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At least 613 records · Page 34Linked to original sources
PubMed1997

L and M cone input into spectral sensitivity functions: a reanalysis.

In order to better understand the nature of long-wavelength (L) and middle-wavelength (M) cone input into spectral sensitivity functions and determine the reliability with which it is possible to predict L:M cone inputs, we developed analytical methods to determine confidence intervals for L:M cone input for spectral sensitivity functions or data transformed to cone-contrast space. Spectral sensitivity functions measured by direct heterochromatic brightness matches are dominated by the L/M opponent channel over most of the spectral range. For detection of large/ long test stimuli, spectral sensitivity functions show a characteristic "notch" at the adapting wavelength, with the L/M opponent channel dominating most of the spectral range. Flicker increment threshold (FIT) spectral sensitivity functions display many of the characteristics of the luminance flicker mechanism described by Stromeyer et al. (1987). [Vision Research, 27, 1113-1137]. Previous modelling of FIT spectral sensitivity functions proposed a 2:1 L:M cone input for most of testing conditions. We show that FIT spectral sensitivity functions are dominated by L cones but show L cone suppression under bright red adapting fields. For the fitted spectral sensitivity functions or simulated data sets, we found small confidence intervals for L:M cone input into the L/M opponent channel and conclude that it is possible to reliably predict L:M cone input ratios. However, for similar data sets of additive spectral sensitivity functions, we found large confidence intervals for L:M cone input ratios and conclude that it is not possible reliably predict L:M cone input into the L/M non-opponent channel using available spectral sensitivity functions.

Adaptation, Ocular↗
PubMed2001

Subretinal transplantation of genetically modified human cell lines attenuates loss of visual function in dystrophic rats.

Royal College of Surgeons rats are genetically predisposed to undergo significant visual loss caused by a primary dysfunction of retinal pigment epithelial (RPE) cells. By using this model, we have examined the efficacy of subretinal transplantation of two independent human RPE cell lines each exhibiting genetic modifications that confer long-term stability in vitro. The two cell lines, a spontaneously derived cell line (ARPE19) and an extensively characterized genetically engineered human RPE cell line (h1RPE7), which expresses SV40 large T (tumor) antigen, were evaluated separately. Both lines result in a significant preservation of visual function as assessed by either behavioral or physiological techniques. This attenuation of visual loss correlates with photoreceptor survival and the presence of donor cells in the areas of rescued photoreceptors at 5 months postgrafting (6 months of age). These results demonstrate the potential of genetically modified human RPE cells for ultimate application in therapeutic transplantation strategies for retinal degenerative diseases caused by RPE dysfunction.

Animals↗
PubMed2004

A method for the automated assessment of temporal characteristics of functional hemispheric lateralization by transcranial Doppler sonography.

BACKGROUND AND PURPOSE: Transcranial Doppler sonography (TCD) can guide and complement investigations based on functional magnetic resonance and positron emission tomography imaging by providing continuous information on cerebral perfusion changes correlated to cerebral activation. So far, however, the role of functional TCD has been limited by a lack of sensitivity. METHODS: Here, the authors present an outline of a method that increases the potential of TCD to detect perfusion changes within a vascular territory. Sensitivity on the order of 1% can be achieved by transformation of Doppler envelope curves, which accounts for systemic quasi-periodic and irregular spontaneous blood flow modulations and artificial disturbances related to the recording. A statistical technique is introduced that allows the automatic detection of time periods of significant hemispheric lateralization in evoked flow studies. Furthermore, an index of laterality is defined quantifying the extent of hemispheric dominance during stimulus processing. RESULTS AND DISCUSSION: The analysis technique described in this article has been successfully employed in recent examinations on vision, motor activation, language, language recovery, and other cognitive tasks. CONCLUSION: The novel functional TCD technique permits valid and reproducible assessments of the temporal characteristics of functional hemispheric lateralization.

Blood Flow Velocity↗
PubMedDate not supplied

[When was modern science born? Homage to the memory of Professor Arturo Rosenblueth at his birth centennial].

The Renaissance savants essentially repelled the scholastic translations and commentaries of the ancient writings. Nevertheless they did not reach a modern vision of the experimental science. Moreover, the education at the universities was not credited for the science's development. In fact, the academic training of students was rather precarious. The first professional associations, such as the "Royal College of Physicians" of London, were not any better. Regarding hermetic influence on the Renaissance thought, the cultured and philosophical reformed magic (the so called white magic) was the equivalent of science at the time. Once the animistic universe, operated by magic, is transformed into the mathematical universe, operated by mechanics, the era of science came into being. This movement began during the post-Renaissance age and gradually progressed following the physical-mathematical orientation of Galileo and his pupils: Borelli, Fabrizi, Santorio, Harvey, etc. They initiated the physiological studies and introduced the quantitative method into the research field. Harvey's circulatory doctrine was the first adequate explication of an organic phenomenon and a starting point for the way toward experimental physiology. However the English physician did not leave completely the pre-scientific era, as can be inferred from his monography on animals reproduction. In this work, some points suggesting the birth of modern scientific reasoning alternate with confused, vague and capricious assertions. In fact, the modern science did not arise suddenly, but was elaborated and sustained slowly starting from the XVII century: the Galileo's century.

History, 15th Century↗
PubMed2006

Perceptual grouping induces non-retinotopic feature attribution in human vision.

The human visual system computes features of moving objects with high precision despite the fact that these features can change or blend into each other in the retinotopic image. Very little is known about how the human brain accomplishes this complex feat. Using a Ternus-Pikler display, introduced by Gestalt psychologists about a century ago, we show that human observers can perceive features of moving objects at locations these features are not present. More importantly, our results indicate that these non-retinotopic feature attributions are not errors caused by the limitations of the perceptual system but follow rules of perceptual grouping. From a computational perspective, our data imply sophisticated real-time transformations of retinotopic relations in the visual cortex. Our results suggest that the human motion and form systems interact with each other to remap the retinotopic projection of the physical space in order to maintain the identity of moving objects in the perceptual space.

Eye Movements↗
PubMed2001

Two-dimensional sound-localization behavior of early-blind humans.

To investigate whether the visual system is crucial for adequate calibration of acoustic localization cues, sound-localization performance of early blind humans was compared with that of sighted controls. Because a potential benefit of vision is mainly expected for targets within the two-dimensional (2D) frontal hemifield, localization was tested within this target range, while using sounds of various durations and spectral content. Subjects were instructed to point, in separate experimental sessions, either with their left arm, or with their nose, in the direction of the perceived target position as accurately as possible. The experiments required the use of all available sound-localization cues such as interaural differences in phase and intensity, as well as the complex spectral shape cues provided by the pinnae. In addition, for long-duration stimuli, subjects could have had access to head motion-induced acoustic feedback. Moreover, the two pointing methods allowed us to assess different response strategies for the two groups. In an additional series, subjects were instructed to respond as quickly as possible. The results show that, in general, 2D sound-localization performance of blind subjects was indistinguishable from that of sighted subjects, both for broad-band noise and for pure tones. In the fast head-pointing task, the latency distributions of both groups were equal. These findings suggest that visual feedback is not required to calibrate the available localization cues--including the idiosyncratic and complex spectral shape cues for elevation. Instead, the localization abilities of blind people show that the putative supervising role of vision may be supported, or taken over, by other non-visual feedback systems. The results do not provide support for the hypothesis that blind people can hypercompensate for the loss of vision in the frontal hemifield by developing superior sound-localization abilities. Despite the general correspondence in localization behavior, some specific differences related to pointing strategies as well as to those between blind and sighted subjects were apparent. Most importantly, the reconstructed origin (bias) of arm pointing was located near the shoulder for the blind subjects, whereas it was shifted and located near the cyclopean eye for the sighted subjects. The results indicate that both early blind and sighted humans adequately transform the head-centered acoustic target coordinates into the required reference frame of either motor system, but that the adopted response strategy may be specific to the subject group and pointer method.

Adult↗
PubMed2001

Adaptation to disparity but not to perceived depth.

The purpose of the present study was to investigate whether adaptation can occur to disparity per se. The adapting stimuli were large random-dot patterns of which the two half-images were transformed such that the depth effects induced by the vertical transformations were nulled by horizontal transformations. Thus, the adapting stimuli were perceptually the same, whereas the disparity fields differed from each other. The adapting stimuli were presented for five minutes. During that period, the percept of a fronto-parallel surface did not change. After the adapting period, subjects perceived a thin untransformed strip as either slanted or curved depending on the adapting transformation. The thin strips provided negligible information about the vertical disparity field. In a forced-choice task we measured the amount of horizontal transformation that was required to null the acquired adaptation. We found that the amounts of horizontal transformation required to perceive the test strip fronto-parallel were significantly different from zero. We conclude that the visual system can adapt to disparity signals in the absence of a perceptual drive.

Adaptation, Physiological↗
PubMed2005

Confocal microscopy used as the definitive, early diagnostic method in Chandler syndrome.

PURPOSE: To report the early, rapid diagnosis of the Chandler variant of the iridocorneal endothelial (ICE) syndrome using confocal light microscopy. METHODS: A 62-year-old man with a long history of unilateral glaucoma reported progressively blurred vision in his right eye. Examination of both eyes included visual acuity, slit-lamp examination, pneumotonometry, visual field, gonioscopy, and confocal microscopy. RESULTS: On examination, visual acuity was 20/80 and 20/20 and the IOPs were 26 and 12. The anterior segment OD revealed 1+ inferior and axial corneal edema, while the OS was normal to biomicroscopy and posterior segment. Chandler syndrome or Fuchs endothelial dystrophy was suspected. In the affected eye, confocal light microscopy clearly showed an "epithelium-like" transformation of the corneal endothelium with irregularly shaped cells and hyperreflective nuclei, establishing the diagnosis of Chandler syndrome. CONCLUSIONS: In the presence of corneal edema or haze, corneal endothelium can be clearly visualized by confocal microscopy. "Epithelium-like" endothelial cells with highly reflective nuclei characteristic of Chandler syndrome were easily identified by confocal light microscopy to establish the diagnosis, despite the presence of corneal edema. Thus, confocal microscopy is a sensitive tool for the rapid, early diagnosis of ICE syndrome and may help distinguish among its variants.

Corneal Diseases↗
PubMed1997

Monocular discrimination of rigidly and nonrigidly moving objects.

We measured thresholds for the monocular discrimination of rigidly and nonrigidly moving objects defined by motion parallax. The retinal projections of rigidly moving objects are subject to certain constraints. By applying smooth 2-D transformations to the projections of rigidly moving objects, we created stimuli in which these constraints were affected. Thresholds for (generic) nonrigid transformations that in theory can be detected from rigid ones by processing pairs of views depended not only on the extent to which the rigidity constraints were affected, but also on the structure and the movement of the simulated object. Nonrigid transformations under which every three successive views had a rigid interpretation were not discriminable from rigid transformations, except in cases where the distortions were very large. Under the rigidity assumption, this would mean that a large class of nonrigidly moving objects is erroneously perceived as rigidly moving.

Adult↗
PubMed2003

Proliferative capacity of the corneal endothelium.

Corneal endothelium is the single layer of cells forming a boundary between the corneal stroma and anterior chamber. The barrier and "pump" functions of the endothelium are responsible for maintaining corneal transparency by regulating stromal hydration. Morphological studies have demonstrated an age-related decrease in endothelial cell density and indicate that the endothelium in vivo either does not proliferate at all or proliferates at a rate that does not keep pace with the rate of cell loss. Lack of a robust proliferative response to cell loss makes the endothelium, at best, a fragile tissue. As a result of excessive cell loss due to accidental or surgical trauma, dystrophy, or disease, the endothelium may no longer effectively act as a barrier to fluid flow from the aqueous humor to the stroma. This loss of function can cause corneal edema, decreased corneal clarity, and loss of visual acuity, thus requiring corneal transplantation to restore normal vision. Studies from this and other laboratories indicate that corneal endothelium in vivo DOES possess proliferative capacity, but is arrested in G1-phase of the cell cycle. It appears that several intrinsic and extrinsic factors together contribute to maintain the endothelium in a non-replicative state. Ex vivo studies comparing cell cycle kinetics in wounded endothelium of young (< 30 years old) and older donors ( > 50 years old) provide evidence that cells from older donors can enter and complete the cell cycle; however, the length of G1-phase appears to be longer and the cells require stronger mitogenic stimulation than cells from younger donors. In vivo conditions per se also contribute to maintenance of a non-replicative monolayer. Endothelial cells are apparently unable to respond to autocrine or paracrine stimulation even though they express mRNA and protein for a number of growth factors and their receptors. Exogenous transforming growth factor-beta (TGF-beta) and TGF-beta in aqueous humor suppress S-phase entry in cultured endothelial cells, suggesting that this cytokine could inhibit proliferation in vivo. In addition, cell-cell contact appears to inhibit endothelial cell proliferation during corneal development and to help maintain the mature endothelial monolayer in a non-proliferative state, in part, via the activity of p27kip1, a known G1-phase inhibitor. The fact that human corneal endothelium retains proliferative capacity has led to recent efforts to induce division and increase the density of these important cells. For example, recent studies have demonstrated that adult human corneal endothelial cells can be induced to grow in culture and then transplanted to recipient corneas ex vivo. The laboratory work that has been conducted up to now opens an exciting new door to the future. The time is right to apply the knowledge that has been gained regarding corneal endothelial cell proliferative capacity and regulation of its cell cycle to develop new therapies to treat patients at risk for vision loss due to low endothelial cells counts.

Animals↗
PubMed1998

Transforming a hospital through growing, not building, an electronic patient record system.

The benefits for any health care provider of successfully introducing an Electronic Patient Record System (EPRS) into their organisation can be considerable. It has the potential to enhance both clinical care and managerial processes, as well as producing more cost-effective care and care programmes across clinical disciplines and health care sectors. However, realising an EPRS's full potential can be a long and difficult process and should not be entered into lightly. Introducing an EPR System involves major personnel, organisational and technological changes. These changes must be interwoven and symbiotic and must be managed so that they grow together in stages towards a vision created and shared by all clinical professional staff, other staff, and managers in that process. The use of traditional "building" or "journey" metaphors inadequately reflects the complexity, uncertainty and, therefore, the unpredictability of the process. We propose that a more useful metaphor may be of "growing" a progressively more united, unified information system and health care organisation. We suggest this metaphor better recognises that the evolutionary process appears to be more organic than predictable and more systemic than mechanistic. An illustration is given of how these organisational clinical and technical issues might evolve and interweave in a hospital setting through a number of stages.

Computer Systems↗
PubMed1989

Computational approaches to color constancy: adaptive and ontogenetic considerations.

Recent computational approaches to color constancy can be realized using a two-stage model of color vision. Adaptation at both the sensor stage and the second or reflectance channel stage is necessary to compute illuminant-invariant reflectance estimates. von Kries adaptation is shown to contribute significantly toward reducing, but not completely eliminating, the need for adaptation at the second stage. Examples of computations from the model using daylight illuminants, simulated natural reflectance functions, and putative human spectral sensitivity functions are shown. The ontogenetic plausibility of these models is also discussed. The role of the second-stage transformation in estimating surface reflectance is compared with the role of opponent color transformations in decorrelating primary receptor outputs.

Aging↗
PubMed2002

Bidirectional reflectance distribution function of specular surfaces with hemispherical pits.

We derive the bidirectional reflectance distribution function for a class of opaque surfaces that are rough on a macroscale and smooth on a microscale. We model this type of surface as a distribution of spherical mirrors. Since our study concerns geometrical optics, it is only the aperture of the concavities that is relevant, not the dimension. The three-dimensional problem is effectively transformed into a much simpler two-dimensional one involving the possibly infinitely many reflections in a spherical mirror. We find that these types of surface show very strong backscattering when the pits are deep but forward scattering when the pits are shallow. Such surfaces also show spectral effects as a result of multiple reflections and polarization effects that are due to the orientation of the effective surface. Both this model and the locally diffuse thoroughly pitted surface model [Int. J. Comput. Vision 31, 129 (1999)] are superior to other models in that they allow for an exact treatment for physically realizable surface geometries.

Journal Article↗
PubMedDate not supplied

Physician-centered management guidelines.

The "Fortune 500 Most Admired" companies fully understand the irreverent premise "the customer comes second" and that there is a direct correlation between a satisfied work force and productivity, service quality, and, ultimately, organizational success. If health care organizations hope to recruit and retain the quality workforce upon which their core competency depends, they must develop a vision strategic plan, organizational structure, and managerial style that acknowledges the vital and central role of physicians in the delivery of care. This article outlines a conceptual framework for effective physician management, a "critical pathway," that will enable health care organizations to add their name to the list of "most admired." The nine principles described in this article are based on a more respectful and solicitous treatment of physicians and their more central directing role in organizational change. They would permit the transformation of health care into a system that both preserves the virtues of the physician-patient relationship and meets the demand for quality and cost-effectiveness.

Efficiency, Organizational↗
PubMed1996

A bilateral model integrating vergence and the vestibulo-ocular reflex.

The majority of previous modelling studies of vergence and the vestibulo-ocular reflex (VOR) have postulated arbitrary structures mainly on the basis of input-output behavioural relationships. Such models were developed following traditional schemes of oculomotor organization, based upon the notion of independence between different oculomotor subsystems. This impedes the simulation of complex binocular interactions and associated central activities. In contrast to preceding studies, the mathematical model for binocular control presented here was developed fully on physiological and anatomical grounds which reflect the organization and functional properties of known vergence and VOR premotor centres. Computer simulations show the model properly simulates the main observed characteristics in the discharge of several premotor and motor nuclei during slow vergence and the VOR in the dark. In particular, the model reproduces the activity profiles of abducens internuclear neurons, secondary vestibular cells, tonic prepositus hypoglossi neurons and ocular motoneurons during vergence and the VOR. It also simulates the activity of mesencephalic neurons whose discharge is modulated by vergence parameters alone. It is shown that given recent neurophysiological and behavioural findings, ocular reflexes cannot be properly modelled as separate independent subsystems whereas a single, unified modelling approach can produce results consistent with observed data. This study also shows how changes in the functional activity of shared pathways in a single two-sided structure produce vergence and conjugate integrators whose function relies on coupled loops across the brainstem: separate, dedicated operators are not necessary to replicate data. This provides evidence that challenges previous studies supporting the existence of separate vergence and conjugate integrators to transform velocity to position signals in the brainstem. A major implication of this study is that it questions the validity of testing conjugate and vergence systems independently, neglecting potential interactions.

Computer Simulation↗
PubMed1989

Is there a constancy for velocity?

Human observers are unable to use disparity information to transform the angular velocity signal into a precise object-based code. The Weber fraction for discriminating changes in objective velocity (cm/sec) is about twice the Weber fraction for discriminating changes in angular velocity (deg/sec), and is substantially higher than predicted from a combination of the errors in judging disparity and angular velocity. By comparison, judgments of the distance traversed by the moving target show excellent size constancy. The discrimination of changes in objective size (cm) is as precise as the discrimination of changes in angular size (deg). The angular velocity signal is useful without transformation into an object-centered signal; it guides eye and body movements, and is the basis of motion parallax judgments. The need to retain this angular signal may explain why there is no efficient mechanism for velocity constancy.

Discrimination, Psychological↗
PubMed1990

Neuro-informatics and the neurology of perception.

In the preceding pages a description was given of spatial behaviour and spatial perception. Orientation in space develops under the guidance of a directional frame of reference. The formative power of this spatial organisation is gravity and anti-gravity by which throughout evolution the architecture of the body and its axial layout are shaped. There are no local signs at the surface of the body enabling us to read from them directions and orientations. Locomotion requires equilibrium. It is carried out by articulated limbs. The nervous system modulates the motor patterns dictated by physical forces. Peripheral control loops keep the limbs manageable. Higher levels of control organize control loops among each other. Their activity has the status of operations. As soon as animals are capable of locomotion orientation is based on relationships with objects, with a stable background in particular. As motor organisation develops spatial relationships become more complex. In the case of active touch the shape of an object is determined by manipulation, which is a form of scanning. Tactile scanning combines the features of sampling, tracking and measuring: f.e. solidity, weight, curvature, size. Manipulation is guided by the spatial frame of reference. The innumerable tactile impulses, many of them not perceived separately, are not the primary material of information. Only by ordering the impressions, by discriminating between noise and relevant events is the tactile experience guided by expectations transformed into information. The organism stops manipulating as soon as the scanning procedure has enabled it to match the shape under investigation with the spatial patterns it has at its disposal.(ABSTRACT TRUNCATED AT 250 WORDS)

Electronic Data Processing↗
PubMed1999

Auditory activation of cortical visual areas in cats after early visual deprivation.

Auditory activation of the primary visual cortex (area 17) and two extrastriate visual cortical areas - the anterolateral lateral suprasylvian area (ALLS) and anteromedial lateral suprasylvian area (AMLS), was investigated in visually impaired cats. Impairment was accomplished shortly after birth by bilateral eyelid suturing (binocularly deprived cats, BD) or bilateral enucleation (binocularly enucleated cats, BE). In BE cats, the optic nerve and chiasm were entirely degenerated. No cortical atrophy or cytoarchitectural malformation was noticed in either BD or BE cats. In both normal and impaired cats we found auditory-responsive cells in the ALLS and AMLS, areas that are considered strictly visual. The most remarkable finding was an increase in the relative number of these auditory cells in the BD and BE cats, which was more prominent in the latter. Some auditory-responsive cells were also found in area 17 of BE cats. On the basis of formal calculation, it is tempting to suggest that the increase in relative number of auditory cells in these areas reflects the transformation of all the visual cells in the ALLS of BD and BE cats into auditory cells. In BE cats, all bimodal cells and an appreciable percentage of non-responsive cells also had transformed to auditory cells. In the AMLS of BD cats, it is primarily the bimodal cells that become auditory cells, whereas in BE cats all the visual and bimodal cells as well as non-responsive cells undergo this transformation. This assumption, however, is one possible interpretation of our results but not the only one. Other modes of neuronal plasticity that might yield similar results in the visually deprived cats can not be ruled out.

Acoustic Stimulation↗