Biomedical subjects

Mark F Bradshaw

Publications and source records attributed to Mark F Bradshaw.

At least 19 recordsLinked to original sources

PubMed2006

The pop out of scene-relative object movement against retinal motion due to self-movement.

An object that moves is spotted almost effortlessly; it "pops out". When the observer is stationary, a moving object is uniquely identified by retinal motion. This is not so when the observer is also moving; as the eye travels through space all scene objects change position relative to the eye producing a complicated field of retinal motion. Without the unique identifier of retinal motion an object moving relative to the scene should be difficult to locate. Using a search task, we investigated this proposition. Computer-rendered objects were moved and transformed in a manner consistent with movement of the observer. Despite the complex pattern of retinal motion, objects moving relative to the scene were found to pop out. We suggest the brain uses its sensitivity to optic flow to "stabilise" the scene, allowing the scene-relative movement of an object to be identified.

Attention↗

PubMed2006

Surface orientation, modulation frequency and the detection and perception of depth defined by binocular disparity and motion parallax.

Binocular disparity and motion parallax provide information about the spatial structure and layout of the world. Descriptive similarities between the two cues have often been noted which have been taken as evidence of a close relationship between them. Here, we report two experiments which investigate the effect of surface orientation and modulation frequency on (i) a threshold detection task and (ii) a supra-threshold depth-matching task using sinusoidally corrugated surfaces defined by binocular disparity or motion parallax. For low frequency corrugations, an orientation anisotropy was observed in both domains, with sensitivity decreasing as surface orientation was varied from horizontal to vertical. In the depth-matching task, for surfaces defined by binocular disparity the greatest depth was seen for oblique orientations. For surfaces defined by motion parallax, perceived depth was found to increase as surface orientation was varied from horizontal to vertical. In neither case was perceived depth for supra-threshold surfaces related to threshold performance in any simple manner. These results reveal clear differences between the perception of depth from binocular disparity or motion parallax, and between perception at threshold and supra-threshold levels of performance.

Cues↗

PubMed2006

2-D tilt and 3-D slant illusions in perception and action tasks.

There is now a well established dissociation between perception and action based primarily on neuropsychological evidence [Milner and Goodale, 1995 The Visual Brain in Action (Oxford: Oxford University Press)]. Although equivocal, an important source of evidence from normal observers is that 'perceptual illusions' may affect the systems differently. We investigated the relative effects of 2-D tilt and 3-D slant illusions in the two domains, using similar tasks to those employed originally by Milner and Goodale. Subjects were required to either post a card through, or set a paddle to match the orientation of, a plane that was presented in two conditions: surrounded by a striped surface tilted between +90 degrees and -90 degrees (2-D tilt contrast), or surrounded by a disparity defined surface slanted in depth between +60 degrees and -60 degrees (3-D depth contrast). For 2-D tilt, action and perception were equally affected by the illusion, whereas in the 3-D condition they were not. Here, the illusion appeared greater in the posting than in the perceptual task. We conclude that, although no qualitative differences exist, there were quantitative differences between perception and action tasks in the binocular condition.

Cues↗

PubMed2006

Stereoscopic acuity and observation distance.

The amount of depth perceived from a fixed pattern of horizontal disparities varies with viewing distance. We investigated whether thresholds for discriminating stereoscopic corrugations at a range of spatial frequencies were also affected by viewing distance or whether they were determined solely by the angular disparity in the stimulus prior to scaling. Although thresholds were found to be determined primarily by disparity over a broad range of viewing distances, they were on average a factor of two higher at the shortest viewing distance (28.5 cm) than at larger viewing distances (57 to 450 cm). We found the same pattern of results when subjects' accommodation was arranged to be the same at all viewing distances. The change in thresholds at close distances is in the direction expected if subjects' performance is limited by a minimum perceived depth.

Accommodation, Ocular↗

PubMed2005

The perception of emotion from body movement in point-light displays of interpersonal dialogue.

We examined whether it is possible to identify the emotional content of behaviour from point-light displays where pairs of actors are engaged in interpersonal communication. These actors displayed a series of emotions, which included sadness, anger, joy, disgust, fear, and romantic love. In experiment 1, subjects viewed brief clips of these point-light displays presented the right way up and upside down. In experiment 2, the importance of the interaction between the two figures in the recognition of emotion was examined. Subjects were shown upright versions of (i) the original pairs (dyads), (ii) a single actor (monad), and (iii) a dyad comprising a single actor and his/her mirror image (reflected dyad). In each experiment, the subjects rated the emotional content of the displays by moving a slider along a horizontal scale. All of the emotions received a rating for every clip. In experiment 1, when the displays were upright, the correct emotions were identified in each case except disgust; but, when the displays were inverted, performance was significantly diminished for some emotions. In experiment 2, the recognition of love and joy was impaired by the absence of the acting partner, and the recognition of sadness, joy, and fear was impaired in the non-veridical (mirror image) displays. These findings both support and extend previous research by showing that biological motion is sufficient for the perception of emotion, although inversion affects performance. Moreover, emotion perception from biological motion can be affected by the veridical or non-veridical social context within the displays.

Adult↗

PubMed2004

The effects of a pre-movement delay on the kinematics of prehension in middle childhood.

The present study examined the effects of a pre-movement delay on the kinematics of prehension in middle childhood. Twenty-five children between the ages of 5 and 11 years made visually open-loop reaches to two different sized objects at two different distances along the midline. Reaches took place either (i) immediately, or (ii) 2 s after the occlusion of the stimulus. In all age groups, reaches following the pre-movement delay were characterised by longer movement durations, lower peak velocities, larger peak grip apertures and longer time spent in the final slow phase of the movement. This pattern of results suggests that the representations that control the transport and grasp component are affected similarly by delay, and is consistent with the results previously reported for adults. Such representations therefore appear to develop before the age of 5.

Biomechanical Phenomena↗

PubMed2004

Binocular cues and the control of prehension.

The present study was designed to assess the importance of binocular information (i.e. binocular disparity and angle of convergence) in the control of prehension. Previous studies which have addressed this question have typically used the same experimental manipulation: comparing prehensile movements executed either under binocular conditions to those executed when one eye was occluded (monocular). However this may not be the correct comparison as in addition to depriving the subject of binocular depth cues. it also deprives the subject of any visual information in one eye. Therefore we determined the prehensile performance when the subject viewed the target object and scene with either (i) two different views (binocular), (ii) two identical views (bi-ocular), or (iii) one view only (monocular). Overall, the qualitative and quantitative performance in the bi-ocular and monocular control conditions was very similar on all the main measures (and different from the performance in the binocular condition). We conclude that the deficits in performance observed found for 'monocular' reaches should be attributed to the lack of local depth information specified by the binocular cues. In addition we speculate that convergence angle and binocular disparity, although involved in both the pre-movement and movement-execution phases of the reach, the cues may be weighted differently in both phases of a prehension movement depending on the behavioural strategy involved.

Adult↗

PubMed2003

Binocular vision and prehension in middle childhood.

Binocular cues have been shown previously to make an important contribution to the control of natural prehensile movements in adults [Visual Cognition 4 (1997) 113, Vision Research 32 (1992) 1513, Neuropsychologia 38 (2000) 1473]. The present study examined the role of binocular vision in the control of prehension in middle childhood. Fourteen children aged 5-6 years, and 16 children aged 10-11 years reached out and grasped different sized objects at different distances, in either binocular or monocular viewing conditions. In contrast to adult data, many of the principal kinematic indices of the children's reaches were unaffected by the removal of binocular information. The older children, like adults, spent an increased amount of time in the final approach to the object when only monocular information was available. However, both peak wrist velocities and peak grip apertures were unaffected by the removal of binocular information and continued to scale with object properties in the normal way. These results suggest that the use of binocular cues to control prehensile movements is not yet mature at the age of 10-11 years.

Child↗

PubMed2003

Geometric and induced effects in binocular stereopsis and motion parallax.

This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.

Depth Perception↗

PubMed2003

The visual control of reaching and grasping: binocular disparity and motion parallax.

The primary visual sources of depth and size information are binocular cues and motion parallax. Here, the authors determine the efficacy of these cues to control prehension by presenting them in isolation from other visual cues. When only binocular cues were available, reaches showed normal scaling of the transport and grasp components with object distance and size. However, when only motion parallax was available, only the transport component scaled reliably. No additional increase in scaling was found when both cues were available simultaneously. Therefore, although equivalent information is available from binocular and motion parallax information, the latter may be of relatively limited use for the control of the grasp. Binocular disparity appears selectively important for the control of the grasp.

Adaptation, Physiological↗

PubMed2003

Endogenous shifts of covert attention operate within multiple coordinate frames: evidence from a feature-priming task.

The locations of visual objects and events in the world are represented in a number of different coordinate frameworks. For example, a visual transient is known to attract (exogenous) attention and facilitate performance within an egocentric framework. However, when attention is allocated voluntarily to a particular visual feature (ie endogenous attention), the location of that feature appears to be variously encoded either within an allocentric framework or in a spatially invariant manner. In three experiments we investigated the importance of location for the allocation of endogenous attention and whether egocentric and/or allocentric spatial frameworks are involved. Primes and targets were presented in four conditions designed to vary systematically their spatial relationships in egocentric and allocentric coordinates. A reliable effect of egocentric priming was found in all three experiments, which suggests that endogenous shifts of attention towards targets defined by a particular feature operate in an egocentric representation of visual space. In addition, allocentric priming was also found for targets primed by their colour or shape. This suggests that attending to targets primed by nonspatial attributes results in facilitation that is localised in more than one coordinate frame of spatial reference.

Analysis of Variance↗

PubMed2003

Attention affects the stereoscopic depth aftereffect.

'Preattentive' vision is typically considered to include several low-level processes, including the perception of depth from binocular disparity and motion parallax. However, doubt was cast on this model when it was shown that a secondary attentional task can modulate the motion aftereffect (Chaudhuri, 1990 Nature 344 60-62). Here we investigate whether attention can also affect the depth aftereffect (Blakemore and Julesz, 1971 Science 171 286-288). Subjects adapted to stationary or moving random-dot patterns segmented into depth planes while attention was manipulated with a secondary task (character processing at parametrically varied rates). We found that the duration of the depth aftereffect can be affected by attentional manipulations, and both its duration and that of the motion aftereffect varied with the difficulty of the secondary task. The results are discussed in the context of dynamic feedback models of vision, and support the penetrability of low-level sensory processes by attentional mechanisms.

Adaptation, Ocular↗

PubMed2003

The role of binocular information in the 'on-line' control of prehension.

Binocular visual information may be involved in the selection of appropriate motor programs before a reach is executed or it may be involved during the movement-execution phase in order to monitor and guide the hand to the target object. Here we introduced binocular information after 0%, 25%, 50% or 75% of the movement-execution phase and determined its effects on the kinematic indices of prehensile movements made to objects of different sizes placed at different distances. Kinematic indices linked to the transport component, such as peak velocity and time-to-peak velocity, were unaffected by the presence of binocular cues whereas later occurring indices, such as peak grip aperture and time in the slow phase, were significantly affected. Although the magnitude of the peak grip was affected by the presence of binocular cues, the time at which it occurred did not change. This pattern of results suggest that the visuo-motor control of prehensile movements utilises both feedforward and feedback strategies and that binocular cues are particularly important for the fine manual adjustments typical of the latter.

Adult↗

PubMed2002

Reaching for virtual objects: binocular disparity and the control of prehension.

Although, in principle, binocular cues provide veridical information about the three-dimensional shape of objects, our perception on the basis of these cues is distorted systematically. The consequences of these distortions may be less serious than they first appear, however, since in everyday life we rarely are required to judge the absolute shape, size or distance of objects. An important exception to this is in the control of prehension, where veridical information about an object to be grasped is required to plan the transport of the hand and to select the most appropriate grip. Here we investigate whether binocular cues provide accurate depth information for the control of prehension using disparity-defined, virtual objects and report that whilst binocular disparity can support prehensile movements, the kinematic indices, which reflect distance-reached and perceived size, show clear biases. These results suggest that accurate metric depth information for the control of prehension is not available from binocular cues in isolation.

Analysis of Variance↗

PubMed2002

A dissociation of perception and action in normal human observers: the effect of temporal-delay.

Neuropsychological results support the proposal that the human visual system is organised into distinct processing pathways, one for conscious perception and one for the control of action. Here, we compare perceptual and action responses following a pre-response-delay. Experiment 1 required participants to reproduce remembered locations and found that although perceptual matches were unaffected by delays of up to 4 s, pointing responses were significantly biased after only 2 s. Experiment 2 examined whether both the transport and grasp components of a natural prehensile movement were similarly affected by delay. Both peak wrist velocities and peak grip-apertures were affected equivalently by delay, suggesting that the two components of a prehensile movement have similar temporal constraints. The results from both experiments are consistent with the general perception-action dichotomy as originally proposed by Milner and Goodale [The visual brain in action, Oxford: Oxford University Press, 1995].

Adult↗

PubMed2002

The stereoscopic anisotropy: individual differences and underlying mechanisms.

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms.

Anisotropy↗

PubMed2002

The stereoscopic anisotropy affects manual pointing.

Although binocular disparity can in principle provide absolute depth information, perceived stereoscopic depth depends on the relative disparities between points and their spatial arrangement. An example of this is the stereoscopic anisotropy--observers typically perceive less depth for stereoscopic surfaces when depth varies in the horizontal direction than in the vertical direction. We investigated whether this anisotropy also affects manual pointing. Participants were presented with stereograms depicting surfaces that were slanted in depth about either a horizontal axis (inclination) or a vertical axis (slant), and were asked either to point to the edge of a surface, or to estimate its inclination or slant. For both tasks, a clear anisotropy was observed, with participants perceiving greater depth, and also pointing out steeper surfaces, for inclined surfaces than for slanted surfaces. We conclude that both perception and the control of action are subject to a similar stereoscopic anisotropy, and that performance on the two tasks relies on similar depth processing mechanisms.

Anisotropy↗

PubMed2002

Binocular information in the control of prehensile movements in multiple-object scenes.

Recent evidence suggests that the visual control of prehension may be less dependent on binocular information than has previously been thought. Studies investigating this question, however, have generally only examined reaches to single objects presented in isolation, even though natural prehensile movements are typically directed at objects in cluttered scenes which contain many objects. The present study was designed, therefore, to assess the contribution of binocular information to the control of prehensile movements in multiple-object scenes. Subjects reached for and grasped objects presented either in isolation or in the presence of one, two or four additional 'flanking' objects, under binocular and monocular viewing conditions. So that the role of binocular information could be clearly determined, subjects made reaches both in the absence of a visible scene around the target objects (self-illuminated objects presented in the dark) and under normal ambient lighting conditions. Analysis of kinematic parameters indicated that the removal of binocular information did not significantly affect many of the major indices of the transport component, including peak wrist velocity. However, peak grip apertures increased and subjects spent more time in the final slow phase of movement, prior to grasping the object, during monocularly guided reaches. The dissociation between effects of binocular versus monocular viewing on transport and grasp parameters was observed irrespective of the presence of flanking objects. These results therefore further question the view that binocular vision is pre-eminent in the control of natural prehensile movements.

Adult↗