Intact automatic avoidance of obstacles in patients with visual form agnosia

Coming to grips with reality: Real grasps, but not pantomimed grasps, resist a simultaneous tilt illusion

Investigations of grasping real, 3D objects subjected to illusory effects from a pictorial background often choose in-flight grasp aperture as the primary variable to test the hypothesis that the visuomotor system resists the illusion. Here we test an equally important feature of grasps that has received less attention: in-flight grasp orientation. The current study tested a variant of the simultaneous tilt illusion using a mirror-apparatus to manipulate the availability of haptic feedback. Participants performed grasps with haptic feedback (real grasps) and without it (pantomime grasps), reaching for the reflection of a real, 3D bar atop a background grating that induced a 1.1° bias in the perceived orientation of the bar in a separate sample of participants. Analysis of the hand's in-flight grasp orientation at early, late, and end stages of the reach showed that at no point were the real grasps biased by the illusion. In contrast, pantomimed grasps were affected by the illusion at the late and end stages of the reach. At each stage, the effect on the real grasps was significantly weaker than the effect of the illusion as measured by the mean point of subjective equality (PSE) in a two-alternative forced-choice task. In contrast, the effect on the pantomime grasps was statistically indistinguishable from the mean PSE at all three stages of the reach. These findings reinforce the idea that in-flight grasp orientation, like grasp aperture to pictorial illusions of target size, is refractory to pictorial backgrounds that bias perceived orientation.

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.

Grasping of Real-World Objects Is Not Biased by Ensemble Perception

The visual system is known to extract summary representations of visually similar objects which bias the perception of individual objects toward the ensemble average. Although vision plays a large role in guiding action, less is known about whether ensemble representation is informative for action. Motor behavior is tuned to the veridical dimensions of objects and generally considered resistant to perceptual biases. However, when the relevant grasp dimension is not available or is unconstrained, ensemble perception may be informative to behavior by providing gist information about surrounding objects. In the present study, we examined if summary representations of a surrounding ensemble display influenced grip aperture and orientation when participants reached-to-grasp a central circular target which had an explicit size but importantly no explicit orientation that the visuomotor system could selectively attend to. Maximum grip aperture and grip orientation were not biased by ensemble statistics during grasping, although participants were able to perceive and provide manual estimations of the average size and orientation of the ensemble display. Support vector machine classification of ensemble statistics achieved above-chance classification accuracy when trained on kinematic and electromyography data of the perceptual but not grasping conditions, supporting our univariate findings. These results suggest that even along unconstrained grasping dimensions, visually-guided behaviors toward real-world objects are not biased by ensemble processing.

When two worlds collide: the influence of an obstacle in peripersonal space on multisensory encoding

Multisensory coding of the space surrounding our body, the peripersonal space, is crucial for motor control. Recently, it has been proposed that an important function of multisensory coding is that it allows anticipation of the tactile consequences of contact with a nearby object. Indeed, performing goal-directed actions (i.e. pointing and grasping) induces a continuous visuotactile remapping as a function of on-line sensorimotor requirements. Here, we investigated whether visuotactile remapping can be induced by obstacles, e.g. objects that are not the target of the grasping movement. In the current experiment, we used a cross-modal obstacle avoidance paradigm, in which participants reached past an obstacle to grasp a second object. Participants indicated the location of tactile targets delivered to the hand during the grasping movement, while a visual cue was sometimes presented simultaneously on the to-be-avoided object. The tactile and visual stimulation was triggered when the reaching hand passed a position that was drawn randomly from a continuous set of predetermined locations (between 0 and 200 mm depth at 5 mm intervals). We observed differences in visuotactile interaction during obstacle avoidance dependent on the location of the stimulation trigger: visual interference was enhanced for tactile stimulation that occurred when the hand was near the to-be-avoided object. We show that to-be-avoided obstacles, which are relevant for action but are not to-be-interacted with (as the terminus of an action), automatically evoke the tactile consequences of interaction. This shows that visuotactile remapping extends to obstacle avoidance and that this process is flexible.

Neural Networks Mediating Perceptual Learning in Congenital Blindness

Despite the fact that complete visual deprivation leads to volumetric reductions in brain structures associated with spatial learning, blind individuals are still able to navigate. The neural structures involved in this function are not fully understood. Our study aims to correlate the performance of congenitally blind individuals (CB) and blindfolded sighted controls (SC) in a life-size obstacle-course using a visual-to-tactile sensory substitution device, with the size of brain structures (voxel based morphometry-VBM-) measured through structural magnetic resonance Imaging (MRI). VBM was used to extract grey matter volumes within several a-priori defined brain regions in all participants. Principal component analysis was utilized to group brain regions in factors and orthogonalize brain volumes. Regression analyses were then performed to link learning abilities to these factors. We found that (1) both CB and SC were able to learn to detect and avoid obstacles; (2) their learning rates for obstacle detection and avoidance correlated significantly with the volume of brain structures known to be involved in spatial skills. There is a similar relation between regions of the dorsal stream network and avoidance for both SC and CB whereas for detection, SC rely more on medial temporal lobe structures and CB on sensorimotor areas.

The target as an obstacle: Grasping an object at different heights

Humans use a stereotypical movement pattern to grasp a target object. What is the cause of this stereotypical pattern? One of the possible factors is that the target object is considered an obstacle at positions other than the envisioned goal positions for the digits: while each digit aims for a goal position on the target object, they avoid other positions on the target object even if these positions do not obstruct the movement. According to this hypothesis, the maximum grip aperture will be higher if the risk of colliding with the target object is larger. Based on this hypothesis, we made a set of two unique predictions for grasping a vertically oriented cuboid at its sides at different heights. For cuboids of the same height, the maximum grip aperture will be smaller when grasped higher. For cuboids whose height varies with grip height, the maximum grip aperture will be larger when grasped higher. Both predicted relations were experimentally confirmed. This result supports the idea that considering the target object as an obstacle at positions other than the envisioned goal positions for the digits is underlying the stereotypical movement patterns in grasping. The goal positions of the digits thus influence the maximum grip aperture even if the distance between the goal positions on the target object does not change.

The visual guidance of action is not insulated from cognitive interference: A multitasking study on obstacle-avoidance and bisection

The Perception-Action Model (PAM) considers the visual system to be divided into two streams defined by their specific functions, a ventral stream for vision and a dorsal stream for action. In this study we investigated two behavioral paradigms which according to PAM represent the two contrasting functions of the ventral and dorsal stream, namely bisection and obstacle-avoidance, respectively. It is an assumption of PAM that while ventral stream processing is ultimately linked with processing in other cognitive systems, dorsal stream processing is insulated from cognition. Accordingly it can be expected that a secondary task will interfere with bisection but not with obstacle-avoidance. We tested this prediction using a rapid serial visual presentation task as our secondary task (RSVP). Contrary to expectations we found significant interference for both bisection and obstacle-avoidance. Our findings suggest that dorsal-stream processing is not insulated from cognitive processes.

Principles of Motor Recovery After Neurological Injury Based on a Motor Control Theory

Problems of neurological rehabilitation are considered based on two levels of the International Classification of Functioning (ICF)-Body Structures and Function level and Activity level-and modulating factors related to the individual and the environment. Specifically, at the Body Structures and Function level, problems addressed include spasticity, muscle weakness, disordered muscle activation patterns and disruptions in coordinated movement. At the Activity level, deficits in multi-joint and multi-segment upper limb reaching movements are reviewed. We address how physiologically well established principles in the control of actions, Threshold Control and Referent Control as outlined in the Equilibrium-Point theory can help advance the understanding of underlying deficits that may limit recovery at each level.

Avoiding unseen obstacles: Subcortical vision is not sufficient to maintain normal obstacle avoidance behaviour during reaching

Previous research found that a patient with cortical blindness (homonymous hemianopia) was able to successfully avoid an obstacle placed in his blind field, despite reporting no conscious awareness of it [Striemer, C. L., Chapman, C. S., & Goodale, M. A., 2009, PNAS, 106(37), 15996-16001]. This finding led to the suggestion that dorsal stream areas, that are assumed to mediate obstacle avoidance behaviour, may obtain their visual input primarily from subcortical pathways. Hence, it was suggested that normal obstacle avoidance behaviour can proceed without input from the primary visual cortex. Here we tried to replicate this finding in a group of patients (N = 6) that suffered from highly circumscribed lesions in the occipital lobe (including V1) that spared the subcortical structures that have been associated with action-blindsight. We also tested if obstacle avoidance behaviour differs depending on whether obstacles are placed only in the blind field or in both the blind and intact visual field of the patients simultaneously. As expected, all patients successfully avoided obstacles placed in their intact visual field. However, none of them showed reliable avoidance behaviour - as indicated by adjustments in the hand trajectory in response to obstacle position - for obstacles placed in their blind visual field. The effects were not dependent on whether one or two obstacles were present. These findings suggest that behaviour in complex visuomotor tasks relies on visual input from occipital areas.

The Effect of Gaze Position on Reaching Movements in an Obstacle Avoidance Task

Numerous studies have addressed the issue of where people look when they perform hand movements. Yet, very little is known about how visuomotor performance is affected by fixation location. Previous studies investigating the accuracy of actions performed in visual periphery have revealed inconsistent results. While movements performed under full visual-feedback (closed-loop) seem to remain surprisingly accurate, open-loop as well as memory-guided movements usually show a distinct bias (i.e. overestimation of target eccentricity) when executed in periphery. In this study, we aimed to investigate whether gaze position affects movements that are performed under full-vision but cannot be corrected based on a direct comparison between the hand and target position. To do so, we employed a classical visuomotor reaching task in which participants were required to move their hand through a gap between two obstacles into a target area. Participants performed the task in four gaze conditions: free-viewing (no restrictions on gaze), central fixation, or fixation on one of the two obstacles. Our findings show that obstacle avoidance behaviour is moderated by fixation position. Specifically, participants tended to select movement paths that veered away from the obstacle fixated indicating that perceptual errors persist in closed-loop vision conditions if they cannot be corrected effectively based on visual feedback. Moreover, measuring the eye-movement in a free-viewing task (Experiment 2), we confirmed that naturally participants’ prefer to move their eyes and hand to the same spatial location.

Non-obstructing 3D depth cues influence reach-to-grasp kinematics

It has been demonstrated that both visual feedback and the presence of certain types of non-target objects in the workspace can affect kinematic measures and the trajectory path of the moving hand during reach-to-grasp movements. Yet no study to date has examined the possible effect of providing non-obstructing three-dimensional (3D) depth cues within the workspace and with consistent retinal inputs and whether or not these alter manual prehension movements. Participants performed a series of reach-to-grasp movements in both open- (without visual feedback) and closed-loop (with visual feedback) conditions in the presence of one of three possible 3D depth cues. Here, it is reported that preventing online visual feedback (or not) and the presence of a particular depth cue had a profound effect on kinematic measures for both the reaching and grasping components of manual prehension-despite the fact that the 3D depth cues did not act as a physical obstruction at any point. The depth cues modulated the trajectory of the reaching hand when the target block was located on the left side of the workspace but not on the right. These results are discussed in relation to previous reports and implications for brain-computer interface decoding algorithms are provided.

Line-bisectioning and obstacle avoidance: evidence for separate strategies

Previous studies have frequently applied a combination of line-bisection tasks (in which participants indicate the middle of a line) and obstacle avoidance tasks (in which participants move their hand between two obstacles) with the aim of revealing perception-action dissociations in certain neurological disorders, such as visual form agnosia and optic ataxia. However, valid conclusions about the underlying processing pathways can only be drawn if participants apply the same strategy in both tasks (i.e. finding the middle between the obstacles). Yet, this assumption has never been tested directly. In this experiment, we investigated whether participants perform obstacle avoidance and line-bisectioning using similar strategies by manipulating the position of the obstacles and the start position of the hand relative to the obstacles. Our results indicate that the lateral hand position during obstacle avoidance does not only vary as a function of obstacle location but also strongly depends on the start position. Moreover, participants showed increased sensitivity to obstacle shifts occurring closer to the hand's start position. In contrast, during line-bisectioning the sensitivity to obstacles shifts was unaffected by the hand's start position. The findings suggest that during obstacle-avoidance the need to keep a safe distance from the obstacles is balanced with the requirement to minimise energetic demands. In contrast, the main intention during line-bisectioning is to move to the perceived midpoint as accurately as possible. The fact that very different constraints underlie trajectory planning in both tasks implies that caution has to be taken when interpreting differences in performance levels.

The influence of object identity on obstacle avoidance reaching behaviour

When reaching for target objects, we hardly ever collide with other objects located in our working environment. Behavioural studies have demonstrated that the introduction of non-target objects into the workspace alters both spatial and temporal parameters of reaching trajectories. Previous studies have shown the influence of spatial object features (e.g. size and position) on obstacle avoidance movements. However, obstacle identity may also play a role in the preparation of avoidance responses as this allows prediction of possible negative consequences of collision based on recognition of the obstacle. In this study we test this hypothesis by asking participants to reach towards a target as quickly as possible, in the presence of an empty or full glass of water placed about half way between the target and the starting position, at 8 cm either left or right of the virtual midline. While the spatial features of full and empty glasses of water are the same, the consequences of collision are clearly different. Indeed, when there was a high chance of collision, reaching trajectories veered away more from filled than from empty glasses. This shows that the identity of potential obstacles, which allows for estimating the predicted consequences of collision, is taken into account during obstacle avoidance.

Two visual streams: Interconnections do not imply duplication of function

Abstract Schenk and McIntosh (S&M) provide a useful review of the perception-action model (PAM), highlighting some of the gaps that need to be filled, and counteracting the erroneous belief held by some that the PAM implies two mutually independent streams. Although we agree with S&M's contention that the functional independence of the two streams has been overestimated, we reject their speculation that "the specializations proposed may be relative rather than absolute." We argue that the contributions made by the two streams are quite distinct, and that establishing how they work together is the key to a full understanding of visually guided behavior.

Optic ataxia as a model to investigate the role of the posterior parietal cortex in visually guided action: evidence from studies of patient M.H

Optic ataxia is a neuropsychological disorder that affects the ability to interact with objects presented in the visual modality following either unilateral or bilateral lesions of the posterior parietal cortex (PPC). Patients with optic ataxia fail to reach accurately for objects, particularly when they are presented in peripheral vision. The present review will focus on a series of experiments performed on patient M.H. Following a lesion restricted largely to the left PPC, he developed mis-reaching behavior when using his contralesional right arm for movements directed toward the contralesional (right) visual half-field. Given the clear-cut specificity of this patient's deficit, whereby reaching actions are essentially spared when executed toward his ipsilateral space or when using his left arm, M.H. provides a valuable "experiment of nature" for investigating the role of the PPC in performing different visually guided actions. In order to address this, we used kinematic measurement techniques to investigate M.H.'s reaching and grasping behavior in various tasks. Our experiments support the idea that optic ataxia is highly function-specific: it affects a specific sub-category of visually guided actions (reaching but not grasping), regardless of their specific end goal (both reaching toward an object and reaching to avoid an obstacle); and finally, is independent of the limb used to perform the action (whether the arm or the leg). Critically, these results are congruent with recent functional MRI experiments in neurologically intact subjects which suggest that the PPC is organized in a function-specific, rather than effector-specific, manner with different sub-portions of its mantle devoted to guiding actions according to their specific end-goal (reaching, grasping, or looking), rather than according to the effector used to perform them (leg, arm, hand, or eyes).

Outsider interference: no role for motor lateralization in determining the strength of avoidance responses during reaching

When reaches are performed toward target objects, the presence of other non-target objects influences kinematic parameters of the reach. A typical observation has been that non-targets positioned ipsilaterally to the acting limb interfere more with the trajectory of the hand than contralateral non-targets. Here, we investigate whether this effect is mediated by motor lateralization or by the relative positioning of the objects with reference to the acting limb. Participants were asked to perform reaches toward physical target objects with their preferred or non-preferred hands while physical non-targets were present in different possible positions in the workspace. We tested both left-handers and right-handers. Our results show that a participant's handedness does not influence reaching behavior in an obstacle avoidance paradigm. Furthermore, no statistically significant differences between the use of the preferred and non-preferred hand were observed on the kinematic parameters of the reaches. We found evidence that non-targets positioned on the outside of the reaching limb influenced the reaching behavior more strongly than non-targets on the inside. Moreover, the type of movement also appeared to play a role, as reaches that crossed the workspace had a stronger effect on avoidance behavior than reaches that were 'uncrossed.' We interpret these results as support for the hypothesis that the avoidance response is determined by keeping a preferred distance between the acting limb in all stages of its reach toward the target and the non-target position. This process is not biased by hand dominance or the hand preference of the actor.

The effect of similarity: non-spatial features modulate obstacle avoidance

The introduction of non-target objects into a workspace leads to temporal and spatial adjustments of reaching trajectories towards a target. If the non-target is obstructing the path of the hand towards the target, the reach is adjusted such that collision with the non-target, or obstacle, is avoided. Little is known about the influence of features which are irrelevant for the execution of the movement on avoidance movements, like color similarity between target and non-target objects. In eye movement studies the similarity of non-targets has been revealed to influence oculomotor competition. Because of the tight neural and behavioral coupling between the gaze and reaching system, our aim was to determine the contribution of similarity between target and non-target to avoidance movements. We performed 2 experiments in which participants had to reach to grasp a target object while a non-target was present in the workspace. These non-targets could be either similar or dissimilar in color to the target. The results indicate that the non-spatial feature similarity can further modify the avoidance response and therefore further modify the spatial path of the reach. Indeed, we find that dissimilar pairs have a stronger effect on reaching-to-grasp movements than similar pairs. This effect was most pronounced when the non-target was on the outside of the reaching hand, where it served as more of an obstacle to the trailing arm. We propose that the increased capture of attention by the dissimilar obstacle is responsible for the more robust avoidance response.

In Manually-Assisted Search, Perception Supervises Rather Than Directs Action

During manually-assisted search, where participants must actively manipulate search items, it has been reported that participants will often select and move the target of search itself without recognizing it (Solman et al., 2012a). In two experiments we explore the hypothesis that this error results from a naturally-arising strategy that decouples perception and action during search, enabling motor interactions with items to outpace the speed of perceptual analysis. In Experiment 1, we report that the error is prevalent for both mouse and touch-screen interaction modes, and is uninfluenced by speeding or slowing instructions – ruling out these task-specific details as causes of the error. In Experiment 2 we manipulate motor speed, and show that reducing the speed of individual movements during search leads to a reduction in error rates. These findings support the conclusion that the error results from incoordination between motor and perceptual processes, with motor processes outpacing perceptual abilities.

How obstructing is an obstacle? The influence of starting posture on obstacle avoidance

The introduction of non-target objects into a workspace leads to temporal and spatial adjustments of reaching trajectories towards a target. Currently, there are two different explanations for this phenomenon: the non-target objects are considered as either physical obstacles to which we maintain a preferred distance (see Tresilian, 1998) or as distractors that interfere with movement planning (see Tipper, Howard, & Jackson, 1997). These components are difficult to disentangle, however. Our aim was to determine the unique contribution of the avoidance of a physical obstacle to the adjustments of reaching trajectories. In this study, we manipulate the degree of physical obstruction by non-target objects while keeping the a priori visual layout of the workspace more or less constant. This is achieved by placing participants in different starting postures with respect to the orientation of their limb segments. Participants reach towards and grasp target objects with non-targets present in the workspace in a frontal and a lateral starting posture. In the frontal conditions participants showed larger movements away from the non-target on the ipsilateral side of the workspace than in the lateral conditions. The results provide evidence for the interpretation that non-targets influence the movement trajectory partly because they are 'obstructing'.

Pathways involved in human conscious vision contribute to obstacle-avoidance behaviour

Human patients with visual field defects following damage to their primary visual cortex (V1) will often misperceive the midpoint of a horizontal line. They tend to shift the midpoint away from the real position towards their blind field. In patients with unilateral neglect, where midpoint shifts can also be observed, these perceptual errors do not lead to errors in an obstacle-avoidance task, which also requires the ability to find the midpoint between two obstacles. This dissociation in neglect patients was taken as evidence that obstacle-avoidance performance is guided by visual information from the dorsal visual stream. Recently it was shown that a patient with hemianopia could avoid an obstacle presented in his blind field. This suggests that obstacle-avoidance behaviour can be guided by subconscious vision alone involving a direct route from extrageniculate structures in the brain to dorsal stream areas. To investigate whether obstacle avoidance relies only on this subconscious route or also uses information from pathways involved in conscious vision, we examined the effect of the hemianopic shift on obstacle-avoidance behaviour. This shift is found in tasks where a conscious visual judgement is required and presumably arises in pathways underlying conscious vision (V1 and ventral stream areas). We compared the performance of six patients with left hemianopia with the performance of six patients with right hemianopia. We found a clear bias in both groups, which also affected obstacle-avoidance performance. It is thus concluded that obstacle avoidance does not bypass the system for conscious vision completely.

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.

Prisms and neglect: what have we learned?

Since Rossetti et al. (1998) reported that prism adaptation (PA) can lead to a substantial reduction of neglect symptoms PA has become a hot topic in neglect-research. More than 280 articles have been published in this area. Not all of those studies investigated the therapeutic potential of this technique, many studies examined the responsiveness to PA as a way to subdivide neglect into separate subsyndromes, other studies focussed on the process of PA itself in an effort to illuminate its underlying neurobiological mechanisms. In this article we will review research in all of these three areas to determine whether and to what extent research on PA in neglect patients has fulfilled its promise as a new way to improve the treatment of neglect, enhance our understanding of this complex syndrome and provide new insights into the neurobiology of sensorimotor learning.

Gait adaptation during obstacle crossing reveals impairments in the visual control of locomotion in Williams syndrome

Recent evidence indicates that individuals with Williams syndrome (WS), a rare genetically based neurodevelopmental disorder, show abnormalities of parietal and cerebellar regions of the brain that may be involved in the visual control of locomotion. Here we examined whether parietal and cerebellar abnormalities contribute to deficits in spatiotemporal characteristics and foot placement variability during obstacle crossing in adults with WS, when compared with an IQ-matched group of adults with Down syndrome (DS) and typically developing adult controls. We used the GAITRite walkway to examine the spatiotemporal characteristics and foot placement variability relative to a small ground-based obstacle in the travel path. We found that adults with WS showed late adjustments to spatiotemporal gait characteristics alongside an exaggerated and more spatially constrained visual guidance of foot positioning in the final steps prior to stepping over the obstacle. In contrast, the adults with DS showed longer step duration and more variable step length and step duration during the crossing and recovery steps after the obstacle, suggestive of cerebellar dysfunction. Although the controls were able to reduce the variability of foot placement across the obstacle crossing trials, both the WS and DS groups did not become more consistent with practice. These findings indicate a less flexible and overly constrained visuomotor system in WS, which is consistent with more widespread and diffuse abnormalities in parietal and cerebellar regions.

Unconscious knowledge: A survey

The concept of unconscious knowledge is fundamental for an understanding of human thought processes and mentation in general; however, the psychological community at large is not familiar with it. This paper offers a survey of the main psychological research currently being carried out into cognitive processes, and examines pathways that can be integrated into a discipline of unconscious knowledge. It shows that the field has already a defined history and discusses some of the features that all kinds of unconscious knowledge seem to share at a deeper level. With the aim of promoting further research, we discuss the main challenges which the postulation of unconscious cognition faces within the psychological community.

Mental blocks: fMRI reveals top-down modulation of early visual cortex when obstacles interfere with grasp planning

When grasping an object, the fingers, hand and arm rarely collide with other non-target objects in the workspace. Kinematic studies of neurological patients (Schindler et al., 2004) and healthy participants (Chapman and Goodale, 2010a) suggest that the location of potential obstacles and the degree of interference they pose are encoded by the dorsal visual stream during action planning. Here, we used a slow event-related paradigm in functional magnetic resonance imaging (fMRI) to examine the neural encoding of obstacles in normal participants. Fifteen right-handed participants grasped a square target object with a thumb-front or thumb-side wrist-posture with (1) no obstacle present, (2) an obstacle behind the target object (interfering with the thumb-front grasp), or (3) an obstacle beside the target object (interfering with the thumb-side grasp). Within a specified network of areas involved in planning, a group voxelwise analysis revealed that one area in the left posterior intraparietal sulcus (pIPS) and one in early visual cortex were modulated by the degree of obstacle interference, and that this modulation occurred prior to movement execution. Given previous reports of a functional link between IPS and early visual cortex, we suggest that the increasing activity in the IPS with obstacle interference provides the top-down signal to suppress the corresponding obstacle coding in early visual areas, where we observed that activity decreased with interference. This is the first concrete evidence that the planning of a grasping movement can modulate early visual cortex and provides a unifying framework for understanding the dual role played by the IPS in motor planning and attentional orienting.

Vision-for-perception and vision-for-action: which model is compatible with the available psychophysical and neuropsychological data?

Westwood and Goodale (this issue) review the evidence for distinct visual streams for action and perception. They argue that, on balance, both the neuropsychological and psychophysical data support this distinction. They claim that critical results were either statistically inconclusive (because they consisted of negative evidence) or based on a suspect "calibration" procedure. Finally, they suggest that explanations dismissing the psychophysical evidence for the TVSH are contradicted by the neuropsychological evidence. We disagree with their assessment. 'Negative evidence' is not necessarily inconclusive. Problems raised by mixed evidence are best dealt with by conducting meta-analytical studies, which so far are only in part consistent with the TVSH. Correction ("calibration") of illusion effects is critical for comparisons across stimuli, studies, and tasks. We furthermore argue that both psychophysical and neuropsychological evidence can be explained without assuming divergent pathways for perception and action.

Stereoscopic vision in the absence of the lateral occipital cortex

Both dorsal and ventral cortical visual streams contain neurons sensitive to binocular disparities, but the two streams may underlie different aspects of stereoscopic vision. Here we investigate stereopsis in the neurological patient D.F., whose ventral stream, specifically lateral occipital cortex, has been damaged bilaterally, causing profound visual form agnosia. Despite her severe damage to cortical visual areas, we report that DF's stereo vision is strikingly unimpaired. She is better than many control observers at using binocular disparity to judge whether an isolated object appears near or far, and to resolve ambiguous structure-from-motion. DF is, however, poor at using relative disparity between features at different locations across the visual field. This may stem from a difficulty in identifying the surface boundaries where relative disparity is available. We suggest that the ventral processing stream may play a critical role in enabling healthy observers to extract fine depth information from relative disparities within one surface or between surfaces located in different parts of the visual field.

The neuroscience of vision-based grasping: a functional review for computational modeling and bio-inspired robotics

The topic of vision-based grasping is being widely studied in humans and in other primates using various techniques and with different goals. The fundamental related findings are reviewed in this paper, with the aim of providing researchers from different fields, including intelligent robotics and neural computation, a comprehensive but accessible view on the subject. A detailed description of the principal sensorimotor processes and the brain areas involved is provided following a functional perspective, in order to make this survey especially useful for computational modeling and bio-inspired robotic applications.

"Real-time" obstacle avoidance in the absence of primary visual cortex

When we reach toward objects, we easily avoid potential obstacles located in the workspace. Previous studies suggest that obstacle avoidance relies on mechanisms in the dorsal visual stream in the posterior parietal cortex. One fundamental question that remains unanswered is where the visual inputs to these dorsal-stream mechanisms are coming from. Here, we provide compelling evidence that these mechanisms can operate in "real-time" without direct input from primary visual cortex (V1). In our first experiment, we used a reaching task to demonstrate that an individual with a dense left visual field hemianopia after damage to V1 remained strikingly sensitive to the position of unseen static obstacles placed in his blind field. Importantly, in a second experiment, we showed that his sensitivity to the same obstacles in his blind field was abolished when a short 2-s delay (without vision) was introduced before reach onset. These findings have far-reaching implications, not only for our understanding of the time constraints under which different visual pathways operate, but also in relation to how these seemingly "primitive" subcortical visual pathways can control complex everyday behavior without recourse to conscious vision.

Action without perception in human vision

In 1992, David Milner and I (Goodale & Milner, 1992) proposed a division of labour in the visual pathways of the primate cerebral cortex between a dorsal stream specialized for the visual control of action and a ventral stream dedicated to constructing our percepts of the visual world. Support for the perception-action distinction has come from neuroimaging experiments, human neuropsychology, and monkey neurophysiology. Differences in the timing and spatial metrics of vision-for-perception and vision-for-action have been studied in human psychophysical experiments, particularly in those that have looked at the way in which each system deals with pictorial illusions. Although the literature is not free from controversy, a large number of studies have found that actions such as grasping and reaching are often unaffected by high-level pictorial illusions, which by definition affect perception. Recent experiments have shown that for actions to escape the effects of such illusions, however, they must be highly practised actions, preferably with the right hand, and must be directed in real time at visible targets. But even though the behavioural evidence suggests that the dorsal and ventral streams make use of different timing, different metrics, and different frames of reference in carrying out their computations, there is a seamless interaction between the two streams in the production of adaptive behaviour. A full understanding of the integrated nature of visually guided behaviour will require that we specify the nature of the interactions and information exchange that occur between these two streams of visual processing.

Self-regulation and the hypothesis of experience-based selection: investigating indirect conscious control

The assumption that the contents of our conscious visual experience directly control our fine-tuned, real-time motor activity has been challenged by neurological and psychophysical evidence that suggest the two processes work semi-independently of each other. Clark [Clark, A. (2001). Visual experience and motor action: Are the bonds too tight? The Philosophical Review, 110, 495-519; Clark, A. (2002). Is seeing all it seems? Action, reason and the grand illusion. Journal of Consciousness Studies, 9, 181-202; Clark, A. (2006). Vision as dance? Three challenges for sensori-motor contingency theory. PSYCHE, 12 (1). Available from http://www.psyche.cs.monash.edu.au] argues that such evidence implies a more indirect relationship between conscious visual experience and motor-control where the function of visual consciousness is not to control action but to select what actions are to be controlled. In this paper, I argue that this type of dynamic also exists at the wider level of self-regulation where conscious intent appears to indirectly control the enactment of the intended behaviour. I argue that by drawing parallels between Clark's proposed dynamic and self-regulation, the former is not only bolstered by a previously unrecognised source of support but our understanding of the latter can help to further elucidate Clark's proposed mechanism of indirect conscious control.

Action Rules: Why the Visual Control of Reaching and Grasping is Not Always Influenced by Perceptual Illusions

It is generally accepted that vision first evolved for the distal control of movement and that perception or ‘representational’ vision emerged much later. Vision-for-action operates in real time and uses egocentric frames of reference and the real metrics of the world. Vision-for-perception can operate over longer time scales and is much more scene-based in its computations. These differences in the timing and metrics of the two systems have been examined in experiments that have looked at the way in which each system deals with visual illusions. Although controversial, the consensus is that actions such as grasping and reaching are often unaffected by high-level pictorial illusions, which by definition affect perception. However, recent experiments have shown that, for actions to escape the effects of such illusions, they must be highly practiced actions, preferably with the right hand, and must be directed in real time at visible targets. This latter finding suggests that some of the critical components of the encapsulated (bottom – up) systems that mediate the visual control of skilled reaching and grasping movements are lateralised to the left hemisphere.

Two visual systems re-viewed

The model proposed by the authors of two cortical systems providing 'vision for action' and 'vision for perception', respectively, owed much to the inspiration of Larry Weiskrantz. In the present article some essential concepts inherent in the model are summarized, and certain clarifications and refinements are offered. Some illustrations are given of recent experiments by ourselves and others that have prompted us to sharpen these concepts. Our explicit hope in writing our book in 1995 was to provide a theoretical framework that would stimulate research in the field. Conversely, well-designed empirical contributions conceived within the framework of the model are the only way for us to progress along the route towards a fully fleshed-out specification of its workings.