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The Scientific Contributions of Paul D. MacLean (1913–2007)


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Paul D. MacLean, a leading brain scientist of the 20th century, died on December 26, 2007. We review his life as a scientist and highlight some of his most important research contributions.
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The Journal of Nervous and Mental Disease Volume 197, Number 1, January 2009
The Scientific Contributions of Paul D. MacLean
John D. Newman, PhD,* and James C. Harris, MD†
Abstract: Paul D. MacLean, a leading brain scientist of the 20th century, died
on December 26, 2007. We review his life as a scientist and highlight some of
his most important research contributions.
Key Words: Limbic system, triune brain.
(J Nerv Ment Dis 2009;197: 3–5)
December 26, 2007 marked the passing of Paul D. MacLean one
of the leading brain scientists of the 20th century and a
long-time member of this Journal’s Editorial Board (Fig. 1). In a
remarkably long and productive career, he established the parame-
ters for investigating the evolution and functioning of the emotional
brain and set the stage for current research in affective and social
neurosciences. He is best known for identifying and naming the
limbic system in 1952, but this was only the beginning of his
research contributions. In 1990, nearly 40 years later, he summa-
rized his lifetime of research in his magnum opus, The Triune Brain
in Evolution (MacLean, 1990; hereafter “TB”). There he discussed
the ramifications of the limbic system concept and his subsequent
findings on brain evolution and behavior culminating in his proposal
of a new brain model he called “the Triune Brain.” His research has
altered our view of Homo sapiens and our understanding of brain
evolution and behavior. Words cannot capture Paul MacLean’s
elegance as a scientist, compassion as a human being and his impact
in inspiring others. Those who spent time with him appreciated these
MacLean’s Work in Historical Perspective
MacLean’s contributions have impacted evolutionary biol-
ogy, neuroethology, clinical neuroscience, neurology, psychiatry,
and the social sciences. He posed 2 critical research questions that
are pertinent to each of these fields: (1) “where do subjective
emotional experiences reside in the brain?” and (2) “is the functional
circuitry of the brain inherited in the evolution of vertebrates and, if
so, how did these circuits evolve?” He was particularly interested in
the evolution of the family, writing “the evolution of the limbic
system is the history of the evolution of mammals, whereas the
history of the evolution of mammals is the history of the evolution
of the family (TB, p. 247).”
MacLean helped establish an approach to neuroscience es-
sentially unknown in the United States at the time he began his
research. In Europe, the field of neuroethology had arisen with
*Laboratory of Comparative Ethology, Eunice Kennedy Shriver National Institute
of Child Health and Human Development, NIH, Poolesville, Maryland, and
†Departments of Psychiatry and Behavioral Sciences and Pediatrics, The
Johns Hopkins University School of Medicine, Baltimore, Maryland.
Supported in part by the Intramural Research Program of the NIH and NICHD.
Send reprint requests to John D. Newman, PhD, Laboratory of Comparative
Ethology, Eunice Kennedy Shriver National Institute of Child Health and
Human Development, NIH, Poolesville, MD 20837. E-mail:
Copyright © 2009 by Lippincott Williams & Wilkins
ISSN: 0022-3018/09/19701-0003
DOI: 10.1097/NMD.0b013e31818ec5d9
primary focus on nonmammalian models. Still, in Zurich, Swit-
zerland, W. R. Hess and his coworkers, using electrical brain
stimulation of indwelling electrodes in cats, were mapping out areas
in the hypothalamus and other parts of the diencephalon from which
coordinated species-typical aggressive and fear-like behavior could
be elicited. MacLean visited the Zurich laboratory in 1956, but did
not refer to the work that came out of that laboratory in his own
publications. Subsequently, with Detlev Ploog, he developed a
laboratory environment where species-typical behavior could be
studied in a primate, the squirrel monkey (MacLean, 1962). Ploog
set up his own laboratory, the Department of Primate Behavior, at
the Max Planck Institute for Psychiatry in Munich. There, he
focused in large measure on squirrel monkey vocal behavior and its
underlying mechanisms (e.g., Ploog, et al., 1975). MacLean returned to
the United States initiating his own research on brain and behavior. His
approach was clearly ethological and he emphasized the importance of
preparing an ethogram, an inventory of the behavior of a species that
thoroughly describes its behaviors and organizes them into categories.
He was particularly interested in identifying behavioral master routines
and subroutines. Although he never used the term ‘neuroethology,’
MacLean was clearly an important early pioneer in this field.
Limbic System
MacLean was aware of Paul Broca’s (Broca, 1878) observation
that the ‘limbic lobe,’ so named because it surrounds the brainstem, is
common to the brains of all mammals. He was intrigued with James
Papez’s (Papez, 1937) proposal that emotional experience is based on
connections linking the hypothalamus with the limbic lobe (MacLean,
1993) and visited Papez to discuss his observations with neurological
patients. MacLean’s curiosity about subjective emotional experience
arose from his earlier work with people with psycho-motor epilepsy
whose symptoms included emotional feelings and viscerosomatic
symptoms (MacLean, 1949). Seeking to account for sensations in-
volved in the visual, olfactory, and auditory auras associated with these
seizures, he reasoned that the external sensory apparatus must have
access to the brain circuits where seizures arose and that cortical
association pathways could potentially connect visual, auditory, and
somatic neocortical areas with the hippocampal gyrus. MacLean am-
plified and reconceptualized Papez’s proposal and suggested that a
circuit consisting of the limbic lobe and its major connections in the
forebrain—the hypothalamus, amygdala, and septum—constituted a
“visceral brain” (MacLean, 1949). Seeking greater clarity, he intro-
duced the term “limbic system” (MacLean, 1952) soon afterward. He
wrote that emotional feelings elude “the grasp of the intellect” (Ma-
cLean, 1949 p. 348) because their origins lie in evolutionary primitive
structures preventing their communication in verbal terms. He wrote
that this underlying neurobiology provides a clue to understanding the
difference between what we feel and what we know (MacLean, 1993).
His research focus on the limbic system led to the establishment of the
Section on Limbic Integration and Behavior at the National Institute of
Mental Health (NIMH). Subsequently, with the creation of the new
NIMH Laboratory of Brain Evolution and Behavior on a rural farm
owned by the NIH near Poolesville, Maryland, MacLean, as laboratory
chief, expanded his research. There he studied brain mechanisms
underlying behavior in animal models under semi-natural conditions.
Newman and Harris The Journal of Nervous and Mental Disease Volume 197, Number 1, January 2009
FIGURE 1. Paul D. MacLean. Photo by Alex MacLean.
Triune Brain
At the Laboratory of Brain Evolution and Behavior, MacLean
embarked on a series of experiments on the effects of targeted brain
lesions on species-typical behavior in lizards (Greenberg, et al.,
1979), and squirrel monkeys (MacLean, 1978), and the role of
limbic cortex on play and maternal behavior in hamsters (Murphy,
et al., 1981). These studies resulted in his triune brain proposal first
fully enunciated in his Hinck’s Memorial Lecture in 1969 (Ma-
cLean, 1973). Tracing brain evolution from reptiles to mammals, he
established that the reptilian forebrain, characterized by greatly
enlarged basal ganglia, resembles the striatopallidal complex of
mammals. Later, describing forebrain structures in an evolutionary
context, MacLean coined the term “protoreptilian formation,” or
“R-complex,” in describing the striatal complex, locating it at the
base of the forebrain in reptiles, birds, and mammals (TB, p. 15).In
the triune brain model he reconceptualized the limbic system as the
“paleomammalian brain.”
In enunciating his triune brain proposal from an evolutionary
perspective, MacLean wrote (1967, p. 375): “There are those who
argue that one has no right to apply behavioral observations on
animals to human affairs, but they may be reminded that man has
inherited the basic structure and organization of 3 brains, 2 of which
are quite similar to those of animals. They evolved somewhat like a
house to which wings and superstructure are added. Despite their
great disparity in structure and chemistry, all 3 must function
together. Man’s brain of oldest heritage is basically reptilian. It
forms the matrix of the brain stem and comprises much of the
so-called reticular system, midbrain, and basal ganglia. Superim-
posed on the reptilian is a structure inherited from lower mammals.
It consists of the 2 oldest forms of cerebral cortex together with
related nuclei of the brain stem. As it is common to the brains of all
mammals, 1 might refer to it as the paleomammalian brain [limbic
brain]. Superimposed on the old mammalian brain and appearing
late in evolution is a more elaborate form of cortex called neocortex
which, together with its associated nuclei of the brain stem, reaches
the most advanced stage of development in man and overshadows
the rest of the brain. It can be characterized as neomammalian in
structure and organization.” Accompanying this text is the now-
famous figure (Fig. 2), showing in schematic side view the 3 basic
types of brains that MacLean proposed as man’s inheritance.
The Family and the Brain
MacLean’s appreciation of the importance of family-related
behavior in the evolution of the mammalian brain became a major
theme in his later writings. He summarized this research in his Adolph
Meyer lecture to the American Psychiatric Association in Toronto,
Canada in 1982 (MacLean, 1985). MacLean cited evidence that the
thalamo-cingulate division of the limbic system was unique to mam-
mals, hence it followed that this structure would mediate behaviors that
were either unique to mammals, or, at the very least, largely absent in
infra-mammalian vertebrates. These behaviors included play, nurs-
ing and maternal care, and audiovocal mother-infant communi-
cation (the separation or isolation cry). Two classic papers
examined his hypotheses about the brain and family life. One
(Murphy, et al., 1981) showed that hamsters deprived from birth
of their neocortex, but sparing limbic cortex, retained both
playfulness when young and maternal care as adults, but that play
was disrupted if the limbic cortex was damaged. A later study
(MacLean and Newman, 1988), involving lesions placed in
midline frontolimbic cortex of adult squirrel monkeys, showed
that such lesions largely eliminated production of the adult form
of the “isolation peep,” which in infants serves as the species-
specific infant cry.
MacLean’s Views on Medical Education and the
Future of Psychiatry
MacLean spoke widely about the implication of brain evolu-
tion and behavior, addressing educators on the importance of pro-
viding an education that took into account the evolution of our
FIGURE 2. From the frontispiece to MacLean, 1990.
© 2009 Lippincott Williams & Wilkins 4
The Journal of Nervous and Mental Disease Volume 197, Number 1, January 2009 Paul D. MacLean
species). He expressed concern that the present emphasis in psychi-
atry on finding a chemical cure for every condition could lead to
patients being “manipulated like machines.” As an antidote to this,
he proposed an evolutionary psychiatry (MacLean, 1985) whose
domain is the subjective self. He proposed that “evolutionary psy-
chiatry will focus on the integrated function of the still evolving
brain (MacLean, 1985).” MacLean emphasized the importance of
empathy as a mark of integrated brain functioning in brain evolution
(Harris, 2003). MacLean concluded a paper on the evolution of
audiovocal communication on an optimistic note (MacLean, 1988):
“Judged by human behavior, it was as though there were a directed
evolution towards the development of a concern not only for the
feelings and future welfare of human beings, but also for all living
It remains to be seen the extent to which MacLean’s work
will continue to stimulate further research and if his ideas will be put
into practice. It was his hope that a national “brain library” be
established to facilitate comparative study. Such a library has come
into being (
neuro.html). It is ironic however, that MacLean’s own extensive col-
lection of brain histology is not part of the museum’s collections,
having been discarded during the last days before closing of his
laboratory at the NIH Animal Center (R. Gelhard, personal communi-
Several symposia have been held to honor MacLean. One,
held in Boston in 1999, resulted in an edited volume of papers whose
contributors were familiar with MacLean’s work (Cory and Gardner,
2002). Subsequently, a special satellite symposium to the Interna-
tional Behavioral Neuroscience Society’s annual meeting held in
Capri, Italy was devoted to MacLean. This led to a special issue of
Physiology and Behavior with papers from attendees, along with
additional invited contributions. The preface to that issue (Lambert
and Gerlai, 2003) contained a fitting expression of appreciation to
MacLean and his work: “A special thanks is extended to this
amazing scientist for opening the door to social and evolutionary
neuroscience so that today’s scientists and practitioners can embark
on many paths to understand more about the brain in its natural
context” (p. 342).
A collection of MacLean’s papers and correspondence is
available at the National Library of Medicine in Bethesda, MD. A
finding aid to the collection is available at
Broca P (1878) Anatomie compare´e des circonvolutions ce´re´brales: Le grand lobe
limbique et la scissure limbique dans la se´rie des mammife´res. Revue
d’Anthropologie. 1:385– 498.
Cory GA, Gardner R (Eds) (2002) The Evolutionary Neuroethology of Paul
MacLean: Convergences and Frontiers. Westport (CT): Praeger.
Greenberg N, MacLean PD, Ferguson JL (1979) Role of the paleostriatum in
species-typical behavior of the lizard (Anolis carolinensis). Brain Res. 172:
229 –241.
Harris JC (2003) Social neuroscience, empathy, brain integration and neurode-
velopmental disorders. Physiol Behav. 79:525–531.
Lambert KG, Gerlai R (2003) The neurobiological relevance of social behavior:
Paul MacLean’s legacy. Physiol Behav. 79:341–342.
MacLean PD (1949) Psychosomatic disease and the ‘visceral brain’: recent
developments bearing on the Papez theory of emotion. Psychosom Med.
11:338 –353.
MacLean PD (1952) Some psychiatric implications of physiological studies on
frontotemporal portion of limbic system (visceral brain). Electroenceph Clin
Neurophysiol. 4:407– 418.
MacLean PD (1962) New findings relevant to the evolution of psychosexual
functions of the brain. J Nerv Ment Dis. 135:289 –301.
MacLean PD (1967) The brain in relation to empathy and medical education.
J Nerv Ment Dis. 144:374 –382.
MacLean PD (1973) A triune concept of the brain and behavior. In. TJ Boag, D
Campbell (Eds), The Hincks Memorial Lectures (pp 6 66), Toronto: Univer-
sity of Toronto Press.
MacLean PD (1978) Effects of lesions of globus pallidus on species-typical
display behavior of squirrel monkeys. Brain Res. 149:175–196.
MacLean PD (1985) Brain evolution relating to family, play and the separation
call. Arch Gen Psychiatry. 42:405– 417.
MacLean PD (1985) Evolutionary psychiatry and the triune brain. Psychol Med.
15:219 –221.
MacLean PD (1988) Evolution of audiovocal communication as reflected by the
therapsid-mammalian transition and the limbic thalamocingulate division. In JD
Newman (Ed), The Physiological Control of Mammalian Vocalization (pp
185–201), New York: Plenum Press.
MacLean PD (1990) The Triune Brain in Evolution: Role in Paleocerebral
Functions. New York: Plenum Press.
MacLean PD (1993) Cerebral evolution of emotion. In M Lewis, JM Haviland
(Eds), Handbook of Emotions (pp. 67– 83). New York: Guilford Press.
MacLean PD, Newman JD (1988) Role of midline frontolimbic cortex in produc-
tion of the isolation cell of squirrel monkeys. Brain Res. 450:111–123.
Murphy MR, MacLean PD, Hamilton SC (1981) Species-typical behavior of
hamsters deprived from birth of the neocortex. Science. 213:459– 461.
Papez JW (1937) A proposed mechanism of emotion. Arch Neurol Psychiatry.
Ploog D, Hupfer D, Ju¨rgens U, Newman JD (1975) Neuroethological studies of
vocalization in squirrel monkeys with special reference to genetic differences of
calling in two subspecies. In Brazier MAB(Ed), Growth and Development of
the Brain (IBRO Monograph series, Vol 1, pp. 231–254), New York: Raven
© 2009 Lippincott Williams & Wilkins 5
... Once the hypothalamus and brainstem were excluded as pivotal structures for engendering emotions, the forebrain became the focus of research. As suggested by Papez [14], based on anatomical considerations, and later modified by McClean [60,61], based on observations in patients with "psycho-motor" epilepsy who often reported sensory auras that were associated with emotional and other experiential phenomena and visceral-somatic symptoms, the cingulate gyrus, hippocampal formation (limbic lobe), mamillary bodies, anterior thalamic nuclei, amygdala and orbitofrontal cortices were thought to be the key structures for emotional experience. The critical research for determining which structure was essential for an emotional experience was initiated by Penfield and associates [62,63] who pioneered the technique of electrically stimulating the forebrain in awake patients undergoing neurosurgical procedures. ...
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... The human brain became exquisitely adept at storing emotionally salient memories, a skill that permitted our forbearers to avoid circumstances of probable danger. The neuroanatomy of anxiety and fear depend on the close cross-talk between emotion, learning, and memory, a reality made most apparent by the shared limbic circuitry subserving these processes [51,52]. Comprised of the hippocampus, amygdala, fornix, mammillary bodies, anterior thalamus and cingulate cortex, the limbic system is fundamental to healthy, and, when functioning maladaptively, pathologic mental life [53]. ...
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Depression stigma is a potential barrier to engagement in and efficacy of depression treatment. This pilot study examined the association of mindfulness with depression stigma among participants in an eight-week mindfulness-based intervention for depressive symptoms. Thirty-one African American women with depressive symptoms were recruited from an urban Federally Qualified Health Center (FQHC) to participate in a mindfulness intervention (M-Body). Mindfulness, depressive symptoms, and depression stigma were assessed at baseline, eight weeks, and 16 weeks. Focus groups were conducted to examine participants’ subjective experiences with the mindfulness intervention. Mindfulness significantly increased from baseline to eight weeks. There was a non-significant decrease in depression from baseline to eight weeks and a significant decrease in depression from baseline to 16 weeks. Depression stigma significantly increased from baseline to eight weeks and significantly decreased from eight to 16 weeks; however, depression stigma did not return to the baseline. An exploratory qualitative analysis of focus group data revealed themes related to direct and indirect factors that may perpetuate and maintain depression stigma. This is one of the first studies to explicitly explore the relationship between mindfulness, depression symptoms, and depression stigma among African American women.
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Resumen Desde siempre ha existido un interés por comprender cómo funciona el cerebro humano su relación con el comportamiento y cognición. Un acercamiento que busca solucionar esta intriga radica en la propuesta de modelos de organización cerebral que tratan de explicar cómo es el trabajo de la masa encefálica. En tal sentido, en este artículo se propone una revisión teórica de los principales modelos. Jackson propone una teoría en donde se consideran tres niveles de estructura cerebral: nivel inferior o medular, nivel medio y nivel superior. McLean postula la existencia de tres cerebros: reptil, paloemamífero/límbico y neo-córtex. El modelo de Wernicke-Geschwind manifiesta la interacción de la estructuras del lenguaje en favor del funcionamiento cerebral. El modelo de organización cerebral de Luria resalta el rol interactivo de tres unidades funcionales: primera, encargada de regular el tono y vigilia; segunda, recibe, procesa y almacena la información; y la tercera, planifica, monitoriza y verifica la actividad mental y comportamental. Esta revisión deja en evidencia la alta complejidad inmersa en el trabajo del cerebro humano. Se cierra este artículo resaltando la necesidad de realizar investigación que pueda generar evidencia empírica en favor de la comprensión de la eficacia de cada uno de los modelos descritos en este trabajo. Palabras clave: modelos de organización cerebral, neuropsicología, Jackson, MacLean, Luria.
Background One of the barriers to effective care in patients with depression is stigma associated with having a mental disorder, which also acts as a barrier to recovery and increases the disability. Aims To study the stigma and disabilities experienced by the patients with depressive disorders seeking treatment in a tertiary care hospital Methodology Fifty patients diagnosed to have depressive disorder as per ICD-10 were recruited by convenient sampling. To measure the stigma, the Discrimination and Stigma Scale -12 was applied. The severity of depression was determined by applying Hamilton Depression Rating Scale (HAMD). The disability was calculated by using WHO Disability Assessment Schedule 2.0 Results Fifty percentages of the participants reported unfair treatment and they experienced discrimination in at least one life domain. There was significant positive correlation between unfair treatment subscale of stigma and disability. Around one fourth of the participants reported to be treated unfairly by their own families. Seventy percent reported to have concealed their mental health problems, 54% have stopped themselves from having a close personal relationship and 32% didn’t apply for work in anticipating discrimination. Experienced and anticipated discrimination were significantly associated with concealing the mental health problem. Conclusion Stigma due to having depression acts as a barrier to vocational & social integration and functional recovery. Concealment of the diagnosis of depression is itself barrier for help seeking and to receiving appropriate treatment. Small sample size and adopting the purposive sampling method are the limitations of the study.
This report deals with an investigation of brain mechanisms underlying species typical behavior. Squirrel monkeys (Saimiri sciureus) variously use a genital display in a show of aggression, courtship, and greeting. One variety consistently displays to its reflection in a mirror, providing a means of systematically testing effects of brain lesions on the incidence and manifestations of the display. Testing has been conducted on 90 animals living in special cages with an automatic device for presenting a mirror. Large bilateral lesions of many structures (e.g., amygdala, superior colliculus) may have only a transient or no effect on the display. Following bilateral lesions of globus pallidus, however, monkeys may show no inclination to display during several months of formal testing. There is no apparent motor deficit if lesions do not involve internal capsule. Tested in an established colony, such animals can successfully fight and defend themselves. Planimetric measurements in 14 cases suggest that the size, rather than locus, of the pallidal lesions is the critical factor. The results indicate that the striatal complex may be essential for certain forms of species typical behavior and associated imitative factors.
In the evolutionary transition from reptiles to mammals three key developments were (1) nursing, conjoined with maternal care, (2) audiovocal communication for maintaining maternaloffspring contact, and (3) play behavior (MacLean, 1985). There is accumulating evidence that the full expression of this family related behavioral triad depended on the evolution of the thalamocingulate division of the limbic system (MacLean, 1986a). This name derives from the fact that the transitional cingulate cortex of the limbic lobe receives its main afferent connections from the thalamus. Significantly, there is no apparent counterpart of this subdivision in the reptilian brain (Clark and Meyer, 1950).
The work of Cannon,¹ Bard,² Penfield,³ Ranson⁴ and others has greatly advanced knowledge of the functions of the hypothalamus. In the light of these researches the connections of the hypothalamus to the medial wall of the cerebral cortex gain a new significance. The following discussion presents some anatomic, clinical and experimental data dealing with the hypothalamus, the gyrus cinguli, the hippocampus and their interconnections. Taken as a whole, this ensemble of structures is proposed as representing theoretically the anatomic basis of the emotions. It is generally recognized that in the brain of lower vertebrates the medial wall of the cerebral hemisphere is connected anatomically and integrated physiologically with the hypothalamus and that the lateral wall is similarly related to the dorsal thalamus (Herrick⁵). These fundamental relations are not only retained but greatly elaborated in the mammalian brain by the further development of the hippocampal formation
Describes evolutionary psychiatry as a field that applies to potentially salutary insights derived from a better understanding of how the psychencephalon has evolved and how it functions. Comparative neuroanatomical findings indicate that the human forebrain has evolved and expanded to its great size while retaining commonalities of 3 neural assemblies that reflect an ancestral relationship to reptiles, early mammals, and late mammals. Radically different in their chemistry and structure, and in an evolutionary sense eons apart, the 3 assemblies constitute an amalgamation of 3 brains in 1, a triune brain.