bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphology and visual pigment content of photoreceptors
from injured goldfish retina
Authors: CAMERON DAVID A. , POWERS MAUREEN K.
Published: July 2002

Adult teleost fish retinas can regenerate neurons
following either surgical or pharmacological injury. The
cellular milieu of the damaged retina within which regenerated
neurons are produced might be different in these two model
systems of retinal injury, and thus the phenotypic attributes
of regenerated neurons in the two model systems might also
differ. To determine if the phenotypic attributes of photoreceptors,
and by extension the recovery of vision, are different
between these two model systems, we compared the visual
pigment content and morphology of photoreceptors derived
from goldfish retinas of both models with control retina.
Visual pigments—which consist of a protein moiety
(opsin) and a chromophore—were analyzed in single,
isolated photoreceptors using microspectrophotometric techniques.
We report that visual pigments and photoreceptor morphologies
in the surgical model closely matched those of native retina.
In contrast, neither photoreceptor morphology nor visual
pigment content matched closely in the pharmacological
model. The results indicate that phenotypic attributes
of photoreceptors can differ significantly between the
two model systems of retinal regeneration, but that in
both systems, rod- and cone-mediated visual functions can
potentially be reestablished.
Fine retinotopic organization of optic terminal arbors in
the tectum of normal goldfish
Authors: WANG ZIREN , MEYER RONALD L.
Published: July 2002

Although the retinotectal projection of goldfish has
long been known to have a high degree of retinotopic order,
the structural basis for this in terms of the precise positioning
of axonal arbors from neighboring retinal ganglion cells
has not been determined. In studying this, a small number
of neighboring retinal ganglion cells was selectively labeled
by a microinjection of DiI into the retina. Following axoplasmic
transport for several days, the tectum was removed and
flat-mounted for fluorescence microscopy. The injection
labeled a small number of axons and their terminal arbors
which ranged in size from 108 × 134 μm to 394
× 331 μm with a mean of 233 × 219 μm.
This mean size corresponds to about 1/15 of the length
of one tectal axis. Although individual arbors labeled
from one small retinal injection were always observed near
the same retinotopic position, they were almost never coextensive.
Overlap between pairs of arbors along the lines of projection
perpendicular to the tectal surface averaged 57% of the
area of a single arbor. These results indicate that neighboring
retinal ganglion cells do not converge onto the same locus
but instead project as a continuous retinotopic array of
partially overlapping terminal fields.
Visual arrestin in Limulus is phosphorylated at
multiple sites in the light and in the dark
Authors: BATTELLE B-A. , ANDREWS A.W. , KEMPLER K.E. , EDWARDS S.C. , SMITH W.C.
Published: July 2002

Arrestins participate in the termination of phototransduction
in both vertebrates and invertebrates. However, the visual
arrestins of invertebrates and vertebrates differ significantly
from one another in that the invertebrate visual arrestins
become phosphorylated rapidly in response to light while
those in the photoreceptors of vertebrates do not. In an
effort to understand the functional relevance of arrestin
phosphorylation, we examined this process in the photoreceptors
of the horseshoe crab Limulus polyphemus. We report
that Limulus visual arrestin can be phosphorylated
at three sites near its C-terminus and show that arrestin
molecules phosphorylated on one, two, and three sites are
normally present in both light- and dark-adapted photoreceptors.
Light adaptation increases the amount of arrestin phosphorylated
at three sites.
Salamander rods and cones contain distinct transducin alpha
subunits
Authors: RYAN JAMES C. , ZNOIKO SERGEY , XU LIN , CROUCH ROSALIE K. , MA JIAN-XING
Published: July 2002

The mammalian retina is known to contain two distinct
transducins that interact with their respective rod and
cone pigments. However, there are no reports of a nonmammalian
species having two distinct transducins. In the present
study, we report the cloning and cellular localization
of two transducin α subunits (Gαt) from the tiger
salamander. Through degenerate polymerase chain reaction
(PCR) and subsequent screening of a salamander retina cDNA
library, we have identified two forms of Gαt. When
compared to existing sequences in GenBank, the cloned subunits
showed high similarity to rod and cone transducins. The
salamander Gαt-1 has 91.2–93.7% amino acid sequence
identity to mammalian rod Gαt subunits and 79.7–80.9%
to mammalian cone Gαts. The salamander Gαt-2 has
86.2–87.9% sequence identity to mammalian cone Gαts
and 78.9–80.9% to mammalian rod Gαts at the amino
acid level. The Gαt-1 cDNA encodes 350 amino acids
while the Gαt-2 cDNA encodes 354 residues, which is
typical for rod and cone Gαts, respectively, and we
thus identified the Gαt-1 as rod and Gαt-2 as cone
Gαt. Sequences identified as effector binding sites
and GTPase activity regions are highly conserved between
the two subunits. Genomic Southern blot analysis showed
that rod and cone Gαt subunits are both encoded by
single-copy genes. Northern blot analysis identified retina-specific
transcripts of 3.0 kb for rod Gαt and 2.6 kb for cone
Gαt. Immunohistochemistry in the flat-mounted salamander
retina demonstrated that rod Gαt is localized to rods,
predominantly in the outer segments; similarly, cone Gαt
is localized to cone outer segments. The results confirm
that the two sequences encode rod and cone transducins
and demonstrate that this lower vertebrate contains two
distinct transducins that are localized specifically to
rod and cone photoreceptors.
Mathematical models for the spatial receptive-field
organization of nonlagged X-cells in dorsal lateral
geniculate nucleus of cat
Authors: EINEVOLL G.T. , HEGGELUND P.
Published: July 2002

Spatial receptive fields of relay cells in dorsal lateral
geniculate nucleus (dLGN) have commonly been modeled as
a difference of two Gaussian functions. We present alternative
models for dLGN cells which take known physiological couplings
between retina and dLGN and within dLGN into account. The
models include excitatory input from a single retinal ganglion
cell and feedforward inhibition via intrageniculate
interneurons. Mathematical formulas describing the receptive
field and response to circular spot stimuli are found both
for models with a finite and an infinite number of ganglion-cell
inputs to dLGN neurons. The advantage of these models compared
to the common difference-of-Gaussians model is that they,
in addition to providing mathematical descriptions of the
receptive fields of dLGN neurons, also make explicit contributions
from the geniculate circuit. Moreover, the model parameters
have direct physiological relevance and can be manipulated
and measured experimentally. The discrete model is applied
to recently published data (Ruksenas et al., 2000) on response
versus spot-diameter curves for dLGN cells and
for the retinal input to the cell (S-potentials). The models
are found to account well for the results for the X-cells
in these experiments. Moreover, predictions from the discrete
model regarding receptive-field sizes of interneurons,
the amount of center-surround antagonism for interneurons
compared to relay cells, and distance between neighboring
retinal ganglion cells providing input to interneurons,
are all compatible with data available in the literature.
Cyclic AMP has no effect on the generation, recovery, or background
adaptation of light responses in functionally intact rod outer segments:
With implications about the function of phosducin
Authors: JINDROVA HANA , DETWILER PETER B.
Published: July 2002

In retinal rods, light exposure decreases the total outer
segment content of both cGMP and cAMP by about 50%. The
functional role of the light-evoked change in cAMP is not
known. It is postulated to trigger changes in the phosphorylation
state of phosducin, a phosphoprotein that is phosphorylated
in the dark by cAMP-dependent protein kinase (PKA) and
dephosphorylated by basal phosphatase activity when PKA
is inhibited by the light-evoked drop in cAMP. In biochemical
studies, dephosphorylated phosducin binds to free βγ
dimer of transducin (Tβγ) and prevents the regeneration
of heterotrimeric transducin by blocking the re-association
of the βγ and α subunits. Phosducin's interaction
with Tβγ is blocked when it is phosphorylated on a single
residue by PKA. To evaluate the effect of the light-evoked fall
in cAMP, functionally intact isolated lizard rod outer
segments were dialyzed in whole-cell voltage clamp with
a standard internal solution and electrical light responses
were recorded with and without adding cAMP to the dialysis
solution. Since the total outer segment content of cAMP
in darkness is ∼5 μM, internal dialysis with solution
containing a much higher concentration (100 μM) of
cAMP (or 8-bromo-cAMP) will overcome the effects of a light-evoked
decrease in its concentration by keeping cAMP-dependent
processes fully activated. Neither cyclic nucleotide had
any influence on the generation, light sensitivity, recovery,
or background adaptation of the flash response. These results
also argue against the participation of phosducin in the
sequence of events that are responsible for these aspects
of rod function. This does not exclude the possibility
of phosducin being involved in adaptation caused by higher
light levels than used in the present study, that is, bleaching
adaptation, or in light-dependent processes other than
phototransduction.
Thalamic control of cat lateral suprasylvian visual area:
Relation to patchy association projections from area 18
Authors: LEE CHOONGKIL , WEYAND THEODORE G. , MALPELI JOSEPH G.
Published: July 2002

In this study, we examined functional contributions
of major subdivisions of the lateral geniculate nucleus
to the cat's lateral suprasylvian visual area (LS)
in relation to the patchy horizontal distributions of association
inputs. Multiple-unit activity driven via the
contralateral eye was assessed during reversible blockade
of the retinotopically corresponding part of layer A, the
C layers as a group, or the medial interlaminar nucleus
(MIN). Inactivating each of these targets reduced activity
at some cortical sites, with inactivation of layer A having,
on average, the largest effect. Activity was rarely abolished
by inactivation of a single target, indicating that most
LS sites receive multiple inputs. Dependence on layer A
was strongly correlated with the horizontal distribution
of association inputs from area 18. Closely spaced injections
of anatomical tracers into extensive regions of area 18
resulted in patches of terminal label in lateral suprasylvian
cortex. Activity inside the patches was relatively dependent
on layer A, whereas that outside the patches was not. Dependence
on the MIN and layer A were negatively correlated,
suggesting that inputs dominated by the MIN and layer A
were concentrated in independent sets of patches. These
results indicate that the anatomically observed patchy
projections reflect the functional consequences of geniculate
lamination. The A layers are high-acuity relays, whereas
the MIN is probably a specialization for dim-light vision
(Lee et al., 1984; Lee et al., 1992). We propose that the
partial overlap of inputs dominated by the A layers and
the MIN allows dynamic shifts in their relative contributions
to LS responses, optimizing the balance of high-acuity
and high-sensitivity channels over a wide range of illumination
conditions.
Distinctive characteristics of subclasses of red–green
P-cells in LGN of macaque
Authors: LANKHEET MARTIN J.M. , LENNIE PETER , KRAUSKOPF JOHN
Published: July 2002

We characterized the chromatic and temporal properties
of a sample of 177 red–green parvocellular neurons
in the LGN of Macaca nemestrina, using large-field
stimuli modulated along different directions through a
white point in color space. We examined differences among
the properties of the four subclasses of red–green
P-cells (on- and off-center, red and green center). The
responses of off-center cells lag the stimulus more than
do those of on-center cells. At low temporal frequencies,
this causes the phase difference between responses of the
two kinds of cells to be considerably less than 180 deg.
For isoluminant modulations the phases of on- and off-responses
were more nearly 180 deg apart. A cell's temporal
characteristics did not depend on the class of cone driving
its center. Red center and green center cells have characteristically
different chromatic properties, expressed either as preferred
elevations in color space, or as weights with which cells
combine inputs from L- and M-cones. Red center cells are
relatively more responsive to achromatic modulation, and
attach relatively more weight to input from the cones driving
the center. Off-center cells also attach relatively more
weight than do on-center cells to input from the class
of cone driving the center.
Saccade-induced activity of dorsal lateral geniculate nucleus
X- and Y-cells during pharmacological inactivation of the
cat pretectum
Authors: FISCHER W.H. , SCHMIDT M. , HOFFMANN K.-P.
Published: July 2002

The influence of neurons projecting from the pretectal
nuclear complex to the ipsilateral dorsal lateral geniculate
nucleus (LGNd) was investigated in awake cats. Responses
from relay cells in the A-laminae of the LGNd were extracellularly
recorded and analyzed during saccadic eye movements and
visual stimulation in association with reversible inactivation
of the ipsilateral pretectum with the GABA agonist, muscimol.
Pretectal inactivation (PTI) resulted in spontaneous nystagmic
eye movements in the dark with slow phases directed away
from the injected side. In the control situation, all Y-cells
and about two thirds of X-cells were excited during saccades
or saccade-like visual stimulation but one third of X-cells
were inhibited. During PTI all recorded X-cells were inhibited,
either during saccades or saccade-like visual stimulation.
The PTI-associated inhibition was stronger than in inhibited
X-cells in control experiments only during saccades but
not during stimulation with a moving pattern while the
eyes were stationary. In Y-cells a reduction in the response
peak width at half-height was seen during PTI, again only
during saccades but not during stimulation with a moving
pattern. These results indicate that during saccades the
pretecto-geniculate pathway has a stronger influence on
X LGNd relay cells than on Y-cells. The findings are discussed
in terms of saccadic suppression and postsaccadic facilitation.
Aberrant photon responses in rods of the macaque monkey
Authors: KRAFT T.W. , SCHNAPF J.L.
Published: July 2002

Recovery from bright light was studied in macaque
rods by measuring the membrane current of single outer
segments. The recovery phase of some responses displayed
a plateau current of about one picoampere lasting for several
seconds. The following evidence suggests these “steps”
are single photon responses of abnormally long duration.
(1) Over a limited range of intensities, step amplitude
remained constant and summed linearly with intensity. The
collecting area for step generation was about 2.6 ×
10−3 μm2. (2) Step duration
varied exponentially with a mean duration of about 6.5
s. (3) Fluctuation analysis of the tail currents was consistent
with the idea that a step is evoked by isomerization of
a single rhodopsin molecule, and that only 1 in 400 isomerizations
leads to a response with a step-like waveform. (4) With
only the distal portion of the outer segment in the electrode,
the polarity of the step response reversed when the proximal
portion of the outer segment was illuminated, indicating
that step generation results from a local change in outer
segment conductance near the site of photon absorption.
(5) The probability of eliciting a step varied with the
wavelength of light in the manner expected from the absorption
spectrum of rhodopsin.
Regulation of cGMP-dependent current in On bipolar cells
by calcium/calmodulin-dependent kinase
Authors: WALTERS R.J. , KRAMER R.H. , NAWY S.
Published: July 2002

The metabotropic receptor mGluR6 is localized to
the dendrites of On bipolar cells and mediates synaptic
input from photoreceptors. The binding of glutamate to
the receptor activates a phosphodiesterase (PDE), which
then hydrolyzes cGMP. A nonselective cationic conductance,
believed to be gated directly by cGMP, is turned off as
a result of the fall in cGMP levels, and the cell hyperpolarizes.
Here we present evidence for regulation of the conductance
by an additional mechanism that it is independent of cGMP.
Whole-cell recordings were obtained from On bipolar cells
in slices of tiger salamander retina. Dialysis of cells
with 1 μM KN-62 or 10 μM KN-93, two inhibitors
of type II calmodulin-dependent protein kinase (CaMKII),
depressed cGMP-dependent currents. This depression persisted
when hydrolysis of cGMP was prevented with IBMX, a broad-spectrum
PDE inhibitor, suggesting that CaMKII acts downstream from
the PDE in the cascade. The depression of cGMP-dependent
currents was probably not due to a direct interaction of
the inhibitors with the channels as neither 1 μM KN-62
or 10 μM KN-93 was found to have any effect on cyclic
nucleotide-gated channels when applied directly to excised
patches of rod outer segments. We propose that phosphorylation
by CaMKII may be an important mechanism for regulating
the cGMP-dependent conductance of On bipolar cells.
Visual acuity losses in pigeons with lesions of the nucleus
of Edinger-Westphal that disrupt the adaptive regulation
of choroidal blood flow
Authors: HODOS WILLIAM , MILLER RONALD F. , GHIM MIMI M. , FITZGERALD MALINDA E.C. , TOLEDO CLAUDIO , REINER ANTON
Published: July 2002

Choroidal blood flow (ChBF) in birds is regulated
by a neural circuit whose components are the retina, the
suprachiasmatic nucleus, the medial division of the Edinger-Westphal
nucleus (EWM), the ciliary ganglion, and the choriod. We
have previously shown that lesions of EWM appear to result
in pathological alterations in the retina. To determine
whether EWM lesions also lead to altered visual functions,
we have examined the effects of EWM lesions on visual acuity
in pigeons. Bilateral lesions of EWM were made electrolytically,
and visual acuity for high-contrast, square-wave gratings
was determined behaviorally about 1 year later and compared
to that of a group of pigeons that had received sham lesions
of EW about 1 year prior to acuity testing. Because lesions
targeting EWM invariably resulted in damage to the adjoining
lateral part of the Edinger-Westphal nucleus (EWL), which
controls pupillary constriction and accommodation, two
additional control groups were studied. In one such control
group, bilateral lesions in the area pretectalis (AP),
which innervates the pupillary control part of EWL and
thereby controls pupillary constriction, were made and
the effects on visual acuity determined about 1 year later.
In the second such control group, the effects of acute
accommodative and pupillary dysfunction on acuity were
studied in pigeons made cycloplegic. The accuracy of all
lesions was later confirmed histologically. The mean acuities
of birds with AP lesions (9.1 ± 1.4 cycles/deg)
and sham lesions (7.1 ± 1.5 cycles/deg) were not
significantly different from normal, based on published
normative data on pigeons. In contrast, pigeons with lesions
that completely destroyed EW bilaterally showed visual
acuity (2.7 ± 0.1 cycles/deg) that was well below
the acuity of the sham and AP-lesion control groups. The
acuity of the cycloplegic pigeons (4.8 ± 0.3 cycles/deg)
and one pigeon with a nearly complete bilateral EWL but
a unilateral EWM lesion (6.4 cycles/deg) indicated that
only about half of the loss with a bilateral EW lesion
could be attributed to accommodative dysfunction. Thus,
bilateral destruction of EWM appears to have led to a loss
in visual acuity. This conclusion suggests that disruption
of adaptive neural regulation of ChBF may impair visual
function. Destruction of EWM was, however, associated with
damage to the somatic components of the oculomotor and
trochlear nuclei. The possibility cannot be excluded that
such damage also contributed to the acuity loss.
Morphology of human retinal ganglion cells with intraretinal
axon collaterals
Authors: PETERSON BETH B. , DACEY DENNIS M.
Published: July 2002

Ganglion cells with intraretinal axon collaterals
have been described in monkey (Usai et al., 1991), cat
(Dacey, 1985), and turtle (Gardiner & Dacey, 1988)
retina. Using intracellular injection of horseradish peroxidase
and Neurobiotin in in vitro whole-mount preparations
of human retina, we filled over 1000 ganglion cells, 19
of which had intraretinal axon collaterals and wide-field,
spiny dendritic trees stratifying in the inner half of
the inner plexiform layer. The axons were smooth and thin
(∼2 μm) and gave off thin (<1 μm), bouton-studded
terminal collaterals that extended vertically to terminate
in the outer half of the inner plexiform layer. Terminal
collaterals were typically 3–300 μm in length,
though sometimes as long as 700 μm, and were present
in clusters, or as single branched or unbranched varicose
processes with round or somewhat flattened lobular terminal
boutons 1–2 μm in diameter. Some cells had a
single axon whereas other cells had a primary axon that
gave rise to 2–4 axon branches. Axons were located
either in the optic fiber layer or just beneath it in the
ganglion cell layer, or near the border of the ganglion
cell layer and the inner plexiform layer. This study shows
that in the human retina, intraretinal axon collaterals
are associated with a morphologically distinct ganglion
cell type. The synaptic connections and functional role
of these cells are not yet known. Since distinct ganglion
cell types with intraretinal axon collaterals have also
been found in monkey, cat, and turtle, this cell type may
be common to all vertebrate retinas.
Chromatic and luminance interactions in spatial contrast signals
Authors: VICTOR JONATHAN D. , PURPURA KEITH P. , CONTE MARY M.
Published: July 2002

We report VEP studies which delineate interactions
between chromatic and luminance contrast signals. We examined
responses to sinusoidal luminance gratings undergoing 4-Hz
square-wave contrast reversal, upon which standing gratings
with various admixtures of luminance and chromatic contrast
were alternately superimposed and withdrawn. The presence
of the standing grating induced a VEP component at the
fundamental frequency of the contrast-reversal grating.
This VEP component appeared without any appreciable lag,
and did not vary in amplitude over the 4 s during which
the standing grating was present. The observed fundamental
response differed from the fundamental component that would
be expected from the known interaction between the luminance
component of the standing grating with the modulated grating
(Bodis-Wollner et al., 1972; Bobak et al., 1988), in three
ways: (1) The fundamental response was not nulled for standing
gratings that were isoluminant or near-isoluminant. (2)
The chromatic dependence of the fundamental response implied
an S-cone input to the interaction. (3) No single mechanism
(driven by a linear combination of cone signals) could
account quantitatively for the size of this response, particularly
when the standing grating strongly modulated two cones
in phase.
Regional differences in GABA and GAD immunoreactivity in
rabbit horizontal cells
Authors: JOHNSON MADELEINE A. , VARDI NOGA
Published: July 2002

Mammalian horizontal cells are believed to be GABAergic
because, in most species, they contain both GABA and glutamic
acid decarboxylase (GAD), and their terminals are presynaptic
to GABA receptors. In adult rabbit, however, GABA and GAD
immunoreactivity have not been consistently demonstrated
in horizontal cells, casting doubts on the assumption that
they too are GABAergic. Here we demonstrate that all rabbit
horizontal cell terminals—dendritic terminals of
type A, and both dendritic and axonal terminals of type
B—immunostain for one isoform of GAD, GAD67.
In addition, we show that type A horizontal cell somas
and primary dendrites in the visual streak (identified
by their immunoreactivity to calbindin) are immunoreactive
for the other GAD isoform, GAD65. Double-labeling
experiments for GAD65 and GABA reveal that every
cell that stains for GAD65 also stains for GABA.
The presence of GAD67 in horizontal cell terminals
suggests that rabbit horizontal cells are GABAergic. The
segregation of the two GAD isoforms to different cell compartments
suggests that GABA is released at different sites, possibly
by two different mechanisms.
The cone/horizontal cell network: A possible site for color
constancy
Authors: KAMERMANS M. , KRAAIJ D.A. , SPEKREIJSE H.
Published: July 2002

Color vision is spectrally opponent, suggesting
that spectrally opponent neurons, such as the horizontal
cells in fish and turtle retinae, play a prominent role
in color discrimination. In the accompanying paper (Kraaij
et al., 1998), it was shown that the output signal of the
horizontal cell system to the cones is not at all spectrally
opponent. Therefore, a role for the spectrally opponent
horizontal cells in color discrimination seems unlikely.
In this paper, we propose that the horizontal cells play
a prominent role in color constancy and simultaneous color
contrast instead of in color discrimination. We have formulated
a model of the cone/horizontal cell network based on measurements
of the action spectra of the cones and of the feedback
signal of the horizontal cell system to the various cone
types. The key feature of the model is (1) that feedback
is spectrally and spatially very broad and (2) that the
gain of the cone synapse strongly depends on the feedback
strength. This makes the synaptic gain of the cones strongly
dependent on the spectral composition of the surround.
Our model, which incorporates many physiological details
of the outer retina, displays a behavior that can be interpreted
as color constancy and simultaneous color contrast. We
propose that the horizontal cell network modulates the
cone synaptic gains such that the ratios of the cone outputs
become almost invariant with the spectral composition of
the global illumination. Therefore, color constancy appears
to be coded in the retina.
Noise removal at the rod synapse of mammalian retina
Authors: van ROSSUM M.C.W. , SMITH R.G.
Published: July 2002

Mammalian rods respond to single photons with a
hyperpolarization of about 1 mV which is accompanied by
continuous noise. Since the mammalian rod bipolar cell
collects signals from 20–100 rods, the noise from
the converging rods would overwhelm the single-photon signal
from one rod at scotopic intensities (starlight) if the
bipolar cell summed signals linearly (Baylor et al., 1984).
However, it is known that at scotopic intensities the retina
preserves single-photon responses (Barlow et al., 1971;
Mastronarde, 1983). To explore noise summation in the rod
bipolar pathway, we simulated an array of rods synaptically
connected to a rod bipolar cell using a compartmental model.
The performance of the circuit was evaluated with a discriminator
measuring errors in photon detection as false positives
and false negatives, which were compared to physiologically
and psychophysically measured error rates. When only one
rod was connected to the rod bipolar, a Poisson rate of
80 vesicles/s was necessary for reliable transmission of
the single-photon signal. When 25 rods converged through
a linear synapse the noise caused an unacceptably high
false positive rate, even when either dark continuous noise
or synaptic noise where completely removed. We propose
that a threshold nonlinearity is provided by the mGluR6
receptor in the rod bipolar dendrite (Shiells & Falk,
1994) to yield a synapse with a noise removing mechanism.
With the threshold nonlinearity the synapse removed most
of the noise. These results suggest that a threshold provided
by the mGluR6 receptor in the rod bipolar cell is necessary
for proper functioning of the retina at scotopic intensities
and that the metabotropic domains in the rod bipolar are
distinct. Such a nonlinear threshold could also reduce
synaptic noise for cortical circuits in which sparse signals
converge.
Zebrafish visual sensitivity is regulated by a circadian clock
Authors: LI LEI , DOWLING JOHN E.
Published: July 2002

We have recently developed a behavioral assay,
based on the escape response of fish to a threatening object,
to analyze quantitatively the visual sensitivity of zebrafish.
During the course of dark adaptation, we measure the threshold
light intensity required to evoke an escape response. Under
a normal light–dark (LD) cycle, thresholds for both
the cone and rod systems are considerably lower in late
afternoon hours than in early morning hours. Over a period
of 24 h, zebrafish are most sensitive to visual stimuli
prior to light off and least sensitive prior to light on.
Under conditions of constant illumination, this rhythm
of visual sensitivity persists for several days but is
gradually lost. In constant light (LL), the rhythm persists
1–2 days; thereafter, visual thresholds at all times
of the day converge at a level similar to thresholds measured
in late afternoon hours in control animals. In constant
darkness (DD), the rhythm persists at least 5 days; thereafter,
it dampens to a level about a half-log unit less sensitive
to that measured in the late afternoon hours in control
animals. These data suggest that visual sensitivity in
zebrafish is regulated by an endogenous circadian clock
which functions to decrease the visual sensitivity.
Dual expression of GABA or serotonin and dopamine in Xenopus
amacrine cells is transient and may be regulated by laminar cues
Authors: HUANG SEN , MOODY SALLY A.
Published: July 2002

Both local cell–cell interactions and lineage
bias have roles in determining the different retina cell
phenotypes. In this study, subpopulations of amacrine cells
that dually express GABA or serotonin (5-HT) and dopamine
(DA) are identified in the early Xenopus tadpole
(stages 42–48) retina. GABA is first detected by
immunocytochemistry in amacrine cells at stage 35/36, 5-HT
at stage 39, and DA at stage 41. As the number of these
subtypes of amacrine cells increases by differentiation,
a subset of them transiently express two neurotransmitters.
GABA/DA double-labeled amacrine cells are detected first
at stage 42, at which time they constitute 52% of the DA-containing
population; this percentage decreases to only 3% by stage
48. 5-HT/DA amacrine cells are detected only at stage 44,
constituting about 20% of the DA-containing cells and 4%
of the small-dim 5-HT-containing cells. Regional location
does not differentially affect the differentiation of these
three types of amacrine cells (DA only, GABA/DA, and 5-HT/DA
cells); each type is found more in the anterior and dorsal
than the posterior and ventral quadrants, and their overall
distribution patterns are statistically indistinguishable.
However, these subtypes of amacrine cells reside in different
sublamina of the inner nuclear layer. DA-only amacrine
cells are located predominantly in the inner sublayer of
the 2–3 cell thick amacrine cell layer, closest to
the inner plexiform and the ganglion cell layers. Both
types of double-labeled cells are located mostly in the
outer sublayer of the amacrine cell layer, closest to other
interneurons in the inner nuclear layer. This distinct
sublaminar location of different neurotransmitter phenotypes
suggests that local laminar cues influence the coexpression
of neurotransmitters in amacrine cells.
Interaction between rod and cone signals in responses of
lateral geniculate neurons in dichromatic marmosets (Callithrix
jacchus)
Authors: WEISS STEFAN , KREMERS JAN , MAURER JOHANNES
Published: July 2002

Parvocellular (P-) and magnocellular (M-) cells
in the marmoset LGN can receive prominent rod input up
to relatively high illuminance levels (Kremers et al.,
1997b). In the present paper, we quantify rod
and cone input strengths under different retinal illuminance
levels. The stimulus was based on the so-called “silent
substitution” method. The activities of P- and M-cells
of dichromatic animals were recorded extracellularly. We
were able to adequately describe the response amplitudes
and phases by a vector summation of rod and cone signals.
At low retinal illuminance levels, the cells' responses
were determined by rod and cone inputs. With increasing
illuminances the strength of the cone input increased relative
to the rod strength. But, we often found significant rod
inputs up to illuminances equivalent to 700 td in the human
eye or more. Rod input strength was more pronounced in
cells with receptive fields at large retinal eccentricities.
The phase differences between rod and cone inputs suggest
that the rod signals lag about 45 ms behind the cone signals.
Perfusion system components release agents that distort
functional properties of rod cyclic nucleotide-gated ion
channels
Authors: CRARY JENNIFER I. , GORDON SHARONA E. , ZIMMERMAN ANITA L.
Published: July 2002

In switching from studying native cyclic nucleotide-gated
(CNG) ion channels in rod cells to studying the corresponding
cloned channels expressed in Xenopus oocytes,
we changed our perfusion system to a more efficient one.
This change involved replacing culture flasks and a small
plexiglass/glass chamber with plastic syringes, metal needles,
and plastic petri dishes. We now report that these new
perfusion system components release agents that distort
or obscure measured functional properties of rod CNG channels.
The magnitude and time course of appearance of the artifacts
vary widely among individual components (e.g. from syringe
to syringe). The effects most resemble voltage-dependent
block of the channels, giving a decrease in current at
positive potentials, and producing distortions of the kinetics
and voltage dependence of channel activation.
Memantine reduces alterations to the mammalian retina, in
situ, induced by ischemia
Authors: OSBORNE NEVILLE N.
Published: July 2002

The aim of the study was to determine whether memantine
could slow down the changes seen in the rabbit and rat
retina following ischemia/reperfusion. A “suction
cup procedure,” which raises the intraocular pressure,
was used to give an ischemic insult to the rabbit retina.
The electroretinogram was recorded before ischemia and
after 2 days of reperfusion. Memantine or saline (10 μl)
was injected into the eye before ischemia. Immunohistochemistry
was used to study the effect of ischemia/reperfusion on
the GABA, ChAT, and αPKC immunoreactivities. Ischemia/reperfusion
injury to the rat retina was induced by raising the intraocular
pressure above the systolic blood pressure for 60 min,
followed by reperfusion of 3–14 days. Memantine (5
mg/kg) or saline was injected i.p. at the onset of ischemia
or reperfusion. Immunohistochemistry was used to study
the effect of ischemia/reperfusion on the ChAT, αPKC,
and Thy-1 immunoreactivities. In addition, morphometric
analysis was carried out to determine the effects of ischemia/reperfusion
on the thickness of the retina. Ischemia for 75 min caused
a change in the nature of the normal GABA and ChAT immunoreactivities
in the rabbit retina and a reduction in the b-wave
of the electroretinogram. When memantine was injected into
the vitreous humour at the onset of an ischemic insult,
the changes in the GABA and ChAT immunoreactivities were
reduced and the recovery of the reduced b-wave
of the electroretinogram after 2 days reperfusion was enhanced
significantly. Ischemia for 60 min followed by 3 days reperfusion
showed a clear change in ChAT immunoreactivity in the rat
retina. The Thy-1 immunoreactivity was only clearly altered
after a reperfusion period of 7 days. Moreover, a measurable
change in the thickness of the inner retinal layers was
detected after 14 days of reperfusion. When given at the
onset of ischemia, memantine counteracted the effect of
ischemia/reperfusion to varying degrees. However, when
memantine was given at the onset of the reperfusion this
was not the case. The combined data show that a single
injection of memantine given i.p. or intravitreally will
protect the retina from a subsequent ischemic insult.
Saccade target selection in macaque during feature and conjunction
visual search
Authors: BICHOT NARCISSE P. , SCHALL JEFFREY D.
Published: July 2002

To gain insight into how vision guides eye movements,
monkeys were trained to make a single saccade to a specified
target stimulus during feature and conjunction search with
stimuli discriminated by color and shape. Monkeys performed
both tasks at levels well above chance. The latencies of
saccades to the target in conjunction search exhibited
shallow positive slopes as a function of set size, comparable
to slopes of reaction time of humans during target present/absent
judgments, but significantly different than the slopes
in feature search. Properties of the selection process
were revealed by the occasional saccades to distractors.
During feature search, errant saccades were directed more
often to a distractor near the target than to a distractor
at any other location. In contrast, during conjunction
search, saccades to distractors were guided more by similarity
than proximity to the target; monkeys were significantly
more likely to shift gaze to a distractor that had one
of the target features than to a distractor that had none.
Overall, color and shape information were used to similar
degrees in the search for the conjunction target. However,
in single sessions we observed an increased tendency of
saccades to a distractor that had been the target in the
previous experimental session. The establishment of this
tendency across sessions at least a day apart and its persistence
throughout a session distinguish this phenomenon from the
short-term (<10 trials) perceptual priming observed
in this and earlier studies using feature visual search.
Our findings support the hypothesis that the target in
at least some conjunction visual searches can be detected
efficiently based on visual similarity, most likely through
parallel processing of the individual features that define
the stimuli. These observations guide the interpretation
of neurophysiological data and constrain the development
of computational models.
Localization of AMPA-selective glutamate receptor subunits
in the cat retina: A light- and electron-microscopic study
Authors: QIN PU , POURCHO ROBERTA G.
Published: July 2002

The distribution of AMPA-selective glutamate receptor
subunits was studied in the cat retina using antisera against
GluR1 and GluR2/3. Both antisera were localized in postsynaptic
sites in the outer plexiform layer (OPL) as well as the
inner plexiform layer (IPL). Immunoreactivity for GluR1
was seen in a subpopulation of OFF cone bipolar cells and
a number of amacrine and ganglion cells. Within the IPL,
processes staining for GluR1 received input from OFF and
ON cone bipolar cells but not from rod bipolars. Labeling
for GluR2/3 was seen in horizontal cells, an occasional
cone bipolar cell, and numerous amacrine and ganglion cells.
In the IPL, GluR2/3 staining was postsynaptic to cone bipolar
cells in both sublaminae. AII amacrine cells which receive
rod bipolar input were also labeled for GluR2/3. With both
antisera, staining was limited to a single member of the
bipolar dyad complex, providing morphological evidence
for functional diversity in glutamatergic pathways.
Visual function in regenerating teleost retina following
cytotoxic lesioning
Authors: MENSINGER ALLEN F. , POWERS MAUREEN K.
Published: July 2002

Teleost fish retinas can regenerate in vivo
in adulthood. Retinal and visual function was assessed
in adult goldfish following comprehensive retinal destruction
by intraocular injection of ouabain. Electroretinograms
(ERGs) and the dorsal light reflex (DLR) were used to evaluate
the return of visual function. ERGs were detectable in
regenerating eyes 50 to 70 days following ouabain injection.
Amplitudes of both a- and b-waves increased
steadily through day 210 following ouabain treatment, at
which time a-wave amplitude was 90% and b-wave
amplitude approached 50% of the contralateral control eye.
The progressive gain observed in the a-wave was
attributed to photoreceptor regeneration. The increase
in b-wave amplitude was attributed to an increase
in the number of inner nuclear layer cells and the number
and efficacy of neuronal connections to or within the inner
retina. The photopic spectral sensitivity of the b-wave
in regenerating retina closely matched the intrafish control
retina, suggesting that the relative numbers of cone photoreceptors
was normal in regeneration. The recovery of the DLR (indicated
by improved postural balance during regeneration) paralleled
electrophysiological gains during retinal regeneration.
Fish displayed a marked longitudinal body imbalance toward
the control eye following retinal destruction. Improvement
in equilibrium was correlated with increasing b-wave
amplitudes. When the b-wave reached 50% of control
amplitude (30 weeks), normal posture was restored. The
return of the ERG indicates that photoreceptors and their
synaptic connections must be functional in regenerating
retina. Failure of the retina to regenerate produced an
abnormal DLR that persisted through 30 weeks and ERGs were
not measurable. The return of normal equilibrium indicates
that the regenerating retina can establish central connections
to the brain, and that the regenerated connections can
mediate functional visual behavior.
A model for the intracortical origin of orientation preference
and tuning in macaque striate cortex
Authors: ADORJÁN P. , LEVITT J. B. , LUND J. S. , OBERMAYER K.
Published: July 2002

We report results of numerical simulations for
a model of generation of orientation selectivity in macaque
striate cortex. In contrast to previous models, where the
initial orientation bias is generated by convergent geniculate
input to simple cells and subsequently sharpened by lateral
circuits, our approach is based on anisotropic intracortical
excitatory connections which provide both the
initial orientation bias and its subsequent amplification.
Our study shows that the emerging response properties are
similar to the response properties that are observed experimentally,
hence the hypothesis of an intracortical generation of
orientation bias is a sensible alternative to
the notion of an afferent bias by convergent geniculocortical
projection patterns. In contrast to models based on an
afferent orientation bias, however, the “intracortical
hypothesis” predicts that orientation tuning gradually
evolves from an initially nonoriented response and a complete
loss of orientation tuning when the recurrent excitation
is blocked, but new experiments must be designed to unambiguously
decide between both hypotheses.
Current source-density analysis of light-evoked field potentials
in rabbit retina
Authors: KARWOSKI CHESTER J. , XU XIJING
Published: July 2002

The technique of current source-density analysis
was applied to several components of the light-evoked field
potentials (electroretinogram) from the retina of the superfused
eyecup of rabbit. The depth distributions of the major
current sources and sinks were: b-wave—sink
at outer plexiform layer, source at inner plexiform layer;
M-wave—sink at inner plexiform layer, source at retinal
surface; and slow PIII—source near outer plexiform
layer, sink at retinal surface. These distributions, along
with the sensitivities of these responses to certain pharmacological
agents, support earlier studies that Müller cells
generate the M-wave and slow PIII, but that depolarizing
bipolar cells directly generate the b-wave.
Horizontal cell glutamate receptor modulation by NO: Mechanisms
and functional implications for the first visual synapse
Authors: MCMAHON DOUGLAS G. , SCHMIDT KARL F.
Published: July 2002

Neurons of the horizontal cell retinal neural network
are subject to modulation by the neurotransmitter nitric
oxide (NO). We have examined the effects of NO on glutamate
receptor function in isolated horizontal cells from the
perch (Perca fluviatilis) using the concentration
ramp technique to simultaneously record receptor current
and agonist concentration. Dose–response curves for
glutamate (0–1 mM) and kainate (0–200 μM)
were measured in the presence and absence of 1–2
mM sodium nitroprusside (SNP), 1 mM 8-Br-cGMP, 100 μM
cyclothiazide or 200 μM dopamine as modulators. SNP
increased the EC50 (i.e. decreased
affinity) for glutamate and increased Imax
(i.e. increased efficacy), whereas 8-Br-cGMP increased
EC50, but not Imax. In the
presence of the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic
acid (AMPA) receptor desensitization blocker cyclothiazide,
the SNP-induced increase in EC50 persisted,
but the increase in Imax was blocked. The
increase in EC50, but not the increase in
Imax, was also observed when the non-desensitizing
agonist kainate (100–200 μM) was applied in the presence
of SNP. When 2 mM SNP and 200 μM dopamine were applied together,
they increased Imax (740 vs. 2455 pA)
and EC50 (422 vs. 682 μM). Our
findings indicate that NO modulates horizontal cell glutamate
responses by reducing the affinity of receptors for glutamate while
simultaneously increasing the maximal current. The shift in affinity
is cGMP-mediated and independent of desensitization. The action
of NO on horizontal cell glutamate receptors is distinct
from, but synergistic with, that of dopamine. Glutamate
receptor modulation by NO qualitatively predicts the action
of NO on horizontal cell light responses in situ
and may alter transmission at visual synapses according
to adaptational conditions.
Cortical processing of second-order motion
Authors: MARESCHAL ISABELLE , BAKER CURTIS L.
Published: July 2002

Neurons in the mammalian visual cortex have been
found to respond to second-order features which are not
defined by changes in luminance over the retina (Albright,
1992; Zhou & Baker, 1993, 1994, 1996; Mareschal &
Baker, 1998a,b). The detection of these
stimuli is most often accounted for by a separate nonlinear
processing stream, acting in parallel to the linear stream
in the visual system. Here we examine the two-dimensional
spatial properties of these nonlinear neurons in area 18
using envelope stimuli, which consist of a high spatial-frequency
carrier whose contrast is modulated by a low spatial-frequency
envelope. These stimuli would fail to elicit a response
in a conventional linear neuron because they are designed
to contain no spatial-frequency components overlapping
the neuron's luminance defined passband. We measured
neurons' responses to these stimuli as a function
of both the relative spatial frequencies and relative orientations
of the carrier and envelope. Neurons' responses to
envelope stimuli were narrowband to the carrier spatial
frequency, with optimal values ranging from 8- to 30-fold
higher than the envelope spatial frequencies. Neurons'
responses to the envelope stimuli were strongly dependent
on the orientation of the envelope and less so on the orientation
of the carrier. Although the selectivity to the carrier
orientation was broader, neurons' responses were clearly
tuned, suggesting that the source of nonlinear input is
cortical. There was no fixed relationship between the optimal
carrier and envelope spatial frequencies or orientations,
such that nonlinear neurons responding to these stimuli
could perhaps respond to a variety of stimuli defined by
changes in scale or orientation.