bluuebunny/crossref_metadata_2025 huggingface dataset:
BEYOND EXPLICIT RULE LEARNING
Authors: DeKeyser Robert M.
Published: July 2002

This study is a fine-grained analysis of extensive empirical data on the automatization of
explicitly learned rules of morphosyntax in a second language. Sixty-one subjects were taught
four morphosyntactic rules and 32 vocabulary items in an artificial language. After they had
reached criterion on a set of metalinguistic tests of grammar and vocabulary, they engaged in
systematic, computer-controlled comprehension and production practice for 8 weeks.
Comprehension practice consisted of choosing between pictures displayed on the computer
screen to match a sentence; production practice consisted of typing the correct sentence
corresponding to a picture. All subjects were taught the same rules and then practiced them, and
all subjects had the same amount of comprehension and production practice, but which rules
were practiced in comprehension and which in production varied between groups. Results show
that the learning of morphosyntactic rules is highly skill-specific and that these skills develop
very gradually over time, following the same power function learning curve as the acquisition of
other cognitive skills. These results are consistent with current skill acquisition theory.
Acquiring basic word order: evidence for data-driven
learning of syntactic structure
Authors: AKHTAR NAMEERA
Published: July 2002

Recent studies indicate that young English-speaking children do not
have a general understanding of the significance of SVO order in
reversible sentences; that is, they seem to rely on verb-specific formulas
(e.g. NPpusher – form of the verb PUSH – NPpushee)
to interpret such sentences (Akhtar & Tomasello, 1997). This finding raises the possibility
that young children may be open to learning non-SVO structures with
novel transitive verbs. To test this hypothesis, 12 children in each of
three age groups (two-year-olds, three-year-olds, and four-year-olds)
were taught novel verbs, one in each of three sentence positions: medial
(SVO), final (SOV), and initial (VSO). The younger age groups were
equally likely to use the novel (non-English) orders spontaneously as to
correct them to SVO order, whereas the oldest children consistently
corrected these structures to SVO order. These results suggest that
English-speaking children's acquisition of a truly general understanding
of SVO order may be a gradual process involving generalization
(learning) from examples. The findings are discussed in terms of recent
data-driven learning accounts of grammar acquisition.
Systoles of hyperbolic 3-manifolds
Authors: ADAMS COLIN C. , REID ALAN W.
Published: July 2002

Let M be a complete hyperbolic n-manifold of finite
volume. By a systole of M we
mean a shortest closed geodesic in M. By the systole length of M we mean the length
of a systole. We denote this by sl (M). In the case when M is closed, the systole length
is simply twice the injectivity radius of M. In the presence of cusps, injectivity radius
becomes arbitrarily small and it is for this reason we use the language of ‘systole
length’.

In the context of hyperbolic surfaces of finite volume, much work has been done
on systoles; we refer the reader to [2, 10–12]
for some results. In dimension 3, little seems known about systoles. The main result in
this paper is the following (see below for definitions):
Reclaiming the Past, Building a Future: Māori Identity in the Plays of Hone Kouka
Authors: Peterson William
Published: July 2002

Hone Kouka's historical plays Nga Tangata Toa and Waiora, created and produced in Aotearoa/New Zealand, one set in the immediate aftermath of World War I, and the other during the great Māori urban migrations of the 1960s, provide fresh insights into the way in which individual Māori responded to the tremendous social disruptions they experienced during the twentieth century. Much like the Māori orator who prefaces his formal interactions with a statement of his whakapapa (genealogy), Kouka reassembles the bones of both his ancestors, and those of other Māori, by demonstrating how the present is constructed by the past, offering a view of contemporary Māori identity that is traditional and modern, rural and urban, respectful of the past and open to the future.
Low-Cost INS/GPS Integration: Concepts and Testing
Authors: Cannon M. E. , Nayak R. , Lachapelle G. , Salychev O. S. , Voronov V. V.
Published: July 2002

The high cost of inertial units is the main obstacle for their inclusion in precision navigation
systems to support a variety of application areas. Standard inertial navigation systems (INS)
use precise gyro and accelerometer sensors; however, newer inertial devices with compact,
lower precision sensors have become available in recent years. This group of instruments,
called motion sensors, is six to eight times less costly than a standard INS. Given their weak
stand-alone accuracy and poor run-to-run stability, such devices are not usable as sole
navigation systems. Even the integration of a motion sensor into a navigation system as a
supporting device requires the development of non-traditional approaches and algorithms.
The objective of this paper is to assess the feasibility of using a motion sensor, specifically
the MotionPak™, integrated with DGPS and DGLONASS information, to provide accurate
position and attitude information, and to assess its capability to bridge satellite outages for
up to 20 seconds. The motion sensor has three orthogonally mounted ‘solid-state’ micro-
machined quartz angular rate sensors, and three high performance linear servo accelerometers
mounted in a compact, rugged package. Advanced algorithms are used to integrate
the GPS and motion sensor data. These include INS error damping, calculated platform
corrections using DGPS (or DGPS/DGLONASS) output, velocity correction, attitude
correction and error model estimation for prediction. This multi-loop algorithm structure is
very robust, which guarantees a high level of software reliability. Vehicular and aircraft test
trials were conducted with the system in land vehicle mode and the results are discussed.
Simulated outages in GPS availability were made to assess the bridging accuracy of the
system. Results show that a bridging accuracy of up to 3 m after 10 seconds in vehicular
mode and a corresponding accuracy of 6 m after 20 seconds in aircraft mode can be
obtained, depending on vehicle dynamics and the specific MotionPak™ unit used. The
attitude accuracy was on the order of 22 to 25 arcmin for roll and pitch, and about 44 arcmin
for heading.
The Use of Landmarks by Clark's Nutcrackers: First Tests of a New Model
Authors: Kamil Alan C. , Goodyear Aleida J. , Cheng Ken
Published: July 2002

Animals use many different mechanisms to navigate in space. The characteristics of the mechanism employed are usually well-suited to the demands of each particular navigational problem. For example, desert ants navigating in a relatively featureless environment use path integration, birds homing or migrating over long distances use compasses of various sorts, salmon returning to their natal stream home on olfactory cues. The study of navigation requires the study of many different taxa confronting different problems. One interesting case involves scatter-hoarding species that use memory to relocate their hidden food. Such animals face the problem of remembering many locations simultaneously. Clark's nutcrackers (Nucifraga columbiana) are an excellent example, and this paper considers their possible use of multiple bearings from landmarks.
Contrasts in conservation attitudes and agricultural practices between farmers operating in wetlands and a plain in Macedonia, Greece
Authors: PYROVETSI MYRTO , DAOUTOPOULOS GEORGE
Published: July 2002

Conservation projects around the world have been impeded by conflicting land uses. In Greece, although agriculture is considered to have significant impacts on wetlands and adversely to have affected conservation, the attitudes of Greek farmers to wetlands and conservation have not been assessed. Data on demographic variables of farm operators, characteristics of the farming operation, irrigation practices, attitudes towards environment and the wetland resources, knowledge on the impact of agriculture on the local environment and opinions on the Common Agricultural Policy reform, were collected through a survey using personal interviews from a random sample of 196 farmers operating in two wetlands, Lakes Mikri Prespa and Kerkini (Ramsar sites), and 141 farmers operating in a plain, in Macedonia, northern Greece.

Analysis of the data revealed that farmers practise crop and stock agriculture more intensively in wetlands than in the plain, and exploit wetland resources excessively. Hunting, fishing and wood harvesting are practised, and lake water is used intensively for irrigation by both groups of farmers, with no care for loss of the resources. Alternative methods of irrigation for improving efficiency of water use or alleviating hydrological pressures on the wetlands are not considered. Sustainability of agriculture is at risk as present practices result in impoverished soils, salinization of the fields and waste of water resources. Wetland farmers seemed to have a more negative attitude toward the wetland resources and seemed to be more ignorant of conservation issues or the impact of their practices on the environment than plain farmers. Moreover, their awareness and willingness to adopt an environmentally-friendly type of farming was very low.
Microbial quality of oysters sold in Western Trinidad and
potential health risk to consumers
Authors: RAMPERSAD F. S. , LALOO S. , LA BORDE A. , MAHARAJ K. , SOOKHAI L. , TEELUCKSINGH J. , REID S. , McDOUGALL L. , ADESIYUN A. A.
Published: July 2002

The prevalence and characteristics of Escherichia coli and
Salmonella spp. as well as counts of
E. coli in raw oysters, condiments/spices, and raw oyster cocktails sampled from 72 vendors
across Western Trinidad were determined. The microbial quality of the water used in the
preparation of raw oysters was also investigated. Of 200 samples each of raw oysters,
condiments/spices and oyster cocktails tested, 154 (77·0%), 89 (44·5%)
and 154 (77·0%) respectively yielded E. coli. The differences were
statistically significant (P=[les ]0·001; χ2=62·91).
The mean E. coli count per g in the ready-to-eat oyster cocktail ranged from
1·5×103±2·7×103 in Couva to
8·7×106±4·9×107 in San Fernando. One hundred and forty-six (73·0%) oyster cocktails contaminated with E. coli had counts that exceeded the
recommended standard of 16 per g. Of a total of 590 E. coli isolates from various sources
tested, 24 (4·1%), 20 (3·4%) and 69 (11·7%) were mucoid,
haemolytic and non-sorbitol fermenters respectively. Twelve (2·0%)
isolates of E. coli were O157 strains, while 92 (46·0%)
of 200 E. coli isolates tested belonged to enteropathogenic serogroups. Ninety (45·0%) and 73
(36·5%) of 200 water samples contained total coliforms and faecal coliforms respectively, with
counts that exceeded 2·2 coliforms per 100 ml. Salmonella
spp. were isolated from 7 (3·5%), 1
(0·5%) and 2 (1·0%) of 200 samples each, of raw oysters, condiments/spices and oyster
cocktails respectively. Oysters pose a health risk to consumers in Trinidad, particularly from
colibacillosis and salmonellosis, and the need for increased public awareness of this hazard
cannot be over-emphasized.
Prevalence of hepatitis B, D and C virus infections among
children and pregnant women in Moldova: additional evidence
supporting the need for routine hepatitis B vaccination of
infants
Authors: DROBENIUC J. , HUTIN Y. J. F. , HARPAZ R. , FAVOROV M. , MELNIK A. , IAROVOI P. , SHAPIRO C. N. , WOODRUFF B. A.
Published: July 2002

Rates of acute hepatitis B are high in Moldova, but the prevalence of chronic infection is
unknown. In 1994, we surveyed children and pregnant women, collected demographic
information, and drew blood for laboratory testing. Among the 439 children (mean age,
5 years), the prevalence of antibody to hepatitis B core antigen (anti-HBc) and hepatitis B
surface antigen (HBsAg) were 17·1 and 6·8%, respectively. Among the 1098 pregnant women
(mean age, 26 years), 52·4% were anti-HBc-positive and 9·7% were HBsAg-positive. Of the
HBsAg-positive pregnant women, 35·6% were hepatitis B e antigen (HBeAg) positive and
18·3% had antibodies to hepatitis D virus. The prevalence of antibody to hepatitis C virus was
1·4% in children and 2·3% in pregnant women. The high HBeAg prevalence among HBsAg-positive pregnant women and the high anti-HBc prevalence among children indicate that both
perinatal and early childhood transmission contribute to the high hepatitis B virus endemicity
in Moldova.
Outbreak of meningococcal disease in western Norway due to
a new serogroup C variant of the ET-5 clone: effect of
vaccination and selective carriage eradication
Authors: SMITH I. , LEHMANN A. K. , LIE L. , DIGRANES A. , CAUGANT D. A. , HØIBY E. A. , FRØHOLM L. O. , HALSTENSEN A.
Published: July 2002

A new sulphonamide resistant (SR) C[ratio ]15[ratio ]P1.7,16 meningococcal strain, a variant of the ET-5
clone, dominated in an outbreak of 22 cases in western Norway commencing in 1995. The first
eight patients were 15–21 years old from the Nordhordland area, initiating a carrier study in
the local high schools. Carriage of SR serogroup C meningococci was detected by routine
methods and treated with a single dose of ofloxacin 400 mg. Of 20 treated carriers, 14
harboured the outbreak strain C[ratio ]15[ratio ]P1.7,16. Vaccination of 4000 children, adolescents and
close contacts of patients was also performed. After the intervention, 14 additional cases of
meningococcal disease occurred, 8 due to the outbreak strain. However, incidence rates
dropped from 180 to 30 per 100000 per year in the student population, but increased from
0 to 13 in the rest of the population in Nordhordland. Carriage eradication is not generally
recommended in Norway. However, tracing and treating meningococcal carriage may have
reduced transmission and disease in this outbreak situation.
Serogroup C meningococcal disease outbreak associated with
discotheque attendance during carnival
Authors: HAURI A. M. , EHRHARD I. , FRANK U. , AMMER J. , FELL G. , HAMOUDA O. , PETERSEN L.
Published: July 2002

In the week following a carnival during 19–24 February 1998, an outbreak of meningococcal
disease occurred in a rural German county. The available isolates belonged to phenotype
C[ratio ]2a[ratio ]P1.2,5 and were clonally related by pulsed-field gel electrophoresis. A case-control study
was done to identify risk factors for the outbreak and to define possible vaccination target
groups. Five persons aged 13–16 years who fell ill during 24–27 February were included in the
study. Four of 5 cases and 10 of 32 controls visited local discotheques (OR = 8·8; P = 0·06).
Cases also visited discotheques more frequently than controls (χ2 for trend, P = 0·0002).
Multiple discotheques during the carnival may have been predominant locations of
transmission in this outbreak. Because this risk factor was limited in time, a mass community
vaccination campaign was not initiated.
Expression of CB2 cannabinoid receptor mRNA in adult rat retina
Authors: LU QINGJUN , STRAIKER ALEX , LU QINGXIAN , MAGUIRE GREG
Published: July 2002

To date, two cannabinoid receptors, CB1 and CB2,
have been cloned. The CB1 receptor has been found in a
variety of tissues, particularly in the brain. CB2 receptor
mRNA is mainly expressed in the immune system, though one
group has found it in mouse cerebellum. Previous immunostaining
studies in our lab demonstrated the presence of CB1 receptors
in the retina though little evidence exists for the presence
of CB2. The putative endogenous ligand for CB2 has been
found in retina, however, suggesting that further study
of CB2 in retina is warranted. Because glutamate is toxic
to retinal ganglion cells in glaucoma and activation of
CB2 receptors may be able to protect neurons from glutamate-induced
death, we examined the expression of CB2 mRNA in adult
rat retina in order to better understand possible neuroprotective
mechanisms relevant to glaucoma. Using in situ
hybridization, we demonstrated that CB2 cannabinoid receptor
messenger RNA was clearly expressed in the adult rat retina,
including the somas of retinal ganglion cells. Antisense
cRNA probe detected strong signals in the retinal ganglion
cell layer, the inner nuclear layer, and the inner segments
of photoreceptor cells. Using reverse transcription polymerase
chain reaction (RT-PCR) in both rat and mouse tissue, we
obtained an RT-PCR product with the same sequence as that
reported for CB2 in the GenBank database, thus confirming
the presence of CB2 mRNA in retina. The presence of CB2
in retina provides new evidence for the presence of CB2
in the central nervous system (CNS) and an excellent model
for its study.
High color-vision sensitivity in macaque and humans
Authors: LOOP MICHAEL S. , CROSSMAN DAVID K.
Published: July 2002

Psychophysical (behavioral) detection thresholds
and color-discrimination thresholds were determined in
a macaque using a two-alternative forced-choice procedure.
On a white background, detection thresholds were determined
for a white increment and three spectral increments: 618,
516, and 456 nm. Intermixed with detection threshold determinations,
color-discrimination thresholds were determined by presenting
the white increment, and one of the spectral increments,
at 1.0 log units above their respective detection thresholds
and dimming both until discrimination performance fell
to threshold. The monkey could discriminate the color of
the increments at detection threshold because the average
color-discrimination threshold was 0.98 ± 0.14 log
attenuation. Because the monkey was much more sensitive
to the spectral increments than the white increment, we
performed an unconventional experiment. We determined the
monkey's detection threshold for the white increment
alone, and with broadband color filters in the
white light path without adjusting the light's intensity.
Insertion of several color filters in the light path lowered
detection thresholds of both the macaque and six human
trichromats. We believe that this improvement in detection
thresholds produced by simply inserting color filters in
a white light path is a threshold manifestation of the
Helmholtz-Kohlrausch effect and suggests that one of color
vision's important evolutionary advantages may be
improved detection sensitivity.
The fountain amacrine cells of the rabbit retina
Authors: WRIGHT LAYNE L. , VANEY DAVID I.
Published: July 2002

(article appeared in Visual Neuroscience (1999),
16, 1145–1156.)

Due to an unfortunate misunderstanding, Figure 3 that
appeared on p. 1149 of Visual Neuroscience 16:6
was not sized to the full two-column measure. A corrected
version of the article, with Figure 3 reshot to full size,
is reproduced on pp. 1145R–1156R, which follows.
Cambridge University Press regrets any inconvenience that
this inadvertent error may have caused.
Regulation of arrestin mRNA levels in Limulus lateral eye:
Separate and combined influences of circadian efferent input and light
Authors: BATTELLE BARBARA-ANNE , WILLIAMS CHRISTOPHER D. , SCHREMSER-BERLIN JERI-LYNN , CACCIATORE CHELSI
Published: July 2002

Most animals experience daily changes in light
and darkness. The retinas of many of these animals show
concomitant rhythmic changes in the levels of mRNAs that
encode proteins involved in the photoresponse. These changes
may be circadian and independent of light, independent
of circadian clocks and regulated by light, or regulated
by a circadian clock and light. We have taken advantage
of the organization of the Limulus visual system
to examine the separate and combined effects of signals
from a circadian clock and light on arrestin mRNA levels
in photoreceptors. The clock that regulates photoreceptors
in the lateral eye of Limulus is in the brain,
and signals from the clock reach the lateral eye via
activation of a well-characterized efferent projection
in the lateral optic nerve. In the experiments described,
clock-driven efferent input to the lateral eye was eliminated
by cutting the lateral optic nerve, and light input to
the lateral eye was eliminated by placing an opaque patch
over the eye. Arrestin mRNA levels were quantified relative
to 18s rRNA with a ribonuclease protection assay. We observed
the following. In lateral eyes exposed to natural diurnal
light and endogenous efferent nerve activity, the level
of arrestin mRNA was higher during the day in the light
than during the night in the dark. Circadian efferent nerve
activity was necessary and sufficient to produce normal
daily fluctuations in the level of arrestin mRNA. Light
influenced arrestin mRNA levels only in eyes with intact
and active efferent projections. We conclude that arrestin
mRNA levels in lateral eye photoreceptors are controlled
entirely by efferent nerve activity, and that light exerts
its effects by modulating this output from the circadian
clock. Light-stimulated changes in arrestin mRNA in the
vertebrate retina may likewise require interactions between
light-driven biochemical cascades and clock output.
Spontaneous activity in developing turtle retinal ganglion
cells: Statistical analysis
Authors: GRZYWACZ NORBERTO M. , SERNAGOR EVELYNE
Published: July 2002

We report on the temporal properties of the spontaneous
bursts of activity in the developing turtle retina. Quantitative
statistical criteria were used to detect, cluster, and
analyze the temporal properties of the bursts. The interburst
interval, duration, firing rate, and number of spikes per
burst varied widely among cells and from burst to burst
in a single cell. Part of this variability was due to the
positive correlation between a burst's duration and
the interburst interval preceding that burst. This correlation
indicated the influence of a refractory period on the bursts'
properties. Further evidence of such a refractoriness came
from the bursts' auto-covariance function, which gives
the tendency of a spike to occur a given amount of time
after another spike. This function showed a positive phase
(between ≈10 ms and 10 s) followed by a negative one
(between 10 s and more than 100 s), suggestive of burst
refractoriness. The bursts seemed to be propagating from
cell to cell, because there was a long (symmetrically distributed)
delay between the activation of two neighbor cells (median
absolute delay = 2.3 s). However, the activity often failed
to propagate from one cell to the other (median safety
factor = 0.59). The number of spikes per burst in neighbor
cells was statistically positively correlated, indicating
that the activity in the two cells was driven by the same
excitatory process. At least two factors contribute to
the excitability during bursts, because the positive phase
of the cross-covariance function (similar to auto-covariance
but for two cells) had a temporally asymmetric fast component
(1–3 ms) followed by a temporally symmetric slow
component (1 ms to 10 s).
Modulation by Zn2+ of GABA responses in bipolar
cells of the mouse retina
Authors: KANEDA M. , ANDRÁSFALVY B. , KANEKO A.
Published: July 2002

The localization of endogenous Zn2+
in the mouse retina was examined histochemically and the
inhibitory action of Zn2+ on GABA-induced responses
was studied in bipolar cells isolated from the mouse retina.
Accumulation of endogenous Zn2+ was detected
in photoreceptors, bipolar, and/or amacrine cells by either
the bromopyridylazo-diethylaminophenol method or the dithizone
method. Under whole-cell recording conditions, GABA induced
a Cl− current in isolated bipolar cells.
The current consisted of two components. The first component
was inhibited completely by application of 100 μM bicuculline,
suggesting that this is a GABAA-receptor mediated
current. The second component was inhibited completely
by 100 μM 3-aminopropyl-(methyl)-phosphinic acid, suggesting
that this is a GABAC-receptor mediated current.
GABAC receptors were present at a higher density
on the axon terminal than on dendrites. Zn2+
inhibited both GABAA and GABAC receptors.
GABAC receptors were more susceptible to Zn2+;
the IC50 for the GABAA
receptor was 67.4 μM and that for the GABAC
receptor was 1.9 μM. These results suggest that Zn2+
modulates the inhibitory interaction between amacrine and
bipolar cells, particularly that mediated by the GABAC
receptor.
Photoreceptors, visual pigments, and ellipsosomes in the
killifish, Fundulus heteroclitus: A microspectrophotometric
and histological study
Authors: FLAMARIQUE IÑIGO NOVALES , HÁROSI FERENC I.
Published: July 2002

The photoreceptor layer of F. heteroclitus was
examined by light and electron microscopy. We identified
four cone visual pigments with maximum absorbance (λmax)
in the UV (363 nm), short (400 nm), middle (463 nm), and
long (563 nm) wavelength regions of the spectrum and a
rod visual pigment that peaked in the middle wavelengths
(503 nm). Electron-dense bodies, ellipsosomes and pseudoellipsosomes,
were present in the distal ellipsoids of long/middle (L/M)
and long/long (L/L) wavelength double cones and in single
short wavelength (S) cones, respectively. The light absorption
of ellipsosomes indicated the presence of reduced cytochrome-c
with the highest optical densities found in the M members
of L/M double cones. By contrast, S cones contained pseudo-ellipsosomes
which had very low optical density. UV cones were present
everywhere as part of square or row mosaics in the retina
of F. heteroclitus. Cone packing was on average
higher for locations in the upper half of the retina while
the highest cone density was found in the centro-ventral
retina. An analysis of potential quantum catches for each
cone type as a function of retinal sector and underwater
irradiance characteristics revealed higher overall quantum
catches for cones in the upper retina when the light field
was assumed homogeneous, and higher quantum catches for
cones in the lower retina when downwelling, horizontal,
and upwelling irradiances were considered separately. At
dusk, quantum catch was highest for M cones and the contribution
to the overall retinal quantum catch by UV and S cones
was much greater than during daylight hours. We propose
that UV and S cones may be used to detect targets of interest
against the background irradiance sensed by double cones.
Striate cortex increases contrast gain of macaque LGN neurons
Authors: PRZYBYSZEWSKI ANDRZEJ W. , GASKA JAMES P. , FOOTE WARREN , POLLEN DANIEL A.
Published: July 2002

Recurrent projections comprise a universal feature
of cerebral organization. Here, we show that the corticofugal
projections from the striate cortex (V1) to the lateral
geniculate nucleus (LGN) robustly and multiplicatively
enhance the responses of parvocellular neurons, stimulated
by gratings restricted to the classical receptive field and
modulated in luminance, by over two-fold in a contrast-independent
manner at all but the lowest contrasts. In the equiluminant
plane, wherein stimuli are modulated in chromaticity with
luminance held constant, such enhancement is strongly contrast
dependent. These projections also robustly enhance
the responses of magnocellular neurons but contrast
independently only at high contrasts. Thus, these
results have broad functional significance at both network
and neuronal levels by providing the experimental basis
and quantitative constraints for a wide range of models
on recurrent projections and the control of contrast gain.
Visual evoked potentials and magnocellular and parvocellular
segregation
Authors: RUDVIN INGER , VALBERG ARNE , KILAVIK BJØRG ELISABETH
Published: July 2002

We have measured visual evoked potentials (VEPs)
to luminance-modulated, square-wave alternating, 3-deg
homogeneous disks for stimulus frequencies ranging from
1 Hz to 16.7 Hz. The aim of the study was to determine
the range of frequencies at which we could reproduce the
two-branched contrast-response (C-R) curves we had seen
at 1 Hz (Valberg & Rudvin, 1997) and which we interpreted
as magnocellular (MC) and parvocellular (PC) segregation.
Low-contrast stimuli elicited relatively simple responses
to luminance increments resulting in waveforms that may
be the signatures of inputs from magnocellular channels
to the visual cortex. At all frequencies, the C-R curves
of the main waveforms were characterized by a steep slope
at low contrasts and a leveling off at 10%–20% Michelson
contrast. This was typically followed by an abrupt increase
in slope at higher contrasts, giving a distinctive two-branched
C-R curve. On the assumption that the low-contrast, high-gain
branch reflects the responsivity of magnocellular-pathway
inputs to the cortex, the high-contrast branch may be attributed
to additional parvocellular activation. While a two-branched
curve was maintained for frequencies up to 8 Hz, the high-contrast
response was significantly compromised at 16.7 Hz, revealing
a differential low-pass filtering. A model decomposing
the measured VEP response into two separate C-R curves
yielded a difference in sensitivity of the putative MC-
and PC-mediated response that, when plotted as a function
of frequency, followed a trend similar to that found for
single cells. Due to temporal overlap of responses, the
MC and PC contributions to the waveforms were hard to distinguish
in the transient VEP. However, curves of time-to-peak (delay)
as a function of contrast often went through a minimum
before the high-contrast gain increase of the corresponding
C-R curve, supporting the notion of a recruitment of new
cell ensembles in the transition from low to high contrasts.
The mosaic of horizontal cells in the macaque monkey retina:
With a comment on biplexiform ganglion cells
Authors: WÄSSLE HEINZ , DACEY DENNIS M. , HAUN TONI , HAVERKAMP SILKE , GRÜNERT ULRIKE , BOYCOTT BRIAN B.
Published: July 2002

To further characterize the H1 and H2 horizontal
cell populations in macaque monkey retinae, cells were
injected with the tracer Neurobiotin following intracellular
recordings. Tracer coupling between cells of the same type
revealed all H1 or H2 cells in small patches around the
injected cell. The mosaics of their cell bodies and the
tiling of the retina with their dendrites were analyzed.
Morphological differences between the H1 and H2 cells observable
in Neurobiotin-labeled patches made it possible to recognize
H1 and H2 cells in retinae immunolabeled for the calcium-binding
proteins parvalbumin and calbindin, and thus to study their
relative spatial densities across the retina. These data,
together with the intracellularly stained patches, show
that H1 cells outnumber H2 cells at all eccentricities.
There is, however, a change in the relative proportions
of H1 and H2 cells with eccentricity: close to the fovea
the ratio of H1 to H2 cells is ∼4 to 1, in midperipheral
retina ∼3 to 1, and in peripheral retina ∼2 to
1. In both the Neurobiotin-stained and the immunostained
retinae, about 3–5% of the H2 cells were obviously
misplaced into the ganglion cell layer. Several features
of the morphology of the misplaced H2 cells suggest that
they represent the so-called “biplexiform ganglion
cells” previously described in Golgi studies of primate
retina.
The sequential processing of visual motion in the human
electroretinogram and visual evoked potential
Authors: KORTH MATTHIAS , RIX RAINER , SEMBRITZKI OTTO
Published: July 2002

Mechanisms of motion vision in the human have been
studied extensively by psychophysical methods but less
frequently by electrophysiological techniques. It is the
purpose of the present investigation to study electrical
potentials of the eye (electroretinogram, ERG) and of the
brain (visual evoked potential, VEP) in response to moving
regular square-wave stripe patterns spanning a wide range
of contrasts, spatial frequencies, and speeds. The results
show that ERG amplitudes increase linearly with contrast
while VEPs, in agreement with the literature, show an amplitude
saturation at low contrast. Furthermore, retinal responses
oscillate with the fundamental temporal stimulus frequency
of the moving pattern while brain responses do not. In
both the retina and the brain, the response amplitudes
are tuned to certain speeds which is in agreement with
the nonlinear correlation-type motion detector. Along the
ascending slopes (which means increasing amplitudes) of
the tuning functions, the ERG curves overlap at all spatial
frequencies if plotted as a function of temporal stimulation
frequency. The ascending slopes of the tuning functions
of the VEP overlap if plotted as a function of speed. The
descending slopes (which means decreasing amplitudes) of
the tuning functions show little (ERG) or no (VEP) overlap
and the waveforms at high speeds approach pattern-offset-onset
responses. These observations suggest that in the retina
motion processing along the ascending slopes of the tuning
curves takes place by coding the temporal stimulation frequency
which depends on the spatial frequency of the moving pattern.
In the brain, however, motion processing is by speed independent
of spatial frequency. Simple calculations show that the
VEP information is decoded from the ERG signal into a speed
signal.
Rod and cone visual cycle consequences of a null mutation
in the 11-cis-retinol dehydrogenase gene in man
Authors: CIDECIYAN ARTUR V. , HAESELEER FRANÇOISE , FARISS ROBERT N. , ALEMAN TOMAS S. , JANG GEENG-FU , VERLINDE CHRISTOPHE L.M.J. , MARMOR MICHAEL F. , JACOBSON SAMUEL G. , PALCZEWSKI KRZYSZTOF
Published: July 2002

Vertebrate vision starts with photoisomerization of the
11-cis-retinal chromophore to all-trans-retinal.
Biosynthesis of 11-cis-retinal is required to
maintain vision. A key enzyme catalyzing the oxidation
of 11-cis-retinol is 11-cis-retinol dehydrogenase
(11-cis-RDH), which is encoded by the RDH5
gene. 11-cis-RDH is expressed in the RPE and not
in the neural retina. The consequences of a lack of 11-cis-RDH
were studied in a family with fundus albipunctatus. We
identified the causative novel RDH5 mutation,
Arg157Trp, that replaces an amino acid residue conserved
among short-chain alcohol dehydrogenases. Three-dimensional
structure modeling and in vitro experiments suggested
that this mutation destabilizes proper folding and inactivates
the enzyme. Studies using RPE membranes indicated the existence
of an alternative oxidizing system for the production of
11-cis-retinal. In vivo visual consequences
of this null mutation showed complex kinetics of dark adaptation.
Rod and cone resensitization was extremely delayed following
full bleaches; unexpectedly, the rate of cone recovery
was slower than rods. Cones showed a biphasic recovery
with an initial rapid component and an elevated final threshold.
Other unanticipated results included normal rod recovery
following 0.5% bleach and abnormal recovery following bleaches
in the 2–12% range. These intermediate bleaches showed
rapid partial recovery of rods with transitory plateaux.
Pathways in addition to 11-cis-RDH likely provide
11-cis-retinal for rods and cones and can maintain
normal kinetics of visual recovery but only under certain
constraints and less efficiently for cone than rod function.
Microtubules in a rod-specific cytoskeleton associated with
outer segment incisures
Authors: ECKMILLER MARION SANGSTER
Published: July 2002

In many vertebrate retinas the outer segments of rod
photoreceptors have multiple incisures, that is, there
are numerous indentations in the highly curved membrane
forming the edge of their disks and in the plasma membrane
enclosing the entire stack of disks. Immunofluorescent
localization of tubulin in amphibian photoreceptors yielded
a novel series of thin, parallel, fluorescent lines in
rod outer segments that extended their full length and
coincided with their multiple incisures. Electron-microscopic
examination of amphibian retinas revealed the structures
responsible for this fluorescence: longitudinally oriented
microtubules were associated with incisures at heights
throughout rod outer segments. These microtubules were
located between the disk rims and the overlying plasma
membrane, in the small cytoplasmic compartment at the mouth
of incisures; the microtubules and membranes were separated
from each other by distances that were uniform, as though
interconnected by filaments described in other studies.
Thus, in amphibian rod outer segments the incisures mark
the site of a cytoskeletal system containing longitudinal
microtubules distinct from those of the ciliary axoneme,
linked by filaments to the adjacent membranes. This cytoskeleton
is expected to be important for the normal structure, function,
and renewal of rod outer segments. In amphibian cone outer
segments, which do not have incisures, the only anti-tubulin
immunofluorescence and the only microtubules were at the
axoneme. These findings may help elucidate the diverse
properties of rods and cones in many vertebrate retinas
and could prove relevant for human retinal degenerations.
A glial palisade delineates the ipsilateral optic projection
in Monodelphis
Authors: MacLAREN ROBERT E.
Published: July 2002

In developing marsupials, the path taken through
the optic chiasm by ipsilaterally projecting retinal ganglion
cells is complicated. Just prior to entry into the chiasm,
ganglion cells destined for the ipsilateral optic tract
separate from the remainder of axons by turning abruptly
downwards to take a position in the ventral part of the
optic nerve. In this report, it is shown that a discrete
population of about 10–15 large glial cells transiently
form a linear array across the prechiasmatic part of the
optic nerve, precisely at this axon turning point. The
distinct morphology of these cells and their novel location
may reflect a specialized role in axon guidance.
Spatial-frequency and orientation tuning in psychophysical
end-stopping
Authors: YU CONG , LEVI DENNIS M.
Published: July 2002

A psychophysical analog to cortical receptive-field
end-stopping has been demonstrated previously in spatial
filters tuned to a wide range of spatial frequencies (Yu
& Levi, 1997a). The current study investigated
tuning characteristics in psychophysical spatial filter
end-stopping. When a D6 (the sixth derivative of a Gaussian)
target is masked by a center mask (placed in the putative
spatial filter center), two end-zone masks (placed in the
filter end-zones) reduce thresholds. This “end-stopping”
effect (the reduction of masking induced by end-zone masks)
was measured at various spatial frequencies and orientations
of end-zone masks. End-stopping reached its maximal strength
when the spatial frequency and/or orientation of the end-zone
masks matched the spatial frequency and/or orientation
of the target and center mask, showing spatial-frequency
tuning and orientation tuning. The bandwidths of spatial-frequency
and orientation tuning functions decreased with increasing
target spatial frequency. At larger orientation differences,
however, end-zone masks induced a secondary facilitation
effect, which was maximal when the spatial frequency of
end-zone masks equated the target spatial frequency. This
facilitation effect might be related to certain types of
contour and texture perception, such as perceptual pop-out.
Temporal diversity in the lateral geniculate nucleus of cat
Authors: WOLFE JONATHAN , PALMER LARRY A.
Published: July 2002

Reverse correlation was used in conjunction with
ternary white noise to estimate the first-order spatiotemporal
receptive-field structure of LGN cells in the anesthetized,
paralyzed cat. Based on a singular-value decomposition
of these data, we conclude that most LGN cells are approximately
space–time separable. An analysis of the timecourses
of the first singular values revealed a strongly bimodal
but continuous distribution of rise times and waveforms.
The two modes represented cells generally associated with
the lagged and nonlagged classes of Mastronarde (1987a,b),
and this was confirmed by their responses to step and sine-modulated
spots in their field centers. The intermediate cells, rather
than appearing to constitute a separate group, smoothly
filled the region between the obviously lagged and nonlagged
cells in every respect. These conclusions are limited to
X-cells although the data from a much smaller population
of Y-cells conform to the same scheme. We conclude that
lagged and nonlagged cells represent the modes of a continuous
and very broad distribution of temporal responses in the
cat LGN.
Dopaminergic amacrine cells in the retina of the possum,
Trichosurus vulpecula
Authors: DANN J.F.
Published: July 2002

The common brush-tailed possum Trichosurus
vulpecula is a small diprotodont marsupial common
to both urban and natural environments. This is the first
analysis of the neurotransmitter content of its retinal
cells and, as the possum is a nocturnal forager, it was
appropriate to begin with the dopaminergic amacrine cells
that form an essential link in the modulation of the rod
pathways subserving nocturnal vision. These results were
compared with those from another diprotodont, the marsupial
wallaby or quokka (Dann, 1996) to establish whether the
dopaminergic systems were similar between these two diprotodont
marsupials and also to compare these findings with those
of other mammals. This study describes a population of
amacrine cells in the possum retina that were immunolabelled
with an antibody raised against tyrosine hydroxylase (TH).
These TH-immunoreactive (IR) cells were located within
the inner nuclear layer (INL) and their dendrites predominantly
ramified within the most sclerad layers of the inner plexiform
layer (IPL). The TH-IR amacrines formed a sparse cell population,
of around 2400 cells, distributed over the entire retina.
There was little evidence of a concentration gradient except
for a slight elevation in density in the naso-temporal
axis in dorsal retina. The formation of rings within the
dendritic plexus, a feature common to TH-IR cells in other
species, was also present in the possum and these appeared
relatively frequently. This latter finding was rather unexpected
since, in the marsupial quokka (Dann, 1996), the TH-IR
dendrites formed fewer rings despite having the same density
of TH-IR amacrines as the possum. This suggests that there
may be subtle differences in the way the rod pathways are
interconnected even within the same marsupial group and
may also be a reflection of relative rod dominance across
species.
Spectral sensitivity of the feedback signal from horizontal
cells to cones in goldfish retina
Authors: KRAAIJ D.A. , KAMERMANS M. , SPEKREIJSE H.
Published: July 2002

The spectral sensitivity of cones in isolated goldfish
retina was determined with whole-cell recording techniques.
Three spectral classes of cones were found with maximal
sensitivities around 620 nm, 540 nm, and 460 nm. UV-cones
were not found because our stimulator did not allow effective
stimulation in the UV range. The spectral sensitivity of
the cones closely matched the cone photopigment absorption
spectra at the long wavelength side of the spectrum, but
deviated significantly at shorter wavelengths. Surround
stimulation induced an inward current in cones due to feedback
from horizontal cells. The spectral sensitivity of this
feedback signal was determined in all three cone classes
and found to be broader than the spectral sensitivity of
the cones recorded from, and to be spectrally nonopponent.
These data are consistent with a connectivity scheme between
cones and horizontal cells in which the three horizontal
cell systems feed back to all cone systems and in which
all horizontal cell systems receive input from more than
one cone system.
Analysis of the postnatal growth of visual cortex
Authors: DUFFY KEVIN R. , MURPHY KATHRYN M. , JONES DAVID G.
Published: July 2002

Development and growth of V1 begins during embryogenesis
and continues postnatally. The growth of V1 has direct
implications on the organization of features such as the
retinotopic map and the pattern of visual cortical columns.
We have examined the postnatal growth and two-dimensional
shape of V1 in macaque monkeys, cats, and rats. The perimeter,
area, and anterior–posterior length of V1 were measured
from unfolded and flattened sections from neonatal and
adult animals from each of these species. Although there
were substantial differences in the overall amount of postnatal
growth, from 18% in macaque monkeys to more than 100% in
cats, in all three species the shape of V1 did not change
during development. Thus, growth of the mammalian visual
cortex is well described as an isotropic expansion, so
the layout of the global features, such as the arrangement
of ocular dominance columns and the retinotopic map, does
not need to change during development. Furthermore, quantification
of the shape confirms the observations that there is a
similar, egg-like oval shape to the visual cortex of these
mammalian species.
Responses of regular spiking and fast spiking cells in turtle
visual cortex to light flashes
Authors: MANCILLA JAIME G. , FOWLER MICHAEL , ULINSKI PHILIP S.
Published: July 2002

Sharp electrodes were used to record light-evoked
postsynaptic potentials (PSPs) from neurons in turtle visual
cortex in an in vitro preparation of the geniculocortical
pathway. Neurons were placed into four groups based on
the firing patterns produced by intracellular current injections:
regular spiking (RS), fast spiking (FS), intrinsic bursting
(IB), and chattering (CH) cells. RS cells have been shown
to be pyramidal cells while FS cells are typically interneurons.
Light stimuli were diffuse, 1-s flashes of 640-nm light
with intensities (I) varying from 0 to 104
photons μm−2 s−1.
The response (R) in each case was the maximal
amplitude of the light-evoked depolarizing PSP. Cells of
all four types showed sigmoidal intensity–response
(IR) functions with a linear rising phase for
stimuli above the intensity threshold followed by saturation
at high light intensities. Responses at high intensities
were variable and some cells showed indications of supersaturation.
Light-evoked PSPs had longer latencies and times-to-peak
response in RS cells than they did in FS cells. RS cells
fired action potentials as much as 200 ms later than did
FS cells. Since responses recorded in RS cells at light
intensities just above threshold are unlikely to involve
contributions from other pyramidal cells, these data indicate
that the geniculocortical or feedforward pathway to pyramidal
cells has a high gain. The fact that FS cells fire well
before RS cells suggests that feedforward inhibition plays
a role in controlling the gain of the geniculocortical
pathway.
Cone contributions to the photopic spectral sensitivity
of the zebrafish ERG
Authors: HUGHES ALAN , SASZIK SHANNON , BILOTTA JOSEPH , DEMARCO PAUL J. , PATTERSON WARREN F.
Published: July 2002

Microspectrophotometry studies show that zebrafish
(Danio rerio) possess four cone photopigments.
The purpose of this study was to determine the cone contributions
to the zebrafish photopic increment threshold spectral-sensitivity
function. Electroretinogram (ERG) b-wave responses
to monochromatic lights presented on a broadband or chromatic
background were obtained. It was found that under the broadband
background condition, the zebrafish spectral-sensitivity
function showed several peaks that were narrower in sensitivity
compared to the cone spectra. The spectral-sensitivity
function was modeled with L − M and M − S opponent
interactions and nonopponent S- and U-cone mechanisms.
Using chromatic adaptation designed to suppress the contribution
of the S-cones, a strong U-cone contribution to the spectral-sensitivity
function was revealed, and the contributions of the S-cones
to the M − S mechanism were reduced. These results
show that the b-wave component of the ERG receives
input from all four cone types and appears to reflect color
opponent mechanisms. Thus, zebrafish may possess the fundamental
properties necessary for color vision.