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PubMed1996

Binocular rivalry with isoluminant stimuli visible only via short-wavelength-sensitive cones.

To test whether the binocular contour rivalry mechanism is tritanopic, we presented isoluminant, rival stimuli visible only via the short-wavelength-sensitive (S) cones. We stimulated only the S cones with violet gratings superimposed on a bright yellow field that adapted the responses of the middle- and long-wavelength-sensitive (M and L) cones. We found that an S-cone grating presented to one eye rivalled with an orthogonal grating presented to the other. Rivalry persisted over a range of luminances and contrasts of the S-cone stimuli, and was greater than could be accounted for by nonrival fading. The spatial spread of rivalry from S-cone stimuli is similar to that for the same stimuli when visible also to the M and L cones (luminance stimuli). We found that an S-cone stimulus would rival with a luminance stimulus, and exploited this to determine the equivalent luminance contrast of S-cone stimuli by putting them in a rivalry competition with luminance stimuli. For rivalry, the equivalent luminance contrast of isoluminant, S-cone stimuli is much less than their S-cone contrast. The existence of rivalry with isoluminant stimuli, along with earlier evidence that such stimuli can support stereopsis, challenges the view that an achromatic channel alone drives certain higher level functions such as depth perception.

Contrast Sensitivity↗
PubMed1995

On the time-course of inhibition in the stereoscopic perception of rows of dots.

Perception of depth difference between two dots is more difficult if additional dots intervene between them. By varying the onset asynchrony (SOA) between the endpoints and one or several intervening dots, we measured the time-course of the process that elevates stereoscopic thresholds. It turned out that adding the intervening dots under these conditions decreased performance to, and often below, the level that was achieved with a total presentation time corresponding to the SOA for intervening dots presented both binocularly and monocularly. This is an indication for an active inhibitory process.

Adult↗
PubMed1989

Principles of visual motion detection.

Motion information is required for the solution of many complex tasks of the visual system such as depth perception by motion parallax and figure/ground discrimination by relative motion. However, motion information is not explicitly encoded at the level of the retinal input. Instead, it has to be computed from the time-dependent brightness patterns of the retinal image as sensed by the two-dimensional array of photoreceptors. Different models have been proposed which describe the neural computations underlying motion detection in various ways. To what extent do biological motion detectors approximate any of these models? As will be argued here, there is increasing evidence from the different disciplines studying biological motion vision, that, throughout the animal kingdom ranging from invertebrates to vertebrates including man, the mechanisms underlying motion detection can be attributed to only a few, essentially equivalent computational principles. Motion detection may, therefore, be one of the first examples in computational neurosciences where common principles can be found not only at the cellular level (e.g., dendritic integration, spike propagation, synaptic transmission) but also at the level of computations performed by small neural networks.

Animals↗
PubMed1990

The color-opponent and broad-band channels of the primate visual system.

Physiological, anatomical and psychophysical studies have identified several parallel channels of information processing in the primate visual system. Two of these, the color-opponent and the broad-band channels, originate in the retina and remain in part segregated through several higher cortical stations. To improve understanding of their function, recent studies have examined the visual capacities of monkeys following selective disruption of these channels. Color vision, fine- but not coarse-form vision and stereopsis are severely impaired in the absence of the color-opponent channel, whereas motion and flicker perception are impaired at high but not low temporal frequencies in the absence of the broad-band channel. The results suggest that the color-opponent channel extends the range of vision in the spatial and wavelength domains, and that the broad-band channel extends it in the temporal domain. Lesion studies also indicate that these channels must reach higher cortical centers through extrastriate regions other than just area V4 and the middle temporal area, and that the analysis performed by these two regions cannot be uniquely identified with specific visual capacities.

Animals↗
PubMed1996

Perception of motion-in-depth in patients with partial or complete cerebral hemispherectomy.

Four patients with functional hemispherectomy, one patient with a complete anatomical hemispherectomy, and one patient with unilateral removal of the temporal, parietal and occipital lobes took part in two sets of experiments designed to investigate their residual sensitivity to motion-in-depth in the hemianopic visual field. Two types of computer-generated visual displays were used; in the first set of experiments, a dot pattern and in the second, a circular checkerboard. These simulated either convergent, divergent or reversed rotational motion. Each set of experiments consisted of two parts; in the first part, electrodermal responses were monitored during stimulus presentation while the subjects performed a simple distracting task. In the second part, subjects were asked to state verbally the direction of stimulus motion. Contrary to expectations, no reliable changes in skin conductance were elicited from any of the subjects by changes in the direction of motion of the component parts of either the dot pattern display or the circular checkerboard display. Furthermore, none of the subjects were able to discriminate the direction of motion of the target patterns when presented in the hemianopic field. The most parsimonious explanation is that the subcortical visual pathways which survive hemispherectomy are unable to process visual information relating to motion in depth.

Adolescent↗
PubMed1987

Three South African studies on the relation between road accidents and drivers' vision.

This report describes three studies of drivers' vision which have recently been carried out in South Africa. In the first, it was found that younger myopes constituted a substantial fraction of those drivers who failed to reach the acuity standards required for a South African driving licence: the acuity of almost all these drivers could be raised to an adequate standard by a proper refractive correction. In the second study, a comparison of the visual characteristics of a group of drivers who had been involved in accidents with those of a control group of accident-free drivers suggested that the former had mean acuities and depth perception which were worse than those of the accident-free group by small but statistically significant amounts. A third study, comparing the visual acuities of a group of randomly selected drivers with those of a group who had been involved in road accidents, showed that a higher proportion of the accident group had acuities which were below that required by the licensing authority.

Accidents, Traffic↗
PubMed1993

Binocular matching in monkey visual cortex: single cell responses to correlated and uncorrelated dynamic random dot stereograms.

This paper presents results obtained from experiments performed on two behaving monkeys (Macaca mulatta) viewing correlated and uncorrelated dynamic random dot stereograms. Extracellular single unit activity was recorded from areas V1 and V2. We found that cells sensitive to correlated stereograms (38%) were also sensitive to uncorrelated stereograms. Regarding their response to both situations, these cells were grouped into two types. The first type (57%), termed "correlation-sensitive cells", responded with statistically significant difference to correlated and uncorrelated stereograms. The second type responded equally to both stereograms (43%), and therefore they were termed "correlation-insensitive cells". Our data suggest that these two types may form two populations with different functions subserving depth perception.

Animals↗
PubMed1995

Transthoracic three-dimensional echocardiography in adult patients with congenital heart disease.

OBJECTIVES: This study sought to assess both the feasibility and potential role of transthoracic three-dimensional echocardiography for the evaluation of adult patients with congenital heart disease. BACKGROUND: The unrestricted views with depth perception provided by three-dimensional echocardiography with dynamic volume-rendered display may enhance visualization of cardiac structures and detection of abnormalities in patients with congenital heart defects. METHODS: We studied 33 patients with various heart defects (mitral valve anomalies in 9, aortic valve anomalies in 5, subaortic membrane in 5, ventricular septal defect in 4, transposition of the great arteries in 3, tetralogy of Fallot in 2, other defects in 5). Cross-sectional images of the specific region of interest were acquired from either the parasternal or apical window with the rotational technique (2 degrees interval with electrocardiographic and respiratory gating) and postprocessed for resampling in cubic format. From these three-dimensional data sets a multitude of cut planes were selected, presented in volume-rendered dynamic display and analyzed by two observers for comparison with standard two-dimensional images to assess their additional information. RESULTS: Three-dimensional reconstruction was possible in all patients. Structures of interest were evaluated from unusual viewpoints, providing both cardiologists and surgeons with immediate feedback. When compared with standard two-dimensional images, additional information was provided for 12 patients (36%). The mitral valve, aortoseptal continuity and interatrial septum were the structures for which three-dimensional echocardiography was most useful. CONCLUSIONS: Transthoracic three-dimensional echocardiography is feasible and facilitates spatial recognition of the intracardiac anatomy in a significant proportion of patients and enhances diagnostic confidence of complex congenital heart disease.

Adolescent↗
PubMed2004

Subregions of human MT complex revealed by comparative MEG and direct electrocorticographic recordings.

OBJECTIVE: To locate the visual motion complex (MT+) and study its response properties in an epilepsy surgery patient. METHODS: A 17-year-old epilepsy patient underwent invasive monitoring with subdural electrodes in the right temporo-parieto-occipital area. MT+ was investigated by cortical electric stimulation and by epicortical visual evoked potentials time-locked to motion onset of sinusoidal gratings (motion VEP). Motion-related visual evoked magnetic field (motion VEF) was also recorded before the electrode implantation to complement the invasive recording. RESULTS: Motion VEPs revealed two subregions within MT+, generating early and late potentials respectively. The early activity with a peak around 130 ms was localized at a single electrode situated immediately caudal to the initial portion of the ascending limb of the superior temporal sulcus (AL-STS). The late activity, peaking at 242-274 ms, was located ventro-rostrally over three electrodes. Among the four electrodes with motion VEPs, cortical stimulation at the most caudal pair elicited motion-in-depth perception involving the whole visual field. In addition to two subregions revealed on the gyral crown, magnetoencephalography (MEG) demonstrated another subregion with a late motion VEF in AL-STS immediately rostral to the electrode with the early motion VEP. CONCLUSIONS: In combination with MEG recording, the present invasive exploration demonstrated human MT+ in a focal area of the temporo-parieto-occipital junction and delineated possible three subregions as indicated by the different latencies and distributions of the motion VEP/VEFs. SIGNIFICANCE: Comparative MEG and direct electrocorticographic recordings delineated possible subregions within the human MT complex.

Adolescent↗
PubMed2005

The ties that keep us bound: Top-down influences on the persistence of shape-from-motion.

The phenomenon of perceptual persistence after the motion stops in shape-from-motion displays (SFM) was used to study the influence of prior knowledge on the maintenance of a percept in awareness. In SFM displays an object composed of discontinuous line segments are embedded in a background of randomly oriented lines. The object only becomes perceptible when the line segments that compose the object and the lines that compose the background move in counterphase. Critically, once the movement of the line segments stops, the percept of the object persists for a short period of time. In the present study, perceptual persistence for digits exceeded that reported for nonsense shapes composed of the same line segments. This result is taken as evidence that the processes involved in the persistence of SFM, and therefore sustained perception, are sensitive to top-down influences.

Adult↗
PubMed2006

Basic laparoscopic surgical training: examination of a low-cost alternative.

OBJECTIVE: "Dry lab" facilities are integral to laparoscopy training, but access is often limited due to the high costs of video-laparoscopy equipment. We assessed the effectiveness of a cheap and simple training model compared to conventional video-laparoscopy for basic training using a randomised, blinded study. METHODS: Thirty-six third-year medical students without previous surgical skills were randomised into two groups: group A students were taught basic laparoscopy skills using a conventional video-laparoscopy pelvic trainer and group B students were taught similar techniques using a cardboard box with a cut-out top to allow light and visualisation. Participants in group B had one eye obscured to reduce their stereoscopic vision. After eight sessions of training amounting to 24h, the two groups were assessed by a blinded adjudicator on set tasks using both the video-laparoscopy pelvic trainer and the cardboard box. Accuracy, timing and depth perception were assessed and the results compared. RESULTS: There was no significant difference in performance scores or times between the two groups in any of the parameters when tested on the cardboard box. However, when assessed on the video trainer, the cardboard box-trained group had significantly faster times with equivalent scores in the majority of tasks. CONCLUSION: For basic laparoscopic training the cardboard box, costing nothing, is a simple and effective alternative, which can be used in conjunction with sophisticated video-laparoscopy equipment costing thousands of dollars.

Adult↗
PubMed2006

Field of view and base of support width influence postural responses to visual stimuli during quiet stance.

We explored the destabilizing effect of visual field motion as the base of support (BOS) and the field of view (FOV) were narrowed. Visual field motion was achieved using an immersive virtual environment (scene) that moved realistically with head motion (natural motion) and translated sinusoidally at 0.1Hz in the fore-aft direction (augmented motion). Natural motion was presented in stereo while augmented motion was presented in both stereo and non-stereo. Subjects viewed scene motion under wide (90 degrees and 55 degrees in the horizontal and vertical directions) and narrow (25 degrees in both directions) FOV conditions while standing flatfooted (100% BOS) and on two blocks (45% and 35% BOS). Head and whole body center of mass (COM) and ankle angle root mean square (RMS) were determined as were head, whole body, and shank COM FFTs. During natural motion, the primary effect emerged in the head RMS which was significantly smaller with a 35% BOS and the wide FOV compared to the narrow FOV. However, the primary effect of augmented motion emerged in the power analysis of head and whole body COM which significantly increased with the wide FOV for a 35% BOS compared to 100% BOS. Statistical analysis indicated an effect of BOS on depth perception for head and whole body RMS; however, post hoc comparisons revealed no significant differences between stereo and non-stereo augmented motion. We conclude that reducing the BOS increased reliance on peripheral visual information to stabilize the head in space even when the augmented visual motion promoted postural instability.

Adult↗
PubMed2006

Pain relief in knee osteoarthritis reduces the propensity to trip on an obstacle.

The pain associated with knee osteoarthritis (OA) has been shown to lead to an increased propensity to trip on an obstacle. Pain-relieving intra-articular injections are widely utilized in the treatment of knee OA. This study examined the effects of pain-relieving intra-articular knee injections on the ability to avoid contacting a suddenly appearing obstacle in patients with knee OA. Obstacle avoidance success rates, pain, body mass index, visual acuity, contrast sensitivity, depth perception, and single-leg stance duration were evaluated in nine patients with painful osteoarthritis of the knee and 14 age-matched healthy control subjects. Obstacle avoidance success rates, pain, and single leg stance duration were tested a second time in the patients with knee OA after they received their injections, which contained a fast-acting local anesthetic to provide rapid pain relief. After receiving the pain-relieving knee injections, patients with knee OA had 48% less pain and were 31% more successful in avoiding stepping on the obstacle. However, after receiving the injection, the obstacle avoidance success rates remained 20% less than those of the healthy controls. The results of this study suggest that knee pain-relief can decrease the propensity of people with painful knee OA to trip and fall over an obstacle. However, pain-relief alone did not return the patients with knee OA in this study to a disease-free risk of tripping.

Accidental Falls↗
PubMed2005

Comparison of skill training with robotic systems and traditional endoscopy: implications on training and adoption.

BACKGROUND: Robotic systems are being used by an increasing number of surgeons. This environment is markedly different from that of traditional surgery and involves videoscopic guidance, remote surgical control, and the loss of haptic feedback. Defining how surgeons learn with these systems is necessary to establish training protocols for this technology. This study compared the learning curve for a robotic surgical system with that of traditional endoscopy in the performance of two standardized skill drills. MATERIALS AND METHODS: Twenty participants (average age 27 +/- 4 years, six females) repeated two standardized endoscopic dexterity and depth perception drills for 15 repetitions with the ZEUS robotic surgical system and manual endoscopic instruments (MAN). A score combining time and precision was given for each repetition. The learning curves and overall performance with and without robotic assistance were compared. RESULTS: For both MAN and ZEUS, improvements in performance were significantly greatest during the first five repetitions (P < 0.01, for both). Participants reached the training curve plateau faster with ZEUS than with conventional instruments (8th versus 10th for both drills). Using robotic assistance, dominant and non-dominant hand performance were statistically similar. The number of errors committed with ZEUS were significantly fewer for drill two (0.09 errors/repetition versus 0.24 errors/repetition, P = 0.002) compared to manual technique. CONCLUSIONS: This study demonstrated that training curves for conventional and robotic-assisted systems are remarkably similar. This should prove useful in the training and education of this new technology. This study further suggested that robotics may increase ambidexterity by improving non-dominant hand performance.

Adult↗
PubMed2004

Visual completion of three-dimensional, chromatic, moving stimuli in humans.

We studied whether visual completion can be produced within three-dimensional (3-D), moving, chromatic objects. Rotating thin and thick discs with red sectors, forming the corners of Kanizsa triangle, were presented on a display. Observers also rotated a hand-held real thin Kanizsa triangle. An illusory, moving contour encompassing an illusory colourful triangle was generally observed. This indicates that the visual system is capable of filling in complex (3-D, moving, chromatic) percepts.

Adult↗
PubMed2004

Spatial-scale interaction in human stereoscopic vision in response to sustained and transient stimuli.

We investigated temporal properties of stereopsis at different spatial scales in dynamic random-dot stereograms (DRS) consisting of (i) the repeated presentation of two image pairs (i.e. sustained presentation) and (ii) single presentations of two image pairs (transient presentation). In dense stereograms perception of depth is possible if the levels of interocular correlation are above a certain threshold. Therefore, detection of correlation is an important step in stereopsis. Across different experiments, we varied the level of interocular correlation as a function of time by alternating correlated and un/anticorrelated random-dot stereograms. Also, we varied their spatial scale (coarse, fine and alternating between coarse and fine). The presence of stereopsis was examined using a forced-choice depth detection task. For both sustained and transient presentations of the stimuli, the results show that (i) stereopsis has similar temporal properties at coarse and fine spatial scales; (ii) interaction between spatial scales depends on their relative sizes. The results indicate a strong inhibitory influence of rivalry at a coarse scale on stereopsis at a fine scale, and just a weak inhibitory influence of rivalry at a fine scale on stereopsis at a coarse scale. This study provides experimental evidence for a hierarchical organisation of spatial scales in human stereoscopic vision based on neural interaction instead of vergence eye movements.

Adult↗
PubMed2005

Interocular suppression is gated by interocular feature matching.

We present a new form of contrast masking in which the target is a patch of low spatial frequency grating (0.46 c/deg) and the mask is a dark thin ring that surrounds the centre of the target patch. In matching and detection experiments we found little or no effect for binocular presentation of mask and test stimuli. But when mask and test were presented briefly (33 or 200 ms) to different eyes (dichoptic presentation), masking was substantial. In a 'half-binocular' condition the test stimulus was presented to one eye, but the mask stimulus was presented to both eyes with zero-disparity. This produced masking effects intermediate to those found in dichoptic and full-binocular conditions. We suggest that interocular feature matching can attenuate the potency of interocular suppression, but unlike in previous work (McKee, S. P., Bravo, M. J., Taylor, D. G., & Legge, G. E. (1994) Stereo matching precedes dichoptic masking. Vision Research, 34, 1047) we do not invoke a special role for depth perception.

Accommodation, Ocular↗
PubMed2006

The perceived transmittance of inhomogeneous surfaces and media.

A series of experiments was performed to determine how the visual system computes the transmittance of inhomogeneous surfaces and media. Previous work (Anderson, B. L. (1999) Stereoscopic surface perception. Neuron, 26, 919-928; Anderson, B. L. (2003) The role of occlusion in the perception of depth, lightness, and opacity. Psychological Review, 110, 762-784) has suggested that the visual system employs a transmittance anchoring principle in determining when transparency is perceived. This principle states that the visual system interprets the highest contrast region along contours and surfaces as a region in plain view and uses this anchor as a reference point for transparency computations. In particular, recent work has shown that the transmittance of homogeneous transparent surfaces is well described by a ratio of contrasts model (Singh, M., & Anderson, B. L. (2002). Toward a perceptual theory of transparency. Psychological Review, 109, 492-519). In this model, the transmittance of a transparent surface is determined by the contrast of a transparent image region normalized by the contrast of the region in plain view. Here, a series of experiments is reported that assesses this model for inhomogeneous transparent surfaces that vary in both space and time. The results of these experiments reveal that transmittance anchoring has both a spatial and temporal component, and that the perceived transmittance of transparent surfaces is well described by a ratio of perceived contrasts model.

Contrast Sensitivity↗