Matthew Heath

The control of goal-directed limb movements: Correcting errors in the trajectory

Abstract A number of recent models of limb control have attempted to explain speed-accuracy trade-off in goal-directed movements on the basis of the characteristics of the muscular impulses that are specified prior to movement initiation. In contrast, studies from our laboratory have demonstrated that, even for very rapid aiming movements, the characteristics of the movement trajectory change with the availability of visual information about the position of the limb and the target during the movement. Moreover, when the movement of the limb is perturbed at movement initiation by an electromagnetic force, performers can rapidly adjust their aiming movements in order to hit the target if visual feedback is available. The performer can also rapidly adjust to unexpected changes in target size and amplitude. Visually based adjustments to the movement trajectory can be either discrete or continuous. PsycINFO classification: 2330; 2343

Can the motor system resolve a premovement bias in grip aperture? Online analysis of grasping the Müller-Lyer illusion.

The goal of the present investigation was to determine the time-course by which the motor system might resolve the context-dependent effects of a visual illusion [i.e., the Müller-Lyer (ML) figure]. Specifically, we asked participants to scale their grip aperture (GA) to the perceived size of an object embedded within a ML figure in advance of closed-loop (CL) and open-loop (OL) grasping movements. As a result, premovement GA was biased in a direction consistent with the perceptual effects of the illusion. We reasoned that such a manipulation might provide a novel opportunity to determine whether the motor system is able to resolve a biased GA immediately following the onset of a response [i.e., in accord with the perception/action model (PAM); Milner and Goodale 1995, The visual brain in action, Oxford University Press], or gradually as the action unfolds [i.e., in accord with the planning/control model (PCM); Glover and Dixon 2002, Percept Psychophys 64:266–278]. It was found that biasing GA in advance of movement resulted in a reliable effect of the ML figure throughout CL and OL trials (i.e., up to 80% of grasping time). Although the present findings appear contrary to the theoretical tenets of the PAM and the PCM, it is proposed that biasing GA in advance of movement leads to offline visual processing and a feedforward mode of grasping control, thus accounting for the illusion-induced effect throughout the grasping response.

Visual feedback schedules influence visuomotor resistance to the Müller-Lyer figures

We examined whether blocked or random visual feedback schedules influence visuomotor resistance to the Müller-Lyer (ML) illusion. Participants completed closed-loop (CL) and open-loop (OL) grasping movements to an object embedded within fins-in and fins-out ML configurations. In the blocked feedback schedule, CL and OL trials were completed in separate blocks of trials, whereas visual conditions were randomly interleaved in the random feedback schedule. The results of the blocked feedback schedule showed that OL, but not CL, trials were influenced in a direction consistent with the perceptual effects of the ML illusion. For the random feedback schedule, however, both CL and OL trials were influenced by the illusion. We have interpreted these results to reflect the fact that participants evoked distinct control strategies based on the predicted availability of visual feedback. Specifically, the refractory nature of CL trials in the blocked feedback schedule suggests that advance knowledge that visual feedback would be available during a response encouraged an online control strategy wherein metrical visual information supported grasping. When visual feedback was unavailable (i.e., blocked OL trials), or could not be predicted in advance of a response (i.e., random CL and OL trials), it is proposed that movements were structured offline via perception-based visual information that was “tricked” by the cognitive properties of the ML illusion.

Time course analysis of closed- and open-loop grasping of the Müller-Lyer illusion.

The authors investigated whether the early or later stages of closed-loop (CL) and open-loop (OL) grasping movements were differentially influenced by the Müller-Lyer (ML) illusion. Participants (N = 21) reached out and grasped small (5 cm) and large (7 cm) objects embedded within fins-in and fins-out ML configurations. Grasping time (GT) was normalized, and absolute grip aperture (GA) as well as scaled illusion effects were computed at 20%, 40%, 60%, and 80% of GT. The results indicated that CL trials were refractory to the illusory array (i.e., from 20% to 80% of GT), whereas OL trials were influenced by the ML figure during that same time. Those findings suggest that CL trials were supported by unitary and metrical visual information, whereas OL trials were entirely supported by perception-based visual information.

A lower visual field advantage for endpoint stability but no advantage for online movement precision

It has been proposed that visually guided reaching movements performed in the lower visual field (LVF) of peripersonal space are more effective and efficient than their upper visual field (UVF) counterparts (Danckert and Goodale 2001). In the present investigation we sought to determine whether this purported visual field asymmetry reflects advantaged processing of online visual feedback. To accomplish that objective, participants performed discrete reaching movements to each of three target locations in the LVF and UVF. In addition, reaches were completed under conditions wherein target location remained constant throughout a reaching response (i.e., control trials) and a separate condition wherein target location unexpectedly perturbed at movement onset (i.e., experimental trials). We reasoned that the target perturbation paradigm would provide a novel means to assess a possible superior–inferior visual field asymmetry for online reaching control. In terms of the impact of a target perturbation, both visual fields demonstrated equal proficiency integrating visual feedback for online limb adjustments. Interestingly, however, the spatial distribution of movement endpoints in the LVF was less than UVF counterparts (cf. Binsted and Heath 2005). Taken together, the present findings suggest that although LVF and UVF reaches readily use visual feedback to accommodate an unexpected target perturbation, reaches in the LVF elicit advantaged spatial benefits influencing the effectiveness of online limb corrections.

Muller-Lyer figures influence the online reorganization of visually guided grasping movements

In advance of grasping a visual object embedded within fins-in and fins-out Müller-Lyer (ML) configurations, participants formulated a premovement grip aperture (GA) based on the size of a neutral preview object. Preview objects were smaller, veridical, or larger than the size of the to-be-grasped target object. As a result, premovement GA associated with the small and large preview objects required significant online reorganization to appropriately grasp the target object. We reasoned that such a manipulation would provide an opportunity to examine the extent to which the visuomotor system engages egocentric and/or allocentric visual cues for the online, feedback-based control of action. It was found that the online reorganization of GA was reliably influenced by the ML figures (i.e., from 20 to 80% of movement time), regardless of the size of the preview object, albeit the small and large preview objects elicited more robust illusory effects than the veridical preview object. These results counter the view that online grasping control is mediated by absolute visual information computed with respect to the observer (e.g., Glover in Behav Brain Sci 27:3–78, 2004; Milner and Goodale in The visual brain in action 1995). Instead, the impact of the ML figures suggests a level of interaction between egocentric and allocentric visual cues in online action control.

Cerebral Specialization for Speech Production in Persons with Down Syndrome

The study of cerebral specialization in persons with Down syndrome (DS) has revealed an anomalous pattern of organization. Specifically, dichotic listening studies (e.g., Elliott & Weeks, 1993) have suggested a left ear/right hemisphere dominance for speech perception for persons with DS. In the current investigation, the cerebral dominance for speech production was examined using the mouth asymmetry technique. In right-handed, nonhandicapped subjects, mouth asymmetry methodology has shown that during speech, the right side of the mouth opens sooner and to a larger degree then the left side (Graves, Goodglass, & Landis, 1982). The phenomenon of right mouth asymmetry (RMA) is believed to reflect the direct access that the musculature on the right side of the face has to the left hemispheres speech production systems. This direct access may facilitate the transfer of innervatory patterns to the muscles on the right side of the face. In the present study, the lateralization for speech production was investigated in 10 right-handed participants with DS and 10 nonhandicapped subjects. A RMA at the initiation and end of speech production occurred for subjects in both groups. Surprisingly, the degree of asymmetry between groups did not differ, suggesting that the lateralization of speech production is similar for persons with and persons without DS. These results support the biological dissociation model (Elliott, Weeks, & Elliott, 1987), which holds that persons with DS display a unique dissociation between speech perception (right hemisphere) and speech production (left hemisphere).

No evidence of a lower visual field specialization for visuomotor control

The lower visual field (loVF) has been hypothesized to demonstrate specialization for skilled, visually guided action. According to Danckert and Goodale (Exp Brain Res 2002; 137:303–308), this visual field asymmetry indirectly suggests that the loVF has privileged connections to visuomotor networks within the dorsal visual pathway. Here we attempted to replicate the loVF advantage during the execution of a discrete aiming movement to targets of various widths (index of difficulty ranging from 1.5 to 5 bits). In addition, we employed trials in which vision of the target object was available or unavailable during the reaching movement to determine whether or not the purported visual field asymmetry reflects enhanced central planning (i.e., feedforward) or online control (i.e., feedback) processes. Reaching trajectories were examined for indicators of online amendments, and movement times and endpoint characteristics were examined to quantify possible visual field asymmetries in relative speed/accuracy trade-offs. In terms of reaching kinematics, it was found that vision of the target during the reaching movement resulted in greater online control of the reaching trajectory; however, no significant main effects or interactions involving visual field were observed. In other words, fixating in the upper or the lower region of peripersonal space did not influence the nature of reaching control (i.e., feedback vs. feedforward). Most importantly, our movement time and endpoint accuracy data elicited a robust speed/accuracy trade-off in both upper and lower regions of working space [cf. Fitts, J Exp Psychol 1954; 48:303–312]. Thus, and contrary to previous findings (such as those reported by Danckert and Goodale), the indices of difficulty coupled with the discrete aiming task used here did not elicit a lower visual field advantage for visually guided action.

Manual asymmetries in bimanual reaching: The influence of spatial compatibility and visuospatial attention

The goal of the present investigation was to explore the possible expression of hemispheric-specific processing during the planning and execution of a bimanual reaching task. Participants (N = 9) completed 80 bimanual reaching movements (requiring simultaneous, bilateral production of arm movements) to peripherally presented targets while selectively attending to either their left or right hand. Further, targets were presented in spatially compatible (ipsilateral to the aiming limb) and incompatible (contralateral to the aiming limb) response contexts. It was found that the left hand exhibited temporal superiority over the right hand in the response planning phase of bimanual reaching, indicating a left hand/right hemisphere advantage in the preparation of a bimanual response. During response execution, and consistent with the view that interhemispheric processing time (Barthelemy & Boulinguez, 2002) or biomechanical constraints (Carey, Hargreaves, & Goodale, 1996) generate temporal delays, longer movement times were observed in response to spatially incompatible target positions. However, no hemisphere-specific benefit was demonstrated for response execution. Based on these findings, we propose lateralized processing is present at the time of response planning (i.e., left hand/right hemisphere processing advantage); however, lateralized specialization appears to be annulled during dynamic execution of a bimanual reaching task.

Relative Processing Demands Influence Cerebral Laterality for Verbal-Motor Integration in Persons with Down Syndrome

The study of cerebral specialization in the Down syndrome (DS) population has revealed an anomalous pattern of organization. In particular, dichotic-listening studies have suggested a left-ear/right hemisphere dominance for speech perception, whereas motor control research has revealed a left hemisphere dominance for executive-motor control. In the present investigation, we employed a recent adaptation of the dichotic listening procedure to examine interhemispheric integration during the performance of a lateralized verbal-motor task. Specifically, using the selective dichotic-listening procedure, participants were required to complete a rapid left or right hand pointing movement to one of two pictorial icons corresponding to the word presented to their precued ear. We observed that persons with DS (N = 17) and age-matched controls (N = 35) exhibited a right-ear advantage (REA) for our dichotic-aiming task. While these results appear to contradict previous dichotic listening studies, we propose that the manifestation of a lateral ear advantage in the DS population may have more to do with the response requirements of the task than with the characteristics or complexity of the stimulus material.