Asymmetries in the spatial localization of transformed targets

Modulation of reaching by spatial attention

Attention is needed to perform goal-directed vision-guided movements. We investigated whether the direction of covert attention modulates movement outcomes and dynamics. Right-handed and left-handed volunteers attended to a spatial location while planning a reach toward the same hemifield, the opposite one, or planned a reach without constraining attention. We measured behavioral variables as outcomes of ipsilateral and contralateral reaching and the tangling of behavioral trajectories obtained through principal component analysis as a measure of the dynamics of motor control. We found that the direction of covert attention had significant effects on the dynamics of motor control, specifically during contralateral reaching. Data suggest that motor control was more feedback-driven when attention was directed leftward than when attention was directed rightward or when it was not constrained, irrespectively of handedness. These results may help to better understand the neural bases of asymmetrical neurological diseases like hemispatial neglect.

Evaluating the efficacy of an iPad® app in determining a single bout of exercise benefit to executive function

We examined the efficacy and feasibility of an iPad® app used at-home in identifying a postexercise benefit to executive function. The iPad® app required simple reaching movements mirror-symmetrical to an exogenously presented target (i.e., antipointing) and is a task that lab-based behavioral and neuroimaging work has shown to provide a valid measure of the response inhibition component of executive function. Fifty English-speaking individuals (18 female, age range 18-26 years of age) completed the iPad® app before and immediately after a 20-min session of heavy-intensity aerobic exercise, and on a separate day completed the app prior to and following a 20-min non-exercise control condition. Results showed antipointing reaction times (RTs) in the exercise condition decreased by an average of 18 ms postexercise (p < 0.001) with an observed large effect size (d_z = 0.90), whereas control condition pre- and post-assessment RTs did not reliably differ (p = 0.12, d_z = 0.22) and were within an equivalence boundary (p < 0.005). Further, pre-assessment exercise and control condition antipointing RTs were within an equivalence boundary (p < 0.05). Accordingly, a simple iPad® app provides the requisite resolution to detect subtle executive function benefits derived from a single bout of exercise.

Antipointing Reaches Do Not Adhere to Width-Based Manipulations of Fitts' (1954) Equation

Reaches with overlapping stimulus-response spatial relations (propointing) adhere to speed-accuracy relations as defined by Paul Fitts' index of difficulty equation (IDFitts: in bits of information). This movement principle is attributed to response mediation via the "fast" visuomotor networks of the dorsal visual pathway. It is, however, unclear whether the executive demands of dissociating stimulus-response spatial relations by reaching mirror-symmetrical to a target (antipointing) elicits similar adherence to Fitts' equation. Here, pro- and antipointing responses were directed to a constant target amplitude with varying target widths to provide IDFitts values of 3.0, 3.5, 4.3, and 6.3 bits. Propointing movement times linearly increased with IDFitts-a result attributed to visually based trajectory corrections. In contrast, antipointing movement times, deceleration times, and endpoint precision did not adhere to Fitts' equation. These results indicate that antipointing renders a "slow" and offline mode of control mediated by the visuoperceptual networks of the ventral visual pathway.

Are there right hemisphere contributions to visually-guided movement? Manipulating left hand reaction time advantages in dextrals

Many studies have argued for distinct but complementary contributions from each hemisphere in the control of movements to visual targets. Investigators have attempted to extend observations from patients with unilateral left- and right-hemisphere damage, to those using neurologically-intact participants, by assuming that each hand has privileged access to the contralateral hemisphere. Previous attempts to illustrate right hemispheric contributions to the control of aiming have focussed on increasing the spatial demands of an aiming task, to attenuate the typical right hand advantages, to try to enhance a left hand reaction time advantage in right-handed participants. These early attempts have not been successful. The present study circumnavigates some of the theoretical and methodological difficulties of some of the earlier experiments, by using three different tasks linked directly to specialized functions of the right hemisphere: bisecting, the gap effect, and visuospatial localization. None of these tasks were effective in reducing the magnitude of left hand reaction time advantages in right handers. Results are discussed in terms of alternatives to right hemispheric functional explanations of the effect, the one-dimensional nature of our target arrays, power and precision given the size of the left hand RT effect, and the utility of examining the proportions of participants who show these effects, rather than exclusive reliance on measures of central tendency and their associated null hypothesis significance tests.

Hemifield or hemispace: what accounts for the ipsilateral advantages in visually guided aiming?

Aiming movements to targets presented on the same side as the reaching limb are faster and more accurate than movements made across the body. These advantages are typically attributed to within-hemisphere sensorimotor control. However, contrary to the within- versus between-hemisphere model, we have shown that some of these advantages tend to go with the side of the movement, rather than the side of the target (Carey et al. Exp Brain Res 112:496-504, 1996; Carey and Otto-de Haart Neuropsychologia 39:894, 2001). Barthélémy and Boulinghez (Exp Brain Res 147:305-312, 2002) acknowledge that our biomechanical account fits data for post-onset movement parameters such as peak velocity and duration, yet they report evidence for some within- versus between-hemisphere contributions to reaction time (RT) advantages. To examine a possible difference between early and late movement kinematics fitting these alternative models, we have dissociated field and space in a different way, which required arm movements with differential inertial consequences, as well as unpredictability of target location in terms of visual field. The data suggest that visual field may contribute some of the variance to hemispatial effects, but only for the right hand. In a second experiment, we used an antipointing task to examine hemispatial versus visual field effects on RTs and to revisit the possible hand difference identified in experiment 1. We found that hemispace accounted for all of the ipsilateral advantages, including RT, for both right and left hands. Results are discussed in terms of the computational requirements of eye-hand coordination in relative unconstrained conditions.

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.

Saccadic-like visuomotor adaptation involves little if any perceptual effects

Studies on visuomotor adaptation provide crucial clues on the functional properties of the human motor system. The widely studied saccadic adaptation paradigm is a major example of such a fruitful field of investigation. Magescas and Prablanc (J Cogn Neurosci 18(1):75-83, 2006) proposed a transposition of this protocol to arm pointing behavior, by designing an experiment in which the informational context of the upper limb visuomotor system is comparable to that of the saccadic system. Subjects were given terminal only visual feedback in a hand pointing task, assumed to produce a purely terminal visual error signal. Importantly, this paradigm has been shown to induce no saccadic adaptation. Although the saccadic adaptation paradigm is known to induce a predominantly motor adaptation with minor sensory effects, the lack of sensory changes has not been tested in its transposition to pointing. The present study was a partial replication of Magescas and Prablanc's (J Cogn Neurosci 18(1):75-83, 2006) study with additional control tests. A first experiment searched for a possible change in the static visual-to-proprioceptive congruency. A second experiment, based on an anti-pointing task, aimed at separating the sensory and motor effects of the adaptation in a dynamic condition. Consistent with most results on saccadic adaptation, we found a predominant adaptation of the motor components, with little if any involvement of the sensory components. Results are interpreted by proposing a causal relationship between the type of error signal and its adaptive effects.

Dissociation between intentional and automatic remapping: different levels of inter-hemispheric transfer

In order to efficiently interact with our environment we need to constantly to update the spatial representation of visual targets for movement. This is required not only when we move our eyes but also when we want to reach toward a location different from the actual physical target (for example symmetrical). These two types of remapping are very different in nature, one being automatic, and the other intentional. However, they both have been shown to involve the posterior parietal cortex (PPC). To further investigate the role of this brain region in automatic and intentional remapping processes and the level of inter-hemispheric transfer of visuo-motor information in these two conditions of reaching, we tested two patients with unilateral optic ataxia (OA) in two different tasks: reaching to a memorised visual target after an intervening eye movement (trans-saccadic remapping) and an anti-reaching task. We showed that lesions of the PPC had different implications for these two tasks. In the trans-saccadic remapping task, movements toward the contralesional field were disrupted, even when the visual target was presented in the ipsilesional field. In contrast, in the anti-reaching task, the patients were mostly impaired in conditions where the target was presented in the contralesional field, even if the movement was executed toward the ipsilesional field. We postulate that the transfer of the visuo-motor information between hemispheres occurs before the parietal cortex in trans-saccadic remapping (transfer of visual information), and at the parietal level or after in anti-reaching (transfer of visuo-motor information).

Antisaccades exhibit diminished online control relative to prosaccades

Convergent evidence suggests that stimulus-driven saccades (i.e., prosaccades) are mediated via online trajectory modifications (e.g., Gaveau et al. 2003). The goal of the present investigation was to determine whether manipulating the cognitive demands of a saccade influences the extent to which the response's trajectory is structured online. To that end, participants completed pro- and antisaccades (i.e., 180 degrees mirror-symmetrical transformation) to target stimuli that were continuously visible (Experiment 1) or occluded (Experiment 2) during the response. To index trajectory modifications, we computed the proportion of variance (R (2) values) explained by the spatial location of the eye at 10% increments of normalized movement time [i.e., 10, 20, ... 80, 90% of movement time (MT)] relative to the saccade's ultimate movement endpoint. The basis for this analysis is that between-task differences in the magnitude of R (2) values reflect differences in the use of feedback for online trajectory amendments. Results indicated that antisaccades produced larger R (2) values (from 40 to 80% of MT) as well as less accurate and more variable endpoints than their prosaccade counterparts. Such a pattern of results indicates that antisaccades were not controlled online to the same degree as prosaccades. In particular, we propose that the cognitive nature of the antisaccade task disrupts the normally online operation of saccade networks and renders a mode of control that is not optimized for feedback-based trajectory amendments.

The antipointing task: vector inversion is supported by a perceptual estimate of visual space

The authors examined whether the visual field-specific endpoint bias of mirror-symmetrical reaching movements (i.e., antipointing) is related to top-down decoupling of the normal spatial relations between target and response (i.e., visuomotor inhibition) or the inversion of target coordinates to a mirror-symmetrical location (i.e., vector inversion). Participants completed pro- and antipointing movements in left and right visual space under conditions in which movement type was performed in separate blocks (i.e., blocked condition) and when randomly interleaved on a trial-by-trial basis (i.e., random condition). Most important, the random condition entailed equivalent premovement inhibition across pro- and antipointing. Propointing produced comparable endpoint accuracy in left and right visual space whereas antipointing under- and overshot target position: a finding characterizing blocked and random conditions. The authors attribute the visual field-specific bias of antipointing to the obligatory nature of the task and the integration of visuoperceptual networks to support vector inversion.

Conceptual binding: integrated visual cues reduce processing costs in bimanual movements

In discrete reaction time (RT) tasks, it has been shown that nonsymmetric bimanual movements are initiated slower than symmetric movements in response to symbolic cues. By contrast, no such RT differences are found in response to direct cues ("direct cue effect"). Here, we report three experiments showing that the direct cue effect generalizes to rhythmical bimanual movements and that RT cost depends on different cue features: 1) symbolic versus direct or 2) integrated (i.e., action of both hands is indicated as one entity) versus dissociated (i.e., action of each hand is indicated separately). Our main finding was that dissociated symbolic cues were most likely processed serially, resulting in the longest RTs, which were substantially reduced with integrated symbolic cues. However, extra RT costs for switching to nonsymmetrical bimanual movements were overcome only when the integrated cues were direct. We conclude that computational resources might have been exceeded when the response needs to be determined for each hand separately, but not when a common response for both hands is selected. This supports the idea that bimanual control benefits from conceptual binding.

Anti-pointing is mediated by a perceptual bias of target location in left and right visual space

We sought to determine whether mirror-symmetrical limb movements (so-called anti-pointing) elicit a pattern of endpoint bias commensurate with perceptual judgments. In particular, we examined whether asymmetries related to the perceptual over- and under-estimation of target extent in respective left and right visual space impacts the trajectories of anti-pointing. In Experiment 1, participants completed direct (i.e. pro-pointing) and mirror-symmetrical (i.e. anti-pointing) responses to targets in left and right visual space with their right hand. In line with the anti-saccade literature, anti-pointing yielded longer reaction times than pro-pointing: a result suggesting increased top-down processing for the sensorimotor transformations underlying a mirror-symmetrical response. Most interestingly, pro-pointing yielded comparable endpoint accuracy in left and right visual space; however, anti-pointing produced an under- and overshooting bias in respective left and right visual space. In Experiment 2, we replicated the findings from Experiment 1 and further demonstrate that the endpoint bias of anti-pointing is independent of the reaching limb (i.e. left vs. right hand) and between-task differences in saccadic drive. We thus propose that the visual field-specific endpoint bias observed here is related to the cognitive (i.e. top-down) nature of anti-pointing and the corollary use of visuo-perceptual networks to support the sensorimotor transformations underlying such actions.

Functional imaging of the parietal cortex during action execution and observation

We used the (14)C-deoxyglucose method to map the functional activity in the cortex of the lateral and medial parietal convexity, the intraparietal and the parietoccipital sulci of monkeys which either reached and grasped a 3D-object or observed the same reaching-to-grasp movements executed by a human. Execution of reaching-to-grasp induced activations in the superior parietal areas SI-forelimb/convexity, PE, PE caudal (PEc); in the intraparietal areas PE intraparietal (PEip), medial intraparietal (MIP), 5 intraparietal posterior, ventral intraparietal (VIP), anterior intraparietal (AIP), lateral intraparietal dorsal; in the inferior parietal areas PF, PFG, PG; in the parietoccipital areas V6, V6A-dorsal; in the medial cortical areas PGm/7m and retrosplenial cortex. Observation of reaching-to-grasp activated areas SI-forelimb/convexity, PE lateral, PEc, PEip, MIP, VIP, AIP, PF, V6, PGm/7m, 31, and retrosplenial cortex. The common activations were stronger for execution than for observation and the interhemispheric differences were smaller for observation than for execution, contributing to the attribution of action to the correct agent. The extensive overlap of parietal networks activated for action execution and observation supports the "mental simulation theory" which assigns the role of understanding others' actions to the entire distributed neural network responsible for the execution of actions, and not the concept of "mirroring" which reflects the function of a certain class of cells in a couple of cortical areas.

Observation of action: grasping with the mind's hand

Engagement of the primary motor cortex (MI) during the observation of actions has been debated for a long time. In the present study, we used the quantitative 14C-deoxyglucose method in monkeys that either grasped 3-D objects or observed the same movements executed by humans. We found that the forelimb regions of the MI and the primary somatosensory (SI) cortex were significantly activated in both cases. Our study resolves a debate in the literature, providing strong evidence for use of MI representations during the observation of actions. It demonstrates that the observation of an action is represented in the primary motor and somatosensory cortices as is its execution. It indicates that in terms of neural correlates, recognizing a motor behavior is like executing the same behavior, requiring the involvement of a distributed system encompassing not only the premotor but also the primary motor cortex. We suggest that movements and their proprioceptive components are stored as motor and somatosensory representations in motor and somatosensory cortices, respectively, and that these representations are recalled during observation of an action.

Processing biases towards the preferred hand: valid and invalid cueing of left- versus right-hand movements

A Posner-like paradigm was employed to investigate the effects of valid and invalid cueing of each hand on reaction time, movement time and peak velocity in an aiming task. Given claims of left hemisphere superiority in movement selection and inhibition (and the privileged within-hemisphere access of the right hand to such systems), it was hypothesised that invalidly cueing the left hand (i.e. right-hand movement precued, left-handed movement required by a go signal) would result in increased reaction time relative to invalid right-hand cueing. The hypothesis was not confirmed as reaction times of both hands were slowed equivalently by invalid cueing. Nevertheless, it was found that the movement duration of the left hand was increased substantially by invalid cueing, while the right hand was unaffected on this measure, suggesting a possible intentional rather than attentional difference between the two hands. These results are discussed in terms of a possible asymmetry of intentional processes related to hand movement and the right-hand advantage in movement duration.

Memory-driven movements in limb apraxia: is there evidence for impaired communication between the dorsal and the ventral streams?

Memory-driven reaching and grasping movements were analysed in patients with left cerebral hemispheric damage and impaired gesture imitation. The dorsal and ventral streams of the visual pathway model of Milner and Goodale (Milner and Goodale, The Visual Brain in Action, 1995) are thought to operate relatively independently. However, cross-connections between the areas of each pathway are likely to enable interactions essential for higher-level praxis. Apraxic errors such as seen in gesture imitation can possibly be understood as arising from a disconnection of the two visual pathways. If the integrated action of the perceptual and visuomotor systems in patients with apraxia is compromised, then we would expect to find indications of impaired motor programming and misreaching in these patients when making movements driven by stored representations. Such a pattern, however, was not found in our sample of apraxic patients. Patients with limb apraxia produced normal movement kinematics and normal end-point accuracy when making memory-driven reaching movements with or without visual guidance of movement. Furthermore, perceptual information about object size and object distance were incorporated as normal in memory-driven grasping movements of these patients.

Hemispatial differences in visually guided aiming are neither hemispatial nor visual

Many studies have found differences in movements made to either side of the body midline. A popular interpretation of these differences has been that movements made by the arm, which is on same side of space in which the visual target appeared, are faster and better organised because they are processed within-hemisphere. Carey et al. (Experimental Brain Research 112 (1996) 496) showed that hemispatial movement differences cannot be accounted for by such a model. Their data suggested that biomechanical factors such as those proposed by Gordon et al. (Experimental Brain Research 99 (1994) 112) could better account for differences in movement duration and several characteristics of velocity and acceleration. The present study examines these arguments by requiring subjects to make rapid pointing movements in two experiments. In the first, results demonstrated that hemispatial effects occurred in pointing movements made without any visual target or vision of the limb. These findings suggest that intra- and inter-hemispheric models are untenable. Gordon et al. argued that hand path direction relative to the long axis of the upper arm accounts for hemispatial effects on kinematics. In the second experiment hand path direction and hemispace were dissociated. Contralateral movements were performed more efficiently than ipsilateral movements, when target and starting positions required an adductive movement to acquire the contralateral target and an abductive movement to acquire the ipsilateral target. These results provide strong support for the Gordon et al. model, although the possible contributions of other dynamic factors and/or differential control of proximal and distal muscles by the central nervous system cannot be ruled out.

Different brain correlates for watching real and virtual hand actions

We investigated whether observation of actions reproduced in three-dimensional virtual reality would engage perceptual and visuomotor brain processes different from those induced by the observation of real hand actions. Participants were asked to passively observe grasping actions of geometrical objects made by a real hand or by hand reconstructions of different quality in 3D virtual reality as well as on a 2D TV screen. We found that only real actions in natural environment activated a visuospatial network including the right posterior parietal cortex. Observation of virtual-reality hand actions engaged prevalent visual perceptual processes within lateral and mesial occipital regions. Thus, only perception of actions in reality maps onto existing action representations, whereas virtual-reality conditions do not access the full motor knowledge available to the central nervous system.

Hemispheric asymmetry and interhemispheric transfer in pointing depend on the spatial components of the movement

The purpose of the present study was to compare the asymmetry and transfer in 3 pointing movements with increasing spatial requirements. The triggering signal was one of four visual targets appearing on the right or left of a central fixation point (FP). The first task consisted in simply removing the arm from the starting platform; the second was a pointing movement towards the FP, and the third was a classical pointing task towards one of the four lateral targets. 20 right-handers (Rhrs) and 20 left-handers (Lhrs) participated in this experiment. In the classical pointing task (task 3), the reaction times were shorter in the Rhrs using their left hand. No such hand-related difference was observed in the Lhrs. No hand asymmetry was observed in the other tasks. In addition, the responses were faster in the uncrossed than in the crossed conditions, in task 3 only. It was concluded that in pointing tasks, both the hemispheric asymmetry and the interhemispheric transfer depend on the spatial requirements of the movement.

Hand, space and attentional asymmetries in goal-directed manual aiming

Two experiments were conducted to explore the interaction of the two cerebral hemispheres in motor control, by examining hand, space and attentional asymmetries in goal-directed aiming. In Experiment 1, right-handed subjects moved to targets more quickly with their right hand than their left hand. In addition, each hand was faster when moving in its own hemispace. Although in a control condition, movements were initiated more quickly with the left hand, visual distractors disrupted left hand performance more than right hand performance. For contralateral aiming, ipsilateral distractors caused the greatest interference. In Experiment 2, when targets and distractors were all presented at the midline, a right hand advantage was found for movement time along with a left hand advantage for reaction time, independent of target and distractor location. Our findings are discussed in terms of a right hemisphere role in movement preparation and the allocation of attention in space, and greater left hemisphere involvement in movement execution.

Reaching to ipsilateral or contralateral targets: within-hemisphere visuomotor processing cannot explain hemispatial differences in motor control

Aiming movements made to visual targets on the same side of the body as the reaching hand typically show advantages as compared to aiming movements made to targets on the opposite side of the body midline in the contralateral visual field. These advantages for ipsilateral reaches include shorter reaction time, higher peak velocity, shorter duration and greater endpoint accuracy. It is commonly hypothesized that such advantages are related to the efficiency of intrahemispheric processing, since, for example, a left-sided target would be initially processed in the visual cortex of the right hemisphere and that same hemisphere controls the motor output to the left hand. We tested this hypothesis by examining the kinematics of aiming movements made by 26 right-handed subjects to visual targets briefly presented in either the left or the right visual field. In one block of trials, the subjects aimed their finger directly towards the target; in the other block, subjects were required to aim their movement to the mirror symmetrical position on the opposite side of the fixation light from the target. For the three kinematic measures in which hemispatial differences were obtained (peak velocity, duration and percentage of movement time spent in deceleration), the advantages were related to the side to which the motor response was directed and not to the side where the target was presented. In addition, these effects tended to be larger in the right hand than in the left, particularly for the percentage of the movement time spent in deceleration. The results are interpreted in terms of models of biomechanical constraints on contralateral movements, which are independent of the hemispace of target presentation.

The influence of target perturbation on manual aiming asymmetries in right-handers

Ten right-handed subjects performed 100 target-aiming movements with each hand. These movements were directed toward a small target on the midline. On 60% of the trials, the target remained stationary. On other randomly placed trials, the target "jumped" to a location 3 cm to the right (20%) or left (20%) of its original position when the cursor had travelled 6.5 cm. Although no hand differences were evident in the control condition, the right hand acquired the new target location more quickly than the left hand when the target was perturbed in either direction. Kinematic data revealed that this advantage was not due to initiating an adjustment to the initial movement more rapidly, but rather less time decelerating the corrective movement. Movement adjustments on perturbed trials were implemented more rapidly in left space than right space independent of the hand doing the aiming. These asymmetries may reflect the differential role of the two cerebral hemispheres in the control of goal-directed movements.