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Biomedical subjects

Julie M Harris

Publications and source records attributed to Julie M Harris.

13 recordsLinked to original sources
PubMed2006

The interaction of eye movements and retinal signals during the perception of 3-D motion direction.

When an object is tracked with the eyes, veridical perception of the motion of that object and other objects requires the brain to take account of and compensate for the eye movement. Here, I explore the effects of version and vergence eye movements on three-dimensional (3-D) motion perception. After demonstrating that eye movement compensation can be poor for detecting small objects moving in depth, I develop two models for how eye movement and visual signals may interact during the perception of 3-D motion direction. The first model assumes that the visual system is aiming to form an explicit representation of 3-D motion. From the results of a second experiment, on 3-D motion direction judgements, I show that this model could only hold with almost perfect 3-D motion compensation, contradicting the results from the first experiment. A second model assumes a much simpler strategy for estimating 3-D motion direction, based on recent experimental work. It predicts that compensation for vergence is not required because the Z-component of 3-D motion is not needed for direction judgements, consistent with the experimental results. This suggests that, for 3-D motion direction discrimination and angle judgements, extraretinal signals from vergence are not used.

Adaptation, Physiological↗
PubMed2006

Accuracy of estimates of food portion size using food photographs--the importance of using age-appropriate tools.

BACKGROUND: In order to obtain a measure of nutrient intake, a measure or estimate of the amount of food consumed is required. Weighing foods imposes a large burden on subjects, often resulting in underreporting. Tools are available to assist subjects in providing an estimate of portion size and these include food photographs. The application of these tools in improving portion size estimation by children has not been investigated systematically. OBJECTIVES: To assess the accuracy with which children are able to estimate food portion sizes using food photographs designed for use with adults, and to determine whether the accuracy of estimates is improved when age-appropriate portion size photographs are provided. DESIGN: Original data from three separate studies, on the accuracy of portion size estimates by adults using food photographs, by children using adult photographs and by children using age-appropriate photographs, are analysed and compared. SUBJECTS: One hundred and thirty-five adults aged 18 to 90 years and 210 children aged 4 to 11 years. RESULTS: Children's estimates of portion sizes using age-appropriate food photographs were significantly more accurate (an underestimate of 1% on average) than estimates using photographs designed for use with adults (an overestimate of 45% on average). Accuracy of children's estimates of portion size using age-appropriate photographs was not significantly different from that of adults. Children overestimated a food's weight by 18% on average and adults underestimated by 5%. CONCLUSIONS: Providing children with food photographs depicting age-appropriate portion sizes greatly increases the accuracy of portion size estimates compared with estimates using photographs designed for use with adults.

Adolescent↗
PubMed2005

Sporting contests: seeing red? Putting sportswear in context.

The shirt colour worn by sportsmen can affect the behaviour of the competitors, but Hill and Barton show that it may also influence the outcome of contests. By analysing the results of men's combat sports from the Athens 2004 Olympics, they found that more matches were won by fighters wearing red outfits than by those wearing blue; they suggest that red might confer success because it is a sign of dominance in many animal species and could signal aggression in human contests. Here we use another data set from the 2004 Olympics to show that similar winning biases occur in contests in which neither contestant wears red, indicating that a different mechanism may be responsible for these effects.

Bias↗
PubMed2005

Using visual direction in three-dimensional motion perception.

The eyes receive slightly different views of the world, and the differences between their images (binocular disparity) are used to see depth. Several authors have suggested how the brain could exploit this information for three-dimensional (3D) motion perception, but here we consider a simpler strategy. Visual direction is the angle between the direction of an object and the direction that an observer faces. Here we describe human behavioral experiments in which observers use visual direction, rather than binocular information, to estimate an object's 3D motion even though this causes them to make systematic errors. This suggests that recent models of binocular 3D motion perception may not reflect the strategies that human observers actually use.

Depth Perception↗
PubMed2004

Binocular vision: moving closer to reality.

Vision science is a truly interdisciplinary field of research, lying at the interface between psychology, computer science and neurobiology. It spans a broad range of research questions, from what visual neurons in the brain do, to the design and use of virtual reality technology. Here, I will take one well-researched area of human vision binocular depth perception, as an exemplar of the various ways in which the processes of human perception can be studied: from basic questions about the sensitivity of binocular vision to understanding how having two eyes helps us function in the natural environment.

Animals↗
PubMed2004

Human observers are biased in judging the angular approach of a projectile.

How do we decide whether an object approaching us will hit us? The optic array provides information sufficient for us to determine the approaching trajectory of a projectile. However, when using binocular information, observers report that trajectories near the mid-sagittal plane are wider than they actually are. Here we extend this work to consider stimuli containing additional depth cues. We measure observers' estimates of trajectory direction first for computer rendered, stereoscopically presented, rich-cue objects, and then for real objects moving in the world. We find that, under both rich cue conditions and with real moving objects, observers show positive bias, overestimating the angle of approach when movement is near the mid-sagittal plane. The findings question whether the visual system, using both binocular and monocular cues to depth, can make explicit estimates of the 3-D location and movement of objects in depth.

Adolescent↗
PubMed2003

Task demands and binocular eye movements.

Humans make rapid movements of their eyes several times a second that enable them to examine objects located at different positions in space with both of their eyes. Much of our understanding of these binocular movements comes from studies using experienced observers performing repetitive, unnatural tasks. But what eye movements are made when naïve observers perform tasks demanding specific binocular visual information? We examined the binocular eye movements produced by observers performing two tasks differing in the visual information needed for their completion. Our motivation for doing this was to examine the role and function of binocular eye movements when making decisions. We considered the fixation strategies adopted by observers, the effects of the task on the dynamics of saccadic eye movements, and the combination of vergence and version in gaze shifts. We report that the task-dependent use of visual information can have a strong influence on the patterns of fixations, whilst not influencing saccade dynamics. Our data provide some support for the notion that observers choose and fixate a notional reference point in the scene when making judgments about depth structure.

Adult↗
PubMed2003

Is neural filling-in necessary to explain the perceptual completion of motion and depth information?

Retinal activity is the first stage of visual perception. Retinal sampling is non-uniform and not continuous, yet visual experience is not characterized by holes and discontinuities in the world. How does the brain achieve this perceptual completion? Fifty years ago, it was suggested that visual perception involves a two-stage process of (i) edge detection followed by (ii) neural filling-in of surface properties. We examine whether this general hypothesis can account for the specific example of perceptual completion of a small target surrounded by dynamic dots (an 'artificial scotoma'), a phenomenon argued to provide insight into the mechanisms responsible for perception. We degrade the target's borders using first blur and then depth continuity, and find that border degradation does not influence time to target disappearance. This indicates that important information for the continuity of target perception is conveyed at a coarse spatial scale. We suggest that target disappearance could result from adaptation that is not specific to borders, and question the need to hypothesize an active filling-in process to explain this phenomenon.

Depth Perception↗
PubMed2003

Poor visibility of motion in depth is due to early motion averaging.

Under a variety of conditions, motion in depth from binocular cues is harder to detect than lateral motion in the frontoparallel plane. This is surprising, as the nasal-temporal motion in the left eye associated with motion in depth is easily detectable, as is the nasal-temporal motion in the right eye. It is only when the two motions are combined in binocular viewing that detection can become difficult. We previously suggested that the visibility of motion-in-depth is low because early stereomotion detectors average left and right retinal motions. For motion in depth, a neural averaging process would produce a motion signal close to zero. Here we tested the averaging hypothesis further. Specifically we asked, could the reduced visibility observed in previous experiments be associated with depth and layout in the stimuli, rather than motion averaging? We used anti-correlated random dot stereograms to show that, despite no depth being perceived, it is still harder to detect motion when it is presented in opposite directions in the two eyes than when motion is presented in the same direction in the two eyes. This suggests that the motion in depth signal is lost due to early motion averaging, rather than due to the presence of noise from the perceived depth patterns in the stimulus.

Depth Perception↗
PubMed2003

Accuracy and precision of binocular 3-D motion perception.

In principle, information for 3-D motion perception is provided by the differences in position and motion between left- and right-eye images of the world. It is known that observers can precisely judge between different 3-D motion trajectories, but the accuracy of binocular 3-D motion perception has not been studied. The authors measured the accuracy of 3-D motion perception. In 4 different tasks, observers were inaccurate, overestimating trajectory angle, despite consistently choosing similar angles (high precision). Errors did not vary consistently with target distance, as would be expected had inaccuracy been due to misestimates of viewing distance. Observers appeared to rely strongly on the lateral position of the target, almost to the exclusion of the use of depth information. For the present tasks, these data suggest that neither an accurate estimate of 3-D motion direction nor one of passing distance can be obtained using only binocular cues to motion in depth. ((c) 2003 APA, all rights reserved)

Discrimination, Psychological↗
PubMed2002

Minimum displacement thresholds for binocular three-dimensional motion.

Thresholds for the detection of motion in depth in the median plane of the head are substantially poorer than those for motion in the frontoparallel plane. This suggests the existence of two independent mechanisms for their detection. Any three-dimensional (3-D) motion can be decomposed into components of motion in the frontoparallel plane and in the median plane of the head. Can human performance for the detection and discrimination of other 3-D motions be predicted by a combination of responses from the two independent mechanisms? Minimum displacement thresholds (d(min)) for the detection of 3-D motion and the direction discrimination of 3-D motion were measured for a wide range of 3-D directions. d(min) data were modelled in terms of the probability summation of a pair of independent motion mechanisms, one responding to motion in the median plane of the head, the second to motion in the frontoparallel plane. Detection of 3-D motion was well predicted by probability summation across a range of 3-D directions. Direction discrimination of 3-D motion was similarly well fit by the probability summation model for multiframe motion displays for some observers. However for two-frame motion displays, direction discrimination for 3-D motion was best fit by a model using only a motion mechanism in the frontoparallel plane. Detection and direction discrimination thresholds for 3-D motion can therefore be explained on the basis of one or two mechanisms, sensitive to motion in the frontoparallel plane and in the median plane of the head.

Depth Perception↗
PubMed2002

Optic flow and scene structure do not always contribute to the control of human walking.

Using displacing prisms to dissociate the influence of optic flow and egocentric direction, previous research (Current Biology 8 (1998) 1191) showed that people primarily use egocentric direction to control their locomotion on foot, rather than optic flow. When wearing displacing prisms, participants followed the curved path predicted by the use of simple egocentric direction, rather than a straight path, as predicted by the use of optic flow. It has previously been suggested that, in rich visual environments, other visual information including optic flow and static scene structure may influence locomotion in addition to direction. Here we report a study where neither scene structure nor optic flow have any influence on the control of walking. Participants wearing displacing prisms walked along a well-lit corridor (containing rich scene structure and flow) and along the same corridor in darkness (no scene structure or flow). Heading errors were not significantly different between the dark and light conditions. Thus, even under conditions of rich scene structure and high flow when walking in a well-lit corridor, participants follow the same curved paths as when these cues are not available. These results demonstrate that there are conditions under which visual direction is the only useful source of visual information for the control of locomotion.

Cues↗