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At least 1,693 records · Page 94Linked to original sources
PubMed2003

Axonal sprouting in the optic nerve is not a prerequisite for successful regeneration.

Axonal sprouting, the production of axons additional to the parent one, occurs during optic nerve regeneration in goldfish and the frog Rana pipiens, with numbers of regenerate axons exceeding normal values four- to sixfold (Murray [1982] J. Comp. Neurol. 209:352-362; Stelzner and Strauss [1986] J. Comp. Neurol. 245:83-103). To determine whether axonal sprouting is a prerequisite for regeneration, the frog Litoria moorei was examined, a species that undergoes successful optic nerve regeneration but with a different time course compared with R. pipiens. Sprouting was assessed, as in goldfish and R. pipiens, from electron microscopic counts between the lesion and chiasm. However, disconnected axons that persist after axotomy would have falsely elevated the counts. The suspected overlap of these two axon populations was confirmed by labeling regenerate axons anterogradely with DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) and disconnected ones retrogradely with DiA (4-4-dihexadecylaminostyrl 1-N methylpyridinium iodide). Numbers of disconnected axons were estimated after preventing regeneration and subtracted from numbers in regenerate nerves. Throughout, the total number of regenerate axons was approximately one third lower than normal (P < 0.05) supporting a previous finding of minimal axonal sprouting in L. moorei (Dunlop et al. [2002] J. Comp. Neurol. 446:276-287). The validity of the subtractive electron microscopic method was confirmed by retrograde labeling to estimate numbers of retinal ganglion cells whose axons had crossed the lesion; values were approximately one third lower than normal. The data suggest that sprouting is not essential for either axon outgrowth or topographic map refinement.

Animals↗
PubMed2007

Connections of eye-saccade-related areas within mesencephalic reticular formation with the optic tectum in goldfish.

Physiological studies demonstrate that separate sites within the mesencephalic reticular formation (MRF) can evoke eye saccades with different preferred directions. Furthermore, anatomical research suggests that a tectoreticulotectal circuit organized in accordance with the tectal eye movement map is present. However, whether the reticulotectal projection shifts with the gaze map present in the MRF is unknown. We explored this question in goldfish, by injecting biotin dextran amine within MRF sites that evoked upward, downward, oblique, and horizontal eye saccades. Then, we analyzed the labeling in the optic tectum. The main findings can be summarized as follows. 1) The MRF and the optic tectum were connected by separate axons of the tectobulbar tract. 2) The MRF was reciprocally connected mainly with the ipsilateral tectal lobe, but also with the contralateral one. 3) The MRF received projections chiefly from neurons located within intermediate and deep tectal layers. In addition, the MRF projections terminated primarily within the intermediate tectal layer. 4) The distribution of labeled neurons in the tectum shifted with the different MRF sites in a manner consistent with the tectal motor map. The area containing these cells was targeted by a high-density reticulotectal projection. In addition to this high-density topographic projection, there was a low-density one spread throughout the tectum. 5) Occasionally, boutons were observed adjacent to tectal labeled neurons. We conclude that the organization of the reticulotectal circuit is consistent with the functional topography of the MRF and that the MRF participates in a tectoreticulotectal feedback circuit.

Animals↗
PubMed1997

Using corneal height maps and polynomial decomposition to determine corneal aberrations.

PURPOSE: To review the use of corneal videokeratoscopic height date, elaborate on the advantages and disadvantages of such data, describe techniques for overcoming the limitations of height data, and demonstrate its use in quantifying the optical properties and aberrations of the cornea. METHODS: The steep sag of the cornea hides fine variations in corneal height that arise naturally or due to disease or surgery. The dynamic range, or ratio of the overall sag to the feature height, is the primary limitation of videokeratoscopic height data. Techniques for removing single or multiple reference surfaces are described in detail, and applications of the methodology to wavefront and raytracing analysis of corneal aberrations arising from radial keratotomy (RK), photorefractive keratectomy (PRK), and keratoconus are described. RESULTS: Removing a single reference surface from the raw corneal height data begins to reveal subtle variations in corneal height. However, expansion of surface height data into a complete set of basis functions provides a sophisticated method for extracting high-order corneal variations. Choosing an orthogonal basis set provides a robust least-squares fit and forms unique expansions of the surface. The resulting coefficients are uncorrelated and form a simple measure of the optical quality. CONCLUSION: Videokeratoscopic height data are useful for analyzing and quantifying corneal deformity arising from disease or refractive surgery and they provide a sophisticated alternative or complement to dioptric power maps.

Cornea↗
PubMed2006

Thalamo-cortical connectivity in children born preterm mapped using probabilistic magnetic resonance tractography.

Our aim was to investigate the feasibility of studying white matter tracts and connections between the thalamus and the cortex in 2-year-old infants who were born preterm by probabilistic magnetic resonance (MR) tractography. Using this approach, we were able to visualize and quantify connectivity distributions in a number of white matter tracts, including the corticospinal tracts, optic radiations, fibers of the genu and splenium of the corpus callosum, superior longitudinal fasciculus and inferior fronto-occipital fasciculus, and to map the distribution within thalamus of fibers connecting to specific cortical regions. In eleven infants with no MR evidence of focal cerebral lesions and appropriate neurodevelopment as shown by general quotient (GQ) scores above 100, we mapped cortical connections to the thalamus that appeared similar to those reported in adults. However, in a proof-of-principle experiment, we examined one further child with marked white matter abnormalities and found that the volume and pattern of thalamo-cortical connections were severely disrupted. This technique promises to be a useful tool for assessing connectivity in the developing brain and in infants with lesions.

Cerebral Cortex↗
PubMed1996

Distributed representation of vibrissa movement in the upper layers of somatosensory cortex revealed with voltage-sensitive dyes.

We have identified large-scale patterns of electrical activity in circuits that occur in response to stimulation of peripheral receptors. Our focus was on primary (S1) vibrissal cortex of anesthetized rat, and we used optical techniques in conjunction with voltage-sensitive dyes to measure depolarization of the upper layers of cortex. Displacement of one vibrissa produced a field of activity that extends over very many cortical columns in S1. There are multiple, focal maxima within this field. A global maximum is located near the center of the field of activity, and, as determined electrically and histologically, this site maps to the cortical column appropriate for the deflected vibrissa. The amplitude of this component attains a steady-state value under continuous stimulation. Additional temporal characteristics are revealed by the response to a single displacement; the signal was triphasic and began with a prompt depolarization that was followed by a transient phase of inhibition and a final phase of long-lasting depolarization. The somatotopy of the other, satellite maxima in the field of activity were established through the reconstruction of the fields of activity produced by individual stimulation of other vibrissae. Local maxima for one vibrissa were seen to overlie the global maximum found for stimulation of nearest- and next-nearest-neighbor vibrissae. In contrast to the amplitude of the global maxima, the amplitude associated with the local maxima was not maintained with either continuous or infrequent but repetitive stimulation. Finally, the field of activity induced by alternate deflection of two neighboring vibrissae was suppressed in amplitude in comparison to the summed amplitudes of the signals elicited by deflection of each vibrissa alone. We suggest that these patterns of activity are a manifestation of the dynamic interaction among neighboring cortical columns.

Adaptation, Physiological↗
PubMed1975

Direction-selective units in the rabbit's nucleus of the optic tract.

A class of direction-selective (DS) units, histologically localized within the nucleus of the optic tract (NOT) was isolated in the rabbit's pretectum. These units typically had a maintained discharge of 25-50 action potentials/sec and large receptive fields (up to 40 X 150 degrees) in the visual streak area of the contralateral eye. They were excited by a visual pattern moving in one direction and inhibited by motion in the opposite direction. The reactions were sustained. Excitatory and inhibitory acceptance angles were each 180 degrees. Most units were excited by anterior motion of the stimulus and reacted to a wide range of velocities (0.01-20 degrees/sec). Random checkerboard patterns (elements 0.8 degrees), grids of black and white stripes (1, 2 and 4 degrees wide) and single black and white edges were all effective, with a decreasing response magnitude in roughly this order. A stimulus area of 2 X 2 degrees was already effective; response increased with area and was maximal for 15 X 15 degrees and larger stimuli. Latency for visual stimulation was 60 +/- 10 (S.D.) msec, for electrical stimulation of the chiasm 2.2 +/- 0.3 (S.D.) msec. Synaptic latency and presynaptic conduction velocity were estimated at 0.7 msec and 13m/sec, respectively. A strong convergence of retinal DS fibers upon NOT units is postulated. Since most properties of NOT units are compatible with those of optokinetic nystagmus, and electrical stimulation of the NOT elicits vigorous nystagmus, these data suggest that these cells are the essential afferent link in the optokinetic reflex arc.

Animals↗
PubMed1981

[Cortical control of afferent flow in the lateral geniculate body in the cat].

In experiments carried out on cats immobilized with d-tubocurarine unit responses within the dorsolateral geniculate nucleus were analyzed following visual cortex (testing) and optic chiasm (conditioning) stimulation. Two types of responses were found indicating the existence of principal and inhibitory cells in the nucleus. Optic chiasm stimulation was found to produce an inhibitory corticofugal effect on the responses of principal cells. A conclusion is made that these effects are mediated by inhibitory cells.

Animals↗
PubMedDate not supplied

Use-dependent plasticity in barrel cortex: intrinsic signal imaging reveals functional expansion of spared whisker representation into adjacent deprived columns.

We used optical imaging of intrinsic cortical signals, elicited by whisker stimulation, to define areas of activation in primary sensory cortex of normal hamsters and hamsters subjected to neonatal follicle ablation at postnatal day seven (P7). Follicle ablations were unilateral, and spared either C-row whiskers or the second whisker arc. This study was done to determine if the intrinsic cortical connectivity pattern of the barrel cortex, established during the critical period, affects the process of representational plasticity that follows whisker follicle ablation. Additionally, we tested the ability to monitor such changes in individual cortical whisker representations using intrinsic signal imaging. Stimulation of a single whisker yielded peak activation of a barrel-sized patch in the somatotopically appropriate location in normal cortex. In both row and arc-spared animals, functional representations corresponding to spared follicles were significantly stronger and more oblong than normal. The pattern of activation differed in the row-sparing and arc-sparing groups, in that the expansion was preferentially into deprived, not spared areas. Single whisker stimulation in row-spared cases preferentially activated the corresponding barrel arc, while stimulation of one whisker in arc-spared cases produced elongated activation down the barrel row. Since whisker deflection normally has a net inhibitory effect on neighboring barrels, our data suggest that intracortical inhibition fails to develop normally in deprived cortical columns. Because thalamocortical projections are not affected by follicle ablation after P7, we suggest that the effects we observed are largely cortical, not thalamocortical.

Animals↗
PubMed1992

Mapping the entry of reactive oxygen metabolites into target erythrocytes during neutrophil-mediated antibody-dependent cellular cytotoxicity.

Transmitted Soret band optical microscopy has been used to image the entry and passage of reactive oxygen metabolites across target erythrocytes. Due to the rapid cytosolic diffusion of hemoglobin in comparison to video rates, it was necessary to use erythrocytes with relatively immobilized hemoglobin. To achieve this, erythrocytes from patients with sickle cell anemia were used. The movement of reactive oxygen metabolites across rabbit IgG-opsonized sickle cells was observed in real time. These observations indicate that reactive oxygen metabolites can enter and cross targets in an asymmetric fashion.

Anemia, Sickle Cell↗
PubMed2003

Design principles and limitations of wave-front guided contact lenses.

The concept of the wave-front guided design of contact lenses is presented from three vantage points: ray optics, wave front aberrations, and optical path-length errors. We argue that the goal of contact lenses is to make all of the optical paths from a distant object to the retina equal in length, regardless of where the path intersects the plane of the eye's pupil. The aberration map of an eye is a prescription for such a lens. Unfortunately, variability of measured aberration maps is a fundamental limit to our knowledge of the true aberration structure of an eye. Variability arises because the eye is a biologic system that changes over time for normal, physiologic reasons. Furthermore, uncertainty in our measurement of the aberration map because of such variable factors, such as alignment of the aberrometer to the eye by the clinician or small fixation errors committed by the patient, will make it difficult to achieve a full measure of success with aberration-correcting contact lenses. The clinical implication of these findings is that multiple measurements of the aberration map should be collected using a protocol that includes realignment of the instrument and then averaging the aberration maps to reduce the level of uncertainty associated with any single measurement.

Contact Lenses↗
PubMed1979

Importance of corpus callosum for visual receptive fields of single neurons in cat superior colliculus.

1. Section of the posterior two-thirds of the corpus callosum eliminates almost completely the response of superior colliculus (SC) neurons to stimulation of the contralateral eye in split-chiasm cats. On the contrary, the responsiveness of SC neurons to stimulation of the contralateral eye is not abolished by a transection of the posterior and tectal commissures leaving the corpus callosum intact. The callosal section also reduces the number of SC receptive fields abutting the vertical meridian in the ipsilateral eye of split-chiasm cats. 2. In cats with intact optic pathways, a similar callosal section abolishes the SC representation of the ipsilateral visual field in the ipsilateral eye and also reduces the number of receptive fields adjoining the vertical meridian in the same eye. In the contralateral eye, the SC representation of the ipsilateral visual field is reduced in extension to about one-fifth of that seen in cats with intact commissures. 3. The results suggest that the corpus callosum is the main pathway for cross-midline communication of visual information at not only the cortical, but also the midbrain level. The corpus callosum may subserve this function because it contains uninterrupted crossed corticotectal projections or because it transmits visual information from one hemisphere to contralateral cortical areas projecting ipsilaterally to SC. The latter hypothesis is more likely but, in any case, the findings imply that the lack of interhemispheric transfer of visual learning in cats with a chiasmatic and callosal section may depend on a midline disconnection of both subcortical and cortical visual centers. 4. The corpus callosum is also responsible for the representation of the ipsilateral visual field of the ipsilateral eye in the cat SC. The SC representation of the ipsilateral visual field in the contralateral eye is due, in minimal part, to direct retinotectal connections from temporal retina and, for the largest part, to the corpus callosum. 5. Finally, the corpus callosum contributes to the representation of the contralateral visual field near the vertical meridian of the temporal retina in both split-chiasm and normal cats. This is probably due to the scarcity of direct retinotectal projections from this part of the retina and to their supplementation by corticotectal neurons influenced by the callosal afferents.

Animals↗
PubMed2005

A stimulation method using odors suitable for PET and fMRI studies with recording of physiological and behavioral signals.

A design for a semi-automatic olfactometric system is described for PET and fMRI experiments. The olfactometer presents several advantages because it enables the use of an 'infinite' number of odorants and the synchronization of stimuli with breathing. These advantages mean that the subject is recorded while breathing normally during olfactory judgment tasks. In addition, the design includes a system for recording the behavioral (rating scale) and physiological (breathing, electrodermal reaction (ED), plethysmography (PL)) signals given by the subject. Both systems present the advantage of being compatible with fMRI magnetic fields since no ferrous material is used in the Faraday cage and signals are transmitted via an optical transmission interface to an acquisition system.

Adult↗
PubMed1986

A physiological measure of shifting connections in the Rana pipiens retinotectal system.

The retinotectal connections of developing Rana tadpoles and froglets have been studied using light-pipe techniques to directly assay the pattern of the projection from the retina to the tectum. The projection site of the retina surrounding the optic nerve head was determined at two different stages of development (late larval and metamorphic frog) on the same animal. Small electrolytic marker lesions were used to mark the tectal sites to which the optic nerve head projected at these two times. Comparison of the positions of the two lesions gives a direct measure of the shift in the projection during the interlesion time interval of one week. The results indicate a shift in the projection of 275 micron week-1 in late larval life. Previous work in Xenopus using the light-pipe techniques indicated a qualitatively similar shift during equivalent stages of development, but significantly smaller in magnitude. In the present study, topographic postsynaptic units could be recorded at all stages investigated, indicating functional synapses between the optic nerve fibres and the tectum. Thus, these studies offer evidence of a significant shift in the functional connection pattern of the amphibian retinotectal map during development, in agreement with the recent anatomical data from other laboratories on the Rana and goldfish visual system.

Animals↗
PubMed2005

Goodness-of-prediction of Zernike polynomial fitting to corneal surfaces.

PURPOSE: To determine the goodness-of-prediction of the fitting routine by measuring the difference between topographic corneal surfaces and their Zernike reconstructions as a function of polynomial order and optical zone size for various corneal conditions. SETTING: Corneal research laboratory in a university eye center. METHODS: Corneal topography maps (N = 253) were obtained from the Louisiana State University Eye Center. A variety of corneal conditions were used: normals; astigmatism; laser in situ keratomileusis, photorefractive keratoplasty (PRK), and radial keratotomy (RK) postoperative cases (myopic spherical corrections); keratoconus suspect; mild, moderate, and severe keratoconus; pellucid marginal degeneration; contact lens-induced corneal warpage; and penetrating keratoplasty. The root-mean-square (RMS) error of the goodness-of-prediction of the Zernike representation of corneal surface elevation was extracted for 4, 6, and 10 mm optical zones, whereas Zernike radial orders were varied from 3 to 14 in 1-order steps. The mean +/- SEM of the RMS error was plotted as a function of Zernike order and compared with criteria for normal surface fits. RESULTS: Fitting accuracy improved as more Zernike terms were included, but some conditions showed significant errors (when compared with normal surfaces), even with many added terms. For a 6 mm optical zone, the normal cornea group had the lowest RMS error and did not require terms above the 4th order to achieve <0.25 microm RMS error. Astigmatism met the 0.25 microm threshold at the 5th order, whereas keratoconus suspect required 7 orders. Laser in situ keratomileusis and PRK met the 0.25 microm threshold at the 8th order, whereas RK required 10 orders. Contact lens-induced corneal warpage and mild keratoconus needed 12 orders to meet the 0.25 microm threshold, whereas pellucid marginal degeneration, moderate and severe keratoconus, and keratoplasty categories were not well fitted even at 14 orders. CONCLUSIONS: A 4th-order Zernike polynomial appeared reliable for modeling the normal cornea, but using a 4th-order fitting routine with an abnormal corneal surface caused a loss of fine-detail shape information. As more Zernike terms were added, the accuracy of the fit improved, and the result approached the minimum error found with normal corneas. Unless sufficient higher-order Zernike terms are included when analyzing irregular surfaces, some diagnostic applications of Zernike coefficients may not be rigorous. This conclusion also suggests that wavefront shape analysis is similarly dependent on the number of orders used. Current surgical corrections based on normal-eye wavefronts may fail to capture all visually relevant aberrations in abnormal eyes, such as those having laser retreatments or experiencing corneal warpage from contact lens wear. A clinical goodness-of-fit or goodness-of-prediction index would indicate whether the number of terms in use has fully accounted for all of the visually significant aberrations present in the eye.

Cornea↗
PubMed1990

Contribution of the nucleus of the optic tract to optokinetic nystagmus and optokinetic afternystagmus in the monkey: clinical implications.

1. The role of the pretectal NOT and the DTN in producing horizontal OKN and OKAN were studied using electrical stimulation and lesions. Positive stimulation sites lay in NOT, DTN, and in a fiber bundle in the pulvinar that is presumably a cortical input to NOT. 2. When the region of NOT was electrically stimulated in darkness, horizontal nystagmus was evoked with ipsilateral slow phases. Eye velocity rose slowly to a steady-state level and was followed by afternystagmus at the end of stimulation. The time constant of rise of stimulus-induced nystagmus was similar to the slow rise of slow-phase eye velocity during OKN. The saturation velocity of the induced nystagmus and the falling time constant of the stimulus afternystagmus were the same as those of OKAN. This suggests that electrical stimulation of NOT and DTN had elicited the slow component of OKN, i.e., that component produced by the velocity storage mechanism in the vestibular system. 3. Consistent with this postulate, activity induced by NOT stimulation could enhance, prolong, or block the slow component of OKN and OKAN depending on whether slow phases were to the same or opposite side. Stimulus-induced activity also interacted with vestibular nystagmus as would OKN and OKAN. 4. Unilateral lesions of NOT and DTN caused a loss of OKAN and the slow rise in OKN to the ipsilateral side. Steady-state velocities of OKN were reduced. The initial jump of OKN slow-phase velocity was the same or somewhat less after lesions but was not lost. 5. Partial lesions of a fiber bundle in the lateral pulvinar caused a transient change in OKN and OKAN, consistent with the idea that it carries activity for the slow component from the cortex to NOT. A lesion of the MRF, just rostral to the superior colliculus, caused a transient loss of the rapid component of OKN. This region appears to carry activity responsible for the initial jump in slow-phase velocity at the onset of stimulation. 6. We conclude that: (a) NOT and probably DTN lie in the indirect pathway that produces the slow component of horizontal OKN and OKAN to the ipsilateral side in the rhesus monkey. This pathway activates the velocity storage mechanism in the vestibular nuclei. (b) At the level of NOT, the pathway responsible for the slow component of OKN and OKAN is anatomically distinct from the pathway responsible for rapid changes in eye velocity at the onset of OKN.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Topographic refinement of the regenerating retinotectal projection of the goldfish in standard laboratory conditions: a quantitative WGA-HRP study.

The topographic precision of the regenerating retinotectal projection of the goldfish was studied between 18 and 524 days (at 20 degrees C) after optic nerve cut, using retrograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP) from one of two standardized tectal injection sites. All labelled ganglion cells in each flat-mounted retina were plotted individually, and their degree of dispersion was assessed by a statistical method based on distance to nearest neighbour. Labelled cells in normal fish were clustered tightly, covering on average only 1.3% of the retina. Early in regeneration (18-28 days) they were widely dispersed, covering up to 75.2%, and they did not begin to form recognizable clusters at appropriate sites until about 35 days after nerve cut. Between 18 and 70 days, the proportion of retina covered by labelled cells fell dramatically, halving about every 14 days. Between 70 and 524 days, no further reduction could be demonstrated: overall, clusters remained significantly larger than normal, though a few individual retinae were virtually normal. Several others, labelled from similar single injections between 56 and 524 days after nerve cut, showed pairs of cell clusters; a sign that persistent errors in topography are common. The very wide initial scatter of labelled cells reflects a striking lack of 'goal-directedness' in regenerative axon growth. Extensive branching in the optic nerve, tract and tectum, for which there is already evidence, must contribute to this. Though uptake of some WGA-HRP by nonsynaptic growth cones cannot be ruled out, other evidence for mislocated functional synapses at early stages encourages us to favour 'trial and error' synapse formation as the likely basis of map refinement.

Animals↗
PubMed2003

Stationary pulses of light in an atomic medium.

Physical processes that could facilitate coherent control of light propagation are under active exploration. In addition to their fundamental interest, these efforts are stimulated by practical possibilities, such as the development of a quantum memory for photonic states. Controlled localization and storage of photonic pulses may also allow novel approaches to manipulating of light via enhanced nonlinear optical processes. Recently, electromagnetically induced transparency was used to reduce the group velocity of propagating light pulses and to reversibly map propagating light pulses into stationary spin excitations in atomic media. Here we describe and experimentally demonstrate a technique in which light propagating in a medium of Rb atoms is converted into an excitation with localized, stationary electromagnetic energy, which can be held and released after a controllable interval. Our method creates pulses of light with stationary envelopes bound to an atomic spin coherence, offering new possibilities for photon state manipulation and nonlinear optical processes at low light levels.

Journal Article↗