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Biomedical subjects

J P Wann

Publications and source records attributed to J P Wann.

At least 19 recordsLinked to original sources
PubMed2001

The role of size and binocular information in guiding reaching: insights from virtual reality and visual form agnosia III (of III).

Reaching out to grasp an object requires information about the size of the object and the distance between the object and the body. We used a virtual reality system with a control population and a patient with visual form agnosia (DF) in order to explore the use of binocular information and size cues in prehension. The experiments consisted of a perceptual matching task in addition to a prehension task. In the prehension task, control participants modified their reach distance in response to step changes in vergence in the absence of any clear reference for relative disparity. Their reach distance was unaffected by equivalent step changes in size, even though they used this information to modify grasp and showed a size bias in a distance matching task. Notably, DF showed the same pattern of results as the controls but was far more sensitive to step changes in vergence. This finding complements previous research suggesting that DF relies predominantly on vergence information when gauging target distance. The results from the perceptual matching tasks confirmed previous findings suggesting that DF is unable to make use of size information for perceptual matching, including distance comparisons. These data are discussed with regard to the properties of the pathways subserving the two visual cortical processing streams.

Agnosia↗
PubMed2000

Heading perception and the allocation of attention.

A central theme in previous studies of heading judgements has been whether the retinal flow field can be decomposed to recover the translation component of locomotion when flow also contains the effects of gaze rotation. We explored not just the effect of moving gaze, but also moving attention away from the locomotor path by presenting the case of fixating a road sign and completing different attentional tasks during locomotion. Heading errors increased significantly with attentional load, in the absence of extra-retinal gaze information. When we introduced extra-retinal gaze information with the same tasks this resulted in a significant improvement in heading judgements. These results lead us to question whether the decomposition argument translates to real-world judgements of locomotor heading. If observers need to closely attend to roadside information it seems that decomposition is ineffective, whereas if they have the latitude to alternate gaze it is unnecessary.

Attention↗
PubMed1999

Visual-proprioceptive mapping in children with developmental coordination disorder.

Developmental coordination disorder (DCD) occurs in a small but significant proportion of children who present with impaired body-eye coordination and show poor acquisition of motor skills. This study investigated the visual-proprioceptive mapping ability of children with DCD from a small selected group, with particular reference to the use of vision in matching tasks. The children with DCD in this study were significantly poorer than control children on all matching tasks. They seemed to have particular difficulty in cross-modal judgements that required the use of visual information to guide proprioceptive judgements of limb position. A distinction is drawn between tasks that can be achieved purely through sensory matching and those that require body-centred spatial judgements, suggesting that it is the latter that posits a particular difficulty for children with DCD.

Case-Control Studies↗
PubMed1999

Weighted combination of size and disparity: a computational model for timing a ball catch.

How do we time hand closure to catch a ball? Binocular disparity and optical looming provide two sources of information about an object's motion in depth, but the relative effectiveness of the two cues depends on ball size. Based on results from a virtual reality ball-catching task, we derive a simple model that uses both cues. The model is sensitive to the relative effectiveness of size and disparity and implicitly switches its response to the cue that specifies the earliest arrival and away from a cue that is lost or below threshold. We demonstrate the model's robustness by predicting the response of participants to some very unusual ball trajectories in a virtual reality task.

Cues↗
PubMed1999

Steering, optic flow, and the respective importance of depth and retinal motion distribution.

Movement through an environment produces an optical spatiotemporal pattern, known as a flow field. When visually guiding movement using a flow field, do humans make use of information about the distance of constituent elements? Employing a novel active steering task, we examined the use of depth (height-in-scene and disparity) and the role of the retinal motion distribution in the perceptual control of heading from flow. We found that retinal motion distribution, rather than depth order, has the primary role in determining the accuracy of steering.

Depth Perception↗
PubMed1998

Guidance of locomotion on foot uses perceived target location rather than optic flow.

What visual information do we use to guide movement through our environment? Self-movement produces a pattern of motion on the retina, called optic flow. During translation, the direction of movement (locomotor direction) is specified by the point in the flow field from which the motion vectors radiate - the focus of expansion (FoE) [1-3]. If an eye movement is made, however, the FoE no longer specifies locomotor direction [4], but the 'heading' direction can still be judged accurately [5]. Models have been proposed that remove confounding rotational motion due to eye movements by decomposing the retinal flow into its separable translational and rotational components ([6-7] are early examples). An alternative theory is based upon the use of invariants in the retinal flow field [8]. The assumption underpinning all these models (see also [9-11]), and associated psychophysical [5,12,13] and neurophysiological studies [14-16], is that locomotive heading is guided by optic flow. In this paper we challenge that assumption for the control of direction of locomotion on foot. Here we have explored the role of perceived location by recording the walking trajectories of people wearing displacing prism glasses. The results suggest that perceived location, rather than optic or retinal flow, is the predominant cue that guides locomotion on foot.

Adult↗
PubMed1998

Improving vision: neural compensation for optical defocus.

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for the defocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid-range (5-25 cycles deg-1) spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two-stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid-range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter-ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Adaptation, Physiological↗
PubMed1998

The contribution of vision and proprioception to judgements of finger proximity.

We sought to determine whether an increase in judged egocentric distance created by increasing vergence-specified distance would be negated when participants pointed at their own finger. It was found that ocular position dominates limb proprioception in the judgement of finger distance in the sagittal plane when vision is available. In contrast, an increase in perceived egocentric distance was largely attenuated by the presence of limb proprioception in reduced visual cue conditions. We conclude that the relative contribution of vergence to perceived distance depends upon the strength of the vergence effort signal when there are other cues present. Furthermore, if the distance percept includes a major contribution from retinal cues, then the visual component will dominate the limb proprioception component. If the visual component is largely determined by vergence information, limb proprioception will make a significant contribution and actually dominate when the vergence effort signal is weak. The results extend previous studies that have found a similar relationship between ocular position and limb proprioception in the perception of a finger's location in the coronal plane.

Adult↗
PubMed1998

Eye movements, prematurity and developmental co-ordination disorder.

Horizontal pursuit eye movements were investigated in two separate groups of children: One group exhibited developmental co-ordination disorder (n = 8) whilst another group of children were born prematurely (n = 8). Both studies found a reduced gain in pursuit eye movements when the respective populations were compared with control groups (n = 32). A difference was also found in the ability of some children to temporally synchronize their tracking response to the stimulus, which was indicative of poor predictive control rather than lags in the control system. We suggest that horizontal eye movements may be a sensitive indicator of more general motor deficits during childhood development.

Adult↗
PubMed1997

Synaesthesia in the normal limb.

We explored the degree to which vision may alter kinaesthetic perception by asking participants to view their hand through a prism, introducing different horizontal deviations, while trying to align their fingers above and below a thin table. When the visual image of one hand was displaced this overwhelmed kinaesthetic judgements and participants reliably reported that they felt their limbs were aligned, even when they were laterally mis-aligned by as much as 10 cm. This effect, however, was mediated by 'visual capture' and when the task was attempted in a darkened room with limb position indicated by an LED taped to the finger, kinaesthesis dominated and participants reported that the LED seemed to become detached from their finger tip. In both light and dark conditions the finger was clearly visible and only the background detail was extinguished. Hence, in perceiving limb position, it appears that we believe in what we see, rather than in what we feel, when the visual background is rich, and in what we feel when the visual background is sparse.

Arm↗
PubMed1997

Looking at the task in hand: vergence eye movements and perceived size.

A retinal afterimage of the hand changes size when the same unseen hand is moved backwards and forwards in darkness. We demonstrate that arm movements per se are not sufficient to cause a size change and that vergence eye movements are a necessary and sufficient condition for the presence of the illusory size change. We review previous literature to illustrate that changing limb position in the dark alters vergence angle and we explain the illusion via this mechanism. A discussion is provided on why altering limb position causes a change in vergence and we speculate on the underlying mechanisms.

Convergence, Ocular↗
PubMed1997

Virtual environments for the rehabilitation of disorders of attention and movement.

The incidence of perceptual-motor disorders arising from stroke is steadily increasing in the population of Europe and USA. This chapter sought to define the role that virtual environments may have in designing remedial programmes for rehabilitation following stroke in the areas of attentional retraining and the reacquisition of perceptuo-motor skills. Principles for the structure of guided learning were identified and emphasis placed on the need to identify when and how virtual environments technology can introduce added value to the therapy situation.

Attention Deficit Disorder with Hyperactivity↗
PubMed1996

Anticipating arrival: is the tau margin a specious theory?

A critical review is presented of previous studies that have examined the use of the tau margin (tau m) in the temporal judgment of arrival in natural contexts. This body of evidence is frequently cited as providing strong support for the use of Tau m for interceptive timing. Critical flaws are demonstrated in either the analysis or interpretation of many of these studies. It is suggested that temporal control in a number of these experiments could have been effected using a relative distance estimate (zeta ratio). Results are also presented and discussed that conflict with the tau m control hypothesis. It is concluded that although the tau m hypothesis provides an appealing account of interceptive timing, its broad acceptance is unwarranted on the evidence available.

Acceleration↗
PubMed1995

Natural problems for stereoscopic depth perception in virtual environments.

The use of virtual reality (VR) display systems has escalated over the last 5 yr and may have consequences for those working within vision research. This paper provides a brief review of the literature pertaining to the representation of depth in stereoscopic VR displays. Specific attention is paid to the response of the accommodation system with its cross-links to vergence eye movements, and to the spatial errors that arise when portraying three-dimensional space on a two-dimensional window. It is suggested that these factors prevent large depth intervals of three-dimensional visual space being rendered with integrity through dual two-dimensional arrays.

Accommodation, Ocular↗
PubMed1993

Time-to-contact judgment in the locomotion of adults and preschool children.

The hypothesis of Lee (1976)--that approach and deceleration toward a surface can be controlled through the rate of change of the optic variable tau--was examined for natural human locomotion. In Experiment 1, 12 adults were asked to perform locomotor tasks that required running at speed and then decelerating so that either the hand or head made a controlled contact with a door. In Experiment 2, 12 preschool children performed a relay-running task that required similar control. In Experiment 3, 12 children and 12 adults ran with a stick as an extension to their arm length and performed the same task. The results supported Lee's hypothesis for the initial phase of approach, but subjects switched to a separate adjustment phase 2 to 3 arm lengths from the target. Children did not adopt an appropriate tau strategy for collision avoidance and appeared unable to modify their approach strategy to allow for a hand-held stick.

Acceleration↗
PubMed1993

Binocular vision in a virtual world: visual deficits following the wearing of a head-mounted display.

The short-term effects on binocular stability of wearing a conventional head-mounted display (HMD) to explore a virtual reality environment were examined. Twenty adult subjects (aged 19-29 years) wore a commercially available HMD for 10 min while cycling around a computer generated 3-D world. The twin screen presentations were set to suit the average interpupillary distance of our subject population, to mimic the conditions of public access virtual reality systems. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of the subjects. The implications of introducing such HMDs into the workplace and entertainment environments are discussed.

Computer Simulation↗
PubMed1993

Motor skill learning in cerebral palsy: movement, action and computer-enhanced therapy.

This chapter discusses the extent to which previous research into movement control can provide key principles on which to model therapy for individuals with severe cerebral palsy. It is suggested that the movement perspective has traditionally stressed the role of implicit knowledge of the dynamic characteristics of the body and that this provides support for the principles of biofeedback training. From a more ecological 'action' perspective, however, it is concluded that previous approaches to biofeedback have been too constrained and do not translate well to functional tasks. A variation of biofeedback training is proposed that provides real-time feedback to shape the subject's action, thereby guiding them through learning contexts that are moulded to fit the individual's capabilities. A pilot study of the efficacy of such an approach in adolescents with cerebral palsy is then presented. The potential of extending computer-enhanced therapy into speculative areas such as virtual reality is also discussed.

Adolescent↗