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We investigated the effect of visual context (i.e., a visual illusion) on the planning of a sequential object manipulation task. Participants (n = 13) had to grasp a rod embedded in a "rod-and-frame" illusion and insert the rod-end into a tight hole in a pre-defined way. The grip type (defined by start posture, either pronated or supinated; and end...
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Citations
... This preference can be predicted from the individual participants' favored contact points in the no-obstacle condition. This consistency implies that grasp planning is based on a common optimality criterion, such as end-state comfort Craje et al. 2008;Zhang and Rosenbaum 2008). Conversely, obstacles placed directly at the favored contact point for the thumb (i.e., in the southwest sector) did not yield this bistable pattern, likely due to anatomical restrictions and the height of the obstacle, such that the palm of the hand would interfere with the obstacle when grasping the object to the left of the obstacle. ...
When reaching to grasp for an object in the absence of obstacles, the choice of contact points is highly consistent within and between healthy humans, suggesting a preplanning of grasping movements (Gilster et al. in Exp Brain Res 217:137–151, 2012). In real life, objects may obstruct the favored contact points at a target object, requiring adjustments to avoid collision. In the present study, we investigated how an obstacle that directly obstructs the favored contact points for two-digit grasping changes the planning and execution of reach-to-grasp movements. Furthermore, we elucidated to what extent an obstacle placed at various angular positions around the target object (thereby not directly obstructing the favored contact points) still influences trajectories, contact points, and time-related parameters. When obstacles directly obstructed favored contact points participants either chose a completely new contact point or grasped the object only slightly away from the favored contact point. Obstacles located near the favored contact points but not directly obstructing them still resulted in a repulsive effect, meaning that contact points were shifted away from the obstacle to ensure sufficient distance to the obstacle. We found that the position of an obstacle even influences the direction in which the fingers set off. This leads to a deviation of the trajectory very early in the time course, yielding longer movement times if the main contact points are obstructed. Taken together, the early significant influence of obstacles on the grasping movement supports the assumption that grasping movements are preplanned.
... Subjects generally have a clear preference for one of the two, depending on the object's orientation, with only a small range of orientations for which both options are equally likely (Stelmach et al. 1994). If the visual information is biased (due to a visual illusion) it is the illusory orientation that determines the choice (Crajé et al. 2008). This suggests that choosing grasping points is based on the same processing of visual information as perceptual judgments. ...
... In the past, we have called this " motor ambiguity " (Wood & Goodale, 2011). Within a bistable state, a final grasp selection is more likely to be influenced by the subtler influences of variables like hysteresis (Kent, Wilson, Plumb, Williams, & Mon-Williams, 2009), visual context (Craj e, van der Kamp, & Steenbergen, 2008; Dyde & Milner, 2002; Glover, Dixon, Castiello, & Rushworth, 2005), and starting hand position (Dijkerman, Mcintosh, Schindler, Nijboer, & Milner, 2009). In this sense, the factors that guide postural decision-making are amplified when the relevant behavior is bistable. ...
... Consistent with the two-visual systems model, adult research suggests a separation in information use for action mode selection and movement control (e.g., Craj e, van der Kamp, & Steenbergen, 2008;Dijkerman, McIntosh, Schindler, Nijboer, & Milner, 2009;van Doorn et al., 2007van Doorn et al., , 2009). More specifically, movement control mainly exploits egocentric sources of information, whereas action mode selection is primarily reliant on allocentric sources of information (Michaels, 2000;Milner & Goodale, 2008;van der Kamp et al., 2003). ...
In human adults the use of visual information for selecting appropriate modes for action appears to be separate from the use of visual information for the control of movements of which the action is composed (Milner & Goodale, [1995] The visual brain in action; [2008] Neuropsychologia 46:774-785). More specifically, action mode selection primarily relies upon allocentric information, whereas movement control mainly exploits egocentric information. In the present study, we investigated to what degree this division is already present in 6- to 10-month-old infants when reaching for moving objects; that is, whether allocentric information is uniquely exploited for action mode selection (i.e., reaching with one or the other hand) or whether it is also used for movement control (i.e., reaching kinematics). Infants were presented with laterally approaching objects at two speeds (i.e., 20 and 40 cm/s) against a stationary or moving background. Background motion affects allocentric information about the object's velocity relative to its background. Results indicated that object speed constrained both infants' action mode selection and movement control. Importantly, however, the influence of background motion was limited to action mode selection and did not extend to movement control. The findings provide further support for the contention that during early development information usage is-at least to some degree-separated for action mode selection and movement control.
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... Dyde and Milner (2002) reported that a tilted reference influenced perception (by the method of adjustment) but not action (distance between thumb and forefinger when grasping the rods). However, Craje et al. (2008) found that the grip orientation used to grasp a rod was affected by a surrounding frame in the same way as an actual tilt of the rod from the vertical. Craje et al. argued that differences in task constraints may have contributed to the different findings (Smeets et al 2002). ...
... Previous investigations have revealed inconsistent effects of a tilted reference frame upon manual behavior, depending on the task (Dyde and Milner 2002;Craje et al. 2008). Our purpose was to see whether a tilted reference frame would affect eye movements in a 'saccade-to-vertical' task. ...
We investigated the effects of a tilted reference frame (i.e., allocentric visual context) on the perception of the gravitational vertical and saccadic eye movements along a planned egocentric vertical path. Participants (n = 5) in a darkened room fixated a point in the center of a circle on an LCD display and decided which of two sequentially presented dots was closer to the unmarked '6 o'clock' position on that circle (i.e., straight down toward their feet). The slope of their perceptual psychometric functions showed that participants were able to locate which dot was nearer the vertical with a precision of 1°-2°. For three of the participants, a square frame centered at fixation and tilted (in the roll direction) 5.6° from the vertical caused a strong perceptual bias, manifest as a shift in the psychometric function, in the direction of the traditional 'rod-and-frame' effect, without affecting precision. The other two participants showed negligible or no equivalent biases. The same subjects participated in the saccade version of the task, in which they were instructed to shift their gaze to the 6 o'clock position as soon as the central fixation point disappeared. The participants who showed perceptual biases showed biases of similar magnitude in their saccadic endpoints, with a strong correlation between perceptual and saccadic biases across all subjects. Tilting of the head 5.6° reduced both perceptual and saccadic biases in all but one observer, who developed a strong saccadic bias. Otherwise, the overall pattern and significant correlations between results remained the same. We conclude that our observers' saccades-to-vertical were dominated by perceptual input, which outweighed any gravitational or head-centered input.
... Two-visual-streams theory, however, is still incomplete when it comes to understanding object segmentation for purposes of movement planning. Although the two streams certainly interact to some extent (especially considering the role of the ventral stream in goal identification), using functional analyses allowed us to show that the perceptual bias in scene segmentation induced by object connectedness did not influence visuomotor processes at any stage of movement, from planning to execution to on-line control-a finding that contradicts previous work (e.g., Crajé, van der Kamp, & Steenbergen, 2008;Franz, 2003;Franz, Gegenfurtner, Bülthoff, & Fahle, 2000;Glover, 2002;Glover, Rosenbaum, Graham, & Dixon, 2004;Mendoza, Hansen, Glazebrook, Keetch, & Elliot, 2005). Instead, our results indicate (at least as far as segmentation of the scene into separate objects is concerned) that the visuomotor system plans movements accurately very early on, despite the fact that goal selection may be based on a biased perceptual representation. ...
The perceptual system parses complex scenes into discrete objects. Parsing is also required for planning visually guided movements when more than one potential target is present. To examine whether visual perception and motor planning use the same or different parsing strategies, we used the connectedness illusion, in which observers typically report seeing fewer targets if pairs of targets are connected by short lines. We found that despite this illusion, when observers are asked to make speeded reaches toward targets in such displays, their reaches are unaffected by the presence of the connecting lines. Instead, their movement plans, as revealed by their movement trajectories, are influenced by the number of potential targets irrespective of whether connecting lines are present or not. This suggests that scene parsing for perception depends on mechanisms that are distinct from those that allow observers to plan rapid and efficient target-directed movements in situations with multiple potential targets.
... Anticipatory planning implies that objects are grasped in such a way that allows Xexibility for upcoming movements. For example , subjects vary the initial grip, i.e., supination or pronation , to the upcoming task (e.g., Crajé et al. 2008; Rosenbaum and Jorgensen 1992; Rosenbaum et al. 1996) or the height of hand placement to the target location (Cohen and Rosenbaum 2004). Recently, Sun et al. (2011) reported that subjects use a less stable grasp when holding an object with a changeable weight (where liquid could be added to or removed from a hand-held object), as such being more Xexible to change the grip force according to object changes. ...
In addition to hand shaping, previous studies have shown that subjects adapt placement of individual digits to object properties such as its weight and center of mass. However, the extent to which digit placement varies based on task context is unknown. In the present study, we investigated where subjects place their digits on a bottle when the upcoming task (lift versus pour) and object content (i.e., amount of liquid: empty, half, and full) were manipulated. Our results showed that subjects anticipated both the upcoming task and content by varying digit placement when grasping the bottle prior to the onset of manipulation. Specifically, subjects increased the vertical distance between the thumb and index finger for pouring but not for lifting. This larger moment arm might have been established to decrease the amount of force required to tilt the bottle. Content also affected digit placement: the digits were placed higher and were wrapped more around the bottle with increasing content. This strategy may maximize grip surface contact, and hence grasp stability. These findings extend previous research showing that grasp planning not only takes place at a macroscopic level (whole-hand position relative to an object), but also at the level of individual digit placement. This finer level of control appears to be sensitive to the expected mechanical properties of the object and how these may affect grasp stability throughout the upcoming manipulation.
... The mechanism selects a grasp orientation based on the intended object manipulation but also allows other factors to bias the realized grasp orientation. The good fit of this simple model suggests that -besides focusing on the criteria, which determine how an action is executed -future research should also focus on further investigating the mechanisms that 4 Grasp selection may be biased by optical illusions (Crajé et al. 2008). However, here we do not want to create an illusion that distorts the perceived orientation of the object that is to be grasped, but want to suggest a certain way of grasping and turning the box to the participants. ...
The grasp orientation when grasping an object is frequently aligned in anticipation of the intended rotation of the object (end-state comfort effect). We analyzed grasp orientation selection in a continuous task to determine the mechanisms underlying the end-state comfort effect. Participants had to grasp a box by a circular handle-which allowed for arbitrary grasp orientations-and then had to rotate the box by various angles. Experiments 1 and 2 revealed both that the rotation's direction considerably determined grasp orientations and that end-postures varied considerably. Experiments 3 and 4 further showed that visual stimuli and initial arm postures biased grasp orientations if the intended rotation could be easily achieved. The data show that end-state comfort but also other factors determine grasp orientation selection. A simple mechanism that integrates multiple weighted biases can account for the data.
... This is more evident in those with right hemiplegia. 29,30 Bimanual training may require more motor planning effort for children with right hemiplegia as activities were generally more complex compared to CIMT training where activities were modified to account for the unimanual approach. Assessment of motor planning abilities, particularly for children with right hemiplegia, may be warranted. ...
To delineate characteristics of best responders in a randomized trial comparing constraint-induced movement therapy (CIMT) to bimanual training for children with unilateral cerebral palsy.
Secondary analysis of a single-blind matched-pairs randomized comparison trial.
Community sporting facilities in 2 Australian capital cities.
Children (n=64; mean age, 10.2±2.7y; 52% boys), matched for age, sex, side of hemiplegia, and upper-limb function, were randomized within pairs to CIMT or bimanual training. Sixty-one children who completed CIMT (n=31) or bimanual training (n=30) were included in this study.
Each intervention was delivered in day camps (total 60h over 10d) using a novel circus theme with goal-directed training.
Change between baseline, 3, and 26 weeks on the Melbourne Assessment of Unilateral Upper Limb Function (MUUL>7.4%), Assisting Hand Assessment (AHA>4 raw score points), and Canadian Occupational Performance Measure (COPM>2 points) defined best responders.
Poorer baseline hand function predicted a best response for unimanual capacity of the impaired upper limb (MUUL) immediately postintervention; however, at 26 weeks the odds of achieving a favorable outcome were 21 times greater for CIMT than bimanual training. A favorable response for bimanual performance (AHA) was predicted by immediate change in Jebsen-Taylor hand function test scores. Age (older), left-sided hemiplegia, and lower-baseline COPM performance scores significantly predicted favorable individualized outcomes.
Secondary analysis of a randomized trial directly comparing 2 upper-limb training models, found children with poorer hand function benefited most. Favorable outcomes for bimanual performance were associated with gains in movement efficiency and older children with left-sided hemiplegia achieved more favorable gains in perceived occupational performance.
... Van Doorn and colleagues (van Doorn et al. 2007), for instance, demonstrated that allocentric information sources (i.e., context-dependent, relative metrics) are exploited for the selection of the action mode, while online movement control relies on egocentric sources of information (i.e., context-independent, absolute metrics). This was attested for by an influence of the Müller-Lyer illusion on the selection of an action mode that was adapted to object length (i.e., the choice to reach one-or two-handed), whereas subsequent movement execution (i.e., hand aperture) appeared immune to the illusion (see also Crajé et al. 2008). The contention of separate visual processes for action mode selection and online movement control is consistent with long standing tradition (e.g., Woodworth 1899), the most recent formulation of which is the twovisual systems model (Glover 2004;Milner and Goodale 2008; see also Dijkerman et al. 2009). ...
Although 5-month-old infants select action modes that are adaptive to the size of the object (i.e., one- or two-handed reaching), it has largely remained unclear whether infants of this age control the ensuing movement to the size of the object (i.e., scaling of the aperture between hands). We examined 5-, 7-, and 9-month-olds' reaching behaviors to gain more insight into the developmental changes occurring in the visual guidance of action mode selection and movement control, and the relationship between these processes. Infants were presented with a small set of objects (i.e., 2, 3, 7, and 8 cm) and a large set of objects (i.e., 6, 9, 12, and 15 cm). For the first set of objects, it was found that the infants more often performed two-handed reaches for the larger objects based on visual information alone (i.e., before making contact with the object), thus showing adaptive action mode selection relative to object size. Kinematical analyses of the two-handed reaches for the second set of objects revealed that inter-trial variance in aperture between the hands decreased with the approach toward the object, indicating that infants' reaching is constrained by the object. Subsequent analysis showed that between hand aperture scaled to object size, indicating that visual control of the movement is adjusted to object size in infants as young as 5 months. Individual analyses indicated that the two processes were not dependent and followed distinct developmental trajectories. That is, adaptive selection of an action mode was not a prerequisite for appropriate aperture scaling, and vice versa. These findings are consistent with the idea of two separate and independent visual systems (Milner and Goodale in Neuropsychologia 46:774-785, 2008) during early infancy.
