Pointing movements are affected by size-contrast illusions

 The influence that the perceived size of visual targets has on the characteristics of pointing movements was investigated in the present study. A size-contrast illusion, known as the Ebbinghaus or Tichener circles, was employed. In this illusion, a target circle surrounded by several smaller circles is perceived to be larger than a target circle of the same physical size surrounded by several larger circles. Movement times of open-loop pointing responses directed to the perceptually smaller target circle were significantly longer than the movement times of pointing responses directed to the perceptually larger target circle. The extent of this difference was similar to that observed when pointing responses were directed at physically different-sized target circles that were not surrounded by other circles. In addition, when the perceptually smaller circle was enlarged so that it appeared to be the same size as the perceptually larger circle, the movement times became equivalent. This evidence supports the contention that the relative rather than the absolute size of the target has a major impact on the control and execution of pointing movements. Such a conclusion contradicts those made previously concerning grasping movements made under similar conditions and implies that pointing responses are more directly influenced by visual perceptual processing than grasping responses. Received: 3 September 1998 / Accepted: 15 December 1998     

Saccadic eye movements to visual and auditory targets

 Recent neurophysiological studies of the saccadic ocular motor system have lent support to the hypothesis that this system uses a motor error signal in retinotopic coordinates to direct saccades to both visual and auditory targets. With visual targets, the coordinates of the sensory and motor error signals will be identical unless the eyes move between the time of target presentation and the time of saccade onset. However, targets from other modalities must undergo different sensory-motor transformations to access the same motor error map. Because auditory targets are initially localized in head-centered coordinates, analyzing the metrics of saccades from different starting positions allows a determination of whether the coordinates of the motor signals are those of the sensory system. We studied six human subjects who made saccades to visual or auditory targets from a central fixation point or from one at 10° to the right or left of the midline of the head. Although the latencies of saccades to visual targets increased as stimulus eccentricity increased, the latencies of saccades to auditory targets decreased as stimulus eccentricity increased. The longest auditory latencies were for the smallest values of motor error (the difference between target position and fixation eye position) or desired saccade size, regardless of the position of the auditory target relative to the head or the amplitude of the executed saccade. Similarly, differences in initial eye position did not affect the accuracy of saccades of the same desired size. When saccadic error was plotted as a function of motor error, the curves obtained at the different fixation positions overlapped completely. Thus, saccadic programs in the central nervous system compensated for eye position regardless of the modality of the saccade target, supporting the hypothesis that the saccadic ocular motor system uses motor error signals to direct saccades to auditory targets. Received: 8 September 1995 / Accepted: 22 November 1996     

Coordination in prehension Information-based coupling of reaching and grasping

Prehension involves the coordination of a reaching and a grasping movement, such that the hand opens and closes in tune with the transport of the hand to the object to be grasped. To investigate this coordination, we focused on the transition from hand opening to hand closing in the grasping component of prehension. Earlier research has suggested that the time taken to close the hand remains constant over varying reaching amplitudes. In the present experiment, in which subjects reached for objects at six different distances and for objects that moved away from them at three different, constant speeds, hand-closure time was found to vary as a function of experimental conditions. Moreover, initiation of hand closure did not occur at a constant value of the (perceptually available) first-order time remaining until contact with the object. However, the variations observed, occurring as a function of initial hand-object distance and object speed, could be accounted for by an abstract dynamical model of perceptually driven postural changes. Received: 25 July 1996 / Accepted: 9 October 1997     

Deafferentation and pointing with visual double-step perturbations

 The capability of reprogramming movement responses following changes in the visual goal has been studied through the double-step paradigm. These studies have shown that: (a) continuous internal feedback-loops correct unconsciously the dynamic errors throughout the movement; (b) proprioceptive information and/or the efference copy have a privileged status among central processes, insuring on-line regulation of the initial motor commands; and (c) generation of the motor program starts after target presentation, and is continuously updated in the direction of the current internal representation of the target, at least until the onset of hand movement. This main corrective process of the initial program appears to be basically independent of visual reafference from the moving hand. However, the agreement with the possibility of a visuomotor loop, based on the comparison of the new updated representation of the target position and on the information from the moving hand, has not determined whether the correcting process is proprioceptive feedback dependent, or whether internal feedback-loops (efferent copies) are responsible for quick corrections of unfolding motor responses. To answer this question, the present experiment investigated the pointing behavior of a deafferented subject, using a double-step paradigm under various conditions of visual feedback and movement initiation. Overall, the present study (a) clearly showed the capacity of the motor system to modify and correct erroneous trajectories on the mere basis of internal feedback-loops and (b) emphasized the crucial role played by the target jump/arm triggering delay and the importance of the eye efferent copy for providing information about the spatial goal of the movement. Received: 10 July 1998 / Accepted: 23 November 1998     

Differential effect of task conditions on errors of direction and extent of reaching movements

 Invariant patterns in the distribution of the endpoints of reaching movements have been used to suggest that two important movement parameters of reaching movements, direction and extent, are planned by two independent processing channels. This study examined this hypothesis by testing the effect of task conditions on variable errors of direction and extent of reaching movements. Subjects made reaching movements to 25 target locations in a horizontal workspace, in two main task conditions. In task 1, subjects looked directly at the target location on the horizontal workspace before closing their eyes and pointing to it. In task 2, arm movements were made to the same target locations in the same horizontal workspace, but target location was displayed on a vertical screen in front of the subjects. For both tasks, variable errors of movement extent (on-axis error) were greater than for movement direction (off-axis error). As a result, the spatial distributions of endpoints about a given target usually formed an ellipse, with the principal axis oriented in the mean movement direction. Also, both on- and off-axis errors increased with movement amplitude. However, the magnitude of errors, especially on-axis errors, scaled differently with movement amplitude in the two task conditions. This suggests that variable errors of direction and extent can be modified independently by changing the nature of the sensorimotor transformations required to plan the movements. This finding is further evidence that the direction and extent of reaching movements appear to be controlled independently by the motor system. Received: 8 October 1996 / Accepted: 14 January 1997     

Updating visual space during passive and voluntary head-in-space movements

The accuracy of our spatially oriented behaviors largely depends on the precision of monitoring the change in body position with respect to space during self-motion. We investigated observers’ capacity to determine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21° laterally. The subjects (n=6) showed small errors (mean=–0.6°) and little variability (mean=0.9°) in determining the position of an extinguished visual-target position when the head (and gaze) remained in a straight-ahead position. This accuracy was preserved when subjects voluntary rotated the head by various magnitudes in the direction of the memorized visual target (head rotations ranged between 5° and 60°). However, when the chair on which the subjects were seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6° and 36°) during the head-on-trunk rotations, the performance was markedly decreased, both in terms of spatial precision (mean error=5.6°) and variability (mean=5.7°). A control experiment showed that the prior knowledge of chair rotation occurrence had no effect on the perceived target position after head-trunk movements. Updating an earth-fixed target position during head-on-trunk rotations could be achieved through both cervical and vestibular signals processing, but, in the present experiment, the vestibular output was the only signal that had the potentiality to contribute to accurate coding of the target position after simultaneous head and trunk movements. Our results therefore suggest that the vestibular output is a noisy signal for the central nervous signal to update the visual space during head-in-space motion. Received: 2 June 1997 / Accepted: 16 March 1998     

Proximodistal structure of early reaching in human infants

Nine infants were tested, at the age of onset of reaching, seated on their parent’s lap and reaching for a small plastic toy. Kinematic analysis revealed that infants largely used shoulder and torso rotation to move their hands to the toy. Many changes in hand direction were observed during reaching, with later direction changes correcting for earlier directional errors. Approximately half of the infants started many reaches by bringing their hands backward or upward to a starting location that was similar across reaches. Individual infants often achieved highly similar peak speeds across their reaches. These results support the hypothesis that infants reduce the complexity of movement by using a limited number of degrees-of-freedom, which could simplify and accelerate the learning process. The proximodistal direction of maturation of the neural and muscular systems appears to restrict arm and hand movement in a way that simplifies learning to reach. Received: 27 July 1998 / Accepted: 26 March 1999     

Effects of spatial attention on directional manual and ocular responses

 The aim of the present study was to investigate how spatial attention influences directional manual and saccadic reaction times. Two experiments were carried out. In experiment 1 subjects were instructed to perform pointing responses toward targets that were located either in the same or the opposite hemifield with respect to the hemifield in which an imperative stimulus was presented. In experiment 2, they were instructed to make saccadic or pointing responses. The direction of the responses was indicated by the shape of the imperative stimulus. Reaction time (RT), movement time, and, in experiment 2, saccadic trajectory were measured. The imperative stimulus location was either cued (endogenous attention) or uncued. In the latter case the imperative stimulus presentation attracted attention (exogenous attention). The main results of the experiments were the following: First, exogenous attention markedly decreased the RTs when the required movement was directed toward the imperative stimulus location. This directional effect was much stronger for pointing than for ocular responses. Second, endogenously allocated attention did not influence differentially RTs of pointing responses directed toward or away the attended hemifield. In contrast, endogenous attention markedly favored the saccadic responses when made away from the cued hemifield. Third, regardless of cueing, the direction of movement affected both pointing and saccadic reaction times. Saccadic reaction times were faster when the required movement was directed upward, while manual reaction times were faster when the movement was directed downward. Fourth, lateralized spatial attention deviated the trajectory of the saccades contralateral to the attention location. This pattern of results supports the notion that spatial attention depends on the activation of the same sensorimotor circuits that program actions in space. Received: 11 June 1996 / Accepted: 26 October 1996     

Multi-joint limbs permit a flexible response to unpredictable events

The human arm is kinematically redundant, which may allow flexibility in the execution of reaching movements. We have compared reaching movements with and without kinematic redundancy to unpredictable double-step targets. Subjects sat in front of a digitising tablet and were able to view an arc of four targets reflected in the mirror as virtual images in the plane of the tablet. They were instructed to move, from a central starting point, in as straight a line as possible to a target. In one-third of trials, the target light switched to one of its neighbours during the movement. Subjects made 60 movements using shoulder, elbow and wrist and then another 60 movements in which only shoulder and elbow movement were allowed. By restraining the wrist, the limb was made non-redundant. The path length was calculated for each movement. In single-step trials, there was no significant difference between path lengths performed with and without wrist restraint. As expected there was a significant increase in path length during double-step trials. Moreover this increase was significantly greater when the wrist was restrained. The variability across both single- and double-step movements was significantly less while the wrist was restrained. Importantly the performance time of the movements did not alter significantly for single-step, double-step or restrained movements. These results suggest that the nervous system exploits the intrinsic redundancy of the limb when controlling voluntary movements and is therefore more effective at reprogramming movements to double-step targets. Received: 24 March 1997 / Accepted: 7 July 1997     

The development of goal-directed reaching in infants II. Learning to produce task-adequate patterns of joint torque

 Nine young infants were followed longitudinally from 4 to 15 months of age. They performed multijoint reaching movements to a stationary target presented at shoulder height. Time-position data of the hand, shoulder, and elbow were collected using an optoelectronic measurement system. In addition, we recorded electromyographic activity (EMG) from arm extensors and flexors. This paper documents how control problems of proximal torque generation may account for the segmented hand paths seen during early reaching. Our analysis revealed the following results: first, muscular impulse (integral of torque) increased significantly between the ages of 20 (reaching onset) and 64 weeks. That is, as infants got older they produced higher levels of mean muscular flexor torque during reaching. Data were normalized by body weight and movement time, so differences are not explained by anthropometric changes or systematic variations in movement time. Second, while adults produced solely flexor muscle torque to accomplish the task, infants generated flexor and extensor muscle torque at shoulder and elbow throughout a reach. At reaching onset more than half of the trials revealed this latter kinetic profile. Its frequency declined systematically as infants got older. Third, we examined the pattern of muscle coordination in those trials that exhibited elbow extensor muscle torque. We found that during elbow extension coactivation of flexor and extensor muscles was the predominant pattern in 67% of the trials. This pattern was notably absent in comparable adult reaching movements. Fourth, fluctuations in force generation, as measured by the rate of change of total torque (NET) and muscular torque (MUS), were more frequent in early reaching (20–28 weeks) than in the older cohort (52–64 weeks), indicating that muscular torque production became increasingly smoother and task-efficient. Our data demonstrate that young infants have problems in generating smooth profiles of proximal joint torques. One possible reason for this imprecision in infant force control is their inexperience in predicting the magnitude and direction of external forces. That infants learned to consider external forces is documented by their increasing reliance on these forces when performing voluntary elbow extensions. The patterns of muscle coordination underlying active elbow extensions were basically the same as during the prereaching phase, indicating that the formation of functional synergies is based on a basal repertoire of innervation patterns already observable in very early, spontaneous movements. Received: 5 January 1996 / Accepted: 19 August 1996     

The development toward stereotypic arm kinematics during reaching in the first 3 years of life

We recorded reaching movements from nine infants longitudinally from the onset of reaching (5th postnatal month) up to the age of 3 years. Here we analyze hand and proximal joint trajectories and examine the emerging temporal coordination between arm segments. The present investigation seeks (a) to determine when infants acquire consistent, adult-like patterns of multijoint coordination within that 3-year period, and (b) to relate their hand trajectory formation to underlying patterns of proximal joint motion (shoulder, elbow). Our results show: First, most kinematic parameters do not assume adult-like levels before the age of 2 years. At this time, 75% of the trials reveal a single peaked velocity profile of the hand. Between the 2nd and 3rd year of life, “improvements” of hand- or joint-related movement units are only marginal. Second, infant motor systems strive to obtain velocity patterns with as few force reversals as possible (uni- or bimodal) at all three limb segments. Third, the formation of a consistent interjoint synergy between shoulder and elbow motion is not achieved within the 1st year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12–15 months of age and continue to develop up to the 3rd year. In summary, the development toward adult forms of multijoint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace 3–4 months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the 2nd year of life. Received: 10 April 1996 / Accepted: 28 May 1997     

The development of postural adjustments during reaching in 6- to 18-month-old infants Evidence for two transitions

 The present study focused on the developmental changes of postural adjustments accompanying reaching movements in healthy infants. We made a longitudinal study of ten infants between 6 and 18 months of age. During each session multiple surface electromyograms of arm, neck, trunk and leg muscles at the right side of the body were recorded during right-handed reaching movements in two positions (”upright sitting” in an infant chair and ”long-leg” sitting without support). Simultaneously the whole session was recorded on video. Comparable data were present from the same infants at 3–5 months. Additionally, 18 infants (8–15 months) were assessed once during similar reaching tasks, but in these infants electromyographic activity of the trunk and neck muscles at both sides of the body were recorded. Our data revealed two transitions in the development of postural adjustments. The first transition was present around 6 months of age. At this age the postural muscles were infrequently activated during reaching movements. At 8 months ample postural activity reappeared and the infants developed the ability to adapt the postural adjustments to task-specific constraints such as arm movement velocity or the sitting position at the onset of the reaching movement. The second transition occurred between 12 and 15 months. Before 15 months the infants did not show consistent anticipatory postural activity, but from 15 months onwards they did, particularly in the neck muscles. Received: 3 March 1998 / Accepted: 5 February 1999     

Effects of geometric joint constraints on the selection of final arm posture during reaching: a simulation study

 Significant debate exists regarding the neural strategies underlying the positioning and orienting of the hand during voluntary reaching movements of the human upper extremity. Some authors have suggested that positioning and orienting are controlled independently, while others have argued that a strong interdependence exists. In an effort to address this uncertainty, our study employed computer simulations to examine the impact of physiological limitations of joint rotation on the proposed independence of hand position and orientation. Specifically, we analyzed the effects of geometric constraints on final arm postures using a 7 degree-of-freedom model of the human arm. For 20 different hand configurations within the attainable workspace, we computed sets of achievable joint angles by applying inverse kinematics. From each set, we then calculated the locus of possible elbow positions for the particular final hand posture. When the joints were allowed 360° of rotation, the loci formed complete circles; however, when joint ranges were limited to physiological values, the extent of the loci decreased to an average arc angle of 54.6° (±27.9°). Imposition of joint limits also led to practically linear relationships between joint angles within a solution set. These theoretical results suggest a requirement for coordinated interaction between control of the joints associated with hand position and those involved with hand orientation in order to ensure attainable joint trajectories. Furthermore, it is conceivable that some of the correlations observed between joint angles in the course of natural reaching movements result from geometric constraints. Received: 7 December 1998 / Accepted: 14 January 1999     

Directional effects of changes in muscle torques on initial path during simulated reaching movements

Adults are able to reach for an object for the first time with appropriate direction, speed, and accuracy. The rules by which the nervous system is able to set muscle activities to accomplish these outcomes are still debated and, indeed, the sensitivity of kinematics to variations in muscle torques is unknown for complex arm movements. As a result, this study used computer simulations to characterize the effects of change in muscle torque on initial hand path. The same change was applied to movements towards 12 directions in the horizontal plane, and changes were systematically manipulated such that: (1) torque amplitude was changed at one joint, (2) timing of torque was changed at one joint, and (3) amplitude and/or timing was changed at two joints. Results showed that simultaneous changes in torque amplitude at shoulder and elbow joints affected initial speed uniformly across direction. These results add to conclusions from previous experimental and modeling work that the simplest rule to produce a desired change in speed for any direction is to scale torque amplitude at both joints. In contrast, all simulations showed nonuniform effects on initial path direction. For some regions of the workspace, initial path direction was little affected by either a ±30% change in amplitude or a ±100-ms change in timing, whereas for other regions the same changes produced large effects on initial path direction. These findings suggest that the range of possible torque solutions to achieve a particular initial path direction varies within the workspace and, consequently, the requirements for an accurate initial path will vary within the workspace. Received: 27 July 1998 / Accepted: 15 April 1999     

Spatio-temporal and kinematic analysis of pointing movements performed by cerebellar patients with limb ataxia

Three patients with cerebellar limb ataxia and three age-matched controls performed arm-pointing movements towards a visual stimulus during an experimental procedure using a double-step paradigm in a three-dimensional space. Four types of trajectories were defined: P1, single-step pointing movement towards the visual stimulus in the initial position S1; P2, double-step pointing movement towards S1; P3, double-step straight pointing movement towards the second position S2; and P4, double-step pointing movement towards S2 with an initial direction towards S1. We found that the cerebellar patients, as well as the controls, were able to modify their motor programs, but with impaired timing, severe anomalies in the direction and amplitude of the changed movement trajectories and alteration of the precision of the pointing movements. Received: 26 February 1997 / Accepted: 13 October 1997     

Comparison of variability of initial kinematics and endpoints of reaching movements

 The accuracy of reaching movements to memorized visual target locations is presumed to be determined largely by central planning processes before movement onset. If so, then the initial kinematics of a pointing movement should predict its endpoint. Our study examined this hypothesis by testing the correlation between peak acceleration, peak velocity, and movement amplitude and the correspondence between the respective spatial positions of these kinematic landmarks. Subjects made planar horizontal reaching movements to targets located at five different distances and along five radially arrayed directions without visual feedback during the movements.The spatial dispersion of the positions of peak acceleration, peak velocity, and endpoint all tended to form ellipses oriented along the movement trajectory. However, whereas the peaks of acceleration and velocity scaled strongly with movement amplitude for all of the movements made at the five target distances in any one direction, the correlations with movement amplitude were more modest for trajectories aimed at each target separately. Furthermore, the spatial variability in direction and extent of the distribution of positions of peak acceleration and peak velocity did not scale differently with target distance, whereas they did for endpoint distributions. Therefore, certain features of the final kinematics are evident in the early kinematics of the movements as predicted by the hypothesis that they reflect planning processes. However, endpoint distributions were not completely predetermined by the Initial kinematics. In contrast, multivariate analysis suggests that adjustments to movement duration help compensate for the variability of the initial kinematics to achieve desired movement amplitude. These compensatory adjustments do not contradict the general conclusion that the systematic patterns in the spatial variability observed in this study reflect planning processes. On the contrary, and consistent with that conclusion, our results provide further evidence that direction and extent of reaching movements are planned and determined in parallel over time. Received: 23 March 1998 / Accepted: 2 September 1998     

The role of posterior parietal cortex in visually guided reaching movements in humans

 Positron emission tomography (PET) was used to identify the brain areas involved in visually guided reaching by measuring regional cerebral blood flow (rCBF) in six normal volunteers while they were fixating centrally and reaching with the left or right arm to targets presented in either the right or the left visual field. The PET images were registered with magnetic resonance images from each subject so that increases in rCBF could be localized with anatomical precision in individual subjects. Increased neural activity was examined in relation to the hand used to reach, irrespective of field of reach (hand effect), and the effects of target field of reach, irrespective of hand used (field effect). A separate analysis on intersubject, averaged PET data was also performed. A comparison of the results of the two analyses showed close correspondence in the areas of activation that were identified. We did not find a strict segregation of regions associated exclusively with either hand or field. Overall, significant rCBF increases in the hand and field conditions occurred bilaterally in the supplementary motor area, premotor cortex, cuneus, lingual gyrus, superior temporal cortex, insular cortex, thalamus, and putamen. Primary motor cortex, postcentral gyrus, and the superior parietal lobule (intraparietal sulcus) showed predominantly a contralateral hand effect, whereas the inferior parietal lobule showed this effect for the left hand only. Greater contralateral responses for the right hand were observed in the secondary motor areas. Only the anterior and posterior cingulate cortices exhibited strong ipsilateral hand effects. Field of reach was more commonly associated with bilateral patterns of activation in the areas with contralateral or ipsilateral hand effects. These results suggest that the visual and motor components of reaching may have a different functional organization and that many brain regions represent both limb of reach and field of reach. However, since posterior parietal cortex is connected with all of these regions, we suggest that it plays a crucial role in the integration of limb and field coordinates. Received: 23 August 1995 / Accepted: 8 August 1996     

Spatial-frequency-related efficacy of visual stabilisation of posture

The present study investigates the efficacy of visual stabilisation of posture for different spatial frequencies of a visual stimulus. Circular sine wave gratings were used to analyse the correlation between perception of motion in depth and stabilisation of fore-aft sway by the mechanism of detecting changes in target size. Body sway was recorded by a force-measuring platform (series A) and, in addition, by simultaneous tracking of infrared markers fixed to the subject’s body (series B). Mean velocity and amplitude (RMS) of body sway were calculated in both sagittal (a–p) and lateral (l–r) planes. Sagittal sway was of least magnitude when viewing contrast gratings with lowest thresholds, whereas higher thresholds resulted in increasing sway parameters. As intended by the design of the stimuli, sagittal sway was correlated closer with the stabilising effect exerted by the different stimuli than was lateral sway. Sway velocity was reduced more efficiently, however, with a lower correlation with the psychophysical transfer function, than was RMS sway. Since sway velocity measured by the platform is suggested to depend to a greter extent on dynamic muscle forces generated at each individual body site the results indicate that visual information can be used to reduce and thereby optimise dynamic muscle action (sway velocity) even though static body sway is either not or less reduced. A comparable economisation of sway velocity but not of RMS sway was also seen at the end of posture investigations, indicative of positive training effects. Received: 28 February 1997 / Accepted: 2 March 1998     

The influence of visual illusions on grasp position

 Visual size illusions have been shown to affect perceived object size but not the aperture of the hand when reaching to those same objects. Thus, vision for perception is said to be dissociated from vision for action. The present study examines the effect of visual-position and visual-shape illusions on both the visually perceived center of an object and the position of a grasp on that object when a balanced lift is required. The results for both experiments show that although the illusions influence both the perceived and the grasped estimates of the center position, the grasp position is more veridical. This partial dissociation is discussed in terms of its implications for streams of visual processing. Received: 17 November 1997 / Accepted: 11 September 1998     

C-Deoxyglucose mapping of the monkey brain during reaching to visual targets

The strategies used by the macaca monkey brain in controlling the performance of a reaching movement to a visual target have been studied by the quantitative autoradiographic ¹⁴C-DG method.Experiments on visually intact monkeys reaching to a visual target indicate that V1 and V2 convey visuomotor information to the cortex of the superior temporal and parietoccipital sulci which may encode the position of the moving forelimb, and to the cortex in the ventral part and lateral bank of the intraparietal sulcus which may encode the location of the visual target. The involvement of the medial bank of the intraparietal sulcus in proprioceptive guidance of movement is also suggested on the basis of the parallel metabolic effects estimated in this region and in the forelimb representations of the primary somatosensory and motor cortices. The network including the inferior postarcuate skeletomotor and prearcuate oculomotor cortical fields and the caudal periprincipal area 46 may participate in sensory-to-motor and oculomotor-to-skeletomotor transformations, in parallel with the medial and lateral intraparietal cortices.Experiments on split brain monkeys reaching to visual targets revealed that reaching is always controlled by the hemisphere contralateral to the moving forelimb whether it is visually intact or ‘blind'. Two supplementary mechanisms compensate for the ‘blindness' of the hemisphere controlling the moving forelimb. First, the information about the location of the target is derived from head and eye movements and is sent to the ‘blind' hemisphere via inferior parietal cortical areas, while the information about the forelimb position is derived from proprioceptive mechanisms and is sent via the somatosensory and superior parietal cortices. Second, the cerebellar hemispheric extensions of vermian lobules V, VI and VIII, ipsilateral to the moving forelimb, combine visual and oculomotor information about the target position, relayed by the ‘seeing' cerebral hemisphere, with sensorimotor information concerning cortical intended and peripheral actual movements of the forelimb, and then send this integrated information back to the motor cortex of the ‘blind' hemisphere, thus enabling it to guide the contralateral forelimb to the target.