Search Results

Tokens for stereopsis

Ian P. Howard and Brian J. Rogers

in Seeing in Depth: Volume 1: Basic Mechanics/ Volume 2: Depth Perception 2-Volume Set

The spatial separation of the eyes causes a difference in the images in the two eyes formed by a solid object. These differences, or binocular disparities, form the basis for stereoscopic vision. ... More

The spatial separation of the eyes causes a difference in the images in the two eyes formed by a solid object. These differences, or binocular disparities, form the basis for stereoscopic vision. This chapter discusses the extent to which each of these differences is used as a basis for stereopsis. Topics covered include feature tokens for stereopsis, monocular occlusion, occlusion as a depth token, stereopsis from illusions, chromostereopsis, and irradiation stereopsis.Less

Three-Dimensional Laser Imaging in an Archaeological Context

Andrew M. Wallace

in Images and Artefacts of the Ancient World

This chapter discusses the strengths and weaknesses of techniques of 3D laser imaging based on time of flight and triangulation. Laser scanning by triangulation is a fully developed technology. ... More

This chapter discusses the strengths and weaknesses of techniques of 3D laser imaging based on time of flight and triangulation. Laser scanning by triangulation is a fully developed technology. Through this method, it is now possible to create faithful 3D datasets from a reasonable range of surface materials. However, laser scanning has some certain drawbacks, restricting their universal applicability. For surfaces with concavities or with intricate details, occlusion may occur due to the necessary separation of the viewpoint between the viewing camera and the laser projector. In addition, the data may be corrupted by false and poor returns caused by variable material reflectance. In the time of flight system, the distance is measured by measuring the time for a focused laser beam to impact on and return from the surface of interest. In this method, the distance is computed and a 3D image can be created if the laser beam is scanned across the target. Although, the time of flight method is a more attractive alternative as it eliminates occlusion, it has disadvantages as well. The time resolution necessary to measure the range to sub-millimetre accuracy is difficult to achieve and the measured time is also affected by the magnitude of the returned signal. In addition to reviewing the pros and cons of this current technology, the chapter also discusses the development of a new approach to time-of-flight laser imaging based on time-correlated single photon counting. This new method has various advantages compared to the old methods. It has the ability to make 3D measurements on distance, poorly reflecting or transparent surfaces.Less

Ideal occlusion

Claire Nightingale and Jonathan Sandy

in Title Pages

Mystery, Measure, and Humility

David E. Cooper

in The Measure of Things: Humanism, Humility, and Mystery

This chapter contemplates the question of how a doctrine of mystery can provide measure for ones' life. Firstly, it explains how the issue it addresses was reached. Secondly, it argues that one may ... More

This chapter contemplates the question of how a doctrine of mystery can provide measure for ones' life. Firstly, it explains how the issue it addresses was reached. Secondly, it argues that one may discern how various conceptions, beliefs, and comportments are consonant to the experience of mystery. Thirdly, it discusses in detail the disconsonant concepts based on confrontation, degradation, and occlusion. The chapter concludes by discussing the finer aspects of humility.Less

Visual Field Defects in Chorioretinal Disorders

Ron A. Adelman and Patricia Pahk

in Visual Fields

Pathologic processes involving the retina or choroid can present with a wide variety of visual field defects. Usually visual field defects of retinal diseases directly ... More

Pathologic processes involving the retina or choroid can present with a wide variety of visual field defects. Usually visual field defects of retinal diseases directly correlate with the fundus findings. Visual field changes are often the result of damage to the retina or scarring but, in conjunction with other clinical findings, they may help narrow the differential diagnosis. Most of the macular lesions result in visual field defects that do not respect the vertical or horizontal midline. Occasionally inflammatory disorders result in visual field defects that do not directly correlate with the retinal findings. For example, patients with multiple evanescent white dot syndrome (MEWDS) may have an enlarged blind spot. Macular disorders can cause central or paracentral scotomas depending on the location of the lesion. Causes of macular pathology include drusen, atrophy from dry age-related macular degeneration (AMD), retinal hemorrhage, choroidal neovascular membrane, macular edema, macular hole, macular scar, pathologic myopia, and macular dystrophies of the retina or choroid. Central serous chorioretinopathy (CSCR) can show a relative defect that is anatomically correlated with the area of subretinal or sub RPE (retinal pigment epithelium) fluid accumulation. Residual pigmentary changes in inactive CSCR can also cause a relative depression in the corresponding visual field. Pathologic myopia can present with a variety of visual field defects depending on the retinal findings, such as posterior staphyloma or choroidal neovascular membrane. AMD may show nonspecific changes in the central or paracentral visual field that correlate with the structural damage to the retina and choroid. Geographic atrophy in dry AMD can cause a dense scotoma correlated with the pattern of the atrophy. Choroidal neovascular membranes can cause a depression in the correlating visual field due to edema or hemorrhage. Disciform scars in endstage AMD can also cause a dense scotoma. Macular holes may cause a small central scotoma. Pattern dystrophies are a family of disorders with a common pathology at the level of the RPE. Butterfly dystrophy, an autosomal dominant disorder, and Sjögren reticular dystrophy, an autosomal recessive disorder, are two examples of pattern dystrophies. Less

Suffering Occluded

Emmanuel Falque

in The Guide to Gethsemane: Anxiety, Suffering, Death

One of the primary features of Christianity is that it accommodates the whole of the suffering of humankind, a suffering that is both moral and physical. Nonetheless the burden of suffering and ... More

One of the primary features of Christianity is that it accommodates the whole of the suffering of humankind, a suffering that is both moral and physical. Nonetheless the burden of suffering and salvation through suffering have often been confused together. Christ does not tell us either what or why he suffers. But he guides us in how he suffers. Closed in on oneself or open to the other: These are the two ways of living suffering, by which suffering becomes a manner of living rather just a condemnation of life.Less

The How, When, and Why of Configural Processing in the Perception of Human Movement

James C. Thompson

in People Watching: Social, Perceptual, and Neurophysiological Studies of Body Perception

While several different neural processes contribute to the perception of biological motion, the responsiveness of a key area, the human superior temporal sulcus (STS), to biological motion appears to ... More

While several different neural processes contribute to the perception of biological motion, the responsiveness of a key area, the human superior temporal sulcus (STS), to biological motion appears to be driven largely by the need to integrate motion information with configural body form information. This need reflects the fact that the human body produces complex patterns of articulated motions at multiple spatial locations that are linked by the underlying human form. Such a global integration of motion and form information would provide several advantages to observers of human movement across a variety of otherwise challenging conditions including brief durations, occlusion, and observer movement.Less

The management of bowel obstruction in advanced gynaecological malignancy

Doreen Oneschuk

in Palliative Care Consultations in Gynaeoncology

This chapter focuses on malignant bowel obstruction (MBO), a common complication in women diagnosed with ovarian cancer, colorectal cancer, and other gynaecological malignancies. MBO generally ... More

This chapter focuses on malignant bowel obstruction (MBO), a common complication in women diagnosed with ovarian cancer, colorectal cancer, and other gynaecological malignancies. MBO generally afflicts 25–42 per cent of patients with advanced ovarian cancer, but rarely occurs in patients with endometrial cancer. MBO develops by the existence of mechanical obstruction from an extrinsic occlusion of the bowel lumen, intraluminal occlusion of the lumen, luminal obstruction due to tumour growth in the bowel wall, and adynamic ileus. Bowel obstruction can also be caused by other non-malignant factors such as adhesion, post-irridation bowel damage, hernias, and inflammatory bowel disease. Management of MBO includes pain management, pharmacological management, and non-pharmacological management. In patients with MBO, pharmacological treatment is generally successful. Only patients who are strictly considered as appropriate candidates for surgery and parenteral nutrition are given such treatments.Less

Vessel wall injury and growth: pharmacological considerations in atherosclerosis and collateral artery function

Norman K. Hollenberg

in The Pharmacology of Vascular Smooth Muscle

Arteries relax in response to some vasodilators only if the endothelium is present. The obligatory role for vasodilatation of a diffusible factor from endothelium termed ‘EDRF’ for ... More

Arteries relax in response to some vasodilators only if the endothelium is present. The obligatory role for vasodilatation of a diffusible factor from endothelium termed ‘EDRF’ for endothelial-dependent relaxing factor, now identified as nitric oxide or a related nitroso-thiol has been demonstrated for many agents, including acetylcholine, bradykinin, substance P, ATP, other adenine nucleotides — and especially important for this chapter — the vasodilator action of 5-HT. Other vasodilator agents, however, such as nitrates, papaverine, isoproterenol, and prostaglandins, do not require endothelium. This chapter focuses on pharmacological considerations in atherosclerosis and in the collateral arteries that grow in response to the vascular occlusion that so often occurs with atherosclerosis.Less

Occlusion therapy for childhood amblyopia: current concepts in treatment evaluation

Merrick J. Moseley and Alistair R. Fielder

in Infant Vision

This chapter, as its title suggests, is concerned primarily with practical issues surrounding amblyopia therapy. Imprecise or inappropriate measurement techniques have limited understandings of ... More

This chapter, as its title suggests, is concerned primarily with practical issues surrounding amblyopia therapy. Imprecise or inappropriate measurement techniques have limited understandings of occlusion therapy. To examine these issues the chapter draws upon observations made in the literature and on some techniques recently developed in the laboratory. Finally, experimental data that indicate how proposals work in practice are presented. The discussions are necessarily restricted in scope and do not address the many neuroscientific contributions that have increased the understandings of this curious condition. Throughout this chapter the attention is focused on the amblyopic child as opposed to the infant and the chapter defines the former as being of an age at which a visual task involving ‘linear’ optotypes can be performed, either by the elicitation of a verbal response or by the child indicating the target seen at distance on a hand-held key card. Also, it makes no specific reference to the taxonomy of amblyopia although it acknowledge this to be an obvious factor likely to influence treatment outcome. At the beginning of this chapter, attention is drawn to the lack of empirical (as opposed to clinical, anecdotal) support for the use of occlusion therapy. It is suggested that this arose because of inadequate or inappropriate measurement of compliance and visual outcome.Less

Multiple object tracking in infants: Four (or so) ways of being discrete

Marian L. Chen and Alan M. Leslie

in The Origins of Object Knowledge

It has been reported that infants represent the continued existence of an object through occlusion and they can track even multiple objects. Many studies have found that infants track the discrete ... More

It has been reported that infants represent the continued existence of an object through occlusion and they can track even multiple objects. Many studies have found that infants track the discrete numerosity of small sets of objects in multiple modalities, but the interpretation of these findings remains controversial. In this chapter, the authors evaluate the available evidence for four theories that have been proposed to explain how infants track discrete quantity namely: object indexing, sets, mental magnitudes/accumulator, and integers. Of the four theories, only the accumulator and the generative integer views posit an irreducibly numerical basis to infants' number representations. The object indexing and set accounts provide a mechanism by which infants might be able to track small numbers of discrete objects and compare them on the basis of their numerical identity without actually being able to count.Less

Eloquent Blood and Deliberative Bodies: The Physiology of Metaphysical Poetry

Michael Schoenfeldt

in Renaissance Transformations: The Making of English Writing 1500-1650

This chapter identifies the occlusions that are created by current organisations of knowledge for Renaissance texts. It argues that the post-Cartesian segregation of cognition from corporeality ... More

This chapter identifies the occlusions that are created by current organisations of knowledge for Renaissance texts. It argues that the post-Cartesian segregation of cognition from corporeality prevents one from stopping the connection between aesthetic practice and physiological concept in the Renaissance. The chapter then tries to determine whether passionate and argumentative poetic expressions provide happiness that is both intellectual and primeval.Less

Grouping and Segmentation of Visual Objects by Baboons (Papio papio) and Humans (Homo sapiens)

Joël Fagot and Isabelle Barbet

in Comparative Cognition: Experimental Explorations of Animal Intelligence

This chapter discusses the results of a series of experiments aimed at investigating visual-information processing in baboons. Two lines of ... More

This chapter discusses the results of a series of experiments aimed at investigating visual-information processing in baboons. Two lines of research on baboons are presented in this chapter. The first one explores the processes of perceptual grouping. In comparison to humans when similarly tested, baboons barely group spatially separated elements into a single percept. The second one addresses the issue of depth perception. Baboons perceive depth when presented with pictorial depth cues, but the study also highlights human–baboon differences in the processing of occlusion cues as indicators of depth. A local precedence effect was observed in baboons, in contrast to humans, who showed a global precedence effect. Experiments further revealed that local precedence in baboons is a direct consequence of their difficulty in overcoming the separation between the local elements, a necessary process for perceiving the whole. Moreover, baboons perceive the corridor illusion, suggesting that they gain depth information from the pictorial cues available in the image background.Less

Cardiac Output Measurement

Patrick Magee and Mark Tooley

in The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA

The pulmonary artery catheter was the mainstay of clinical cardiac output measurement for many years, but because of its relatively invasive nature and the lack of ... More

The pulmonary artery catheter was the mainstay of clinical cardiac output measurement for many years, but because of its relatively invasive nature and the lack of improvement of clinical outcome with its use, it is now seldom used in a modern clinical environment. Any perceived accuracy of the technique is now considered unnecessary in the face of the risks of its use, and with the introduction of newer non-invasive techniques. Nevertheless, it is worth describing, partly because of its historical interest, and partly because of the technologies involved. A catheter passed into the right atrium from an easily accessible central vein can be passed through the right ventricle and out into the pulmonary arterial tree while the vascular waveforms are visualised. Figure 13.1 shows the waveforms as they appear to the user. A small balloon at the tip of the catheter allows it to be flow directed and wedged in a pulmonary arterial vessel. At this point the pulsatile waveform is lost and the tip of the catheter is looking ahead, down the pulmonary arterial tree towards the left atrium, a system with a relatively low pressure drop from one end to the other, the flow in that vessel having been brought temporarily to a standstill. Thus the pulmonary artery occlusion pressure (PAOP) or pulmonary capillary wedge pressure (PCWP) can be considered a reasonably accurate representation of left atrial pressure or left ventricular filling pressure. This assumes that there is no pulmonary vascular disease, such as pulmonary hypertension, or mitral valve disease, in which case PAOP would not be an accurate representation of left atrial pressure. If the catheter is placed in the apical region of the pulmonary vascular tree, the excess of the alveolar pressure in inspiration over pulmonary capillary pressure becomes significant, and the latter is a less accurate reflection of left atrial pressure. The balloon should not be over-inflated for fear of rupturing the pulmonary artery, and this is one of its perceived risks that has led to less usage. Once the measurement has been made, the balloon should be deflated so that the pulmonary arterial waveform is once again visible, if necessary withdrawing the catheter a bit to achieve this; failure to do so would result in regional lack of perfusion and may result in ischaemia. Less

Advanced Trauma Life Support (ATLS), Polytrauma, Limb Salvage, and UK Trauma Guidelines

Alex Trompeter and David Elliott (eds)

in Trauma for the FRCS (Tr + Orth) Examination

Functional Visual Loss.

Thomas J. Walsh

in Visual Fields

Functional loss of vision or visual fields can present some of the most difficult diagnostic challenges, but even more difficult to diagnose are patients who have, in ... More

Functional loss of vision or visual fields can present some of the most difficult diagnostic challenges, but even more difficult to diagnose are patients who have, in addition to functional visual loss, an organic disease. No matter how functional the symptoms appear, the clinician has to go that extra step to find any true pathology. This is why different field techniques may be more appropriate than the more sophisticated techniques. A tangent screen, for instance, may be better to control the patient’s response than a Humphrey Visual Field Analyzer. Remember the functional patients get sick, too. Patients with functional field loss can be placed into one of three general groups: neurasthenics, hysterics, or malingerers. The neurasthenic patient usually has many complaints, not limited to the visual system or to one set of visual symptoms such as field loss. The complaints, as well as the degree of field or visual defect, frequently vary from one examination to another, as well as during an examination as fatigue increases. The spiral field is often found in neurasthenic patients. The patient with hysteria, on the other hand, usually has a single ocular complaint. This chapter discusses the hysterical loss of acuity or fields. The usual field defect is a severe tubular type of contraction. The spiral field is sometimes seen in these patients. The malingerer may be the most difficult patient with functional loss, particularly if he has previously been examined by another physician and has acquired more experience with field testing than he had when first examined. The spiral field is not limited to functional loss; it can also be seen as a fatigue phenomenon in sick patients when the field testing is unduly prolonged. Its essential feature is that the field becomes progressively smaller as a specific isopter is tested for a second and third time with the same size of test object. shows all the points that were tested with a 3-mm white test object. Less

Elevated IOP and Intentional Tube Occlusion

Ronald L. Gross

in Complications of Glaucoma Surgery

Intrinsic to glaucoma surgery using a tube shunt is the management of early postoperative hypotony. This consideration is unavoidable in all cases when ... More

Intrinsic to glaucoma surgery using a tube shunt is the management of early postoperative hypotony. This consideration is unavoidable in all cases when using a tube shunt without an intrinsic valve and must still be considered in tube shunts that contain a valve, as the valve may not function as anticipated. Unfortunately, in the attempt to avoid hypotony and its associated complications, we are faced with elevated intraocular pressure (IOP) and its associated difficulties. However, the attempt to control IOP is not the only consideration when anticipating intentional tube occlusion. Additional factors such as technical complexity of the procedure, predictability of IOP in the early postoperative period, potential to reverse occlusion either partially or completely, and the impact on the long-term function of the tube shunt must be considered. The desired endpoint when occluding a tube intentionally is the complete prevention of flow to the tube shunt reservoir. The standard ways to occlude the tube are an external encircling ligature or an internal occluding suture, otherwise known as an “obturator” or a “ripcord,” or some combination thereof. With the external suture technique, prior to placing the reservoir, a 7–0 or 8–0 polyglactin 910 (Vicryl™ , Ethicon, Inc., Somerville, New Jersey) suture is tightly tied around the tube approximately 4–6 mm from the reservoir (Figure 35.1). It is anticipated that this suture will dissolve in about one month, opening the tube. However, the timing of opening may be highly variable between individuals, and that variability may be problematic. Alternatively, a 9–0 polypropylene suture can be placed around the tip of an anterior chamber tube with release performed by laser lysis. To prevent the polypropylene suture from floating freely in the anterior chamber after release, a pass should be made through the wall of the tube during placement. Alternatively, with the “ripcord” technique, the end of a 3–0 polypropylene suture without the needle is threaded into the distal opening of the tube at the reservoir for a distance of 4–6 mm. Less

Ophthalmology

Venki Sundaram (ed.)

in Oxford Assess and Progress: Clinical Specialties

Ophthalmology principally aims to prevent visual loss, restore visual function, and relieve ocular discomfort. The majority of the pathology can be directly visualized and thus requires proficient ... More

Ophthalmology principally aims to prevent visual loss, restore visual function, and relieve ocular discomfort. The majority of the pathology can be directly visualized and thus requires proficient ocular examination techniques and visual recognition skills. Another distinguishing aspect of ophthalmology is the overlap between medical and surgical conditions. Common systemic diseases such as diabetes and hypertension have ocular features, and diseases involving every organ of the body can have ocular manifestations. A thorough medical knowledge is paramount, as is the ability to collaborate with other medical teams. Intraocular surgery for conditions such as cataract is technically challenging, as ocular tissues are so delicate. It therefore requires high levels of fine hand–eye coordination. As an ophthalmologist, you will be faced both with acute eye conditions, some of which are sight-threatening and require prompt diagnosis and management, and with chronic conditions, which require monitoring and treatment for many years. You will be exposed to patients of all ages, from premature babies to the elderly, so good communication with a wide range of patient groups and their families is essential. Patients often say that what they fear most is losing their sight. Therefore, empathy and support for patients with debilitating visual impairment are imperative. The questions in this chapter will test your knowledge of acute emergency ophthalmic presentations and the understanding and interpretation of ophthalmic examination, as well as ocular conditions that have systemic associations. In addition, questions relating to ophthalmic risk factors, communication, and probity are included. Eye problems can be daunting to many medical students and doctors. Through practice in examining patients and recognizing key conditions, confidence can be gained in how best to manage these patients and, importantly, when to refer them to other specialties. Ophthalmology incorporates a unique and appealing mix of medical and surgical conditions. It is a rapidly advancing specialty with recent significant advances in diagnostic and therapeutic options. It also provides an opportunity for a good work–life balance. Less