ScienceWeek

SCIENCEWEEK

Briefs in the Sciences

May 23, 2003

Vol. 7 - Number 21B

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CONTENTS:

1. Hearing and Hearing Loss

2. On Tactile Perception

3. On the Stability of Synaptic Connections

4. Skin Cancers and Organ Transplantation

5. On Wolfgang Pauli (1900-1958)

6. Isotopes and the Early Solar System

7. On Signals Traveling Faster than the Speed of Light

8. On the Thickness of the Continents

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1. HEARING AND HEARING LOSS

J. Am. Med. Assoc. 2003 289:1976

The following points are made by B. Yueh et al:

1) Hearing loss is the third most prevalent chronic condition in
older Americans, after hypertension and arthritis; between 25%
and 40% of the population aged 65 years or older is hearing
impaired. The prevalence rises with age, ranging from 40% to 66%
in patients older than 75 years and more than 80% in patients
older than 85 years.

2) The healthy ear is an exquisitely sensitive organ. It
processes sound frequencies ranging from 20 Hz to 20 kHz. It
detects sounds as soft as 0.0002 dynes/cm^(2) (0 dB) and can
tolerate stimuli up to a million times more intense (200
dynes/cm^(2) or 120 dB) for limited periods of exposure. The ear
is particularly sensitive to signals between 500 and 4000 Hz,
which includes the frequencies most important for speech
processing.

3) The ear is composed of the external ear, the middle ear, and
the inner ear. The external ear consists of the pinna (auricle)
and the external auditory canal. Its function is thought to be
largely protective, although its physical configuration may
provide moderate (5-15 dB) passive augmentation of sounds at the
upper range of speech processing frequencies.

4) The middle ear is bounded laterally by the tympanic membrane
(eardrum) and medially by the osseous labyrinth, which is the
bone-encased structure that houses the end organs of hearing
(cochlea) and balance (semicircular canals). The healthy middle
ear is an air-filled cleft that contains the 3 ossicles (malleus,
incus, and stapes) that transduce vibrations from the tympanic
membrane to the oval window of the fluid-filled cochlea. The
substantially larger area of the tympanic membrane, compared with
that of the oval window, and the relatively minor mechanical gain
from the ossicular configuration combine to amplify sound
pressures by 20 to 30 dB (approximately the difference between a
whispered voice and normal conversational speech).

5) The inner ear includes the cochlea, the vestibular apparatus,
and the vestibulocochlear (acoustic) nerve (cranial nerve VIII).
The fluid channels within the cochlea are stimulated by the
vibrating stapes footplate through the membranous oval window at
the base of the cochlea. These fluid-filled channels (scala
vestibuli, tympani, and media) are lined by hair cells, which are
organized tonotopically (by sound frequency) in a coiled, spiral
shape. The base of the cochlea responds to high-frequency sounds,
and the apex responds to low-frequency sounds. Inner hair cells
are innervated by a rich array of afferent nerve fibers (10-20
fibers per hair cell) that synapse with auditory division of the
vestibulocochlear nerve at the spiral ganglion.

6) The two major forms of hearing loss are conductive and
sensorineural disorders. Conductive hearing losses usually
involve abnormalities of the middle and external ear, and
generally have a mechanical cause (eg, perforated eardrum, fluid
in the middle ear, disarticulations of the ossicular chain,
cerumen accumulation). As a result, treatment is often surgical
(eg, repair of the perforated eardrum, drainage of fluid-filled
middle ear, reconstruction of the ossicular chain, removal of
cerumen). However, more than 90% of hearing loss is sensorineural
(nerve deafness), which typically results from permanent damage
to the hair cells of the cochlea. 

7) Sensorineural loss related to aging, or presbycusis, is the
most common cause of hearing loss in the US. This type of hearing
loss is typically gradual, bilateral, and characterized by high-
frequency hearing loss. Patients with presbycusis typically have
difficulty filtering background noise, which makes listening
especially challenging in common social settings. Because no
known treatment is available for damaged hair cells, presbycusis
is typically treated with amplification devices, such as hearing
aids. Profound deafness can be treated with cochlear
implantation, which bypasses the hair cells to stimulate the
vestibulocochlear nerve directly. 

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2. ON TACTILE PERCEPTION

Current Biology 2003 13:R170

The following points are made by P. Haggard et al:

1) The reliability of bodily sensation implies accurate
transmission of peripheral information to the higher brain
centers of conscious perception. Higher cortical regions which
underlie tactile perception also provide several top down
influences which modulate perception: so the brain constructs our
sense of the body, rather than passively receiving it.

2) Bodily sensation is unique in its neurophysiological basis.
The body has many different classes of sensory receptor, each
transducing a specific type of stimulus.

3) Tactile perception may have a special role in body
representation, because the skin forms the interface between the
body and the outside world. Other sensory systems, notably pain
and body position sense, also contribute to body representation.
Nociception lacks the spatial specificity of touch, and
proprioceptive contributions to body representation are difficult
to dissociate from the tactile and motor events normally
correlated with them. So the brain's processing of touch is
perhaps the clearest way to study the construction of our sense
of our own body.

4) The structure and function of the peripheral and subcortical
somatosensory system is well known. Tactile information is
conveyed to the primary somatosensory cortex (SI) of the
contralateral hemisphere. Here, tactile perception and body
representation begin to converge. SI contains a somatotopic map
of the contralateral side of the body. Early studies emphasized
its role as a veridical, organized projection, faithfully
transmitting peripheral inputs. For example, intracranial
stimulation of sites in the SI map produces sensation on the
corresponding body part. More recent studies suggest that SI
processes may be modulated by context, in particular the general
perceptual experience of the body provided by other senses such
as vision.

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3. ON THE STABILITY OF SYNAPTIC CONNECTIONS

Current Biology 2003 13:R180

The following points are made by M.P. Meyer et al:

1) How long do individual synaptic connections persist in mature
nervous systems? Despite progress made towards answering this
question for peripheral synapses, until recently technical
restraints have prevented direct measurements of the stability of
synaptic connections in the intact central nervous system.

2) Patterns of synaptic connection underlie all aspects of brain
function, from perception to learning and memory. Initial
establishment of synaptic connections is believed to occur
independently of experience. This is followed by "critical
periods" during which synaptic connections in the developing
nervous system can be permanently modified by experience. For
example, the somatosensory cortex of rodents receives sensory
input from the whiskers, which are arranged in a regular pattern
across either side of the snout. This topographic pattern is
recapitulated in the cortex in the form of clusters of neurons,
termed "barrels", that respond preferentially to a single
principle whisker. Sensory deprivation applied early in postnatal
life, by trimming all but one whisker on one side of the face,
has shown that the area of cortex driven by the spared whisker is
enlarged in the adult, but the overall layout of the barrel field
remains unaltered. The visual cortex is also organized in a
topographic fashion, and it has been demonstrated that monocular
deprivation during development causes the cortical map for eye
preference to shift in favor of the open eye. Such "ocular
dominance" plasticity can also only be induced during a critical
period of map development.

3) The closure of critical periods defines the point when
experience can no longer cause large-scale rearrangements of
topographic maps, although the adult brain must retain some
capacity to reorganize its functional connections in response to
experience and injury. There are several possible levels at which
such modifications could occur. Models of use-dependent
potentiation and depression -- LTP and LTD -- assume synaptic
transmission is plastic while patterns of connectivity are
unaltered. There are also reports of rearrangements at the level
of axonal and dendritic arbors in visual and somatosensory
cortices following major alterations in peripheral sensory
inputs. A third possibility is that dendritic spines might
mediate experience-dependent remodeling of neuronal circuits by
forming or eliminating synaptic connections, without large-scale
rearrangements of dendritic or axonal arbors.

Notes:

During the past several decades, the detailed anatomy of nerve
cell dendrites has been a focus of much research, in particular
the often-present parts of dendrites called "dendritic spines".
These spines are small (1 to 2 microns) thorn-like protuberances
along the length of a dendrite, and there is evidence that such
spines may be important components in many kinds of neural
microcircuits. In the human nervous system, dendritic spines are
especially prominent in the cerebellar cortex, basal ganglia, and
cerebral cortex, and in the cerebral cortex approximately 80
percent of all excitatory synapses are evidently made onto
dendritic spines, whereas only approximately 30 percent of
inhibitory synapses are made onto dendritic spines. Currently,
neurobiologists have ascribed literally dozens of different
functions to dendritic spines, and investigations of these
structures are underway in many laboratories.

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4. SKIN CANCERS AND ORGAN TRANSPLANTATION

New Engl. J. Med. 2003 348:1681

The following points are made by S. Euvrard et al:

1) Long-term survival after organ transplantation is increasing.
As a result, many patients have long-term complications of
transplantation. Adequate graft function requires lifelong
immunosuppressive treatment, and the resultant modification of
the immune system is associated with an increased risk of various
cancers, particularly those involving viruses. Skin cancers are
the most common malignant conditions in transplant recipients and
account for substantial morbidity and mortality in such patients.

2) Squamous-cell and basal-cell carcinomas account for more than
90 percent of all skin cancers in transplant recipients. The
incidence of these carcinomas increases with the duration of
immunosuppressive therapy, ultimately affecting 50 percent or
more of white transplant recipients. For example, the cumulative
incidence of skin cancer in transplant recipients in Queensland,
Australia, increases from 7 percent after 1 year of
immunosuppressive therapy to 82 percent after 20 years. Among
Dutch transplant recipients, the incidence of skin cancer at one
year is 0.2 percent and the long-term incidence is 41 percent.

3) Squamous-cell carcinoma is the most common skin cancer in
transplant recipients, occurring 65 to 250 times as frequently as
in the general population. The incidence of basal-cell carcinomas
is reportedly increased by a factor of 10 in transplant
recipients. The risk appears to increase linearly for basal-cell
carcinomas and exponentially for squamous-cell carcinomas; thus,
the ratio of squamous-cell to basal-cell carcinomas in patients
without transplants (1:4) is reversed in transplant recipients.
The relative risk of squamous-cell carcinoma after
transplantation is higher for men than for women, except for
cancers of the lip. Curiously, skin cancers appear to be
extremely rare in Japanese patients with transplants.

Notes:

The skin cancer called "basal-cell carcinoma", rare in Blacks and
Asians, is the most common malignant skin tumor in Whites. This
cancer arises from the undifferentiated basal keratinocytes of
the epidermis. Such cancers rarely metastasize but may be highly
invasive locally, when they are called "rodent ulcers". The
lesions are prevalent in fair-skinned persons and on areas of
skin that receive the greatest exposure to sunlight. Treatment
with inorganic arsenical drugs and exposure to ionizing radiation
(e.g., x-rays) may also be contributing factors.

The skin cancer called "squamous-cell carcinoma" is less common
than basal-cell carcinoma but has a higher rate of metastasis.
Squamous-cell carcinoma is common in children with xeroderma
pigmentosum, who are unable to repair DNA damage caused by
ultraviolet radiation. In most persons, such an inability to
repair DNA damage is due to a deficiency of an endonuclease
enzyme. In adults, squamous-cell carcinoma rarely occurs in the
absence of an external cause, and protracted exposure to sunlight
is the usual cause of this disease. Chronic scarring from burns,
as well as reactions to vaccinations, radiation dermatitis, and
chronic ulceration may also be risk factors for squamous-cell
carcinoma of the skin.

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5. ON WOLFGANG PAULI (1900-1958)

Science 2003 300:252

The following points are made by Cathryn Carson:

1) Wolfgang Pauli's friend Paul Ehrenfest (1880-1933) nicknamed
him "God's whip". For three generations of theoretical
physicists, Pauli was a provoker, a critic, and also a clarifier.
He was baptized antimetaphysical, he once said, by his godfather
Ernst Mach (1838-1916), and the sharpness of his judgment was
visible early. He had picked up special and general relativity as
a high-school student in Vienna, and two months after receiving
his doctorate he published his famous 237-page review of the
subject, which induced Einstein to comment, "No one studying this
mature, grandly conceived work would believe that the author is a
man of twenty-one."

2) In picking his central domain, the young Pauli then followed
the guidance of Arnold Sommerfeld (1868-1951), his teacher in
Munich, and dove into the contradictions of quantum physics in
the years around 1920. It was a shrewd choice. In addition to
framing the exclusion principle that describes how electrons are
allocated to atomic energy levels (which earned him the 1945
Nobel Prize in Physics), Pauli helped lay the foundations of the
quantum theory of fields, proposed the neutrino, and proved the
spin-statistics theorem.

3) From the 1920s through the 1950s -- he died at age 58 -- Pauli
tracked the unfolding of quantum mechanics and quantum field
theory with a biting wit and a keen sense for where difficulties
lay hidden. At the same time, as some of his colleagues knew
well, he was deeply interested in dream interpretation,
archetypes, the unconscious, non-Western religions, and the
history of science. His curiosity accompanied a friendship with
C.G. Jung (1875-1961) that began with Pauli's psychoanalytic
treatment by a Jung disciple in Zurich. Pauli's contemporaries
understood that his physics and the rest of his life cohered.

Notes:

Consider an old black-and-white faded photograph: A soccer ball
is flying directly at the camera, almost arrived and large in
view, the soccer ball made fuzzy by its movement, and in the
background, obviously the kicker of the soccer ball, his kicking
foot still raised, is a rather pudgy middle-aged fellow in baggy
trousers, white shirt and tie, grinning with malicious delight at
his aimed kick (the ball actually slammed into the lens of the
expensive Speed Graphic camera), a pudgy fellow with a round face
who might be your uncle, the one who sells real estate, or the
local butcher who chuckles as he chops meat on the chopping
block. But a physiognomy is merely a mask, and the kicker of the
soccer ball, this grinning round-faced fellow facing you with his
foot raised and his back to the shore of Lake Lucerne
(Vierwaldstaetter See) in Switzerland in 1950, the kicker of the
ball is neither your uncle nor the local butcher, but one of
those ephemeral sparks of great genius thrown up by the grinding
gears of history -- the theoretical physicist Wolfgang Pauli.

Pauli never published much, certainly not as much as his more
competitive contemporary physicists, but his influence on modern
physics was profound and enduring. He is most remembered for two
major contributions: the exclusion principle and his prediction
of the existence of the neutrino, but there were many other
contributions of lasting significance. Pauli exemplifies what
might be called "collegial science": his influence on his
contemporary physicists derived primarily from conversations and
letters.

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6. ISOTOPES AND THE EARLY SOLAR SYSTEM

Science 2003 300:265

The following points are made by Ernst Zinner:

1) The isotopic composition of ancient meteorites indicates that
short-lived, now extinct radioisotopes existed in the early Solar
System. Recent measurements on meteorites provide evidence that
several of these short-lived isotopes came from a stellar source
shortly before our Solar System formed. It is thus likely that a
stellar event triggered the formation of the Solar System, and
recent evidence points to a supernova explosion.

2) Short-lived nuclides can potentially shed light on a variety
of processes in the early Solar System. Some of them may have
been formed in stellar sources, connecting the birth of the Solar
System to the presence of nearby stars. Others may have been
produced in the Solar System, providing information about the
early Sun. The nuclides may also serve as chronometers with a
resolution of less than a million years. And two nuclides -- Al-
26 and Fe-60 -- may have been powerful heat sources for the
melting of early planetary bodies.

3) Because the short-lived isotopes no longer exist, they can
only be traced through their daughter isotopes. The ratios of
short-lived isotopes relative to stable isotopes of the same
elements in early Solar System objects change with time, because
different radioisotopes decay at different rates. The ratios for
different isotopes should therefore ideally be measured at the
same time, but this is not always possible. The reason is that
the original presence of a short-lived isotope is indicated by
excesses in the daughter isotope in samples from various
meteorites. Measurements of such excesses require a large
parent/daughter ratio. There are no samples where this is the
case for many different short-lived nuclides.

Notes:

During the past two centuries, astronomers have considered two
types of theories for the origin of our Solar System planets.
Catastrophic theories proposed that these planets formed from
some improbable cataclysm such as the collision of the Sun and
another star, while gradualist theories proposed that the planets
formed naturally with the Sun. At the present time, as a result
of evidence accumulated during the past five decades, the
gradualist idea is the consensus idea, and nearly all astronomers
now believe that planets form naturally as a by-product of star
formation.

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7. ON SIGNALS TRAVELING FASTER THAN THE SPEED OF LIGHT

Nature 2003 422:271

The following points are made by M. Buettiker and S. Washburn:

1) A controversy has arisen from experiments in which pulses of
light and microwaves appear to tunnel through a barrier at speeds
faster than a reference pulse moves through a vacuum
("superluminal tunneling"). This is an apparent violation of
Einstein's founding assumption of relativity, the assumption that
nothing can move faster than light through a vacuum. What does
"faster" really mean in this context? As each pulse is in fact a
"packet" of waves, the central issue is the definition of the
speed of the wave packet.

2) Tunnelling occurs when a wave impinges on a thin barrier of
opaque material and some small amount of the wave "leaks" through
to the other side. The superluminal experiments that caused
controversy were performed with a lattice of layers of
transparent and opaque materials arranged so that waves of some
frequencies are reflected (through destructive interference) but
other frequencies pass through the lattices. All materials behave
like this to some extent: the glass in a window is transparent to
optical frequencies but relatively opaque to infrared
frequencies.

3) In all cases of superluminal tunneling, the pulse that emerges
from the tunnelling process is greatly attenuated, and "front-
loaded" -- only the leading edge of the incident pulse survives
the tunnelling event without being severely attenuated to the
point that it cannot be detected. If we measure the speed by the
peak of the pulse, it looks faster than the incident pulse. But
that is just an artefact of our definition of speed as marked by
the arrival of the peak in the pulse.

Notes:

An experimental "light pulse" has a finite duration, and in
theory (the so-called "bandwidth theorem") this requires an
infinite number of waves of different frequency to be added
together. The shorter the desired pulse, the larger the bandwidth
of frequencies that must be used. Theoretically, all light pulses
are therefore formed by a packet of waves of different frequency,
each of which has a different amplitude and phase. The speed of
individual waves is called the "phase velocity", and the velocity
at which the peak of the wave packet propagates is called the
"group velocity". In a vacuum, the phase and group velocities are
identical, but in a highly absorbing or dispersive medium the
phase and group velocities are usually different. A so-called
"negative group velocity" results when the phases of the
different frequency components are shifted by the medium through
which they travel in a way such that the wave packet they form at
the exit is brought forward in time compared with the same pulse
traveling through a vacuum.

During the past two decades, using various techniques,
researchers have demonstrated the transmission of certain light
pulses over short distances with apparent velocities greater than
the speed of light in a vacuum (c), the effect known as
"superluminal light propagation". The experimental conditions are
extremely specific, and the results difficult of general
interpretation. But despite ballyhoo in the popular media
concerning these experiments, the rule in physics remains intact:
no mass can travel faster than (c), and no information or signal
can be transmitted faster than (c). Any violation of this rule
would imply a violation of both Einstein's theory of special
relativity and the principle of causality -- and the rule has not
yet been demonstrated to be violated anywhere.

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8. ON THE THICKNESS OF THE CONTINENTS

Nature 2003 422:707

The following points are made by Yuancheng Gung et al:

1) For decades there has been a vigorous debate about the depth
extent of continental roots. The analysis of heat-flow, mantle-
xenolith and electrical-conductivity data all indicate that the
coherent, conductive part of continental roots (the
"tectosphere") is at most 200 250 km thick. Some global seismic
tomographic models agree with this estimate, but others suggest
that a much thicker zone of high velocities lies beneath
continental shields, reaching a depth of at least 400 km.

2) The authors demonstrate that this disagreement can be
reconciled by taking into account seismic anisotropy. The authors
show that significant radial anisotropy, with horizontally
polarized shear waves travelling faster than those that are
vertically polarized, is present under most cratons in the depth
range 250 400 km --similar to that found under ocean basins at
shallower depths of 80 250 km. 

3) The authors propose that, in both cases, the anisotropy is
related to shear in a low-viscosity asthenospheric channel,
located at different depths under continents and oceans. The
seismically defined "tectosphere" is then at most 200 250 km
thick under old continents. The "Lehmann discontinuity", observed
mostly under continents at about 200 250 km, and the "Gutenberg
discontinuity", observed under oceans at depths of about 60 80
km, may both be associated with the bottom of the lithosphere,
marking a transition to flow-induced asthenospheric anisotropy.

Notes:

The term "xenolith" refers in general to an inclusion of a
preexisting rock in an igneous rock.

Seismic tomography is a technique similar to medical x-ray
tomography, except that seismic velocities are imaged.

The term "shear wave" (S-wave) refers to an elastic body wave in
which particles oscillate about a fixed point but in a direction
perpendicular to the direction of propagation of the wave energy.

The term "craton" refers to the core of a continent. The
existence of these small and ancient cores of continents has long
been a puzzle. Cratons were apparently created during the
Archaean time-frame, 4 billion to 2.5 billion years ago, and they
form the oldest parts of Earth's tectonic plates.

The term "lithosphere" refers to the outer layer of the Earth,
comprising the crust and upper mantle, and extending to a depth
of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer. "Plate tectonics" is the current consensus theory
that the Earth's lithosphere is broken into fairly rigid plates,
seven or eight major plates and many smaller plates, and that
convection within the underlying less rigid "asthenosphere"
causes the plates (and the associated continents and crust) to
move relative to each other.

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