Postural sway and perceived comfort in pointing tasks.

In this study, we explored relations between indices of postural sway and perceived comfort during pointing postures performed by standing participants. The participants stood on a force plate, grasped a pointer with the dominant (right) hand, and pointed to targets located at four positions and at two distances from the body. We quantified postural sway over 60-s intervals at each pointing posture, and found no effects of target location or distance on postural sway indices. In contrast, comfort ratings correlated significantly with indices of one of the sway components, trembling. Our observations support the hypothesis that rambling and trembling sway components involve different neurophysiological mechanisms. They also suggest that subjective perception of comfort may be more important than the actual posture for postural sway.

End-state comfort trumps handedness in object manipulation.

A goal of research on human perception and performance is to explore the relative importance of constraints shaping action selection. The present study concerned the relative importance of two constraints that have not been directly contrasted: (1) the tendency to grasp objects in ways that afford comfortable or easy-to-control final postures; and (2) the tendency to grasp objects with the dominant rather than the nondominant hand. We asked participants to reach out and grasp a horizontal rod whose left or right end was to be placed into a target after a 90° rotation. In one condition, we told participants which hand to use and let them choose an overhand or underhand initial grasp. In another condition, we told participants which grasp to use and let them choose either hand. Participants sacrificed hand preference to perform the task in a way that ensured a comfortable or easy to control thumb-up posture at the time of object placement, indicating that comfort trumped handedness. A second experiment confirmed that comfort was indeed higher for thumb-down postures than thumb-up postures. A third experiment confirmed that the choice data could be linked to objective performance differences. The results point to the importance of identifying constraint weightings for action selection and support an account of hand selection that ascribes hand preference to sensitivity to performance differences. The results do not support the hypothesis that hand preference simply reflects a bias to use the dominant hand.

Choosing actions.

Actions that are chosen have properties that distinguish them from actions that are not. Of the nearly infinite possible actions that can achieve any given task, many of the unchosen actions are irrelevant, incorrect, or inappropriate. Others are relevant, correct, or appropriate but are disfavored for other reasons. Our research focuses on the question of what distinguishes actions that are chosen from actions that are possible but are not. We review studies that use simple preference methods to identify factors that contribute to action choices, especially for object-manipulation tasks. We can determine which factors are especially important through simple behavioral experiments.

Is handedness just response bias?

Although most people prefer to use the right hand for unimanual tasks, it is unknown whether handedness arises from response bias. Whether it does is a question inspired by signal detection theory. We drew on the framework of signal detection theory to assess bias and sensitivity in hand choice by asking right-handers to choose between two tasks-one performed with the left hand, and one performed with the right. For some participants, the left-hand task stayed constant while the right-hand task was varied. For other participants, the right-hand stayed constant while the left-hand task was varied. We reasoned that if hand preference only reflected bias, participants would choose the generally preferred hand to the same degree, regardless of whether its task was constant or varied. Our participants followed this strategy only to some extent, suggesting that bias and sensitivity both contribute to hand preference.

End-state comfort and joint configuration variance during reaching.

This study joined two approaches to motor control. The first approach comes from cognitive psychology and is based on the idea that goal postures and movements are chosen to satisfy task-specific constraints. The second approach comes from the principle of motor abundance and is based on the idea that control of apparently redundant systems is associated with the creation of multi-element synergies stabilizing important performance variables. The first approach has been tested by relying on psychophysical ratings of comfort. The second approach has been tested by estimating variance along different directions in the space of elemental variables such as joint postures. The two approaches were joined here. Standing subjects performed series of movements in which they brought a hand-held pointer to each of four targets oriented within a frontal plane, close to or far from the body. The subjects were asked to rate the comfort of the final postures, and the variance of their joint configurations during the steady state following pointing was quantified with respect to pointer endpoint position and pointer orientation. The subjects showed consistent patterns of comfort ratings among the targets, and all movements were characterized by multi-joint synergies stabilizing both pointer endpoint position and orientation. Contrary to what was expected, less comfortable postures had higher joint configuration variance than did more comfortable postures without major changes in the synergy indices. Multi-joint synergies stabilized the pointer position and orientation similarly across a range of comfortable/uncomfortable postures. The results are interpreted in terms conducive to the two theoretical frameworks underlying this work, one focusing on comfort ratings reflecting mean postures adopted for different targets and the other focusing on indices of joint configuration variance.

Hemispheric differences in the control of limb dynamics: a link between arm performance asymmetries and arm selection patterns.

Human handedness has been described and measured from two perspectives: handedness inventories rate hand preferences, whereas other tests examine motor performance asymmetries. These two measurement approaches reflect a major controversy in a literature that defines handedness as either a preference or an asymmetry in sensorimotor processing. Over the past decade, our laboratory has developed a model of handedness based on lateralization of neural processes. This model attributes distinct control processes to each hemisphere, which in turn lead to observable interlimb sensorimotor performance asymmetries. We now hypothesize that arm preference, or choice, may depend on the interaction between sensorimotor performance asymmetries and the given task. The purpose of this study is to examine whether arm selection is linked to interlimb performance asymmetries during reaching. Right-handed subjects made choice and nonchoice reaches to each of eight targets (d = 3.5 cm) arranged radially (r = 13 cm) around a midline starting position. We displaced each cursor (one associated with each hand) 30 cm to the midline start circle to ensure that there were no hemispace-related geometric, mechanical, or perceptual biases to use either arm for the two midline targets. The three targets on each side of the midline received mostly reaches from the ipsilateral arm, a tendency previously described as a "hemispace bias." However, the midline targets, which were equidistant from each hand, received more dominant arm reaches. Dominant arm hand paths to these targets were straighter and more accurately directed. Inverse dynamics analyses revealed a more proficient dominant arm strategy that exploited intersegmental dynamics to a greater extent than did the nondominant arm. These findings suggest that sensorimotor asymmetries in dynamic coordination might explain limb choices. We discuss the implications of these results for theories of action selection, models of handedness, and models of neural lateralization.

Imagined actions aren't just weak actions: task variability promotes skill learning in physical practice but not in mental practice.

Early research on visual imagery led investigators to suggest that mental visual images are just weak versions of visual percepts. Later research helped investigators understand that mental visual images differ in deeper and more subtle ways from visual percepts. Research on motor imagery has yet to reach this mature state, however. Many authors have implicitly subscribed to the view that motor images are just weak versions of physical actions. We tested this view by comparing motor learning in variable practice conditions with motor learning in constant practice conditions when participants either physically or mentally practiced golf-putting. We found that physical and mental practice both resulted in significant learning but that variable practice was only better than constant practice when participants practiced physically. This outcome was not predicted by the hypothesis that motor imagery is just a weaker form of real-action experience.

Psychologically distinct classes of motor behavior inferred from individual differences: evidence from a sequential stacking task.

A number of studies have demonstrated regularities in how individuals select and perform single object manipulations, but little work has been concerned with the manipulation of multiple objects. To this end, the authors asked participants to stack a set of linearly spaced containers onto various goal locations. Our aim was to determine whether participants adopted specific strategies to complete this task. We focused on whether the distance between the objects, the goal location of the objects, or both, determined the classes of movement sequences that individuals used to perform the task. The results showed that some individuals tended to use one hand for lifting and releasing the containers whereas other individuals tended to use both hands for lifting and releasing the containers. Those participants who tended to use one hand varied which hand was used according to the goal location of the containers but not the distance between containers. The emergence of these individual differences provides a new basis for inferring psychologically distinct classes of motor behavior.

Visual priming of inverted and rotated objects.

Object images are identified more efficiently after prior exposure. Here, the authors investigated shape representations supporting object priming. The dependent measure in all experiments was the minimum exposure duration required to correctly identify an object image in a rapid serial visual presentation stream. Priming was defined as the change in minimum exposure duration for identification as a function of prior exposure to an object. Experiment 1 demonstrated that this dependent measure yielded an estimate of predominantly visual priming (i.e., free of name and concept priming). Experiments 2 and 3 demonstrated that although identification [corrected] was sensitive to orientation, visual priming was relatively invariant with image inversion (i.e., an image visually primed its inverted counterpart approximately as much as it primed itself). Experiment 4 demonstrated a similar dissociation with images rotated 90 degrees off the upright. In all experiments, the difference in the magnitude of priming for identical or rotated-inverted priming conditions was marginal or nonexistent. These results suggest that visual representations that support priming can be relatively insensitive to picture-plane manipulations, although these manipulations have a substantial effect on object identification.