Current Biology Vol 17 No 20
them a remarkable illustration
of the power of the evolutionary
process to create varied biological
types from a single ancestral form.
Moreover, their close phylogenetic
proximity to humans makes them
invaluable subjects for comparative
study. Presently, the power of this
comparative framework is being
realized in the study of primate
genomics and cognition. Because
the toothcombed primates are the
sister clade to the Anthropoidea,
comparisons between the two
have the unique ability to reveal
primate- specific traits that almost
certainly originated with the
ancestral primate species. The
current project to sequence the
genome of the gray mouse lemur,
Microcebus murinus (http://
is a preliminary but crucial
step towards understanding
the changes to the mammalian
genome that characterize the
primate genome. This comparison
offers the singular opportunity
for genomicists to recognize
those traits that are diagnostic of
primates, separate from all other
mammals. It is an essential first
step towards identifying those
genomic traits that are unique to
humans. In the same way, ground
breaking studies of lemur cognition
are showing that lemurs have
abilities for list memorization and
numerosity discrimination that
are similar to those of monkeys.
This latter finding, in particular, is
revolutionary as it demonstrates
that the higher cognitive functions
thought to uniquely characterize
anthropoid primates were almost
certainly present in the earliest
primates — mammals that first
evolved some 80 million years
ago. Without doubt, comparative
studies of lemurs and humans will
continue to refine and revolutionize
our understanding of primate
evolution and biology, from
genotype to phenotype.
What does the future hold
for lemurs? At first glance, the
future does not look very good
for lemurs in Madagascar, or for
the habitats in which they reside.
Forests are being destroyed at
an alarming rate, and to be a
lemur — any lemur — is to be an
endangered species. There may
be light at the end of the tunnel,
however. Madagascar’s current
president, Marc Ravalomanana,
is as committed to biodiversity
preservation as any president
in Madagascar’s history. In
September 2003, he announced
to the world his commitment to
triple the amount of Madagascar’s
protected areas within the
following five years. Labeled as
‘the Durban Vision’, the plan is
approaching its targeted year
for realization. Big strides have
been made towards achieving
the stated goals. Moreover, in
June 2007, the World Heritage
Committee has named a significant
proportion of Madagascar’s
eastern rainforests as one of three
new UNESCO World Heritage List
sites. Thus, we can hope that the
global coordination of captive
lemur breeding programs, and
the protection of Madagascar’s
remaining natural habitats, will
together provide a stable future for
these fascinating primates.
Where can I find out more about
lemurs and Madagascar?
Horvath, J.E., and Willard, H.F. (2007). Primate
comparative genomics: lemur biology and
evolution. Trends Genet. 23, 173–182.
Karanth, K.P., Delefosse, T.,
Rakotosamimanana, B., Parsons, T.J., and
Yoder, A.D. (2005). Ancient DNA from giant
extinct lemurs confirms single origin of
Malagasy primates. Proc. Natl. Acad. Sci.
USA 102, 5090–5095.
Merritt, D., MacLean, E.L., Jaffe, S., and
Brannon, E.M. (2007). A comparative
analysis of serial ordering in ring-tailed
lemurs (Lemur catta). J. Comp. Pyschol.,
The Duke Lemur Center: http://lemur.duke.
The Madagascar Fauna Group: http://www.
Yoder, A.D. (2003). Phylogeny of the lemurs. In
The Natural History of Madagascar, S.M.
Goodman and J. Benstead, eds. (Chicago:
University of Chicago Press),
Yoder, A.D., and Nowak, M. (2006). Has
vicariance or dispersal been the
predominant biogeographic force in
Madagascar? only time will tell. Annu. Rev.
Ecol. Evol. Systemat. 37, 405–431.
Yoder, A.D., Rasoloarison, R.M., Goodman,
S.M., Irwin, J.A., Atsalis, S., Ravosa, M.J.,
and Ganzhorn, J.U. (2000). Remarkable
species diversity in Malagasy mouse
lemurs (Primates, Microcebus). Proc. Natl.
Acad. Sci. USA 97, 11325–11330.
Yoder, A.D., and Yang, Z. (2004). Divergence
dates for Malagasy lemurs estimated
from multiple gene loci: geological and
evolutionary context. Mol. Ecol. 13,
Department of Biology, Duke University,
Box 90338, Durham, North Carolina
The amygdala is a complex
structure involved in a wide range
of normal behavioral functions and
psychiatric conditions. Not so long
ago it was an obscure region of the
brain that attracted relatively little
scientific interest. Today it is one
of the most heavily studied brain
areas, and practically a household
word. Art critics are explaining
the impact of a painting by its
direct impact on the amygdala;
essential oils are said to alter
mood by affecting the amygdala;
and there is a website where
you can unleash your creativity
by clicking your amygdala, and
thereby popping your frontal
cortex. In this Primer, I will focus
on the scientific implications
of the research, discussing the
anatomical structure, connectivity,
cellular properties and behavioral
functions of the amygdala.
The amygdala was first
recognized as a distinct brain
region in the early 19th century.
The name, derived from the Greek,
was meant to denote an almond-
like shape structure in the medial
temporal lobe. Like most brain
regions, the amygdala is not a
single mass but is composed
of distinct subareas or nuclei
(Figure 1). The almond shaped
area that gives the amygdala its
name was really only one of these
nuclei, the basal nucleus, rather
than the whole structure.
Nuclei within brain areas
like the amygdala are typically
distinguished on the basis of
histological criteria such as the
density, configuration, shape and
size of stained cells, the trajectory
of fibers, and/or chemical
signatures (Figure 1). Recently,
more subtle measures, such as
microscopic features of processes
(axons and dendrites) have also
been used. There has been much
debate about how the amygdala
should be partitioned on the basis
of the various criteria, and how the
Current Biology Vol 17 No 20
amygdala. Both conditioned stimuli
and emotional faces produce
strong amygdala activation
when presented unconsciously,
emphasizing the importance
of the amygdala as an implicit
information processor and its role
in unconscious memory. Findings
regarding the human amygdala
are mainly at the level of the whole
region rather than nuclei (Figure 6).
Structural and/or functional
changes in the amygdala are
associated with a wide variety of
psychiatric conditions in humans.
These include various anxiety
disorders (PTSD, phobia and panic),
depression, schizophrenia, and
autism, to name a few. This does
not mean that amygdala causes
these disorders. It simply means
that in people who have these
disorders alterations occur in the
amygdala. Because each of these
disorders involves fear and anxiety
to some extent, the involvement
of the amygdala in some of these
disorders may be related to the
increased anxiety in these patients.
Not so long ago the amygdala
was a neglected area of the brain,
attracting much less scientific
interest than other regions such
as the neocortex, hippocampus,
or cerebellum. In recent years,
though, scientists have turned
their attention to the amygdala,
revealing its structural organization,
physiological mechanisms, and
functions, both in animals and
humans. Recent studies have
also implicated the amygdala in
a variety of psychiatric disorders.
In spite of this progress much
remains unknown, especially about
behavioral functions. However, the
broad base of knowledge obtained
in recent years provides a firm
foundation upon which to build on
in future work.
J. Aggleton, ed. (2000). The Amygdala: A
Functional Analysis, 2nd Edition. (Oxford:
Oxford University Press).
Cardinal, R.N., and Everitt, B.J. (2004).
Neural and psychological mechanisms
underlying appetitive learning: links to
drug addiction. Curr. Opin. Neurobiol. 14,
Charney, D. (2003). Neuroanatomical circuits
modulating fear and anxiety behaviors.
Acta Psychiat. Scand. Suppl. 417, 38–50.
Davidson, R., and Erwin, W. (1999). The
functional neuroanatomy of emotion
and affective style. Trends Cogn. Sci. 3,
Dolan, R.J., and Vuilleumier, P. (2003).
Amygdala automaticity in emotional
processing. Ann. NY Acad. Sci. 985,
Dudai, Y. (2006). Reconsolidation: the
advantage of being refocused. Curr. Opin.
Neurobiol. 16, 174–178.
Davis, M., and Whalen, P.J. (2001). The
amygdala: vigilance and emotion. Mol.
Psychiatry 6, 13–34.
Holland, P.C., and Gallagher, M. (2004).
Amygdala-prefrontal interactions in
reward expectancy. Curr. Opin. Neurobiol.
Lamprecht, R., and Dudai, Y. (2000). The
amygdala in conditioned taste aversion:
It’s there, but where. In The Amygdala,
J. Aggleton, ed. (Oxford; Oxford
University Press), pp. 310–331.
Lang, P.J., Davis, M., and Ohman, A. (2000).
Fear and anxiety: animal models and
human cognitive psychophysiology.
J. Affect. Disord. 61, 137–159.
LeDoux, J.E. (1996). The Emotional Brain.
(New York: Simon and Schuster).
LeDoux, J.E. (2000). Emotion circuits in the
brain. Annu. Rev. Neurosci. 23, 155–184.
Maren, S. (2001). Neurobiology of Pavlovian
fear conditioning. Annu. Rev. Neurosci.
Maren, S., and Quirk, G.J. (2004). Neuronal
signaling of fear memory. Nat. Rev.
Neurosci. 5, 844–852.
McGaugh, J.L. (2003). Memory and Emotion:
The Making of Lasting Memories.
(London: The Orion Publishing Group).
Nader, K. (2003). Memory traces unbound.
Trends Neurosci. 26, 465-466.
Ohman, A., and Mineka, S. (2002). Fears,
phobias, and preparedness: toward an
evolved module of fear and fear learning.
Psychol. Rev. 108, 483–522.
Phelps, E.A., and LeDoux, J.E. (2005).
Contributions of the amygdala to emotion
processing: from animal models to human
behavior. Neuron 48, 175–187.
Pitkänen, A., Savander, V., and LeDoux, J.E.
(1997). Organization of intra-amygdaloid
circuitries in the rat: an emerging
framework for understanding functions
of the amygdala. Trends Neurosci. 20,
Rauch, S.L., Shin, L.M., and Phelps,
E.A. (2006). Neurocircuitry models
of posttraumatic stress disorder and
extinction: human neuroimaging
research–past, present, and future. Biol.
Psychiatry 60, 376–382.
Rodrigues, S.M., Schafe, G.E., and LeDoux,
J.E. (2004). Molecular mechanisms
underlying emotional learning and
memory in the lateral amygdala. Neuron
Rolls, E. (2005). Emotion Explained. (Oxford:
Oxford University Press).
Sah, P., Farber, E.S., Lopez De Armentia, M.,
and Power, J. (2003). The amygdaloid
complex: anatomy and physiology.
Physiol. Rev. 83, 803–834.
Samson, R.D., Duvarci, S., and Pare, D.
(2005). Synaptic plasticity in the central
nucleus of the amygdala. Rev. Neurosci.
Shinnick-Gallagher, P., Pitkänen, A., Shekhar,
A., and Cahill, L., eds. (2003). The
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Center for Neural Science, New York
University, 4 Washington Place, New
York, New York 10003, USA.
organiser of a sea
Jens H. Fritzenwanker2,
and Ulrich Technau2,*
In 1924 Hilde Mangold and
Hans Spemann transplanted
the dorsal blastopore lip of an
amphibian embryo to a host
embryo’s ventral side. This
experiment revealed that the
dorsal blastopore lip can act
as an ‘organiser’ to induce a
secondary body axis . The
organiser experiment has
fueled research in vertebrate
developmental biology until
today [2,3]. While an organiser
might have been present in
the chordate ancestor , it is
not clear how widespread the
principle of the blastoporal
organiser is and what its
evolutionary roots are. Here, we
examined the organising activity
of different parts of embryos of
the sea anemone Nematostella
vectensis, a representative
of the basal animal phylum
Cnidaria, which has retained
many ancestral traits. We show
by transplantation of small
parts of the gastrula embryo
that the blastopore lip — but
not tissue from other parts of
the embryo — is able to act as
an organiser and to induce the
formation of a secondary body
axis with high efficiency.
We analysed the inductive
capacity of different parts of the
gastrula embryo of Nematostella
by transplanting a vitally labeled
small piece of the blastopore
lip, the pre- endodermal plate
or the aboral blastocoel roof
blastoderm to the blastocoel
roof of unlabeled host gastrula
embryos. The size of the
to the equivalent of 10–20%
of the circumference of the
blastopore lip (about 20–30
μm diameter). We found that