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At least 811 records · Page 45Linked to original sources
PubMed2004

Apparent 3-D image perceived from luminance-modulated two 2-D images displayed at different depths.

An apparent 3-D image can be perceived from only two 2-D images displayed at different depths, when an observer views them from the direction in which they are overlapped. The two 2-D images are created from an original 2-D image by dividing its luminance according to independently obtained depth information. Subjective test results show that (1) an apparent 3-D image is perceived and (2) the perceived depth continuously varies according to the change in luminance ratio between the two 2-D images.

Distance Perception↗
PubMed2004

Interpolating sampled contours in 3D: perturbation analyses.

In four experiments, observers interpolated parabolic sampled contours confined to planes in three-dimensional space. Each sampled contour consisted of eight visible points, placed irregularly along the otherwise invisible parabolic contour. Observers adjusted an additional point until it fell on the contour. We sought to determine how each visible point influenced interpolation by measuring the effect of slightly perturbing its location. Influence fell rapidly to zero as distance from the interpolated point increased, indicating that human visual interpolation of parabolic contours is local. We compare the measured influence for human observers to that predicted by three standard interpolation algorithms. The results were inconsistent with a fit of a quadratic to the points, but were reasonably consistent with a cubic spline and most consistent with an algorithm that minimizes the variance of angles between neighboring line segments defined by the sampled points.

Algorithms↗
PubMed2004

The attentional field has a Mexican hat distribution.

We assessed the interference by distracter letters on target discrimination as a function of the distance between incompatible distracters and target. The slope of the response time-distance function supports a Mexican hat pattern of attentional modulation in the visual field. We relate the results to our recent finding of neural activity suppression in primary visual cortex coding locations in the vicinity of an attended region [Muller, N. G., & Kleinschmidt, A. (2004). The attentional 'spotlight's' penumbra: Center-surround modulation in striate cortex. Neuroreport, 15(6), 977-980]. As behavioral performance parallels activity modulation of primary visual cortex but not other areas we propose that perceptual capacities are determined by attentional response properties of V1.

Adult↗
PubMed2005

Systematic distortions of perceptual stability investigated using immersive virtual reality.

Using an immersive virtual reality system, we measured the ability of observers to detect the rotation of an object when its movement was yoked to the observer's own translation. Most subjects had a large bias such that a static object appeared to rotate away from them as they moved. Thresholds for detecting target rotation were similar to those for an equivalent speed discrimination task carried out by static observers, suggesting that visual discrimination is the predominant limiting factor in detecting target rotation. Adding a stable visual reference frame almost eliminated the bias. Varying the viewing distance of the target had little effect, consistent with observers underestimating distance walked. However, accuracy of walking to a briefly presented visual target was high and not consistent with an underestimation of distance walked. We discuss implications for theories of a task-independent representation of visual space.

Computer Simulation↗
PubMed2005

The interaction of shape- and location-based priming in object categorisation: evidence for a hybrid "what + where" representation stage.

The relationship between part shape and location is not well elucidated in current theories of object recognition. Here we investigated the role of shape and location of object parts on recognition, using a classification priming paradigm with novel 3D objects. In Experiment 1, the relative displacement of two parts comprising the prime gradually reduced the priming effect. In Experiment 2, presenting single-part primes in locations progressively different from those in the composite target had no effect on priming. In Experiment 3, manipulating the relative position of composite prime and target strongly affected priming. Finally, in Experiment 4 the relative displacement of single-part primes and composite targets did influence response time. Together, these findings are best interpreted in terms of a hybrid theory, according to which conjunctions of shape and location are explicitly represented at some stage of visual object processing.

Adolescent↗
PubMed2005

Polarization contrast of zooplankton: a model for polarization-based sighting distance.

Transparency is commonly used by zooplankton for camouflage in open waters. Polarization vision allows planktivorous animals to increase their prey's detectability. Polarization properties of zooplankton were analyzed by measuring changes in the transmitted light. The transmitted light was subjected to depolarization and phase retardance, resulting in a species-specific polarization contrast between animal and background; from 5% in Corycaeus sp. to 92% in Undinula vulgaris (Copepoda). This contrast diminishes exponentially with distance, reaching 50% of the inherent value at 1 and 2m, for moderately turbid and clear waters, respectively. However, at reactive distances of planktivorous fishes this contrast is reduced by less than 20%.

Animals↗
PubMed2005

Children with developmental dyslexia show a left visual "minineglect".

We investigated the performance of children with developmental dyslexia on a visual line bisection task. Dyslexic children did not show the overestimation of the left visual field (pseudoneglect) characteristic of normal adult vision. These results suggest that children with developmental dyslexia present selective deficits in visual attention, probably involving neural structures located in the right posterior parietal cortex.

Adult↗
PubMed2005

Prolongation of latency of horizontal saccades in elderly is distance and task specific.

The present study examined horizontal saccades in healthy subjects: 9 adults (20-32 years) and 10 aged subjects (63-83 years), under gap (fixation target extinguishes prior to target onset) and overlap (fixation stays on after target onset). The gap paradigm is known to promote fast initiation of saccades while the overlap paradigm promotes voluntary saccades with longer latency. In real life we perform saccades at various distances. In this study each paradigm was run at three viewing distances-20, 40 and 150 cm, corresponding to a convergence angle of 17.1 degrees, 8.6 degrees and 2.3 degrees, respectively. Eye movements were recorded with the Chronos video eye tracker or with the photoelectric IRIS. The main findings are: (i) increase in latency of saccades with age, with distance and with the overlap condition; (ii) evidence for interaction between these factors, indicating the following anomaly: in the gap condition and at near, aged subjects show short latencies similar to those of young adults; (iii) express type of latencies (between 80 and 120 ms) occur most frequently at near in the gap condition and at similar rates in young (25%) and aged subjects (20%). The specificity of close distance combined with the gap for triggering short latency saccades could be related to both attention and oculomotor fixation disengagement. The strength of coupling between fixation-eye movement control and visual attention control varies for different locations in space, and its decline with aging can be also different.

Adult↗
PubMed2007

Spatial and temporal properties of the illusory motion-induced position shift for drifting stimuli.

The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabor's carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Distance Perception↗
PubMed2003

Restricting intersection visibility to reduce approach speeds.

This paper reports the field test of a visual restriction treatment for a rural intersection with a high rate of injury crashes. A human factors analysis of the asymmetric pattern of crashes at the site suggested that most of the crashes were the result of anticipatory decision-making occasioned by visual characteristics of the eastbound approach to the intersection. The field test examined the effectiveness of a visual restriction treatment directed at eliminating drivers' anticipatory decision-making. The treatment consisted of a hessian screen erected along the eastbound approach to the intersection beginning 125 m prior to intersection and ending 25 m prior to intersection. Over 2 days of testing, approximately 300 drivers' reactions at the intersection were observed and their responses to a brief survey recorded. The test indicated a 23% reduction in the 80th percentile and mean approach speeds and elimination of all approach speeds over 57 km/h following introduction of the treatment. Survey results showed that the treatment was visually acceptable to the majority of drivers using the intersection and did not affect its perceived safety. Follow-on analyses compared speed data before the treatment, and 2, 21, and 37 weeks after installation of the treatment. These analyses showed that approach speeds remained low; 30% lower than pre-treatment speeds for both the 80th percentile and the average approach speeds. Of perhaps the greatest significance, no crash resulting in serious injury or death has occurred at the intersection since installation of the treatment to the present time.

Acceleration↗
PubMed1997

Unilateral neglect and space constancy during passive locomotion.

Space constancy was investigated in seven blindfolded left-neglect patients by driving them along routes involving one or two, left or right, 90 degrees turns. At the end of each route patients had to indicate its starting point while still blindfolded. On average, no considerable left/right differences were found in pointing accuracy. The entailments of this finding for the understanding of neglect phenomena are briefly discussed.

Adult↗
PubMed2002

Far bias on the radial line bisection task: measuring perceptual-attentional and motor-intentional bias in normal subjects.

Normal subjects usually err distally when bisecting radial lines below eye level in midsagittal space (far bias). To learn if a radial line bisection bias may be characterized as primarily perceptual-attentional or primarily motor-intentional, we had subjects bisect lines while watching their hand and the line under two video monitoring conditions. In the DIRECT condition, proximal (near) and distal (far) on the video monitor corresponded with the workspace where subjects bisected lines. In the INDIRECT condition, we reversed proximal and distal as they appeared on the monitor. Thus, in the indirect condition, distal hand movement appeared proximal, and vice versa. In the DIRECT condition, subjects erred toward far space (mean 2.075 mm). In the INDIRECT condition, however, subjects erred proximally (mean 1.59 mm, near bias). Distal-proximal reversal of errors over the group of subjects is consistent with a primarily perceptual-attentional bias. In lower body space, normal visual-perceptual-attentional systems may be more biased toward far space than are kinesthetic-motor systems.

Adolescent↗
PubMed1992

Pure topographical disorientation due to a deep-seated lesion with cortical remote effects.

Lesions producing pure topographical disorientation syndromes are classically located either in the right parietal region either in the right parahippocampal gyrus. The patient described in the present study was admitted to hospital after sudden onset of a left hemiparesis. The lesion at CT scan was located in the posterior limb of the right internal capsule. Neuropsychological assessment was normal except for the presence of a major topographical disorientation and of mnestic disturbances for visuo-spatial material leading us to attribute topographical disorientation to a specific loss of topographical memory. Regional cerebral blood flow measurements disclosed a right parietal hypoperfusion. This remote cortical effect could account for the presence of the neuropsychological disorders.

Aged↗
PubMed1994

The neglected page.

Normal subjects bisect vertical lines above the true midpoint. We posited that the portion of the page farthest from the line or the page's geometric center, though not the focus of attention, would still engage attention. Therefore, subjects should make bisection errors in the direction of the distant portion or geometric center. We expected the greatest superior bisection errors to be made when lines were placed at the bottom of the page and the smallest errors to be made when the lines were placed at the top of the page. Ten young, normal subjects bisected vertical lines placed in 1 of 3 positions on a page (top, center, bottom) with the page located in 1 of 3 spatial locations (superior, middle, inferior). The results of our study were consistent with our predictions. Thus, although the page on which a line is bisected is often ignored, it is not neglected.

Adult↗
PubMed1994

Independent properties of normal hemispheric specialization predict some characteristics of visuo-spatial neglect.

We derive some predictions about the character of left visuo-spatial neglect from two independent characteristics of normal hemispheric specialization: the contralateral orientation bias of each hemisphere and the association of local and panoramic processing with the left and right hemispheres, respectively. The predictions are then tested in three sets of experiments that manipulate focal and panoramic attention in estimations of the midpoints of lines and outline squares. The results from one patient are consistent with the model and with previously reported data from the same patient (Halligan and Marshall, 1993a) on a different task (line cancellation).

Attention↗
PubMed1988

How long is a piece of string? A study of line bisection in a case of visual neglect.

We report a case of severe left visual neglect consequent upon extensive infarction in the territory of the right middle cerebral artery. A detailed analysis is given of the patient's performance on line bisection. The stimuli cover a wider range of lengths than is usually employed in such studies (11'' to 1''). The magnitude of the patient's rightward displacement of his transections is linearly related to the length of the stimulus line, such that the longer the line, the greater the left 'neglect'. At line length 2'', the patient's transections are relatively accurate, and at length 1'', the transections cross the objective midpoint to give a reliable left displacement (= 'right neglect'). On some occasions, these latter transections are placed beyond the leftmost point of the stimulus line. We interpret the data in terms of two constructs: (a) an 'attentional boundary' placed slightly to the left of the objective midline; (b) 'representational completion' that extends to the attentional boundary.

Adult↗
PubMed2002

Experiencing nearby locations together in time: the role of spatiotemporal contiguity in children's memory for location.

Three studies investigated how experiencing nearby locations together in time influences memory for location. Seven-, 9-, and 11-year-old children and adults learned 20 object locations in a small-scale space. The space was divided into regions by lines or walls. In Study 1, participants learned the locations either region by region or in a random order. Following learning, participants replaced the objects without the aid of the dots marking the locations and the boundaries subdividing the space. They replaced the objects in any order they chose. After experiencing the locations in random orders during learning, only adults underestimated distances between locations belonging to the same group (i.e., region). Conversely, 9- and 11-year-old children and adults who had experienced the locations region by region during learning underestimated these distances. These findings suggest that experiencing nearby locations together in time increases the weight children assign to categorical information in their estimates of location. Results from Studies 2 and 3 in which participants learned the locations region by region and then replaced the objects region by region (Study 2) or in a random order (Study 3) were similar, highlighting the importance of spatiotemporal cues in memory for location. (c) 2002 Elsevier Science (USA).

Adult↗