimulation. Brain 25 Melzack R. Phantom limbs, the self and the brain. Canadian Psychology
1937j60:389-443. 1989;30,1-16.
2 Penfield W, Rasmussen T. The cerebral corltex of man. New York: 26 Dennett DC. Brainstornms. Philosophical essayNs on miind and psychology.
Macmillan, 1950:44, 56, 214-15. Hassocks, Sussex: Harvester Press 1979:57-59.
3 Penfield W, Jasper H. Epilepsy and the fiunctional aniatomyii, of the humolani 27 Pugh GE. The biological origin of human values. London: Routledge and
brain. London: J and A Churchill, 1954:105-106; 159. Kegan Paul, 1978:140.
4 Bates JAV. Stimulation of the medial surface of the human cerebral hemi- 28 Welker C. Receptive fields of barrels in the somatosensory neocortex of
sphere after hemispherectomv. Brain 1953;76:405-47. the rat. 7 Comtparative Neuirol 1976;166: 173-190.
333