Delayed reaching and grasping in patients with optic ataxia

The contribution of semantic distance knowledge to size constancy in perception and grasping when visual cues are limited

To achieve a stable perception of object size in spite of variations in viewing distance, our visual system needs to combine retinal image information and distance cues. Previous research has shown that, not only retinal cues, but also extraretinal sensory signals can provide reliable information about depth and that different neural networks (perception versus action) can exhibit preferences in the use of these different sources of information during size-distance computations. Semantic knowledge of distance, a purely cognitive signal, can also provide distance information. Do the perception and action systems show differences in their ability to use this information in calculating object size and distance? To address this question, we presented 'glow-in-the-dark' objects of different physical sizes at different real distances in a completely dark room. Participants viewed the objects monocularly through a 1-mm pinhole. They either estimated the size and distance of the objects or attempted to grasp them. Semantic knowledge was manipulated by providing an auditory cue about the actual distance of the object: "20 cm", "30 cm", and "40 cm". We found that semantic knowledge of distance contributed to some extent to size constancy operations during perceptual estimation and grasping, but size constancy was never fully restored. Importantly, the contribution of knowledge about distance to size constancy was equivalent between perception and action. Overall, our study reveals similarities and differences between the perception and action systems in the use of semantic distance knowledge and suggests that this cognitive signal is useful but not a reliable depth cue for size constancy under restricted viewing conditions.

Assessment and recovery of visually guided reaching deficits following cerebellar stroke

The cerebellum is known to play an important role in the coordination and timing of limb movements. The present study focused on how reach kinematics are affected by cerebellar lesions to quantify both the presence of motor impairment, and recovery of motor function over time. In the current study, 12 patients with isolated cerebellar stroke completed clinical measures of cognitive and motor function, as well as a visually guided reaching (VGR) task using the Kinarm exoskeleton at baseline (∼2 weeks), as well as 6, 12, and 24-weeks post-stroke. During the VGR task, patients made unassisted reaches with visual feedback from a central 'start' position to one of eight targets arranged in a circle. At baseline, 6/12 patients were impaired across several parameters of the VGR task compared to a Kinarm normative sample (n = 307), revealing deficits in both feed-forward and feedback control. The only clinical measures that consistently demonstrated impairment were the Purdue Pegboard Task (PPT; 9/12 patients) and the Montreal Cognitive Assessment (6/11 patients). Overall, patients who were impaired at baseline showed significant recovery by the 24-week follow-up for both VGR and the PPT. A lesion overlap analysis indicated that the regions most commonly damaged in 5/12 patients (42% overlap) were lobule IX and Crus II of the right cerebellum. A lesion subtraction analysis comparing patients who were impaired (n = 6) vs. unimpaired (n = 6) on the VGR task at baseline showed that the region most commonly damaged in impaired patients was lobule VIII of the right cerebellum (40% overlap). Our results lend further support to the notion that the cerebellum is involved in both feedforward and feedback control during reaching, and that cerebellar patients tend to recover relatively quickly overall. In addition, we argue that future research should study the effects of cerebellar damage on visuomotor control from a perception-action theoretical framework to better understand how the cerebellum works with the dorsal stream to control visually guided action.

Spatial bias in anti-saccade endpoints following bilateral dorsal posterior parietal lesions

Anti-saccades are eye movements in which the saccade is executed in the opposite direction of a visual target and are often hypometric. Because the visual target and saccade goal are decoupled, it has been suggested that competition between the two locations occurs and needs to be resolved. It has been hypothesized that the hypometria of anti-saccades reflects this spatial competition by revealing a bias towards the visual target. To confirm that this hypometria is not simply due to reduced gain, we tested 10 healthy subjects on three different anti-saccade spatial configuration tasks: 90° away across hemifields, 90° away within the same hemifield and 180° away (classic, diagonally opposite). Specifically, we examined whether saccade endpoints showed evidence for the visual target location's interference with anti-saccade programming and execution processes. Among other neural substrates involved in anti-saccades production, the dorsal posterior parietal cortex (PPC) has been implicated in the spatial inhibition of contralateral visual target. To gain insight into the neural processes involved in spatial competition during anti-saccades, we also tested one patient with a bilateral dorsal PPC lesion. In all spatial configurations, we observed that anti-saccade endpoints demonstrated a spatial bias towards the visual target for all participants, likely due to an incomplete inhibition of the visual target location. This spatial bias was exacerbated in our patient, which suggests that the dorsal PPC contributes to the amalgamation of the two competing spatial representations.

Eye movement patterns in complex tasks: Characteristics of ambient and focal processing

Analyzing the time course of eye movements during scene viewing often indicates that people progress through two distinct modes of visual processing: an ambient mode, which is associated with overall spatial orientation in a scene, followed by a focal mode, which requires central vision of an object. However, the shifts between ambient and focal processing modes have mainly been identified relative to changes in the environment, such as relative to the onset of various visual stimuli but also following scene cuts or subjective event boundaries in dynamic stimuli. The results so far do not allow conclusions about the nature of the two processing mechanisms beyond the influence of externally triggered events. It remains unclear whether people shift back and forth from ambient to focal processing also based on internal triggers, such as switching between different tasks while no external event is given. The present study therefore investigated ambient to focal processing shifts in an active task solving paradigm. The Rubik’s Cube task introduced here is a multi-step task, which can be broken down into smaller sub-tasks that are performed serially. The time course of eye movements was analyzed at multiple levels of this Rubik’s Cube task, including when there were no external changes to the stimuli but when internal representations of the task were hypothesized to change (i.e., switching between different sub-tasks). Results suggest that initial ambient exploration is followed by a switch to more focal viewing across various levels of task processing with and without external changes to the stimuli. More importantly, the present findings suggest that ambient and focal eye movement characteristics might serve as a probe for the attentional state in task processing, which does not seem to be influenced by changes in task performance.

Machine learning methods detect arm movement impairments in a patient with parieto-occipital lesion using only early kinematic information

Patients with lesions of the parieto-occipital cortex typically misreach visual targets that they correctly perceive (optic ataxia). Although optic ataxia was described more than 30 years ago, distinguishing this condition from physiological behavior using kinematic data is still far from being an achievement. Here, combining kinematic analysis with machine learning methods, we compared the reaching performance of a patient with bilateral occipitoparietal damage with that of 10 healthy controls. They performed visually guided reaches toward targets located at different depths and directions. Using the horizontal, sagittal, and vertical deviation of the trajectories, we extracted classification accuracy in discriminating the reaching performance of patient from that of controls. Specifically, accurate predictions of the patient's deviations were detected after the 20% of the movement execution in all the spatial positions tested. This classification based on initial trajectory decoding was possible for both directional and depth components of the movement, suggesting the possibility of applying this method to characterize pathological motor behavior in wider frameworks.

Eye-hand coordination: memory-guided grasping during obstacle avoidance

When reaching to grasp previously seen, now out-of-view objects, we rely on stored perceptual representations to guide our actions, likely encoded by the ventral visual stream. So-called memory-guided actions are numerous in daily life, for instance, as we reach to grasp a coffee cup hidden behind our morning newspaper. Little research has examined obstacle avoidance during memory-guided grasping, though it is possible obstacles with increased perceptual salience will provoke exacerbated avoidance maneuvers, like exaggerated deviations in eye and hand position away from obtrusive obstacles. We examined the obstacle avoidance strategies adopted as subjects reached to grasp a 3D target object under visually-guided (closed loop or open loop with full vision prior to movement onset) and memory-guided (short- or long-delay) conditions. On any given trial, subjects reached between a pair of flanker obstacles to grasp a target object. The positions and widths of the obstacles were manipulated, though their inner edges remained a constant distance apart. While reach and grasp behavior was consistent with the obstacle avoidance literature, in that reach, grasp, and gaze positions were biased away from obstacles most obtrusive to the reaching hand, our results reveal distinctive avoidance approaches undertaken depend on the availability of visual feedback. Contrary to expectation, we found subjects reaching to grasp after a long delay in the absence of visual feedback failed to modify their final fixation and grasp positions to accommodate the different positions of obstacles, demonstrating a more moderate, rather than exaggerative, obstacle avoidance strategy.

Card posting does not rely on visual orientation: A challenge to past neuropsychological dissociations

A common set of tasks frequently employed in the neuropsychological assessment of patients with visuomotor or perceptual deficits are the card-posting and the perceptual orientation matching tasks. In the posting task, patients have to post a card (or their hand) through a slot of varying orientations while the matching task requires them to indicate the slot's orientation as accurately as possible. Observations that damage to different areas of the brain (dorsal vs. ventral stream) is associated with selective impairment in one of the tasks - but not the other - has led to the suggestion that different cortical pathways process visual orientation information for perception versus action. In three experiments, we show that this conclusion may be premature as posting does not seem to rely on the processing of visual orientation information but is instead performed using obstacle avoidance strategies that require an accurate judgement of egocentric distances between the card's and the slot's edges. Specifically, we found that while matching is susceptible to the oblique effect (i.e., common perceptual orientation bias with higher accuracy for cardinal than oblique orientations), this was not the case for posting, neither in immediate nor in memory-guided conditions. In contrast to matching, posting errors primarily depended on biomechanical demands and reflected a preference for performing efficient and comfortable movements. Thus, we suggest that previous dissociations between perceptual and visuomotor performance in letter posting tasks are better explained by impairments in egocentric and allocentric spatial processing than by independent visual processing systems.

The posterior parietal cortex processes visuo-spatial and extra-retinal information for saccadic remapping: A case study

Optimally collecting information and controlling behaviour require that we constantly scan our visual environment through eye movements. How the dynamic interaction between short-lived retinal images and extra-retinal signals of eye motion results in our subjective experience of visual stability remains a major issue in Cognitive Neuroscience. The present study aimed to assess and determine the nature of the contribution of the posterior parietal cortex (PPC) to the saccadic remapping mechanisms which contribute to such perceptual visual constancy. Perceptual responses in transsaccadic visual localization tasks were measured in a patient presenting with a PPC lesion and manifesting optic ataxia in the left hemifield with no neglect. Two perceptual localization tasks, each with versus without an intervening saccade, were used: the saccadic suppression of displacement (SSD) task (Ostendorf, Liebermann, & Ploner, 2010) and the peri-saccadic flash localization (LOC) task (Zimmerman & Lappe, 2010). Compared to a group of age-matched healthy subjects, the patient showed a specific pattern of perceptual deficits in the ataxic (left) hemifield. First, a significant impairment occurred in the stationary eye conditions, attesting for an alteration of visuo-spatial encoding. Second, in the saccade conditions, an additional perceptual deficit (an error of ~5° along the saccade direction) was observed in both tasks and mainly in conditions where extra-retinal signals are thought to be critically involved, revealing a constant underestimation by extra-retinal signals of the saccade size, despite preserved saccade accuracy. These findings highlight a crucial role of the PPC in saccadic remapping processes underlying perceptual visual constancy and provide empirical evidence for models such as Ziesche and Hamker's (2014).

Attention attracts action in healthy participants: An insight into optic ataxia?

Patients with optic ataxia following lesions to superior parts of the posterior parietal cortex make large errors when reaching to targets in the peripheral visual field. These errors are characterised by a contraction, or attraction, towards the point of fixation. These patients also have a reduced ability to allocate visual attention away from the point of fixation, but it is unclear whether the core symptom of misreaching is related to these attentional problems. In neurologically-intact adults, we tested the effect of an attention-demanding dual-task performed at fixation upon visually-guided reaching to peripheral targets. The dual task was associated with delayed movement initiation, and a shortened deceleration phase of movement suggesting a reduced ability to benefit from online control. It also induced a small but consistent shift of reaching endpoints towards the side of fixation. Our experimental restriction of visual attention thus impaired both the programming and control of reaching, and induced a spatial pattern of errors that was qualitatively reminiscent of optic ataxia, albeit much less severe. These findings are consistent with a close functional link between attention and action in the healthy brain, and suggest that attentional disturbances could be a core component of optic ataxia following parietal lesions.

Egocentric and allocentric spatial representations in a patient with Bálint-like syndrome: A single-case study

Previous studies suggested that egocentric and allocentric spatial representations are supported by neural networks in the occipito-parietal (dorsal) and occipito-temporal (ventral) streams, respectively. The present study aimed to explore the integrity of ego- and allo-centric spatial representations in a patient (GP) who presented bilateral occipito-parietal damage consistent with the picture of a Bálint-like syndrome. GP and healthy controls were asked to provide memory-based spatial judgments on triads of objects after a short (1.5sec) or long (5sec) delay. The results showed that GP's performance was selectively impaired in the Ego/1.5sec delay condition. As a whole, our findings suggest that GP's spared ventral stream could generate short- and long-term allocentric representations. Furthermore, the stored perceptual representation processed within the ventral stream might have been used to generate long-term egocentric representation. Conversely, the generation of short-term egocentric representation appeared to be selectively undermined by the damage of the dorsal stream.

Time-resolved decoding of planned delayed and immediate prehension movements

Different contexts require us either to react immediately, or to delay (or suppress) a planned movement. Previous studies that aimed at decoding movement plans typically dissociated movement preparation and execution by means of delayed-movement paradigms. Here we asked whether these results can be generalized to the planning and execution of immediate movements. To directly compare delayed, non-delayed, and suppressed reaching and grasping movements, we used a slow event-related functional magnetic resonance imaging (fMRI) design. To examine how neural representations evolved throughout movement planning, execution, and suppression, we performed time-resolved multivariate pattern analysis (MVPA). During the planning phase, we were able to decode upcoming reaching and grasping movements in contralateral parietal and premotor areas. During the execution phase, we were able to decode movements in a widespread bilateral network of motor, premotor, and somatosensory areas. Moreover, we obtained significant decoding across delayed and non-delayed movement plans in contralateral primary motor cortex. Our results demonstrate the feasibility of time-resolved MVPA and provide new insights into the dynamics of the prehension network, suggesting early neural representations of movement plans in the primary motor cortex that are shared between delayed and non-delayed contexts.

When the affordances disappear: Dynamical and computational explanations of optic ataxia

Two options fuel the debate on the cognitive processes underlying the perception of affordances. On the one hand, the ecological theory of affordance fits with the methodological assumptions of the dynamical systems theory of cognition. On the other hand, it is nowadays common to conceive the perception of affordances within a computational framework. This article defends the explanatory power of a computational approach and aims to extend the concept of affordance beyond the boundaries of the dynamical systems theory of cognition. For that purpose, I consider the case of patients suffering from optic ataxia, a condition in which some aspects of visual guidance over reaching with the hand are lost following a lesion in the left parietal cortex. Etiological considerations, indeed, reveal that a computational approach to the perception of affordances allows for an explanation of ataxic behavior that is not available to the dynamical systems theory.

Memory for Spatial Locations in a Patient with Near Space Neglect and Optic Ataxia: Involvement of the Occipitotemporal Stream

Previous studies suggested that the occipitoparietal stream orients attention toward the near/lower space and is involved in immediate reaching, whereas the occipitotemporal stream orients attention toward the far/upper space and is involved in delayed reaching. In the present study, we investigated the role of the occipitotemporal stream in attention orienting and delayed reaching in a patient (GP) with bilateral damage to the occipitoparietal areas and optic ataxia. GP and healthy controls took part in three experiments. In the experiment 1, the participants bisected lines oriented along radial, vertical, and horizontal axes. GP bisected radial lines farther, and vertical lines more above, than the controls, consistent with an attentional bias toward the far/upper space and near/lower space neglect. The experiment 2 consisted of two tasks: (1) an immediate reaching task, in which GP reached target locations under visual control and (2) a delayed visual reaching task, in which GP and controls were asked to reach remembered target locations visually presented. We measured constant and variable distance and direction errors. In immediate reaching task, GP accurately reached target locations. In delayed reaching task, GP overshot remembered target locations, whereas the controls undershot them. Furthermore, variable errors were greater in GP than in the controls. In the experiment 3, GP and controls performed a delayed proprioceptive reaching task. Constant reaching errors did not differ between GP and the controls. However, variable direction errors were greater in GP than in the controls. We suggest that the occipitoparietal damage, and the relatively intact occipitotemporal region, produced in GP an attentional orienting bias toward the far/upper space (experiment 1). In turns, the attentional bias selectively shifted toward the far space remembered visual (experiment 2), but not proprioceptive (experiment 3), target locations. As a whole, these findings further support the hypothesis of an involvement of the occipitotemporal stream in delayed reaching. Furthermore, the observation that in both delayed reaching tasks the variable errors were greater in GP than in the controls suggested that in optic ataxia is present not only a visuo- but also a proprioceptivo-motor integration deficit.

Errors in interception can be predicted from errors in perception

It has been hypothesised that our actions are less susceptible to visual illusions than our perceptual judgements because similar information is processed for perception and action in separate pathways. We test this hypothesis for subjects intercepting a moving object that appears to move at a different speed than its true speed due to an illusion. The object was a moving Gabor patch: a sinusoidal grating of which the luminance contrast is modulated by a two-dimensional Gaussian. We manipulated the patch's apparent speed by moving the grating relative to the Gaussian. We used separate two-interval forced choice discrimination tasks to determine how moving the grating influenced ten people's judgements of the object's position and velocity while they were fixating. Based on their perceptual judgements, and knowing that our ability to correct for errors that arise from relying on incorrect judgements are limited by a sensorimotor delay of about 100 msec, we predicted the extent to which subjects would tap ahead of or behind similar targets when trying to intercept them at the fixation location. The predicted errors closely matched the actual errors that subjects made when trying to intercept the targets. This finding does not support the two visual streams hypothesis. The results are consistent with the idea that the extent to which an illusion influences an action tells us something about the extent to which the action relies on the percept in question.

The dorsal visual stream revisited: Stable circuits or dynamic pathways?

In both macaque and human brain, information regarding visual motion flows from the extrastriate area V6 along two different paths: a dorsolateral one towards areas MT/V5, MST, V3A, and a dorsomedial one towards the visuomotor areas of the superior parietal lobule (V6A, MIP, VIP). The dorsolateral visual stream is involved in many aspects of visual motion analysis, including the recognition of object motion and self motion. The dorsomedial stream uses visual motion information to continuously monitor the spatial location of objects while we are looking and/or moving around, to allow skilled reaching for and grasping of the objects in structured, dynamically changing environments. Grasping activity is present in two areas of the dorsal stream, AIP and V6A. Area AIP is more involved than V6A in object recognition, V6A in encoding vision for action. We suggest that V6A is involved in the fast control of prehension and plays a critical role in biomechanically selecting appropriate postures during reach to grasp behaviors. In everyday life, numerous functional networks, often involving the same cortical areas, are continuously in action in the dorsal visual stream, with each network dynamically activated or inhibited according to the context. The dorsolateral and dorsomedial streams represent only two examples of these networks. Many others streams have been described in the literature, but it is worthwhile noting that the same cortical area, and even the same neurons within an area, are not specific for just one functional property, being part of networks that encode multiple functional aspects. Our proposal is to conceive the cortical streams not as fixed series of interconnected cortical areas in which each area belongs univocally to one stream and is strictly involved in only one function, but as interconnected neuronal networks, often involving the same neurons, that are involved in a number of functional processes and whose activation changes dynamically according to the context.

How Humans Reach: Distinct Cortical Systems for Central and Peripheral Vision

Lesions of the posterior parietal cortex in humans can produce a specific disruption of visually guided hand movements termed optic ataxia. The fact that the deficit mainly occurs in peripheral vision suggests that reaching in foveal and extrafoveal vision relies on two different anatomical substrates. Using fMRI in healthy subjects, the authors demonstrated the existence of two systems, differently modulated by the two reaching conditions. Reaching in central vision involves a restricted network, including the medial intraparietal sulcus (mIPS) and the caudal part of the dorsal premotor cortex (PMd). Reaching in peripheral vision engages a more extensive network, including the parieto-occipital junction (POJ). Interestingly, POJ corresponds to the site of the lesion overlap that the authors recently found to be responsible for optic ataxia. These two sets of results converge to show that there is not a unique cortical network for reaching control but instead two systems engaged in reaching to targets in the central and peripheral visual field.

The Extrastriate Body Area Computes Desired Goal States during Action Planning

How do object perception and action interact at a neural level? Here we test the hypothesis that perceptual features, processed by the ventral visuoperceptual stream, are used as priors by the dorsal visuomotor stream to specify goal-directed grasping actions. We present three main findings, which were obtained by combining time-resolved transcranial magnetic stimulation and kinematic tracking of grasp-and-rotate object manipulations, in a group of healthy human participants (N = 22). First, the extrastriate body area (EBA), in the ventral stream, provides an initial structure to motor plans, based on current and desired states of a grasped object and of the grasping hand. Second, the contributions of EBA are earlier in time than those of a caudal intraparietal region known to specify the action plan. Third, the contributions of EBA are particularly important when desired and current object configurations differ, and multiple courses of actions are possible. These findings specify the temporal and functional characteristics for a mechanism that integrates perceptual processing with motor planning.

Memory-guided reaching in a patient with visual hemiagnosia

The two-visual-systems hypothesis (TVSH) postulates that memory-guided movements rely on intact functions of the ventral stream. Its particular importance for memory-guided actions was initially inferred from behavioral dissociations in the well-known patient DF. Despite of rather accurate reaching and grasping movements to visible targets, she demonstrated grossly impaired memory-guided grasping as much as impaired memory-guided reaching. These dissociations were later complemented by apparently reversed dissociations in patients with dorsal damage and optic ataxia. However, grasping studies in DF and optic ataxia patients differed with respect to the retinotopic position of target objects, questioning the interpretation of the respective findings as a double dissociation. In contrast, the findings for reaching errors in both types of patients came from similar peripheral target presentations. However, new data on brain structural changes and visuomotor deficits in DF also questioned the validity of a double dissociation in reaching. A severe visuospatial short-term memory deficit in DF further questioned the specificity of her memory-guided reaching deficit. Therefore, we compared movement accuracy in visually-guided and memory-guided reaching in a new patient who suffered a confined unilateral damage to the ventral visual system due to stroke. Our results indeed support previous descriptions of memory-guided movements' inaccuracies in DF. Furthermore, our data suggest that recently discovered optic-ataxia like misreaching in DF is most likely caused by her parieto-occipital and not by her ventral stream damage. Finally, multiple visuospatial memory measurements in HWS suggest that inaccuracies in memory-guided reaching tasks in patients with ventral damage cannot be explained by visuospatial short-term memory or perceptual deficits, but by a specific deficit in visuomotor processing.

A perception-based ERP reveals that the magnitude of delay matters for memory-guided reaching

Delayed action research has suggested that perceptual information about a visual stimulus decays over several seconds. With event-related potential (ERP) methodology, one should be able to track the time course of the electrophysiological processes associated with this decay. Recently, Cruikshank et al. (J Vis 12:29, 2012) found that the N170 ERP component reflected ventral stream processes linked to motor planning and perception for action. Specifically, the N170 was larger for actions that relied on perceptual-based information. However, the delay interval was very short (tens of ms). Behavioral and neuroimaging studies suggest that when longer delays are employed, reactivation of ventral areas is necessary in order to access a stored representation of the target's characteristics. Therefore, the N170 may reflect not only the perception-for-action processes, but also the accuracy of the representation. In order to test this, we traced the time course of the N170 in memory-guided reaching when 1-, 2-, and 3-s delays separated target occlusion and response initiation. During reach initiation, the N170 was more negative and peaked earlier for the 1 s than the 2- and 3-s delays and correlated significantly with performance at the longest delay. These results suggest that the neural mechanisms involved in movement planning change for delays beyond 1 s. The smaller N170 may reflect an impoverished visual perceptual representation in the ventral stream. To our knowledge, these are the first electrophysiological results to suggest that there is decay of visual perceptual information that occurs with increasing time.

Decoupled visually-guided reaching in optic ataxia: differences in motor control between canonical and non-canonical orientations in space

Guiding a limb often involves situations in which the spatial location of the target for gaze and limb movement are not congruent (i.e. have been decoupled). Such decoupled situations involve both the implementation of a cognitive rule (i.e. strategic control) and the online monitoring of the limb position relative to gaze and target (i.e. sensorimotor recalibration). To further understand the neural mechanisms underlying these different types of visuomotor control, we tested patient IG who has bilateral caudal superior parietal lobule (SPL) damage resulting in optic ataxia (OA), and compared her performance with six age-matched controls on a series of center-out reaching tasks. The tasks comprised 1) directing a cursor that had been rotated (180° or 90°) within the same spatial plane as the visual display, or 2) moving the hand along a different spatial plane than the visual display (horizontal or para-sagittal). Importantly, all conditions were performed towards visual targets located along either the horizontal axis (left and right; which can be guided from strategic control) or the diagonal axes (top-left and top-right; which require on-line trajectory elaboration and updating by sensorimotor recalibration). The bilateral OA patient performed much better in decoupled visuomotor control towards the horizontal targets, a canonical situation in which well-categorized allocentric cues could be utilized (i.e. guiding cursor direction perpendicular to computer monitor border). Relative to neurologically intact adults, IG's performance suffered towards diagonal targets, a non-canonical situation in which only less-categorized allocentric cues were available (i.e. guiding cursor direction at an off-axis angle to computer monitor border), and she was therefore required to rely on sensorimotor recalibration of her decoupled limb. We propose that an intact caudal SPL is crucial for any decoupled visuomotor control, particularly when relying on the realignment between vision and proprioception without reliable allocentric cues towards non-canonical orientations in space.

Automatic online control of motor adjustments in reaching and grasping

Following the princeps investigations of Marc Jeannerod on action-perception, specifically, goal-directed movement, this review article addresses visual and non-visual processes involved in guiding the hand in reaching or grasping tasks. The contributions of different sources of correction of ongoing movements are considered; these include visual feedback of the hand, as well as the often-neglected but important spatial updating and sharpening of goal localization following gaze-saccade orientation. The existence of an automatic online process guiding limb trajectory toward its goal is highlighted by a series of princeps experiments of goal-directed pointing movements. We then review psychophysical, electrophysiological, neuroimaging and clinical studies that have explored the properties of these automatic corrective mechanisms and their neural bases, and established their generality. Finally, the functional significance of automatic corrective mechanisms-referred to as motor flexibility-and their potential use in rehabilitation are discussed.

Guidelines and quality measures for the diagnosis of optic ataxia

Since the first description of a systematic mis-reaching by Bálint in 1909, a reasonable number of patients showing a similar phenomenology, later termed optic ataxia (OA), has been described. However, there is surprising inconsistency regarding the behavioral measures that are used to detect OA in experimental and clinical reports, if the respective measures are reported at all. A typical screening method that was presumably used by most researchers and clinicians, reaching for a target object in the peripheral visual space, has never been evaluated. We developed a set of instructions and evaluation criteria for the scoring of a semi-standardized version of this reaching task. We tested 36 healthy participants, a group of 52 acute and chronic stroke patients, and 24 patients suffering from cerebellar ataxia. We found a high interrater reliability and a moderate test-retest reliability comparable to other clinical instruments in the stroke sample. The calculation of cut-off thresholds based on healthy control and cerebellar patient data showed an unexpected high number of false positives in these samples due to individual outliers that made a considerable number of errors in peripheral reaching. This study provides first empirical data from large control and patient groups for a screening procedure that seems to be widely used but rarely explicitly reported and prepares the grounds for its use as a standard tool for the description of patients who are included in single case or group studies addressing optic ataxia similar to the use of neglect, extinction, or apraxia screening tools.

Posterior cortical atrophy: visuomotor deficits in reaching and grasping

Posterior Cortical Atrophy (PCA) is a rare clinical syndrome characterized by the predominance of higher-order visual disturbances such as optic ataxia, a characteristic of Balint's syndrome. Deficits result from progressive neurodegeneration of occipito-temporal and occipito-parietal cortices. The current study sought to explore the visuomotor functioning of four individuals with PCA by testing their ability to reach out and grasp real objects under various viewing conditions. Experiment 1 had participants reach out and grasp simple, rectangular blocks under visually- and memory-guided conditions. Experiment 2 explored participants' abilities to accurately reach for objects located in their visual periphery. This investigation revealed that PCA patients demonstrate many of the same deficits that have been previously reported in other individuals with optic ataxia, such as “magnetic misreaching”—a pathological reaching bias toward the point of visual fixation when grasping peripheral targets. Unlike many other individuals with optic ataxia, however, the patients in the current study also show symptoms indicative of damage to the more perceptual stream of visual processing, including abolished grip scaling during memory-guided grasping and deficits in face and object identification. These investigations are the first to perform a quantitative analysis of the visuomotor deficits exhibited by patients with PCA. Critically, this study helps characterize common symptoms of PCA, a vital first step for generating effective diagnostic criteria and therapeutic strategies for this understudied neurodegenerative disorder.

Gaze strategies during visually-guided versus memory-guided grasping

Vision plays a crucial role in guiding motor actions. But sometimes we cannot use vision and must rely on our memory to guide action-e.g. remembering where we placed our eyeglasses on the bedside table when reaching for them with the lights off. Recent studies show subjects look towards the index finger grasp position during visually-guided precision grasping. But, where do people look during memory-guided grasping? Here, we explored the gaze behaviour of subjects as they grasped a centrally placed symmetrical block under open- and closed-loop conditions. In Experiment 1, subjects performed grasps in either a visually-guided task or memory-guided task. The results show that during visually-guided grasping, gaze was first directed towards the index finger's grasp point on the block, suggesting gaze targets future grasp points during the planning of the grasp. Gaze during memory-guided grasping was aimed closer to the blocks' centre of mass from block presentation to the completion of the grasp. In Experiment 2, subjects performed an 'immediate grasping' task in which vision of the block was removed immediately at the onset of the reach. Similar to the visually-guided results from Experiment 1, gaze was primarily directed towards the index finger location. These results support the 2-stream theory of vision in that motor planning with visual feedback at the onset of the movement is driven primarily by real-time visuomotor computations of the dorsal stream, whereas grasping remembered objects without visual feedback is driven primarily by the perceptual memory representations mediated by the ventral stream.

Has brain imaging discovered anything new about how the brain works?

There have now been roughly 130,000 papers on fMRI. While these have clearly contributed to our understanding of the functional anatomy of the human brain, it is less clear that they have changed the way in which we think about the brain. The issue, in other words, is whether they have established new principles about how the brain works. In this paper we offer as an example one new principle, partly to lay down the criteria that are required for establishing a new principle, and partly to encourage others to offer other principles. Our example concerns the flexible flow of information through the cortex that must occur according to the demands of the task or current context. We suggest that this flexibility is achieved by feedback connections from the prefrontal and parietal cortex, and that these include connections to sensory and motor areas. However, the nature of the selective effect differs. The parietal cortex can select both within and across processing streams. By across streams we mean that it can have the same influence on different streams, for example the dorsal and ventral visual systems. However, only the prefrontal cortex can also select between processing streams. The difference between the prefrontal and parietal effects is due to their different positions within the processing hierarchy.

The role of the caudal superior parietal lobule in updating hand location in peripheral vision: further evidence from optic ataxia

Patients with optic ataxia (OA), who are missing the caudal portion of their superior parietal lobule (SPL), have difficulty performing visually-guided reaches towards extra-foveal targets. Such gaze and hand decoupling also occurs in commonly performed non-standard visuomotor transformations such as the use of a computer mouse. In this study, we test two unilateral OA patients in conditions of 1) a change in the physical location of the visual stimulus relative to the plane of the limb movement, 2) a cue that signals a required limb movement 180° opposite to the cued visual target location, or 3) both of these situations combined. In these non-standard visuomotor transformations, the OA deficit is not observed as the well-documented field-dependent misreach. Instead, OA patients make additional eye movements to update hand and goal location during motor execution in order to complete these slow movements. Overall, the OA patients struggled when having to guide centrifugal movements in peripheral vision, even when they were instructed from visual stimuli that could be foveated. We propose that an intact caudal SPL is crucial for any visuomotor control that involves updating ongoing hand location in space without foveating it, i.e. from peripheral vision, proprioceptive or predictive information.

Distinct cortical networks support the planning and online control of reaching-to-grasp in humans

A number of brain imaging studies have identified regions involved in the planning and control of complex actions. Here we attempt to contrast activity related to planning and online control in the human brain during simple reaching and grasping movements. In four conditions, participants did one of the following: passively observed a grasp target; planned a grasping movement without executing; planned and then executed a grasp; or immediately executed a grasp. Neural activity was measured using functional magnetic resonance imaging and activity in the various conditions compared. Two large, independent networks of brain activity were identified: (i) a planning network including the premotor cortex, basal ganglia, anterior cingulate, posterior medial parietal area, superior parietal occipital cortex and middle intraparietal sulcus; and (ii) a control network including sensorimotor cortex, the cerebellum, the supramarginal gyrus and the superior parietal lobule. These findings provide evidence that the planning and control of even simple reaching and grasping actions use different brain regions, including different parts of the frontal and parietal lobes.

The dual loop model: its relation to language and other modalities

The current neurobiological consensus of a general dual loop system scaffolding human and primate brains gives evidence that the dorsal and ventral connections subserve similar functions, independent of the modality and species. However, most current commentators agree that although bees dance and chimpanzees grunt, these systems of communication differ qualitatively from human language. So why is language unique to humans? We discuss anatomical differences between humans and other animals, the meaning of lesion studies in patients, the role of inner speech, and compare functional imaging studies in language with other modalities in respect to the dual loop model. These aspects might be helpful for understanding what kind of biological system the language faculty is, and how it relates to other systems in our own species and others.

Is visual processing in the dorsal stream accessible to consciousness?

There are two highly interconnected clusters of visually responsive areas in the primate cortex. These two clusters have relatively few interconnections with each other, though those interconnections are undoubtedly important. One of the two main clusters (the dorsal stream) links the primary visual cortex (V1) to superior regions of the occipito-parietal cortex, while the other (the ventral stream) links V1 to inferior regions of the occipito-temporal cortex. According to our current understanding of the functional anatomy of these two systems, the dorsal stream's principal role is to provide real-time 'bottom-up' visual guidance of our movements online. In contrast, the ventral stream, in conjunction with top-down information from visual and semantic memory, provides perceptual representations that can serve recognition, visual thought, planning and memory offline. In recent years, this interpretation, initially based chiefly on studies of non-human primates and human neurological patients, has been well supported by functional MRI studies in humans. This perspective presents empirical evidence for the contention that the dorsal stream governs the visual control of movement without the intervention of visual awareness.

Reaction times for allocentric movements are 35 ms slower than reaction times for target-directed movements

Many movements that people perform every day are directed at visual targets, e.g., when we press an elevator button. However, many other movements are not target-directed, but are based on allocentric (object-centered) visual information. Examples of allocentric movements are gesture imitation, drawing or copying. Here, show a reaction time difference between these two types of movements in four separate experiments. In Exp. 1, subjects moved their eyes freely and used direct hand movements. In Exp. 2, subjects moved their eyes freely and their movements were tool-mediated (computer mouse). In Exp. 3, subjects fixated a central target and the visual field in which visual information was presented was manipulated. Experiment 4 was identical to Exp. 3 except for the fact that visual information about targets disappeared before movement onset. In all four experiments, reaction times in the allocentric task were approximately 35 ms slower than they were in the target-directed task. We suggest that this difference in reaction time between the two tasks reflects the fact that allocentric, but not target-directed, movements recruit the ventral stream, in particular lateral occipital cortex, which increases processing time. We also observed an advantage for movements made in the lower visual field as measured by movement variability, whether or not those movements were allocentric or target-directed. This latter result, we argue, reflects the role of the dorsal visual stream in the online control of movements in both kinds of tasks.

The cognitive neuroscience of prehension: recent developments

Prehension, the capacity to reach and grasp, is the key behavior that allows humans to change their environment. It continues to serve as a remarkable experimental test case for probing the cognitive architecture of goal-oriented action. This review focuses on recent experimental evidence that enhances or modifies how we might conceptualize the neural substrates of prehension. Emphasis is placed on studies that consider how precision grasps are selected and transformed into motor commands. Then, the mechanisms that extract action relevant information from vision and touch are considered. These include consideration of how parallel perceptual networks within parietal cortex, along with the ventral stream, are connected and share information to achieve common motor goals. On-line control of grasping action is discussed within a state estimation framework. The review ends with a consideration about how prehension fits within larger action repertoires that solve more complex goals and the possible cortical architectures needed to organize these actions.

Memory for Time Distinguishes between Perception and Action

Our experience of time is unlike that of other features of the sensory world such as colour, movement, touch, or sound because there is no unique receptor system through which it is received. However, since time can be perceived, remembered, estimated, and compared in a way analogous to other sensory experiences, it should perhaps be subject to some of the same architectures or principles that have advanced understanding in these other domains. By adapting a task designed to test visual memory within a perception/action framework we investigated whether memory for time is affected by the use to which temporal information is put. When remembering a visual or auditory duration for subsequent motor production, storage is biased by a delay of up to 8 s. When the same duration is remembered for subsequent perception, however, there is no such effect of delay on memory. The results suggest a distinction in temporal memory that parallels the perception/action dichotomy in vision.