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PubMed1994

The cerebral activity related to the visual perception of forward motion in depth.

We have used the technique of PET to chart the areas of human cerebral cortex specifically responsive to an optical flow stimulus simulating forward motion in depth over a flat horizontal surface. The optical flow display contained about 2000 dots accelerating in radial directions away from the focus of expansion, which subjects fixated at the centre of the display monitor. Dots remained of constant size, but their density decreased from the horizon, lying across the middle of the screen, to the foreground at the lower screen margin; the top half of the display was void. For the control stimulus the dot motions were randomized, removing any sensation of motion in depth and diminishing the impression of a flat terrain. Comparison of the regional cerebral blood flow (rCBF) elicited by the optical flow and control stimuli was thus intended to reveal any area selectively responsive to the radial velocity field that is characteristic of optical flow in its simplest natural form. Six subjects were scanned, and analysed as a group. Four subjects were analysed as individuals, their PET data being co-registered with MRIs of the cerebrum to localize rCBF changes to individual gyri and sulci. There were three main areas of activation associated with optical flow: the dorsal cuneus (area V3) and the latero-posterior precuneus (or superior parietal lobe) in the right hemisphere, and the occipito-temporal ventral surface, in the region of the fusiform gyrus, in both hemispheres. There was no significant activation of V1/V2, nor of V5. These results show that higher stages of motion take place in both the 'dorsal' and 'ventral' visual pathways, as these are commonly conceived, and that both may be fed by area V3. The information potentially derivable from optical flow concerns the direction of heading, and the layout of the visual environment, a form of three-dimensional structure-from-motion. The perceptual division of labour between the various activated areas cannot be directly inferred, though it is a reasonable supposition that the parietal activation reflects the utility of optic flow for guiding self-motion.

Adult↗
PubMed1992

Propagation of colour information across borders.

When a part of an image is stabilized on the retina, the part fades to the same colour as its unstabilized surround. The apparent colour of an empty field is therefore assumed to be equal to the colour of its borders. Here, we will show that a similar phenomenon can take place even when the border of a centre field is still visible. A grey square patch with a black frame was surrounded by a coloured background. When the frame had such crossed disparity that the frame appeared in front of the patch, the colour of the patch became perceptually similar to that of the background, indicating that the colour of an empty field is not determined by the colour of its borders alone.

Color Perception↗
PubMed1990

Is the quality of vision with contact lenses adequate? Not in all instances.

There are several circumstances in which contact lenses (CLs) provide vision inferior to that possible with spectacles. Over time, both soft and rigid gas-permeable (RGP) lenses accumulate deposits that can cloud vision. RGP lenses are especially prone to scratching; some varieties may even become crazed, giving a spiderweb quality to vision. Certain uncommon corneal complications associated with lens wear--infection, neovascularization, ulcers, and severe epithelial edema--may also impair vision, sometimes permanently. CLs often provide less than optimal vision for patients with presbyopia or severe astigmatism. Difficulties with axis stabilization arise with both soft and rigid toric lenses, and these can take several attempts to resolve. Monovision fitting currently offers the best option for presbyopes, but it has an effect on stereopsis, compromising depth perception. Furthermore, patients often require considerable time before adapting to monovision lens use. Finally, even under normal circumstances, certain inherent features of CLs can diminish visual acuity. The water in soft lenses diffuses light; the loss of visual crispness is particularly notable with high-water-content lenses. On the other hand, the small diameter of RGP lenses often contributes to ghosting--a phenomenon in which the edge of the lenses is visible whenever the pupils are widely dilated.

Astigmatism↗
PubMed1996

The future of minimally invasive surgery.

When it was first introduced, minimally invasive surgery longer to perform than open surgeries, limited the number of instruments that could be used simultaneously; restricted the manipulation of instruments, reduced the surgeon's depth perception, decreased eye/hand coordination and eliminated the ability to palpate tissue directly. However, recent advances have corrected some of these deficits and minimally invasive surgery eventually will be used for all gynecological procedures and most general surgery, chest and coronary artery bypass procedures.

Clinical Competence↗
PubMed1986

Parameters of contact lens-spectacle telescopic systems and considerations in prescribing.

Contact lens-spectacle telescopic systems provide an alternative to conventional spectacle-mounted telescopes. There is a need to describe the parameters of such systems, the interrelations between them, and the use of such relations in selecting values for prescribing. Graphical representation of ocular (contact) lens power, objective (spectacle) lens power, separation (vertex distance), and magnification are presented. Skillful prescribing involves selection of the best set of parameters to achieve the desired visual goal in spite of the practical limitations imposed by facial features, lens availability, and appearance. Additional factors (not graphically represented) are induced vergence amplification, altered depth perception, and apparent motion, all unique to telescopic systems. One patient is described and selection of appropriate parameters is discussed. General application of the selection process is described.

Accommodation, Ocular↗
PubMed1991

Effect of chromatic aberration on isoluminance stereothreshold.

The results of a previous experiment have shown that stereothreshold varies as a function of the luminance difference between a target and its background. When the luminance of the target is the same as that of the background (isoluminance) the stereothreshold was elevated by a factor of 3 as compared with a situation where there was maximum luminance difference between the target and the background. Generally, if the target and the background have the same color, stereothreshold at a particular luminance difference level was lower (stereoacuity better) than if the target and the background have different colors. This indicates a possible effect of chromatic aberration on stereothreshold which could be responsible for the 3-fold increase in threshold at isoluminance. This possibility has been investigated in this experiment. The results show that even though stereothresholds are sensitive to chromatic aberration this factor cannot explain the elevated stereothreshold at isoluminance.

Color Perception↗
PubMed2002

The pitfalls of laparoscopic surgery: challenges for robotics and telerobotic surgery.

After its debut in 1988, laparoscopic cholecystectomy rapidly became the standard of care for cholelithiasis, yet very few surgeons use minimally invasive techniques for other abdominal operations. Why do most surgeons continue to perform traditional open gastrointestinal operations? We believe that the answer to this question lies in the fact that advanced laparoscopic operations are difficult to learn, perform, and master. A number of inherent pitfalls of laparoscopy hinder the performance of these operations even after the surgeon has accumulated years of experience. These pitfalls include an unstable video camera platform, limited motion (degrees of freedom) of straight laparoscopic instruments, two-dimensional imaging, and poor ergonomics for the surgeon. Inexperienced or bored laparoscopic camera-holders move the camera frequently and rotate it away from the horizon. The long, straight laparoscopic instruments are limited in their motion by the fixation enforced by the abdominal wall trocars. Similarly, the standard two-dimensional video imaging used in most laparoscopic operations impedes the surgeon's depth perception, compounding the limitations of laparoscopic instruments. In addition, surgeons are forced to assume ergonomically awkward stances in performing many laparoscopic operations. These four factors hinder a surgeon's efforts to learn and to perform advanced laparoscopic operations, significantly lengthening the learning curve. The articles presented in this issue suggest that robotics and telerobotics offer solutions to these nagging pitfalls of laparoscopic surgery.

Cholecystectomy, Laparoscopic↗
PubMed1979

Depth sensitivity of binocular cortical neurons of behaving monkeys.

Activity of neurons if foveal striate and prestriate cortex of trained rhesus monkeys was recorded with metal microelectrodes. While animals fixated a small spot at a given fixation distance (38 or 57 cm), bright or dark bars moving across a frontoparallel plane were presented at different depths in a range of +/- 10 cm about the fixation distance. Almost all cells showed binocular interaction. Neurons with balanced ocularity (approximately equal monocular responses) usually facilitated each other and were tuned to depth around the plane of fixation often with inhibitory flanks nearer and further. Neurons with unbalanced ocularity either inhibited each other or had asymmetric depth sensitivity profiles, i.e. activation by stimuli in front and suppression by stimuli behind the fixation plane (near cells) or vice versa (far cells). Thus striate and prestriate cortex of the monkey contains four subsets of binocular cells which may contribute to depth perception.

Animals↗
PubMed1982

Sensory and non-sensory visual disorders in man and monkey.

The posterior third of the cerebral cortex in monkeys consists of a patchwork of visual areas in each of which there is a 'map' of the retina. The details of the 'map' vary considerably from one area to another and one notable variation concerns the optimal visual feature to which the cells respond. Orientation, disparity, colour and movement are emphasized in separate areas that appear to be concerned with sensory analysis. Their existence and the possibility that brain damage is occasionally restricted chiefly to one such area may explain the rare highly selective visual sensory impairments that can follow posterior cerebral damage in man. Other areas are notable for having little or no retinotopic representation. Here the cells may have huge receptive fields and complex trigger features. When such regions are removed, the animal's visual sensory abilities are intact but its recognition of patterns and objects is not. This condition resembles human visual agnosia.

Agnosia↗
PubMed1997

Perceptual categories for spatial layout.

The central problems of vision are often divided into object identification and localization. Object identification, at least at fine levels of discrimination, may require the application of top-down knowledge to resolve ambiguous image information. Utilizing top-down knowledge, however, may require the initial rapid access of abstract object categories based on low-level image cues. Does object localization require a different set of operating principles than object identification or is category determination also part of the perception of depth and spatial layout? Three-dimensional graphics movies of objects and their cast shadows are used to argue that identifying perceptual categories is important for determining the relative depths of objects. Processes that can identify the causal class (e.g. the kind of material) that generates the image data can provide information to determine the spatial relationships between surfaces. Changes in the blurriness of an edge may be characteristically associated with shadows caused by relative motion between two surfaces. The early identification of abstract events such as moving object/shadow pairs may also be important for depth from shadows. Knowledge of how correlated motion in the image relates to an object and its shadow may provide a reliable cue to access such event categories.

Depth Perception↗
PubMed2003

Time course of perceptual grouping by color.

Does perceptual grouping operate early or late in visual processing? One position is that the elements in perceptual layouts are grouped early in vision, by properties of the retinal image, before perceptual constancies have been determined. A second position is that perceptual grouping operates on a postconstancy representation, one that is available only after stereoscopic depth perception, lightness constancy, and amodal completion have occurred. The present experiments indicate that grouping can operate on both a preconstancy representation and a postconstancy representation. Perceptual grouping was based on retinal color similarity at short exposure durations and based on surface color similarity at long durations. These results permit an integration of the preconstancy and postconstancy positions with regard to grouping by color.

Color Perception↗
PubMed1984

Representation of three-dimensionally reconstructed objects in electron microscopy by surfaces of equal density.

A surface representation algorithm has been developed to visualize the results of three-dimensional reconstructions from projections in electron microscopy. It produces an easily recognizable image of the reconstructed particle and facilitates comprehension of the three-dimensional structure. Thus building of physical models can be avoided in many cases. If such a model is needed the algorithm can be used to establish the correct particle boundary in advance. Some further effort has been made to represent long fibres and visualize inner structures of objects, which are obscured by outer high-density regions. Shading may be used to enhance the depth perception from single views.

Animals↗
PubMed2005

Vision and falls.

Falls occur in about one third of older people living independently in the community every year. This can lead to significant physical, psychological and financial costs to the individual and the community. While the risk factors for falls in older people are multifactorial, poor vision is considered to be an important contributing factor. The aim of this review is to evaluate current research linking impaired visual function with falls and to review current intervention strategies for the prevention of falls in older individuals. The evidence from the current literature indicates that impairment of visual functions, such as visual acuity, contrast sensitivity, visual fields and depth perception, is associated with an increased risk of falls. Recent studies have also demonstrated that falls can be reduced following cataract surgery as a visual intervention. Optometrists need to be aware of these associations and through appropriate treatment, referral and/or education, they can play a major role in optimising visual function in older people, as part of a multidisciplinary approach to falls prevention.

Accidental Falls↗
PubMed1999

Binocular phase interactions in area 21a of the cat.

1. Binocular interactions related to retinal disparity were investigated in single neurons in area 21a of extrastriate cortex in the anaesthetized cat using sinusoidal luminance gratings. 2. The responses of approximately two-thirds of neurons were profoundly modulated by a relative phase difference between identical drifting gratings presented to each eye. This modulation included both facilitatory and inhibitory interocular interactions. The selectivity for binocular disparity was about twice as sharp as the selectivity for monocular spatial position. 3. Significant phase modulation was retained in many neurons at interocular orientation differences exceeding 45 deg. The response suppression associated with stimulation at a phase shift 180 deg from the optimum was stronger than the response suppression to an interocular orientation difference of 90 deg. 4. The proportion of phase modulated neurons and the potency of modulation in area 21a neurons exceed that reported for phase-selective complex cells in area 17. Neurons in area 21a show sharp disparity tuning that is relatively insensitive to changes in orientation and monocular position, which suggests that this extrastriate region has a role in stereoscopic depth perception.

Analysis of Variance↗
PubMed1996

Cognitive visual dysfunction in a child with cerebral damage.

Visual impairment as a sequel to cortical damage can be associated with complex disorders in cognitive visual function. Such dysfunction can be difficult to diagnose with certainty, particularly in children, owing to the child's problems of awareness and communication. The authors describe the case of a 13-year-old girl whose cerebral damage is apparently accompanied by complex cognitive visual disorders. Though she has a good binocular visual acuity, she cannot see rapidly moving objects. She has difficulties with orientation, recognising faces and cognitive depth perception. Despite an absence of detailed psychometric testing, a clinical diagnostic approach such as that used in this case, based on a CT scan and careful history-taking and observation, may be helpful in planning educational strategies.

Adolescent↗
PubMed1998

Visual outcome at 5 years of newborn infants at risk of cerebral visual impairment.

Visual development at 5 years of age was tested in a group of 39 children who had shown severe neonatal encephalopathy or perinatal brain lesions, documented by medical history, cranial ultrasound, or MRI. In all children, grating acuity was tested during the first 2 years of life. The assessment protocol at 5 years included various visual functions (grating and resolution acuity, visual field size, depth perception, optokinetic nystagmus, and ocular motility), and neurological and cognitive development. The majority of the children showed visual disorders of different type and degree, which were not due to ophthalmological abnormalities. Visual defects correlated well with the results of early visual assessment and of neuroimaging. Visual outcome could be predicted by grating acuity at 1 to 2 years in 27 of the 39 children, by neonatal cranial ultrasound in 26 of the 32 cases examined by this technique, and by later MRI in 23 out of 27. Moreover, a significant correlation was found between visual, motor, and cognitive impairment.

Brain↗
PubMed1994

Correlates of impaired function in older women.

OBJECTIVE: To determine the factors associated with impaired function in older women. DESIGN: Cross-sectional analysis of baseline data collected for a multicenter, prospective study of risk factors for osteoporotic fractures. SETTING: Four clinical centers in Portland, Oregon, Minneapolis, Minnesota, Baltimore, Maryland, and the Monongahela Valley, Pennsylvania. PARTICIPANTS: A total of 9,704 ambulatory, non-black women, aged 65 years and older, recruited from population-based listings. MEASUREMENTS: Independent variables, including demographic and historical information (medical conditions, health habits, and medications) and physiologic measures (anthropometry, blood pressure, mental status, vision, and neuromuscular performance) were obtained from a baseline questionnaire, interview, and examination. Measurement of function was assessed by self-reported ability to perform six physical and instrumental activities of daily living (ADL) and impaired function (dependent variable) was defined as difficulty performing three or more physical and instrumental ADLs. RESULTS: In order of decreasing strength of association, hip fracture, osteoarthritis, parkinsonism, slower walking speed, lower hip abduction force, back pain, greater Quetelet index, osteoporosis, former alcohol use, stroke, never drinking alcohol, lower mental status, use of anxiolytics and/or sleeping medications, inability to hold the tandem position, postural dizziness, cataracts, greater waist to hip ratio, lower physical activity in the past year, greater lifetime cigarette consumption, and lower grip strength were independently associated with impaired function in multivariate analyses. Age, low educational level, diabetes, current heavy alcohol use, postural hypotension, depth perception, and contrast sensitivity were not independent predictors. A combination of neuromuscular performance measures, including decreased muscle strength and impaired balance and gait, appeared to account for the effect of age on disability. CONCLUSION: A combination of many factors, including medical conditions, health habits such as obesity, smoking, alcohol abstinence, and physical inactivity, and direct measures of neuromuscular performance are associated with impaired function in older women.

Activities of Daily Living↗