Biomedical subjects

David A Westwood

Publications and source records attributed to David A Westwood.

16 recordsLinked to original sources

PubMed2006

Differential effects of advance semantic cues on grasping, naming, and manual estimation.

In the three experiments reported here, we systematically investigated when and/or how prior semantic information about a target-object would affect the programming and execution of grasping movements. A name-length association was first created during a practice phase while participants performed one of the three tasks: grasping, naming, or manual estimation of an object's length upon an auditory presentation of its name. In the subsequent test phase, in the majority of trials, these names correctly indicated the upcoming object for a grasping task or a control naming task (Experiment 1A and 1B), and a grasping task or a manual estimation of object length (Experiment 2). In both experiments, a name was occasionally incongruent or a neutral cue (a burst of white noise) was presented instead. Although in the grasping task, semantic cues (both congruent and incongruent) consistently facilitated movement-onset time (indicating a preparatory-set like effect), the impact of these cues on the formation of grasping aperture was less consistent or absent. The experiments also demonstrated that naming and perceptual estimation of object length were affected differently from grasping by the same semantic cues, with the congruent cues facilitating and the incongruent cues slowing down response onset. These findings reinforce and extend the proposal that the transformations of visual and semantic information for perceptually-driven tasks are quite different from the transformations of the same information for the programming and control of object-directed action.

Adolescent↗

PubMed2006

Opposite perceptual and sensorimotor responses to a size-weight illusion.

The perceptual size-weight illusion (SWI) occurs when two different-sized objects with equal mass are lifted in sequence: the smaller object is consistently reported to feel heavier than the larger object even after repeated lifting attempts. Here we explored the relationship between sensorimotor and perceptual responses to a SWI in which the smaller of the two target objects in fact weighed slightly less (2.7 N) than the larger object (3.2 N). For 20 consecutive lifts, participants consistently reported that the small-light object felt heavier than the large-heavy object; however, concurrently measured lifting dynamics showed exactly the opposite pattern: peak grip force, peak grip force rate, peak load force, and peak load force rate were all significantly greater for the large-heavy object versus the small-light object. The difference in peak load rate between the two objects was greatest for the initial lift but decreased significantly beyond that point, suggesting that the sensorimotor system used sensory feedback to correct for initial over- and underestimations of object mass. Despite these adjustments to lifting dynamics over the early trials, the difference between the judged heaviness of the two objects did not change. The findings clearly demonstrate that the sensorimotor and perceptual systems utilize distinctly different mechanisms for determining object mass.

Female↗

PubMed2005

The visual control of stepping operates in real time: Evidence from a pictorial illusion.

Recent studies have suggested important similarities between the visual control of upper and lower limb actions despite the different effectors, time scales, and regions of space associated with the two types of actions. Drawing on previous findings from upper limb studies, we show here that the effect of a Muller-Lyer illusion on stepping amplitude is greatly increased when the target display is occluded as compared to when it is visible during the reaction time period (i.e., brief delay and 3-s delay conditions versus full vision and open-loop conditions). The present results suggest that the control of stepping is mediated by different systems when the target is visible as compared to when it is occluded immediately before movement initiation; when target vision is not available during the reaction time period, the control of stepping is more highly dependent upon a non-veridical, perceptual representation of the target environment. These findings are consistent with the idea that, much like grasping, vision during the reaction time period is critical for the optimal control of stepping; that is, the visual control of stepping operates in real time.

Adult↗

PubMed2005

No evidence for visuomotor priming in a visually guided action task.

Craighero et al. showed that grasping movements were initiated more quickly when the goal object shared the same orientation as a previously seen 'prime' object. Because the goal object was never visible in these experiments, however, it is unclear whether the data should be construed as evidence for a general visuomotor priming effect (as the authors contend), or only as evidence for a more specific priming effect on memory-guided actions. In Experiment 1, we demonstrated that memory-guided but not visually guided grasping can be primed by passive viewing of a prime object. In Experiment 2, we compared the effects of a prime object on the grasping and naming of a visible target object. Participants were faster to name the target when its shape was the same as the prime, consistent with well-established perceptual priming effects. Under the identical set of testing parameters, however, reaction time for grasping was unaffected by the orientation or the shape of the prime. In Experiment 3, participants grasped the goal object after either viewing or grasping a prime object. Reaction time for grasping was unaffected by the visual features of the prime in both tasks. Taken together, these results are consistent with the view that perceptual memory -- which presumably underlies visual priming effects -- is largely irrelevant for programming the metrics of actions to visible objects. Visually guided actions are programmed in real-time by dedicated visuomotor modules that appear to be insensitive to the priming effects that are a hallmark of visual perception.

Adolescent↗

PubMed2005

Action control: independent effects of memory and monocular viewing on reaching accuracy.

Evidence suggests that perceptual networks in the ventral visual pathway are necessary for action control when targets are viewed with only one eye, or when the target must be stored in memory. We tested whether memory-linked (i.e., open-loop versus memory-guided actions) and monocular-linked effects (i.e., binocular versus monocular actions) on action arise from a common mechanism as suggested by evidence from neuropsychology and psychophysics. Participants viewed targets with either one eye or two (vision: monocular versus binocular) and then reached to touch targets in open-loop and memory-guided conditions (condition: open-loop versus 0, 500, 1000, and 1500 ms delays). Results showed that memory-linked and monocular-linked increases in radial and variable movement error were additive (i.e., main effects of vision and condition, but no interaction). This suggests that the two effects on visuomotor control arise from separate mechanisms, in contrast to evidence from psychophysics and neuropsychology suggesting a common underlying mechanism.

Adult↗

PubMed2005

Dual routes to action: contributions of the dorsal and ventral streams to adaptive behavior.

More than a decade ago, Goodale and Milner proposed that our perceptual experience of the world depends on visual processing that is fundamentally distinct from that mediating the moment-to-moment visual control of our actions. They mapped this distinction between vision-for-perception and vision-for-action onto the two prominent visual pathways that arise from early visual areas in the primate cerebral cortex: a ventral "perception" pathway projecting to inferotemporal cortex and a dorsal "action" pathway projecting to posterior parietal cortex. In the years since these ideas were first put forward, visual neuroscience has advanced rapidly on several fronts. In this chapter, we examine the perception-action distinction in the light of some of these developments, giving particular emphasis to the differences in the way the two streams process visual information and the way they interact in the production of adaptive behavior.

Adaptation, Physiological↗

PubMed2004

An evolving view of duplex vision: separate but interacting cortical pathways for perception and action.

In 1992, Goodale and Milner proposed a division of labour in the visual pathways of the primate cerebral cortex between a dorsal stream specialised for the visual control of action and a ventral stream dedicated to the perception of the visual world. In the years since this original proposal, support for the perception-action hypothesis has come from neuroimaging experiments, human neuropsychology, monkey neurophysiology, and human psychophysical experiments. Indeed, some of the strongest support for this hypothesis has come from behavioural experiments showing that visually guided actions are largely refractory to perceptual illusions. Although controversial, the findings from this literature both support the original hypothesis and suggest important modifications. The ongoing challenge for neurobiologists is to map these behavioural findings onto their corresponding neural substrates.

Animals↗

PubMed2004

Two distinct modes of control for object-directed action.

There are multiple routes from vision to action that play a role in the production of visually guided reaching and grasping. What remain to be resolved, however, are the conditions under which these various routes are recruited in the generation of actions and the nature of the information they convey. We argue in this chapter that the production of real-time actions to visible targets depends on pathways that are separate from those mediating memory-driven actions. Furthermore, the transition from real-time to memory-driven control occurs as soon as the intended target is no longer visible. Real-time movements depend on pathways from the early visual areas through to relatively encapsulated visuomotor mechanisms in the dorsal stream. These dedicated visuomotor mechanisms, together with motor centers in the premotor cortex and brainstem, compute the absolute metrics of the target object and its position in the egocentric coordinates of the effector used to perform the action. Such real-time programming is essential for the production of accurate and efficient movements in a world where the location and disposition of a goal object with respect to the observer can change quickly and often unpredictably. In contrast, we argue that memory-driven actions make use of a perceptual representation of the target object generated by the ventral stream. Unlike the real-time visuomotor mechanisms, perception-based movement planning makes use of relational metrics and scene-based coordinates. Such computations make it possible, however, to plan and execute actions upon objects long after they have vanished from view.

Humans↗

PubMed2004

The control of memory-guided reaching movements in peripersonal space.

The goal of the present investigation was to explore the putative contributions of feedforward- and feedback-based processes in the control of memory-guided reaching movements. Participants (N = 4) completed an extensive number of reaching movements (2700) to 3 midline targets (20, 30, 40 cm) in 6 visual conditions: full-vision, open-loop, and four memory-guided conditions (0, 200, 400, and 600 ms of delay). To infer limb control, we used a regression technique to examine the within-trial correspondence between the spatial position of the limb at peak acceleration, peak velocity, peak deceleration, and the ultimate movement endpoint. A high degree of within-trial correspondence would suggest that the final position of the limb was largely specified prior to movement onset and not adjusted during the action (i.e., feedforward control); conversely, a low degree of within-trial correspondence would suggest that movements were modified during the reaching trajectory (i.e., feedback control). Full-vision reaches were found to be more accurate and less variable than open-loop and memory-guided reaches. Moreover, full-vision reaches demonstrated only modest within-trial correspondence between the spatial position of the limb at each kinematic marker and the ultimate movement endpoint, suggesting that reaching accuracy was achieved by adjusting the limb trajectory throughout the course of the action. Open-loop and memory-guided movements exhibited strong within-trial correspondence between final limb position and the position of the limb at peak velocity and peak deceleration. This strong correspondence indicates that the final position of the limb was largely determined by processes that occurred before the reach was initiated; errors in the planning process were not corrected during the course of the action. Thus, and contrary to our previous findings in a video-based aiming task, it appears that stored target information is not extensively (if at all) used to modify the trajectory of reaching movements to remembered targets in peripersonal space.

Adult↗

PubMed2003

A haptic size-contrast illusion affects size perception but not grasping.

Object features (e.g. size, shape and orientation) are relevant for recognition and identification, but also for the control of manual actions. Converging evidence suggests a dissociation between the visual systems that mediate object perception and object-directed action. Here we present evidence suggesting that a similar dissociation might exist in the haptic domain. We demonstrate that a haptic variation of a size-contrast illusion influences the perceived size of a target object, but not the degree to which the hand is opened when that object is the target of a grasping movement. This finding is consistent with the view that object perception is "scene-based" and takes into consideration not only the size of the target object but also the sizes of other nearby objects. In contrast, the control of object-directed action is primarily driven by the absolute size of the target object independent of the relative sizes of other objects in the environment, suggesting a "actor-based" frame of reference. The present findings suggest that dissociations between action and perception are not unique to the visual system, but might instead reflect a general organizational principle of sensory processing.

Adult↗

PubMed2003

The influence of visual motion on fast reaching movements to a stationary object.

One of the most important functions of vision is to direct actions to objects. However, every time that vision is used to guide an action, retinal motion signals are produced by the movement of the eye and head as the person looks at the object or by the motion of other objects in the scene. To reach for the object accurately, the visuomotor system must separate information about the position of the stationary target from background retinal motion signals-a long-standing problem that is poorly understood. Here we show that the visuomotor system does not distinguish between these two information sources: when observers made fast reaching movements to a briefly presented stationary target, their hand shifted in a direction consistent with the motion of a distant and unrelated stimulus, a result contrary to most other findings. This can be seen early in the hand's trajectory (approximately 120 ms) and occurs continuously from programming of the movement through to its execution. The visuomotor system might make use of the motion signals arising from eye and head movements to update the positions of targets rapidly and redirect the hand to compensate for body movements.

Eye Movements↗

PubMed2003

No evidence for accurate visuomotor memory: systematic and variable error in memory-guided reaching.

The authors explored whether the motor system has access to highly accurate information about the aiming environment after visual occlusion. Participants (N = 14) reached to 1 of 3 midsagittal targets in 4 visual conditions (open-loop, brief-delay, 500-ms delay, and 2,000-ms delay). In all conditions, the aiming environment was first viewed for 2,000 ms. Movements were cued immediately after the initial viewing period in the open-loop and brief-delay conditions. Vision was not occluded until movement onset in the open-loop condition, whereas vision was occluded coincidentally with the movement cue in the brief-delay condition. In the 2 longer delay conditions, the movement was cued following a 500- or a 2,000-ms no-vision delay period. Participants overshot the target in the open-loop condition, but that tendency was significantly reduced in the 3 delay conditions. Moreover, endpoint variability was greater in the 3 delay conditions than in the open-loop condition. A speed-accuracy tradeoff account could not explain the differences between open-loop and delayed reaching. Those findings suggest that the motor system does not have access to highly accurate information about the aiming environment for any appreciable period of time following visual occlusion, consistent with the view that the visuomotor system operates in real time.

Acoustic Stimulation↗

PubMed2003

Perceptual illusion and the real-time control of action.

Participants were cued by an auditory tone to grasp a target object from within a size-contrast display. The peak grip aperture was unaffected by the perceptual size illusion when the target array was visible between the response cue and movement onset (vision trials). The grasp was sensitive to the illusion, however, when the target array was occluded from view when the response was cued (occlusion trials). This was true when the occlusion occurred 2.5 s before the response cue (delay), but also when the occlusion coincided with the response cue (no-delay). Unlike previous experiments, vision and occlusion trials were presented in random sequence. The results suggest that dedicated, real-time visuomotor mechanisms are engaged for the control of action only after the response is cued, and only if the target is visible. These visuomotor mechanisms compute the absolute metrics of the target object and therefore resist size-contrast illusions. In other situations (e.g. prior to the response cue, or if the target is no longer visible), a perceptual representation of the target object can be used for action planning. Unlike the real-time visuomotor mechanisms, perception-based movement planning makes use of relational metrics, and is therefore sensitive to size-contrast illusions.

Cues↗

PubMed2003

Can a visual representation support the online control of memory-dependent reaching? Evident from a variable spatial mapping paradigm.

We investigated whether a representation of a visual target can be stored in memory and used to support the online control of reaching movements. To distinguish between the use of a stored target representation for movement planning versus online control, we employed a novel movement environment in which participants could not fully plan their action in advance of movement initiation; that is, the spatial mapping between the movement of a computer mouse and the on-screen movement of a cursor was randomly varied from trial to trial. As such, participants were required to use online control to reach the target position. Reaches were examined in full-vision and three memory-dependent conditions (0, 2, and 5 s of delay). Absolute constant error did not accumulate between full-vision and brief delay trials (i.e., the 0-s delay), suggesting a stored representation of the visual target can be used for online control of reaching given a sufficiently brief delay interval. Longer delay trials (2 and 5 s) were less accurate and more variable than brief delay trials; however, the residual accuracy of these memory-dependent actions suggests that the motor system may have access to a stored representation of the visual target for online control processes for upwards of 5 s following target occlusion.

Adult↗

PubMed2002

Grasping two-dimensional images and three-dimensional objects in visual-form agnosia.

Visually guided prehension is controlled by a specialized visuomotor system in the posterior parietal cortex. It is not clear how this system responds to visual stimuli that lack three-dimensional (3D) structure, such as two-dimensional (2D) images of objects. We asked a neurological patient with visual-form agnosia (patient D.F.) to grasp 3D objects and 2D images of the same objects and to estimate their sizes manually. D.F.'s grip aperture was scaled to the sizes of the 2D and 3D target stimuli, but her manual estimates were poorly correlated with object size. Control participants demonstrated appropriate size-scaling in both the grasping and manual size-estimation tasks, but tended to use a smaller peak aperture when reaching to grasp 2D images. We conclude that: (1) the dorsal stream grasping system does not discriminate in a fundamental way between 2D and 3D objects, and (2) neurologically normal participants might adopt a different visuomotor strategy for target objects that are recognized to be ungraspable. These findings are consistent with the view that the dorsal grasping system accesses a pragmatic, spatial representation of the target object, whereas the ventral system accesses a more comprehensive, volumetric description of the object.

Adult↗

PubMed2002

Manual asymmetries in tool-use: implications for apraxia.

Previous examinations of manual performance asymmetries have utilised relatively simple movements such as goal-directed aiming and rapid finger-tapping. In the present research, three-dimensional kinematic analyses were employed to examine manual asymmetries in the production of transitive limb gestures. Transitive limb gestures involve the use of tools and/or objects and are complex movements used in the evaluation of limb apraxia. Ten healthy right hand dominant participants produced two transitive limb gestures in response to verbal command both with (tool-use) and without (pantomime) the appropriate tool. Analyses revealed a dominant (right) hand advantage, although the magnitude of this advantage was influenced by movement context (pantomime vs tool-use). This study provides the first evidence that manual asymmetries are elicited during the production of transitive limb gestures, and mandates that future kinematic analyses of limb apraxia consider the impact of pre-morbid manual asymmetries when contrasting the performance of left and right hemisphere damaged patients.

Journal Article↗