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The goal of the present investigation was to test the somatosensory processing model's (SPM) assertion that tactile actions and perceptions are mediated via egocentric and allocentric frames of reference, respectively (Dijkerman & de Hann's 2007: Behavioral and Brain Sciences). To accomplish that objective, Experiment 1 required that participants use their right hand to grasp and manually estimate differently sized objects placed on the forearm and palm of their left hand. Following each manual estimation trial, participants grasped the target object to equate tasks (i.e., grasping vs. manual estimation) for terminal tactile feedback. Notably, the different object locations (i.e., forearm and palm) were used to examine whether location-specific differences in mechanoreceptor density impacts the percept of object size (i.e., Weber's illusion). In addition, we computed just-noticeable-difference (JND) scores to determine whether grasping and manual estimations adhere to, or violate, the allocentric principles of Weber's law. Results for the grasping task elicited a null expression of Weber's illusion and JNDs for this task violated Weber's law. Results for the manual estimation task similarly exhibited a null expression of Weber's illusion; however, JNDs for the palm but not the forearm condition adhered to Weber's law. Experiment 2 showed that withdrawing terminal tactile feedback during forearm condition manual estimations resulted in responses that adhered to Weber's law. Thus, results provide some support for the SPM's contention that grasping and manual estimations are mediated via ego- and allocentric frames of reference, respectively. However, results further indicate that the dissociation is not complete and is, in part, influenced by the sensory consequences (i.e., terminal tactile feedback) associated with the response.

An issue of continued debate in the visuomotor control literature surrounds whether a 2D object serves as a representative proxy for a 3D object in understanding the nature of the visual information supporting grasping control. In an effort to reconcile this issue, we examined the extent to which aperture profiles for grasping 2D and 3D objects adheres to, or violates, the psychophysical properties of Weber’s law. Specifically, participants grasped differently sized 2D and 3D objects (20, 30, 40, and 50 mm of width) and we computed the just-noticeable-difference scores associated with aperture profiles at decile increments of normalized grasping time. The aperture profiles for 2D objects showed an early through late (i.e., 10% through 90%) adherence to Weber’s law, whereas the late stages of grasping 3D objects (i.e., >50% of grasping time) produced a fundamental violation of the law’s principles. As such, results suggest that grasping a 2D object is a top-down and cognitive task mediated via relative visual information. In contrast, the enriched shape information provided by a 3D object (i.e., stereoscopic vergence and disparity cues) allows for later aperture specification via absolute (Euclidean) visual information. Most notably, our results establish that the dimensional properties of an object influence the visual information mediating motor output, and further indicate that 2D and 3D objects are not representative proxies for one another in understanding the visual control of grasping.

Pantomime-grasps entail a response to an area adjacent to (i.e., spatially dissociated pantomime-grasp), or previously occupied by (i.e., no-target pantomime-grasp) a target. Previous work has reported that pantomime-grasps differ kinematically from naturalistic grasps (i.e., grasping a physical target object) - a result taken to evince that pantomime-grasps are perception-based and mediated via relative visual information. However, such actions differ not only in terms of their visual properties, but also because the former precludes haptic feedback related to a target's absolute size. The current study provides four experiments examining whether experimenter-induced haptic feedback influences the information mediating spatially dissociated and no-target pantomime-grasps. Just-noticeable-difference scores were computed to determine whether grasps adhered to, or violated, the relative psychophysical properties of Weber's law. Spatially dissociated pantomime-grasps performed with haptic feedback adhered to Weber's law (Experiments 1-3), whereas their no-target pantomime-grasp counterparts violated the law (Experiment 4). Accordingly, we propose that the top-down demands of decoupling stimulus-response relations in spatially dissociated pantomime-grasping renders aperture shaping via a visual percept that is not directly influenced by the integration of haptic feedback. In turn, the decreased top-down demands of no-target pantomime-grasps allows haptic feedback to serve as a reliable sensory resource supporting an absolute visuo-haptic calibration.

The production of transitive limb gestures is optimized when the appropriate tool can be physically manipulated. Little research has addressed the independent contributions of visual and nonvisual sources of sensory information to this phenomenon. In this study, 12 control, 37 LHD, and 50 RHD stroke patients performed transitive limb gestures to pantomime (to verbal command with the object visible) and object manipulation. Performance was more accurate in the object manipulation condition, suggesting that haptic and kinesthetic cues are important for transitive gesture production. Various patterns of performance were observed in the stroke groups, indicating that selective damage to the haptic/kinesthetic processing system is possible and common following unilateral stroke.

The model of apraxia proposed by Roy (1996) states that three patterns of apraxia should be observed across pantomime and imitation conditions. In the present analysis the frequency and severity of each pattern of apraxia were examined in a consecutive sample of left-(LHD) and right-hemisphere-damaged (RHD) patients during the production of intransitive limb gestures. The results indicated that a significant proportion of LHD and RHD patients were selectively impaired in formulating the ideational component of intransitive limb gestures.

The present study was designed to examine the frequency and severity of apraxia in patients with left- or right-hemisphere stroke in both pantomime and imitation conditions and to compare the frequency of apraxia in each stroke group across the three patterns of apraxia described in Roy's model (Roy, 1996). Ninety-nine stroke patients and 15 age-matched healthy adults performed eight transitive gestures to pantomime and to imitation. Gestural performance was quantified as accuracy on five performance dimensions; a composite score, an arithmetic combination of the five performance dimensions, was used as an index of the overall accuracy. Analyses revealed a comparable proportion of patients in each stroke group were classified as apraxic in the imitation condition, but a higher proportion of left stroke patients were apraxic in the pantomime condition. The severity of apraxia in each stroke group and the performance dimensions affected were, however, comparable. Analyses of the patterns of apraxia (pantomime alone, imitation alone or apraxia in both conditions) revealed a higher frequency of apraxia in both stroke groups for the pattern reflecting apraxia in both conditions, indicating that a disruption at the movement execution stage of gesture performance was most common.

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.

In this investigation participants formulated a grip aperture (GA) consistent with the size of an object embedded within a Muller-Lyer (ML) figure prior to initiating visually guided grasping movements. The accuracy of the grasping response was emphasized to determine whether or not the visuomotor system might resolve the premovement bias in GA very early in the response, as predicted by the perception/action model (PAM: Milner & Goodale, 1995), or more gradually as the action unfolds over time, as predicted by the planning/control model (PCM: Glover & Dixon, 2002). Grasping time (GT) was normalized and GA was measured at 11 time points: a premovement GA and ten time points beginning at 10% of GT and ending at 100% of GT. Not surprisingly, premovement GA was influenced in a direction consistent with the perceptual effects of the ML figure (cf. Westwood, Heath, & Roy, 2000). Interestingly, however, the bias in GA was not attenuated during the response, that is, a reliable illusory effect was observed for upwards of 90% of GT. Only at 100% of GT did we find resolution of the illusory effect. Based on these results, it is proposed that shaping GA in advance of response execution leads to a feedforward mode of grasping control in which the spatiotemporal characteristics of GA are controlled via an offline and context-dependent planning mechanism (i.e., the ventral visual pathway).

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.

Sixteen right-handed male adults performed a pointing task without vision. The participant's arm was moved passively to one of four targets which was subsequently pointed to following a delay of 1, 2, or 10 s. Our previous research on visual memory for target location showed a rapid decay which was comparable for both hands. The present study of memory for kinesthetic target location found decay for the left hand only. These findings suggest two memory stores for target location information, one visual which decays over time but is accessible to both hands and another based on kinesthetic information which is more stable and limb specific.