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PubMed1998

Is visual anticipation of collision during self-motion related to perceptual style?

We have previously shown that during self-motion in car driving situations, the perception of another car's trajectory relies both on global visual information such as the optical flow field, and on local visual information such as the optical motion of the other car and the relative optical motion of the other car with respect to fixed elements in the environment. Here, we studied the environmental factors that contribute to perceptual judgements in relation to the observer's perceptual style (visual-field dependence vs. visual-field independence). In an experiment, observers were presented with visual scenes corresponding to the curvilinear self-motion of a driver approaching an intersection where another vehicle was arriving perpendicularly. The factors manipulated were the presence or absence of a spatial reference point (road sign near the intersection), environmental complexity ("road" or "spot" scenes), and the degree of field dependence/independence. Nine field-independent (FI) subjects and seven field-dependent (FD) subjects were asked to predict whether the other vehicle would reach the intersection before or after they would. Their responses were analyzed. Overall, subjects' judgements were more accurate with road environments and with a road sign, suggesting that the relative motion of the other vehicle with respect to fixed elements in the environment provides additional useful information. FI subjects were significantly more accurate than FD subjects, suggesting that the former are better at picking up relevant dynamic information in a complex environment.

Adult↗

PubMed2004

Concurrent validity of the stroke drivers screening assessment.

OBJECTIVE: To determine the concurrent validity of the Stroke Drivers Screening Assessment (SDSA). DESIGN: Comparison of the SDSA with criterion standards. SETTING: Subjects' homes in the community in the United Kingdom. PARTICIPANTS: Ninety-three stroke patients (age range, 22-83y) who were referred for assessment of fitness to drive or who had been driving before their stroke. Participants were assessed a median of 5 months post stroke. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Patients were assessed on the SDSA and tests of visuospatial ability (Visual Object and Space Perception [VOSP] Battery), executive abilities (Stroop Neuropsychological Screening Test, Trail-Making Test [TMT], Cognitive Estimates Test [CET]), and visual memory (Recognition Memory Tests [RMT], Verbal Descriptions of Road Signs [VDRS]). RESULTS: The SDSA subtests all correlated significantly with the Stroop test (P<.001) and TMT (P<.001), which suggests that they measure executive abilities and attention. The SDSA Dot Cancellation (DC) also correlated significantly with the VDRS (P<.01). The SDSA Square Matrices (SM) test correlated significantly with the VOSP cube analysis (P< or =.01) and the RMT faces subtest (P<.001), which suggests that the SM test also measures visuospatial abilities and visual memory. The SDSA Road Sign Recognition (RSR) test also correlated significantly with the VOSP cube analysis (P<.05), which suggests that the RSR test also measures visuospatial abilities. Factor analysis produced a 2-factor solution with DC time, SM compass, and RSR all loading on factor 1, together with the Stroop and TMT. This factor was interpreted as "executive abilities and attention." The RMT faces subtest and CET loaded onto a second factor. CONCLUSIONS: The SDSA seems to measure predominantly attention and executive abilities, which have previously been shown to be important determinants of safety to drive. This may account for the high predictive validity of the SDSA.

Adult↗

PubMed2002

Human neural systems for face recognition and social communication.

Face perception is mediated by a distributed neural system in humans that consists of multiple, bilateral regions. The functional organization of this system embodies a distinction between the representation of invariant aspects of faces, which is the basis for recognizing individuals, and the representation of changeable aspects, such as eye gaze, expression, and lip movement, which underlies the perception of information that facilitates social communication. The system also has a hierarchical organization. A core system, consisting of occipitotemporal regions in extrastriate visual cortex, mediates the visual analysis of faces. An extended system consists of regions from neural systems for other cognitive functions that can act in concert with the core system to extract meaning from faces. Of regions in the extended system for face perception, the amygdala plays a central role in processing the social relevance of information gleaned from faces, particularly when that information may signal a potential threat.

Attention↗

PubMed2001

Objects and attention: the state of the art.

What are the units of attention? In addition to standard models holding that attention can select spatial regions and visual features, recent work suggests that in some cases attention can directly select discrete objects. This paper reviews the state of the art with regard to such 'object-based' attention, and explores how objects of attention relate to locations, reference frames, perceptual groups, surfaces, parts, and features. Also discussed are the dynamic aspects of objecthood, including the question of how attended objects are individuated in time, and the possibility of attending to simple dynamic motions and events. The final sections of this review generalize these issues beyond vision science, to other modalities and fields such as auditory objects of attention and the infant's 'object concept'.

Attention↗

PubMed2003

Motion and edge sensitivity in perception of object unity.

Although much evidence indicates that young infants perceive unitary objects by analyzing patterns of motion, infants' abilities to perceive object unity by analyzing Gestalt properties and by integrating distinct views of an object over time are in dispute. To address these controversies, four experiments investigated adults' and infants' perception of the unity of a center-occluded, moving rod with misaligned visible edges. Both alignment information and depth information affected adults' and infants' perception of object unity in similar ways, and infants perceived object unity by integrating information about object features over time. However, infants perceived a moving, misaligned, three-dimensional object as indeterminate in its connectedness, whereas adults perceived it as connected behind the occluder. These findings indicate that the effectiveness of common motion in specifying unified surfaces across an occluder is reduced by misalignment of edges. Alignment information enhances perception of object unity either by serving directly as information for unity or by optimizing the detectability of motion-carried information for unity. In addition, young infants are able to retain information about edge orientation over short intervals in determining connectedness via a process of spatiotemporal integration.

Female↗

PubMed2004

"Mind the gap": the size-distance dissociation in visual neglect is a cueing effect.

There is a growing body of evidence that the processes mediating the allocation of spatial attention within objects may be separable from those governing attentional distribution between objects. In the neglect literature, a related proposal has been made regarding the perception of (within-object) sizes and (between-object) distances. This proposal follows observations that, in size-matching and bisection tasks, neglect is more strongly expressed when patients are required to attend to the sizes of discrete objects than to the (unfilled) distances between objects. These findings are consistent with a partial dissociation between size and distance processing, but a simpler alternative must also be considered. Whilst a neglect patient may fail to explore the full extent of a solid stimulus, the estimation of an unfilled distance requires that both endpoints be inspected before the task can be attempted at all. The attentional cueing implicit in distance estimation tasks might thus account for their superior performance by neglect patients. We report two bisection studies that address this issue. The first confirmed, amongst patients with left visual neglect, a reliable reduction of rightward error for unfilled "gap" stimuli as compared with solid lines. The second study assessed the cause of this reduction, deconfounding the effects of stimulus type (lines vs. gaps) and attentional cueing, by applying an explicit cueing manipulation to line and gap bisection tasks. Under these matched cueing conditions, all patients performed similarly on line and gap bisection tasks, suggesting that the reduction of neglect typically observed for gap stimuli may be attributable entirely to cueing effects. We found no evidence that a spatial extent, once fully attended, is judged any differently according to whether it is filled or unfilled.

Adult↗

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↗

PubMed1984

Sorting of hemifield presented temporal and spatial stimuli.

An experiment was performed, using a sorting task (choice reaction-time), to study the processing of stimuli, which differed along a temporal or a spatial dimension, presented to the right or to the left visual hemifield. The results indicate more accurate responses and shorter reaction times to a temporal stimulus when it appears in the right-visual-field than when it appears in the left-visual-field. Conversely, more accurate responses and shorter reaction times are found to a spatial stimulus when it appears in the left-visual-field than when it appears in the right-visual-field. In addition to this major interaction, three more interactions are found, all of which involve response direction and one or two other stimulus variables. The results are consistent with the hypothesized hemispheric functional specificity, i.e., that the initial processing of the temporal dimension of visual stimuli is performed better by the left hemisphere than by the right hemisphere and that the converse is true for the initial processing of the spatial dimension of visual stimuli.

Discrimination Learning↗

PubMed1985

Individual use of judgmental dimensions and hemispheric specificity.

The present study investigated the relationship of hemispheric functional specificity to a subject's use of judgmental dimensions when discriminating temporal and spatial visual stimuli using a multidimensional scaling (MDS) paired-comparison paradigm. The major purposes of the study were: To identify the judgmental dimensions used in discriminating unidimensional (temporal or spatial) and multidimensional (combinations of temporal and spatial parameters) stimuli presented to the two visual hemifields; to study the relationship of the use of judgmental dimensions to the visual hemifields; to investigate whether the use of judgmental dimensions in discriminating unidimensional stimuli can be used to predict their use in discriminating multidimensional stimuli. Subjects used two dimensions in discriminating the spatial and temporal unidimensional stimuli: A dimension whose scaling paralleled the physically interval-scaled stimuli; a dimension, in which the extreme values were located on one end of the scale, while the mid-values are located at the other end of the scale. There is significantly greater use of the spatial dimension when spatial stimuli are presented to the left visual field (LVF) then when presented to the right visual field (RVF). Conversely, there is significantly greater use of the temporal dimension when temporal stimuli are presented to the RVF then when they are presented to the LVF. Three perceptual dimensions were used to judge multidimensional stimuli: Spatial-to-temporal; stimulus quality; apparent movement. Two groups of subjects were identified who differed in their relative use of the temporally and spatially scaled unidimensional stimuli presented to the RVF. These two groups differed with respect to their relative use of a spatial-to-temporal dimension when multidimensional stimuli were presented to the RVF.

Cues↗

PubMed1996

Congenital visual agnosia and prosopagnosia in a child: a case report.

We present an eight years old child, L.G. with congenital agnosia and prosopagnosia. This is a special case of perceptual deficits in a child which are discrete and exist in the context of a very high verbal intelligence. L.G. was administered an extensive battery of tests of cognitive functioning. He has intact basic visual skills, although his visual analysis is sometimes slow. L.G. can read, write and do math at age level or above. Four normal eight years old boys were used as controls on a selection of the perceptual tests, administered to L.G., which did not have normative data. L.G.'s object recognition skills bear the hallmarks of adult apperceptive agnosia. His visual memory and imagery are normal. Tests of face processing skills reveal, unlike adult prosopagnosics, severe deficits in addition to the familiar face recognition problem. L.G.'s agnosia and prosopagnosia are compared to the relevant literature.

Agnosia↗

PubMed2002

Hemispheric asymmetries for simple visual judgments in the split brain.

While it is commonly noted that the right cerebral hemisphere is specialized for visuospatial processing, the scope and nature of this specialization remain somewhat ill defined. Our previous research with callosotomy ('split-brain') patients has suggested that the asymmetry may be limited to conditions that have an explicit spatial component. To investigate this we compared the performance of the divided hemispheres of two callosotomy patients on four simple visual-matching tasks. These tasks were orientation discrimination, vernier offset discrimination, size discrimination, and luminance discrimination. In each task, two stimuli were presented briefly to one visual hemifield and the patient was asked to discriminate whether they were the same or different. The first three tasks (orientation, vernier, and size) were all spatial in nature and were performed better by the right hemisphere. The luminance discrimination task, which is non-spatial, was performed equivalently by the two hemispheres. These results support the view that the fundamental difference in visual function between the hemispheres is in the ability to perform spatial discriminations.

Cognition↗

PubMed2002

Neural response to emotional faces with and without awareness: event-related fMRI in a parietal patient with visual extinction and spatial neglect.

This study examined whether differential neural responses are evoked by emotional stimuli with and without conscious perception, in a patient with visual neglect and extinction. Stimuli were briefly shown in either right, left, or both fields during event-related fMRI. On bilateral trials, either a fearful or neutral left face appeared with a right house, and it could either be extinguished from awareness or perceived. Seen faces in left visual field (LVF) activated primary visual cortex in the damaged right-hemisphere and bilateral fusiform gyri. Extinguished left faces increased activity in striate and extrastriate cortex, compared with right houses only. Critically, fearful faces activated the left amygdala and extrastriate cortex both when seen and when extinguished; as well as bilateral orbitofrontal and intact right superior parietal areas. Comparison of perceived versus extinguished faces revealed no difference in amygdala for fearful faces. Conscious perception increased activity in fusiform, parietal and prefrontal areas of the left-hemisphere, irrespective of emotional expression; while a differential emotional response to fearful faces occurring specifically with awareness was found in bilateral parietal, temporal, and frontal areas. These results demonstrate that amygdala and orbitofrontal cortex can be activated by emotional stimuli even without awareness after parietal damage; and that substantial unconscious residual processing can occur within spared brain areas well beyond visual cortex, despite neglect and extinction.

Aged↗

PubMed2002

Lateralized spatial processes and their lexical implications.

Numerous lateralized tasks have been identified as involving spatial processing, but the extent to which they use the same spatial process is largely unknown. The present research investigated relationships among significant asymmetries from seven visuo-spatial and two verbal tasks, combining samples from previous factor analytic experiments with those from three new studies (combined N=789). All of the spatial intercorrelations were negligible (r<0.20), unlike previous outcomes finding robust correlations within certain clusters of verbal asymmetries. However, lateral differences involving spatial quantitative processing on the one hand and figure-ground separation on the other, showed significant but independent correlations with one from visual lexical processing (word recognition). The results support three major conclusions. First, there are at least several and possibly many lateralized spatial processes. Second, hemispheric processes follow a modular architecture in preference to a diffuse or parallel distributed architecture. Finally, the dissociated correlations are consistent with a visual lexical processing model having both occipital and parietal components, potentially reconciling a current controversy over the cerebral localization of language.

Adult↗

PubMed2003

Eye-movement patterns do not mediate size distortion effects in hemispatial neglect: looking without seeing.

Over the last decade a range of studies have shown that some patients with hemispatial neglect subjectively underestimate the size of objects presented in their contralesional hemispace. Recently, it has been suggested that the effect is simply due to either hemianopia [Brain 124 (2001) 527], or the combination of neglect and hemianopia [Neurology 52 (1999) 1845]. In the current study we asked right hemisphere lesioned patients with and without neglect and hemianopia as well as healthy controls to judge either two horizontal or vertical lines presented simultaneously in right and left hemispace and monitored their eye movements. Three out of the six patients showed the predicted size distortion effect for horizontal lines. We found no evidence that the effect was mediated by eye movements. The two neglect patients who showed the strongest left side underestimation showed symmetrical (left, right) scanning of the lines both in terms of number of fixations and fixation time, yet they still failed to judge the relative size veridically. In addition, we did not find strong evidence for a link with hemianopia. We therefore propose that the effect reflects a computational/representational failure of processing for horizontal extent.

Aged↗

PubMed2003

When does the Titchener Circles illusion exert an effect on grasping?. Two- and three-dimensional targets.

This study used the Titchener Circles illusion to investigate the functional dissociation of the dorsal visuomotor and ventral perceptual systems. In order to investigate the visual requirements for an action to be driven by the dorsal stream, two-dimensional (2D) and three-dimensional (3D) targets were compared. Thirteen subjects made visual open loop manual estimations or grasping actions towards 2D and 3D versions of the illusion. No illusion effect was found for immediate grasping, but the illusion did influence manual estimation, irrespective of whether the target was 2D or 3D. It is suggested that the underlying representations used to drive grasping actions towards 2D targets are fundamentally the same as those used to drive natural grasping actions. While stereoscopically specified depth may be important to the dorsal visuomotor system, it does not appear to be necessary in order for an action to be based on dorsal representations. It is suggested that an action must be goal directed in order for that action to be driven by dorsal stream processes.

Adolescent↗

PubMed1997

Cognitive and perceptual influences on visual line bisection: psychophysical and chronometric analyses of pseudoneglect.

Perceptual and cognitive influences on line bisection were isolated using a tachistoscopic forced-choice paradigm. Pretransected lines were presented for 150 msec at four orientations ([symbol: see text],-,/ and [symbol: see text]). Subjects made either 'left-right' (for -,/, and [symbol: see text] lines) or 'above-below' (for [symbol: see text],/and [symbol: see text] lines) discriminations in response to each line stimulus, depending upon perceived transector location relative to veridical line midpoint. Median response time and point of subjective equality (P.S.E.) were computed for each treatment condition. P.S.E.s in 'left-right' conditions were significantly left of veridical; response time maxima were similarly displaced. Azimuthal pseudoneglect was greatest for horizontal lines. P.S.E.s in 'above-below' conditions were displaced above veridical, and response time maxima were similarly displaced. Altitudinal pseudoneglect was greatest for negative diagonal lines [symbol: see text]. Azimuthal pseudoneglect significantly exceeded altitudinal pseudoneglect. 'Left-right' responses (mean = 478.3 msec) were significantly faster than 'above-below' responses (mean = 504.6 msec). We conclude that scanning eye and/or gross limb movements do not account for pseudoneglect, and that a significant component must be purely perceptual. Chronometric and psychometric measures of pseudoneglect are in remarkable agreement. The effects of altitudinal and azimuthal pseudoneglect are neither separable nor additive, suggesting the existence of independent mechanisms governing the allocation of spatial attention to objects of differing orientation. The slopes of the psychometric functions for lines of cardinal orientation are significantly steeper than for diagonal lines, which may reflect a processing conflict between these putatively independent mechanisms at diagonal line orientations. Decision context significantly modulates the magnitude of pseudoneglect for physically identical stimuli, perhaps reflecting the selective differential engagement of the vertical or horizontal attentional mechanisms. There are significant individual differences in line bisection performance, even in a very homogeneous sample of strongly right-handed subjects.

Analysis of Variance↗

PubMed2000

Comparing extra-retinal information about distance and direction.

The idea that extra-retinal information about the orientation of the eyes could be used to judge an object's distance has a long history, and has been the issue of considerable debate throughout this century. We here show that the poor performance in comparison with judgements of direction has geometrical rather than physiological reasons, and discuss why previous studies have misled us into believing that information about distance is even poorer than the geometry predicts.

Convergence, Ocular↗

PubMed1997

Pitfalls in estimating motion detector receptive field geometry.

A number of psychophysical investigations have used spatial-summation methods to estimate the receptive field (RF) geometry of motion detectors by exploring how psychophysical thresholds change with stimulus height and/or width. This approach is based on the idea that an observer's ability to detect motion direction is strongly determined by the relationship between the stimulus geometry (height and width) and the RF of the activated motion detectors. Our results show that previous estimates of RF geometry can depend significantly on stimulus position in the visual field as well as on the stimulus height-to-width ratio. The data further show that RF estimates depend on the stimulus in a manner that is inconsistent with basic predictions derived from current motion detector models. Hence previous estimates of height, width, and height-to-width ratios of motion detector RFs are inaccurate and unreliable. This inaccuracy/unreliability is attributed to a number of sources. These include incorrect fixed-parameter values in model fits, as well as the confounding of physiological spatial summation area through combined use of contrast thresholds and Gaussian-windowed stimuli. A third source of error is an asymmetric variation of spatiotemporal correlation in the stimulus as either its height or width is varied (and the other dimension held constant). Most importantly, a fourth source of unreliability is attributed to the existence of a nonlinear, nonmonotonic distribution of motion detectors in the visual field that has been previously described and is a natural result of visual anatomy.

Contrast Sensitivity↗