Visuomotor Transformation

Figure 1: Example of written and spoken same and different-modality prime and target sentences in the Different-Different (DD) Different-Same (DS), Same-Different (SD), and Same-Same (SS) conditions. Participants: 60 students (41 females; mean age: 22.31, SD: 2.05). Task: lexical decision task (LDT) on concept-property target pairs presented either visually or aurally (see figure for an example of the experimental conditions).

The planning of visually guided reaches is accomplished by independent specification of extent and direction. We investigated whether this separation of extent and direction planning for well practiced movements could be explained by differences in the adaptation to extent and directional errors during motor learning. We compared the time course and generalization of adaptation with two types of screen cursor transformation that altered the relationship between hand space and screen space. The first was a gain change that induced extent errors and required subjects to learn a new scaling factor. The second was a screen cursor rotation that induced directional errors and required subjects to learn new reference axes. Subjects learned a new scaling factor at the same rate when training with one or multiple target distances, whereas learning new reference axes took longer and was less complete when training with multiple compared with one target direction. After training to a single target, subjects were able to transfer learning of a new scaling factor to previously unvisited distances and directions. In contrast, generalization of rotation adaptation was incomplete; there was transfer across distances and arm configurations but not across directions. Learning a rotated reference frame only occurred after multiple target directions were sampled during training. These results suggest the separate processing of extent and directional errors by the brain and support the idea that reaching movements are planned as a hand-centered vector whose extent and direction are established via learning a scaling factor and reference axes.

To further understand visuomotor transformations in reaching, we compared adaptation to display rotation and altered gain in planar movements. Healthy subjects moved a cursor on a screen by moving an indicator on a horizontal digitizing tablet with their unseen hand. Adaptation to rotation was less complete and was accompanied by markedly increased directional variability. Adaptation training on a single target generalized broadly for gain change, but poorly for rotation. We propose that the difficulty in adapting to rotation arises from the substantial demands on short-term working memory imposed by the need to determine the new reference direction. Adaptation to gain change makes more modest demands on short-term memory to recalibrate the visuomotor scaling factor.

We review a new computational model developed to understand how evidence about stimulus salience in visual search is translated into a saccade command. The model uses the activity of visually responsive neurons in the frontal eye field as evidence for stimulus salience that is accumulated in a network of stochastic accumulators to produce accurate and timely saccades. We discovered that only when the input to the accumulation process is gated could the model account for the variability in search performance and predict the dynamics of movement neuron discharge rates. This union of cognitive modeling and neurophysiology indicates how the visual-motor transformation can occur and provides a concrete mapping between neuron function and specific cognitive processes.

In this time-resolved functional magnetic resonance imaging (fMRI) study, we aimed to trace the neuronal correlates of covert planning processes that precede visually guided motor behavior. Specifically, we asked whether human posterior parietal cortex has prospective planning activity that can be distinguished from activity related to retrospective visual memory and attention. Although various electrophysiological studies in monkeys have demonstrated such motor planning at the level of parietal neurons, comparatively little support is provided by recent human imaging experiments. Rather, a majority of experiments highlights a role of human posterior parietal cortex in visual working memory and attention. We thus sought to establish a clear separation of visual memory and attention from processes related to the planning of goal-directed motor behaviors. To this end, we compared delayed-response tasks with identical mnemonic and attentional demands but varying degrees of motor planni...

In this time-resolved functional magnetic resonance imaging (fMRI) study, we aimed to trace the neuronal correlates of covert planning processes that precede visually guided motor behavior. Specifically, we asked whether human posterior parietal cortex has prospective planning activity that can be distinguished from activity related to retrospective visual memory and attention. Although various electrophysiological studies in monkeys have demonstrated such motor planning at the level of parietal neurons, comparatively little support is provided by recent human imaging experiments. Rather, a majority of experiments highlights a role of human posterior parietal cortex in visual working memory and attention. We thus sought to establish a clear separation of visual memory and attention from processes related to the planning of goal-directed motor behaviors. To this end, we compared delayed-response tasks with identical mnemonic and attentional demands but varying degrees of motor planni...

We report the case of M.B. who demonstrated severe optic ataxia with the right hand following stroke in the left hemisphere. The clinical picture may shed light on both the pathological characteristics of reaching and grasping actions, and potential rehabilitation strategies for optic ataxia. First, M.B. demonstrated a dissociation between severely impaired reaching and relatively spared grasping and tool use skills and knowledge, which confirms that grasping may be more intermingled with non-motoric cognitive mechanisms than reaching. Besides, M.B.'s reaching performance was sensitive to movement repetition. We observed a substitution effect: Reaching time decreased if M.B. repeatedly reached toward the same object but increased when object identity changed. This may imply that not only object localization but also object

Sensory-motor learning is commonly considered as a mapping process, whereby sensory information is transformed into the motor commands that drive actions. However, this directional mapping, from inputs to outputs, is part of a loop; sensory stimuli cause actions and vice versa. Here, we explore whether actions affect the understanding of the sensory input that they cause. Using a visuo-motor task in humans, we demonstrate two types of learning-related behavioral effects. Stimulusdependent effects reflect stimulus-response learning, while action-dependent effects reflect a distinct learning component, allowing the brain to predict the forthcoming sensory outcome of actions. Together, the stimulus-dependent and the actiondependent learning components allow the brain to construct a complete internal representation of the sensory-motor loop.

This study integrated functional connectivity measures using resting-state fMRI and behavioral data from a single-case observation of patient (PER) one year after right-hemispheric hemorrhage in the intraparietal sulcus and superior parietal lobule (IPS/SPL). PER showed no sign of clinical neglect. Her behavioral performance in the visuo-manual pointing task and in the letter discrimination task under conditions of endogenous and exogenous attentional cueing was compared between the left (affected) and right (unaffected/control) peripheral visual fields. The resting-state fMRI demonstrated an imbalance between the right and left hemispheric frontoparietal functional connectivity within the dorsal attentional and motor networks. Although the frontal and occipital cortices were not structurally damaged, specific fronto-occipital functional connectivity was imbalanced, which was strongly associated with the behavioral changes. First, the activity in the right frontal eye field showed w...

Visual control of locomotion typically involves both detection of current egomotion as well as anticipation of impending changes in trajectory. To determine if there are distinct neural systems involved in these aspects of steering control we used a slalom paradigm, which required participants to steer around objects in a computer simulated environment using a joystick. In some trials the whole slalom layout was visible (steering "preview" trials) so planning of the trajectory around future waypoints was possible, whereas in other trials the slalom course was only revealed one object at a time (steering "near" trials) so that future planning was restricted. In order to control for any differences in the motor requirements and visual properties between "preview" and "near" trials, we also interleaved control trials which replayed a participants' previous steering trials, with the task being to mimic the observed steering. Behavioral and fMRI results confirmed previous findings of superior parietal lobe (SPL) recruitment during steering trials, with a more extensive parietal and sensorimotor network during steering "preview" compared to steering "near" trials. Correlational analysis of fMRI data with respect to individual behavioral performance revealed that there was increased activation in the SPL in participants who exhibited smoother steering performance. These findings indicate that there is a role for the SPL in encoding path defining targets or obstacles during forward locomotion, which also provides a potential neural underpinning to explain improved steering performance on an individual basis.

Visuomotor transformation processes are essential when accurate reaching movements towards a visual target have to be performed. In contrast, those transformations are not needed for similar, but nonvisually guided, arm movements. According to previous studies, these transformations are carried out by neuronal populations located in the parietal and frontal cortical areas (the so-called "dorsal visual stream"). However, it is still debated whether these processes are mediated by the sequential and/or parallel activation of the frontoparietal areas. To investigate this issue, we designed a task where the same visual cue could represent either the target of a reaching/pointing movement or the go-signal for a similar but non-targeting arm movement. By subtracting the event-related potentials (ERPs) recorded from healthy subjects performing the two conditions, we identified the brain processes underlying the visuomotor transformations needed for accurate reaching/pointing movements. We then localized the generators by means of cortical current density (CCD) reconstruction and studied their dynamics from visual cue presentation to movement onset. The results showed simultaneous activation of the parietal and frontal areas from 140 to 260 ms. The results are interpreted as neural correlates of two critical phases of visuomotor integration, namely target selection and movement selection. Our findings suggest that the visuomotor transformation processes required for correct reaching/pointing movements do not rely on a purely sequential activation of the frontoparietal areas, but mainly on a parallel information processing system, where feedback circuits play an important role before movement onset.

We studied a patient (J.J.) with bilateral damage to those regions of the posterior parietal cortex (PPC) thought to be involved in prism adaptation. We demonstrated, for the first time in a parietal patient, that J.J. was unable to adapt to the visual perturbation induced by the optical prisms with either hand within four times the number of trials required by healthy adult subjects. We offer a novel account for the role of the PPC in prism adaptation: that the reach direction to the veridical target location specified in extrinsic limb-based coordinates must be de-coupled from the gaze direction to the perceived target location specified by intrinsic oculocentric coordinates in order to produce spatially accurate movements. This spatial discrepancy between gaze direction and reach direction may provide the necessary training signal required by the cerebellum to update the current internal model used to maintain spatial congruency between visual and proprioceptive maps of peripersonal space. The hypothesis is discussed in relation to recent disconnectionist accounts of optic ataxia.

Figure 1: Example of written and spoken same and different-modality prime and target sentences in the Different-Different (DD) Different-Same (DS), Same-Different (SD), and Same-Same (SS) conditions. METHOD Participants: 60 students (41 females; mean age: 22.31, SD: 2.05). Task: lexical decision task (LDT) on concept-property target pairs presented either visually or aurally (see figure 1 for an example of the experimental conditions).

Parietal lesions in humans can produce a specific disruption of visually guided hand movement, termed optic ataxia. The fact that the deficit mainly occurs in peripheral vision suggests that reaching in foveal and extrafoveal vision rely on two different neural substrates. In the present study, we have directly tested this hypothesis by event-related fMRI in healthy subjects. Brain activity was measured when participants reached toward central or peripheral visual targets. Our results confirm the existence of two systems, differently modulated by the two conditions. Reaching in central vision involved a restricted network including the medial intraparietal sulcus (mIPS) and the caudal part of the dorsal premotor cortex (PMd). Reaching in peripheral vision activated in addition the parietooccipital junction (POJ) and a more rostral part of PMd. These results show that reaching to the peripheral visual field engages a more extensive cortical network than reaching to the central visual field.

The fact that English has a specific part of speech, the comparative inflection of the adjective, to identify a binary category (higher, lower, etc.) raises the possibility that the tendency to form binary subdivisions is a fundamental process in human thought. Maybe, as with some primitive counting systems ("one, two, many"), two subdivisions are as many specific categories as we can easily cope with.

Several behavioral studies show that semantic content influences reach-to-grasp movement responses. However, not much is known about the influence of motor activation on semantic processing. The present study aimed at filling this gap by examining the influence of pre-activated motor information on a subsequent lexical decision task. Participants were instructed to observe a prime object (e.g., the image of a frying pan) and then judge whether the following target was a known word in the lexicon or not. They were required to make a keypress response to target words describing properties either relevant (e.g., handle) or irrelevant (e.g., ceramic) for action or unrelated to the prime object (e.g., eyelash). Response key could be located on the same side as the depicted action-relevant property of the prime object (i.e., spatially compatible key) or on the opposite side (i.e., spatially incompatible key). Results showed a facilitation in terms of lower percentage errors when the targe...

In two experiments we investigate whether different decision tasks were influenced by object manipulability. In Experiment 1, participants had to categorize objects represented by drawings or by words into artefacts or natural kinds. Natural objects received faster responses than artefacts, probably because the latter activate functional information that interferes with task responses. In Experiment 2, manipulability was made relevant to the task by asking participants to categorize items into two categories depending on whether or not they could be picked up and put inside a backpack. The disadvantage of artefacts over natural kinds was still found. Intriguingly, now an effect of manipulability was also found, but only with natural kinds, probably due to the fact that they convey information associated both with action ("how") and function ("what for"). The same pattern of results found with drawings and words suggests that also words activate motor information on how to grasp objects.

The dorsal visual processing stream subserves object-directed action, whereas the ventral visual processing stream subserves visual object recognition. Little is known about how information computed by dorsal-stream structures influences object recognition. We used continuous flash suppression to functionally separate information computed by the dorsal stream from that computed by the ventral stream. We show that information originating from the dorsal stream influences not only decisions requiring the selection of superordinate category labels, but also decisions that entail the selection of a basic-level object. We further show that information computed by the dorsal stream does not carry specific functional information about objects. Our results indicate that the dorsal stream, in isolation from the ventral stream, is agnostic as to the identity of the objects that it processes. We suggest that structures within the dorsal visual processing stream compute motor-relevant informati...

Processing within the dorsal visual stream subserves object-directed action, whereas visual object recognition is mediated by the ventral visual stream. Recent findings suggest that the computations performed by the dorsal stream can nevertheless influence object recognition. Little is known, however, about the type of dorsal stream information that is available to assist in object recognition. Here we present a series of experiments that explored different psychophysical manipulations known to bias the processing of a stimulus toward the dorsal visual stream in order to isolate its contribution to object recognition. We show that elongatedshaped stimuli, regardless of their semantic category and familiarity, when processed by the dorsal stream, elicit visuomotor grasp-related information that affects how we categorize manipulable objects. Elongated-shaped stimuli may reduce ambiguity during grasp preparation by providing a coarse cue to hand shaping and orientation that is sufficient to support action planning. We propose that this dorsal-stream based analysis of elongation along a principal axis is the basis for how the dorsal visual object processing stream can affect categorization of manipulable objects.

Information about an object's shape, its surface material, and how it can be grasped, among other information, becomes available soon after that object engages the visual system. Convergent evidence from neuroimaging and neuropsychology studies indicates that different types of information are differentially relevant to object recognition. For instance, brain-damaged patients may show deficits in their ability to reach, grasp or manipulate objects but retain the ability to identify these objects (e.g.

Background Our ability to acquire, refine and adapt skilled limb movements is a hallmark of human motor learning that allows us to successfully perform many daily activities. The capacity to acquire, refine and adapt other features of motor performance, such as visual search, eye-hand coordination and visuomotor decisions, may also contribute to motor learning. However, the extent to which refinements of multiple behavioral features and their underlying neural processes independently contribute to motor learning remains unknown. In the current study, we used an ethological approach to test the hypothesis that practice-related refinements of multiple behavioral features would be independently predictive of motor learning. Methods Eighteen healthy, young adults used an upper-limb robot with eye-tracking to practice six trials of a continuous, visuomotor task once a week for six consecutive weeks. Participants used virtual paddles to hit away 200 “Targets” and avoid hitting 100 “Distra...

Switching between competing grasp postures incurs costs on speeded performance. We examined switch costs between lift versus use actions under task conditions that required subjects to identify familiar objects. There were no asymmetrical interference effects, though reliable costs occurred when the same object required a different action on consecutive trials. In addition, lift actions were faster to objects targeted for a prospective use action than objects irrelevant to this intended goal. The benefit of a lift-then-use action sequence was not merely due to the production of two different actions in short order on the same object; use actions to an object marked for the distal goal of a lift action were not faster than use actions applied to another object. We propose that the intention to use an object facilitates the prior action of lifting it because the motor sequence lift-then-use is habitually conscripted to enact the proper function of an object.

The phenomenon of object-centred unilateral neglect suggests that some neural networks process spatial information relative to reference objects. To examine object-centred information processing, we measured regional cerebral blood flow in 11 normal subjects with PET. During each PET scan, a subject viewed a sample stimulus followed by a cue on a video screen. The sample consisted of two polygons, termed 'objects', each located in a corner of the screen. A small target spot appeared in a corner of each polygon. There were two tasks: the visuomotor task and the matching-to-sample task. In the visuomotor task, the subject moved a joystick in a direction indicated by either the location of the target spot inside the object (if object-centred coordinates were operative) or the location of the object relative to the video screen (if screen-centred coordinates were operative). In the matching-to-sample task, the subject moved the

The phenomenon of object-centred unilateral neglect suggests that some neural networks process spatial information relative to reference objects. To examine object-centred information processing, we measured regional cerebral blood flow in 11 normal subjects with PET. During each PET scan, a subject viewed a sample stimulus followed by a cue on a video screen. The sample consisted of two polygons, termed 'objects', each located in a corner of the screen. A small target spot appeared in a corner of each polygon. There were two tasks: the visuomotor task and the matching-to-sample task. In the visuomotor task, the subject moved a joystick in a direction indicated by either the location of the target spot inside the object (if object-centred coordinates were operative) or the location of the object relative to the video screen (if screen-centred coordinates were operative). In the matching-to-sample task, the subject moved the

Previous studies showed that the sequential verification of different sensory modality properties for concepts (e.g., BLENDER-loud; BANANA-yellow) incurs a processing cost, known as the modality-switch effect (Pecher et al. 2003; 2004). We assessed the influence of the mode of presentation of stimuli on the modality-switch effect in a property verification priming paradigm. Participants were required to perform a property verification task on a target sentence (e.g., “butter is yellowish”, “leaves rustle”) presented either visually or aurally after having been presented with a prime sentence (e.g., “the light is flickering”, “the sound is echoing”) that could either share both, one or none of the target’s mode of presentation and content modality. Results showed that the presentation and the content-driven effects were not cumulative. We conclude that the MSE is a two-fold effect which can occur at two different levels of information processing (i.e., perceptual and semantic).

In three experiments we assessed whether priming a hand shape activated motor information. Primes consisted of photographs of hands displaying one of three postures (precision, power, open hand). Targets consisted of photographs (Experiment 1 and 3) or words (Experiment 2) of objects, artifacts and natural kinds, manipulable with a precision (pencil) or with a power grip (bottle). Participants had to categorize objects into artifacts or natural kinds by pressing a different key. They had to respond to target-objects only when the targets followed the precision and the power primes, while they didn't have to respond when the targets followed the open hand (catch-trial). In Experiments 1 and 2, artifacts were processed slower than natural kinds, and natural kinds graspable with a power grip were processed faster than those graspable with a precision grip. These results confirm that visual primes activate general motor information on objects. However, only in Experiment 3, in which a motor training phase lead participants to associate a specific visual prime with a motor action, we found an interaction between Kind of Prime (precision, power) and Kind of Grip (precision, power grip). Results suggest that vision and motor information are strictly interwoven and support theories according to which object concepts are grounded in sensorimotor experience.

Movements made in response to sensory cues require the brain to transform sensory information into an appropriate motor output. Sensorimotor mappings used in daily life range from direct or bstandardQ to highly complex. In bnonstandardQ sensorimotor tasks, the visual stimulus guiding an action is often not the target of the action. A common example of such a nonstandard mapping is the use of a computer mouse on a horizontal surface to guide an object visible on a vertical monitor. The present study used event-related BOLD fMRI to examine how patterns of brain activity vary as sensorimotor mappings become progressively more complicated. We observed significantly different patterns of cortical activity depending on the level of dissociation between a sensory input and a required motor response. Our results suggest the presence of a functional network generally involved in performing the type of nonstandard sensorimotor tasks examined. This putative network includes regions of the primary motor cortex, medial motor areas, the superior parietal lobule (SPL), and the lateral premotor cortex. The extent of activity in active areas varied depending on the characteristics of the particular sensorimotor mapping used in performing a task. Furthermore, in addition to this putative network, specific task-related areas of activity were observed.

Critical thinking (CT) is important for consensus building. Although the practice of CT using debate is widely used to improve open-minded thought, the cognitive processes underlying this improvement remain poorly understood. Here, functional magnetic resonance imaging (fMRI) was employed to assess how neural responses while considering another's opinion are changed by CT practice and to determine whether the cortical network showing activation changes related to personality traits is relevant to consensus building. A total of 52 healthy participants were divided into three groups for an intervention; the participants read another's reasoning regarding a controversial issue and judged whether this person's viewpoint was affirmative during fMRI measurements. The intervention required them to prepare speech texts from a designated viewpoint based on both themselves and others. Compared to the control group, the group who took another's perspective showed enhanced activation of the right parieto-frontal network, that has been implicated in belief update, cognitive reappraisal of emotion, and self-perspective inhibition. Additionally, activation of the orbitofrontal cortex was negatively correlated with a stubbornness index. The results provide the first neural evidence of the effects of CT practice and support the notion that open-minded thought underlies the benefits of this practice.

Previous studies suggest that horizontal reaching movements are planned vectorially with independent specification of direction and extent. The transformation from visual to hand-centered coordinates requires the learning of a task-specific reference frame and scaling factor. We studied learning of a novel reference frame by imposing a screen-cursor rotation and learning of a scaling factor by imposing a novel gain. Previous work demonstrates that rotation and gain learning have different time courses and patterns of generalization. Here we used PET to identify and compare brain areas activated during rotation and gain learning, with a baseline motor-execution task as the subtracted control. Previous work has shown that the time courses of rotation and gain adaptation have a short rapid phase followed by a longer slow phase. We therefore also sought to compare activations associated with the rapid and slower phases of adaptation. We isolated the rapid phase by alternating opposite v...

The present study examined possible parallels between the structure of human visuospatial abilities and the organization of the neural systems. Forty-eight participants were tested on seven speeded visuospatial tasks. Three of these tasks were constructed so as to rely primarily on known ventral stream functions and four were constructed so as to rely primarily on known dorsal stream functions. Both sets of tasks spanned approximately the same range of diculty as indexed by both the speed and accuracy of decision making. Factor analysis of response times on the seven tasks revealed only two signi®cant factors. The putative ventral stream tasks all loaded heavily on one factor (mean loading=0.843) but only weakly on the other factor (mean loading=0.222); the putative dorsal stream tasks showed the opposite pattern in that they all loaded heavily on the second factor (mean loading=0.828) but only weakly on the ®rst factor (mean loading=0.229). These ®ndings are consistent with the hypothesis that human visuospatial abilities can be classi®ed using categories based on the specializations of underlying neural structures and systems. #

Forty-eight older adults were tested on a battery of seven speeded visuospatial tasks that were developed by Chen et al. [4] to measure the functions of the ventral and dorsal neural processing streams. Principal components analysis revealed only one factor with an eigenvalue greater than 1.0, and all of the tasks loaded heavily on this general factor. These results are in contrast to those reported in a previous study of young adults in which principal components analysis revealed two factors with eigenvalues greater than 1.0 [4]. Importantly, for young adults the second principal component was a bipolar factor which grouped the tasks based on the neural processing stream (i.e. ventral versus dorsal) whose function they had been designed to assess. The age-related difference in the factor structure of visuospatial abilities apparent from the present results may be interpreted as reflecting an age-related dedifferentiation of the neural processing streams consistent with the results of recent neuroimaging studies .

Reaching Grasping Saccades Visuomotor Perceptual choice Ataxia a b s t r a c t Expansion of the dorsal pulvinar in humans and its anatomical connectivity suggests its involvement in higher-order cognitive and visuomotor functions. We investigated visuomotor performance in a 31 year old patient (M.B.) with a lesion centered on the medial portion of the dorsal pulvinar (left > right) due to an atypical Sarcoidosis manifestation.

Across cultures, speakers produce iconic gestures, which add -through the movement of the speakers' hands -a pictorial dimension to the speakers' message. These gestures capture not only the motor content but also the visuospatial content of the message. Here, we provide first evidence for a direct link between the representation of perceptual information and the motor system that can account for these observations. Across four experiments, participants' hand movements captured both shapes that were directly perceived, and shapes that were only implicitly activated by unrelated semantic judgments of object words. These results were obtained even though the objects were not associated with any motor behaviors that would match the gestures the participants had to produce. Moreover, implied shape affected not only gesture selection processes but also their actual execution -as measured by the shape of hand motion through space -revealing intimate links between implied shape representation and motor output. The results are discussed in terms of ideomotor theories of action and perception, and provide one avenue for explaining the ubiquitous phenomenon of iconic gestures.

In two experiments we investigate whether different decision tasks were influenced by object manipulability. In Experiment 1, participants had to categorize objects represented by drawings or by words into artefacts or natural kinds. Natural objects received faster responses than artefacts, probably because the latter activate functional information that interferes with task responses. In Experiment 2, manipulability was made relevant to the task by asking participants to categorize items into two categories depending on whether or not ...

gyri (BA 32,31), the inferior and middle frontal gyri (BA 6,9,10), the superior temporal sulcus and the subcortical grey matter (caudate and thalamus). The critical cortical activations were found in the right-sided intraparietal sulcus (IPS), anterior cingulate cortex (ACC), and right-sided lateral prefrontal cortex (PFC). These three regions were entered into the DCM. Comparison on group level revealed superiority of dynamic causal models in which IPS and alternatively PFC served as input regions as compared to a model in which ACC is assumed to receive external inputs. No significant difference was observed between fully connected models with IPS (model 1) and PFC (model 2) as input regions. Subsequent analysis of intrinsic connectivity within two investigated models (1,2) disclosed significant parallel forward connections from IPS to the frontal areas (model 1) and from PFC to ACC and IPS (model 2).

We measured regional brain activity using fMRI as normal subjects performed a task of working memory (WM). Working memory refers to the process of actively holding information 'on-line' in the mind and using that information to guide behaviour. When a WM task was compared with a non-WM control task the following brain regions showed increased activation: the dorsolateral prefrontal cortex (DLPFC), frontal motor regions and the intraparietal sulcus. The same regions showed increased activity when we compared a high load WM task (subjects hold five items in WM) with a low load WM task (subjects hold only two items in WM). Our findings add to the evidence that the DLPFC plays a role in maintaining information in WM and suggest that DLPFC activation increases as a function of WM load (the amount of information held on-line.

We measured regional brain activity using fMRI as normal subjects performed a task of working memory (WM). Working memory refers to the process of actively holding information 'on-line' in the mind and using that information to guide behaviour. When a WM task was compared with a non-WM control task the following brain regions showed increased activation: the dorsolateral prefrontal cortex (DLPFC), frontal motor regions and the intraparietal sulcus. The same regions showed increased activity when we compared a high load WM task (subjects hold five items in WM) with a low load WM task (subjects hold only two items in WM). Our findings add to the evidence that the DLPFC plays a role in maintaining information in WM and suggest that DLPFC activation increases as a function of WM load (the amount of information held on-line.

The integration of multisensory information to produce movement specific representations of corporeal and peripersonal space is thought to take place in the posterior parietal cortex (PPC) 1, 2 . Both human and primate lesion studies of reaching movements have shown that the PPC may code the same movement in both visual (extrinsic) and postural (intrinsic) coordinates 3, 4 , and it is suggested that the superior lobule of the PPC may be responsible for dynamically maintaining an estimate of the current postural state of the body 4 . Brain imaging evidence in support of this suggestion was recently demonstrated in a study by our research group 5 . Using eventrelated functional magnetic resonance imaging (fMRI) we further investigated this suggestion by examining regions of brain activation associated with executing reaching movements directed to novel vs. repeated non-visual (postural) target locations. Our findings showed a change in BOLD activation signal in the precuneus (medial) region of the superior parietal cortex. This is consistent with previous suggestions that the medial aspect of the PPC may play a key role in dynamically maintaining a representation of the current state estimate of limb posture 4,5 .

To elucidate cortical mechanisms of visuomotor integration, we recorded whole-scalp neuromagnetic signals from six normal volunteers while they were viewing a black dot moving linearly at the speed of 4 degrees /sec within a virtual rectangle. The dot changed its direction randomly once every 0.3-2 sec. The subject either (1) fixated a cross in the center of the screen (eye fixation task), (2) followed the moving dot with the eyes (eye pursuit task), or (3) followed the dot with both the eyes and the right index finger (eye-finger pursuit task). Prominent magnetic signals, triggered by the changes of the direction of the dot, were seen in all conditions, but they were clearly enhanced by the tasks and were strongest during the eye-finger pursuit task and over the anterior inferior parietal lobule (aIPL). Source modeling indicated activation of aIPL [Brodmann's area (BA) 40], the posterosuperior parietal lobule (SPL; BA 7), the dorsolateral frontal cortex (DLF; BA 6), and the occ...

The phenomenon of object-centred unilateral neglect suggests that some neural networks process spatial information relative to reference objects. To examine object-centred information processing, we measured regional cerebral blood flow in 11 normal subjects with PET. During each PET scan, a subject viewed a sample stimulus followed by a cue on a video screen. The sample consisted of two polygons, termed 'objects', each located in a corner of the screen. A small target spot appeared in a corner of each polygon. There were two tasks: the visuomotor task and the matching-to-sample task. In the visuomotor task, the subject moved a joystick in a direction indicated by either the location of the target spot inside the object (if object-centred coordinates were operative) or the location of the object relative to the video screen (if screen-centred coordinates were operative). In the matching-to-sample task, the subject moved the

Adaptation of reaching movements to visuomotor transformations is generally thought to involve implicit or procedural learning. However, there is evidence that explicit or cognitive processes can also play a role (Redding and Wallace, 2006 [31]). For example, the early phase of adaptation to a visuomotor rotation appears to involve spatial working memory processes linked to mental rotation (Anguera et al., 2010 [11]). Since it is known that cognitive processes like mental rotation lead to larger reaction times (Georgopoulos and Massey, 1987 [12]), here we explored the relation between reaction time (RT) and reach error reduction. Two groups of subjects adapted their reaching movements to a 60 • visuomotor rotation either without RT constraints or with RT limited to 350 ms. In the unconstrained group, we found that adaption rate varied widely across subjects and was strongly correlated with RT. Subjects who decreased hand direction error (DE) rapidly exhibited prolonged RTs whereas little RT cost was seen in subjects who decreased DE gradually. RTs were also correlated with after-effects seen when the visuomotor rotation was removed. Subjects with the longest RTs exhibited the smallest after-effects. In the RT constrained group, all subjects exhibited gradual DE adaptation and large after-effects, similar to the fast responders in the free group. These results suggest that adaptation to a visuomotor rotation can involve processes that produce faster error reductions without increasing after-effects, but at an expense of larger reaction times. Possible candidates are processes related to spatial working memory, and more specifically, to mental rotation.