Perceptual deficits of object identification: apperceptive agnosia.

It is argued here that apperceptive object agnosia (generally now known as visual form agnosia) is in reality not a kind of agnosia, but rather a form of "imperception" (to use the term coined by Hughlings Jackson). We further argue that its proximate cause is a bilateral loss (or functional loss) of the visual form processing systems embodied in the human lateral occipital cortex (area LO). According to the dual-system model of cortical visual processing elaborated by Milner and Goodale (2006), area LO constitutes a crucial component of the ventral stream, and indeed is essential for providing the figural qualities inherent in our normal visual perception of the world. According to this account, the functional loss of area LO would leave only spared visual areas within the occipito-parietal dorsal stream - dedicated to the control of visually-guided actions - potentially able to provide some aspects of visual shape processing in patients with apperceptive agnosia. We review the relevant evidence from such individuals, concentrating particularly on the well-researched patient D.F. We conclude that studies of this kind can provide useful pointers to an understanding of the processing characteristics of parietal-lobe visual mechanisms and their interactions with occipitotemporal perceptual systems in the guidance of action.

Human neuroimaging reveals the subcomponents of grasping, reaching and pointing actions.

Although the neural underpinnings of visually guided grasping and reaching have been well delineated within lateral and medial fronto-parietal networks (respectively), the contributions of subcomponents of visuomotor actions have not been explored in detail. Using careful subtraction logic, here we investigated which aspects of grasping, reaching, and pointing movements drive activation across key areas within visuomotor networks implicated in hand actions. For grasping tasks, we find activation differences based on the precision required (fine > coarse grip: anterior intraparietal sulcus, aIPS), the requirement to lift the object (grip + lift > grip: aIPS; dorsal premotor cortex, PMd; and supplementary motor area, SMA), and the number of digits employed (3-/5- vs. 2-digit grasps: ventral premotor cortex, PMv; motor cortex, M1, and somatosensory cortex, S1). For reaching/pointing tasks, we find activation differences based on whether the task required arm transport ((reach-to-point with index finger and reach-to-touch with knuckles) vs. point-without-reach; anterior superior parietal lobule, aSPL) and whether it required pointing to the object centre ((point-without-reach and reach-to-point) vs. reach-to-touch: anterior superior parieto-occipital cortex, aSPOC). For point-without-reach, in which the index finger is oriented towards the object centre but from a distance (point-without-reach > (reach-to-point and reach-to-touch)), we find activation differences that may be related to the communicative nature of the task (temporo-parietal junction, TPJ) and the need to precisely locate the target (lateral occipito-temporal cortex, LOTC). The present findings elucidate the different subcomponents of hand actions and the roles of specific brain regions in their computation.

Gender differences in non-standard mapping tasks: A kinematic study using pantomimed reach-to-grasp actions.

Comparison between real and pantomimed actions is used in neuroscience to dissociate stimulus-driven (real) as compared to internally driven (pantomimed) visuomotor transformations, with the goal of testing models of vision (Milner & Goodale, 1995) and diagnosing neuropsychological deficits (apraxia syndrome). Real actions refer to an overt movement directed toward a visible target whereas pantomimed actions refer to an overt movement directed either toward an object that is no longer available. Although similar, real and pantomimed actions differ in their kinematic parameters and in their neural substrates. Pantomimed-reach-to-grasp-actions show reduced reaching velocities, higher wrist movements, and reduced grip apertures. In addition, seminal neuropsychological studies and recent neuroimaging findings confirmed that real and pantomimed actions are underpinned by separate brain networks. Although previous literature suggests differences in the praxis system between males and females, no research to date has investigated whether or not gender differences exist in the context of real versus pantomimed reach-to-grasp actions. We asked ten male and ten female participants to perform real and pantomimed reach-to-grasp actions toward objects of different sizes, either with or without visual feedback. During pantomimed actions participants were required to pick up an imaginary object slightly offset relative to the location of the real one (which was in turn the target of the real reach-to-grasp actions). Results demonstrate a significant difference between the kinematic parameters recorded in male and female participants performing pantomimed, but not real reach-to-grasp tasks, depending on the availability of visual feedback. With no feedback both males and females showed smaller grip aperture, slower movement velocity and lower reach height. Crucially, these same differences were abolished when visual feedback was available in male, but not in female participants. Our results suggest that male and female participants should be evaluated separately in the clinical environment and in future research in the field.

Representational content of occipitotemporal and parietal tool areas.

It is now established that the perception of tools engages a left-lateralized network of frontoparietal and occipitotemporal cortical regions. Nevertheless, the precise computational role played by these areas is not yet well understood. To address this question, we used functional MRI to investigate the distribution of responses to pictures of tools and hands relative to other object categories in the so-called "tool" areas. Although hands and tools are visually not alike and belong to different object categories, these are both functionally linked when considering the common role of hands and tools in object manipulation. This distinction can provide insight into the differential functional role of areas within the "tool" network. Results demonstrated that images of hands and tools activate a common network of brain areas in the left intraparietal sulcus (IPS), left lateral occipitotemporal cortex (LOTC) and ventral occipitotemporal cortex (VOTC). Importantly, multivoxel pattern analysis revealed that the distribution of hand and tool response patterns in these regions differs. These observations provide support for the idea that the left IPS, left LOTC and VOTC might have distinct computational roles with regard to tool use. Specifically, these results suggest that while left IPS supports tool action-related computations and VOTC primarily encodes category specific aspects of objects, left LOTC bridges ventro occipitotemporal perception-related and parietal action-related representations by encoding both types of object information.

Coding of attention across the human intraparietal sulcus.

There has been concentrated debate over four decades as to whether or not the nonhuman primate parietal cortex codes for intention or attention. In nonhuman primates, certain studies report results consistent with an intentional role, whereas others provide support for coding of visual-spatial attention. Until now, no one has yet directly contrasted an established motor "intention" paradigm with a verified "attention" paradigm within the same protocol. This debate has continued in both the nonhuman primate and healthy human brain and is subsequently timely. We incorporated both paradigms across two distinct temporal epochs within a whole-parietal slow event-related human functional magnetic resonance imaging experiment. This enabled us to examine whether or not one paradigm proves more effective at driving the neural response across three intraparietal areas. As participants performed saccadic eye and/or pointing tasks, discrete event-related components with dissociable responses were elicited in distinct sub-regions of human parietal cortex. Critically, the posterior intraparietal area showed robust activity consistent with attention (no intention planning). The most contentious area in the literature, the middle intraparietal area produced activation patterns that further reinforce attention coding in human parietal cortex. Finally, the anterior intraparietal area showed the same pattern. Therefore, distributed coding of attention is relatively more pronounced across the two computations within human parietal cortex.

Reprint of: Visual processing of words in a patient with visual form agnosia: A behavioural and fMRI study.

Patient D.F. has a profound and enduring visual form agnosia due to a carbon monoxide poisoning episode suffered in 1988. Her inability to distinguish simple geometric shapes or single alphanumeric characters can be attributed to a bilateral loss of cortical area LO, a loss that has been well established through structural and functional fMRI. Yet despite this severe perceptual deficit, D.F. is able to "guess" remarkably well the identity of whole words. This paradoxical finding, which we were able to replicate more than 20 years following her initial testing, raises the question as to whether D.F. has retained specialized brain circuitry for word recognition that is able to function to some degree without the benefit of inputs from area LO. We used fMRI to investigate this, and found regions in the left fusiform gyrus, left inferior frontal gyrus, and left middle temporal cortex that responded selectively to words. A group of healthy control subjects showed similar activations. The left fusiform activations appear to coincide with the area commonly named the visual word form area (VWFA) in studies of healthy individuals, and appear to be quite separate from the fusiform face area (FFA). We hypothesize that there is a route to this area that lies outside area LO, and which remains relatively unscathed in D.F.

Visual processing of words in a patient with visual form agnosia: a behavioural and fMRI study.

Patient D.F. has a profound and enduring visual form agnosia due to a carbon monoxide poisoning episode suffered in 1988. Her inability to distinguish simple geometric shapes or single alphanumeric characters can be attributed to a bilateral loss of cortical area LO, a loss that has been well established through structural and functional fMRI. Yet despite this severe perceptual deficit, D.F. is able to "guess" remarkably well the identity of whole words. This paradoxical finding, which we were able to replicate more than 20 years following her initial testing, raises the question as to whether D.F. has retained specialized brain circuitry for word recognition that is able to function to some degree without the benefit of inputs from area LO. We used fMRI to investigate this, and found regions in the left fusiform gyrus, left inferior frontal gyrus, and left middle temporal cortex that responded selectively to words. A group of healthy control subjects showed similar activations. The left fusiform activations appear to coincide with the area commonly named the visual word form area (VWFA) in studies of healthy individuals, and appear to be quite separate from the fusiform face area (FFA). We hypothesize that there is a route to this area that lies outside area LO, and which remains relatively unscathed in D.F.

Patient DF's visual brain in action: Visual feedforward control in visual form agnosia.

Patient DF, who developed visual form agnosia following ventral-stream damage, is unable to discriminate the width of objects, performing at chance, for example, when asked to open her thumb and forefinger a matching amount. Remarkably, however, DF adjusts her hand aperture to accommodate the width of objects when reaching out to pick them up (grip scaling). While this spared ability to grasp objects is presumed to be mediated by visuomotor modules in her relatively intact dorsal stream, it is possible that it may rely abnormally on online visual or haptic feedback. We report here that DF's grip scaling remained intact when her vision was completely suppressed during grasp movements, and it still dissociated sharply from her poor perceptual estimates of target size. We then tested whether providing trial-by-trial haptic feedback after making such perceptual estimates might improve DF's performance, but found that they remained significantly impaired. In a final experiment, we re-examined whether DF's grip scaling depends on receiving veridical haptic feedback during grasping. In one condition, the haptic feedback was identical to the visual targets. In a second condition, the haptic feedback was of a constant intermediate width while the visual target varied trial by trial. Despite this incongruent feedback, DF still scaled her grip aperture to the visual widths of the target blocks, showing only normal adaptation to the false haptically-experienced width. Taken together, these results strengthen the view that DF's spared grasping relies on a normal mode of dorsal-stream functioning, based chiefly on visual feedforward processing.

Neural correlates of object size and object location during grasping actions.

The visuo-motor channel hypothesis (Jeannerod, 1981) postulates that grasping movements consist of a grip and a transport component differing in their reliance on intrinsic vs. extrinsic object properties (e.g. size vs. location, respectively). While recent neuroimaging studies have revealed separate brain areas implicated in grip and transport components within the parietal lobe, less is known about the neural processing of extrinsic and intrinsic properties of objects for grasping actions. We used functional magnetic resonance imaging adaptation to examine the cortical areas involved in processing object size, object location or both. Participants grasped (using the dominant right hand) or passively viewed sequential pairs of objects that could differ in size, location or both. We hypothesized that if intrinsic and extrinsic object properties are processed separately, as suggested by the visuo-motor channel hypothesis, we would observe adaptation to object size in areas that code the grip and adaptation to location in areas that code the transport component. On the other hand, if intrinsic and extrinsic object properties are not processed separately, brain areas involved in grasping may show adaptation to both object size and location. We found adaptation to object size for grasping movements in the left anterior intraparietal sulcus (aIPS), in agreement with the idea that object size is processed separately from location. In addition, the left superior parietal occipital sulcus (SPOC), primary somatosensory and motor area (S1/M1), precuneus, dorsal premotor cortex (PMd), and supplementary motor area (SMA) showed non-additive adaptation to both object size and location. We propose different roles for the aIPS as compared with the SPOC, S1/M1, precuneus, PMd and SMA. In particular, while the aIPS codes intrinsic object properties, which are relevant for hand preshaping and force scaling, area SPOC, S1/M1, precuneus, PMd and SMA code intrinsic as well as extrinsic object properties, both of which are relevant for digit positioning during grasping.

DF's visual brain in action: the role of tactile cues.

Patient DF, an extensively-tested woman with visual form agnosia from ventral-stream damage, is able to scale her grip aperture to match a goal object's geometry when reaching out to pick it up, despite being unable to explicitly distinguish amongst objects on the basis of their different geometries. Using evidence from a range of sources, including functional MRI, we have proposed that she does this through a functionally intact visuomotor system housed within the dorsal stream of the posterior parietal lobe. More recently, however, Schenk (2012a). The Journal of Neuroscience, 32(6), 2013-2017; Schenk (2012b). Trends in Cognitive Sciences, 16(5), 258-259. has argued that DF performs well in visually guided grasping, not through spared and functioning visuomotor networks in the dorsal stream, but because haptic feedback about the locations of the edges of the target is available to calibrate her grasps in such tasks, whereas it is not available in standard visual perceptual tasks. We have tested this 'calibration hypothesis' directly, by presenting DF with a grasping task in which the visible width of a target varied from trial to trial while its actual width remained the same. According to the calibration hypothesis, because haptic feedback was completely uninformative, DF should be unable to calibrate her grip aperture in this task. Contrary to this prediction, we found that DF continued to scale her grip aperture to the visual width of the targets and did so well within the range of healthy controls. We also found that DF's inability to distinguish shapes perceptually is not improved by providing haptic feedback. These findings strengthen the notion that DF's spared visuomotor abilities are driven largely by visual feedforward processing of the geometric properties of the target. Crucially, these findings also indicate that simple tactile contact with an object is needed for the visuomotor dorsal stream to be engaged, and accordingly enables DF to execute visually guided grasping successfully. This need for actions to have a tangible endpoint provides an important new modification of the Two Visual Systems theory.

Structural and functional changes across the visual cortex of a patient with visual form agnosia.

Loss of shape recognition in visual-form agnosia occurs without equivalent losses in the use of vision to guide actions, providing support for the hypothesis of two visual systems (for "perception" and "action"). The human individual DF received a toxic exposure to carbon monoxide some years ago, which resulted in a persisting visual-form agnosia that has been extensively characterized at the behavioral level. We conducted a detailed high-resolution MRI study of DF's cortex, combining structural and functional measurements. We present the first accurate quantification of the changes in thickness across DF's occipital cortex, finding the most substantial loss in the lateral occipital cortex (LOC). There are reduced white matter connections between LOC and other areas. Functional measures show pockets of activity that survive within structurally damaged areas. The topographic mapping of visual areas showed that ordered retinotopic maps were evident for DF in the ventral portions of visual cortical areas V1, V2, V3, and hV4. Although V1 shows evidence of topographic order in its dorsal portion, such maps could not be found in the dorsal parts of V2 and V3. We conclude that it is not possible to understand fully the deficits in object perception in visual-form agnosia without the exploitation of both structural and functional measurements. Our results also highlight for DF the cortical routes through which visual information is able to pass to support her well-documented abilities to use visual information to guide actions.

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).

Why do the eyes prefer the index finger? Simultaneous recording of eye and hand movements during precision grasping.

Previous research investigating eye movements when grasping objects with precision grip has shown that we tend to fixate close to the contact position of the index finger on the object. It has been hypothesized that this behavior is related to the fact that the index finger usually describes a more variable trajectory than the thumb and therefore requires a higher amount of visual monitoring. We wished to directly test this prediction by creating a grasping task in which either the index finger or the thumb described a more variable trajectory. Experiment 1 showed that the trajectory variability of the digits can be manipulated by altering the direction from which the hand approaches the object. If the start position is located in front of the object (hand-before), the index finger produces a more variable trajectory. In contrast, when the hand approaches the object from a starting position located behind it (hand-behind), the thumb produces a more variable movement path. In Experiment 2, we tested whether the fixation pattern during grasping is altered in conditions in which the trajectory variability of the two digits is reversed. Results suggest that regardless of the trajectory variability, the gaze was always directed toward the contact position of the index finger. Notably, we observed that regardless of our starting position manipulation, the index finger was the first digit to make contact with the object. Hence, we argue that time to contact (and not movement variability) is the crucial parameter which determines where we look during grasping.

Optic ataxia affects the lower limbs: evidence from a single case study.

Optic ataxia represents a spatial impairment of visually guided reaching following bilateral or unilateral damage to the posterior parietal cortex that is independent of purely motor or visual deficits. Research to date has focused on reaching actions performed with the upper limbs but has neglected to explore whether or not optic ataxia affects the lower limbs, that is, whether it is effector-specific. We asked patient M.H., who suffers from unilateral optic ataxia from left hemispheric damage, and eight age-matched controls, to perform leg movements by stepping down from a wooden block towards a visually presented target. Steps were performed using the left or the right leg, in conditions of central fixation or free viewing. Patient M.H. performed significantly worse than controls. His errors in step accuracy were most pronounced when stepping into the visual periphery (during central fixation), particularly while using the contralesional right foot towards the contralesional right hemispace. This behaviour is consistent with M.H.'s impairments in optic ataxia previously recorded for reaching and grasping actions with the upper limbs. The lesion affecting M.H.'s brain is quite large, encompassing functional areas associated with visuomotor transformations performed with different effectors such as arm and eye (superior parietal-occipital cortex and medial intraparietal sulcus). Our data suggest that optic ataxia is not completely effector-specific, and that neurons encoding visuomotor transformations for both arm and leg are probably both affected by the damage. Our results support the notion that lesions affecting the medial portion of the left posterior parietal cortex similarly affect different effectors (arm and leg) when visually guided actions are directed towards the same contralesional hemispace. In addition they may help explain why patients with optic ataxia have been reported to have difficulties in certain aspects of visually guided locomotion.

Closely overlapping responses to tools and hands in left lateral occipitotemporal cortex.

The perception of object-directed actions performed by either hands or tools recruits regions in left fronto-parietal cortex. Here, using functional MRI (fMRI), we tested whether the common role of hands and tools in object manipulation is also reflected in the distribution of response patterns to these categories in visual cortex. In two experiments we found that static pictures of hands and tools activated closely overlapping regions in left lateral occipitotemporal cortex (LOTC). Left LOTC responses to tools selectively overlapped with responses to hands but not with responses to whole bodies, nonhand body parts, other objects, or visual motion. Multivoxel pattern analysis in left LOTC indicated a high degree of similarity between response patterns to hands and tools but not between hands or tools and other body parts. Finally, functional connectivity analysis showed that the left LOTC hand/tool region was selectively connected, relative to neighboring body-, motion-, and object-responsive regions, with regions in left intraparietal sulcus and left premotor cortex that have previously been implicated in hand/tool action-related processing. Taken together, these results suggest that action-related object properties shared by hands and tools are reflected in the organization of high-order visual cortex. We propose that the functional organization of high-order visual cortex partly reflects the organization of downstream functional networks, such as the fronto-parietal action network, due to differences within visual cortex in the connectivity to these networks.

Functional magnetic resonance adaptation reveals the involvement of the dorsomedial stream in hand orientation for grasping.

Reach-to-grasp actions require coordination of different segments of the upper limbs. Previous studies have examined the neural substrates of arm transport and hand grip components of such actions; however, a third component has been largely neglected: the orientation of the wrist and hand appropriately for the object. Here we used functional magnetic resonance imaging adaptation (fMRA) to investigate human brain areas involved in processing hand orientation during grasping movements. Participants used the dominant right hand to grasp a rod with the four fingers opposing the thumb or to reach and touch the rod with the knuckles without visual feedback. In a control condition, participants passively viewed the rod. Trials in a slow event-related design consisted of two sequential stimuli in which the rod orientation changed (requiring a change in wrist posture while grasping but not reaching or looking) or remained the same. We found reduced activation, that is, adaptation, in superior parieto-occipital cortex (SPOC) when the object was repeatedly grasped with the same orientation. In contrast, there was no adaptation when reaching or looking at an object in the same orientation, suggesting that hand orientation, rather than object orientation, was the critical factor. These results agree with recent neurophysiological research showing that a parieto-occipital area of macaque (V6A) is modulated by hand orientation during reach-to-grasp movements. We suggest that the human dorsomedial stream, like that in the macaque, plays a key role in processing hand orientation in reach-to-grasp movements.

The magic grasp: motor expertise in deception.

BACKGROUND: Most of us are poor at faking actions. Kinematic studies have shown that when pretending to pick up imagined objects (pantomimed actions), we move and shape our hands quite differently from when grasping real ones. These differences between real and pantomimed actions have been linked to separate brain pathways specialized for different kinds of visuomotor guidance. Yet professional magicians regularly use pantomimed actions to deceive audiences. METHODOLOGY AND PRINCIPAL FINDINGS: In this study, we tested whether, despite their skill, magicians might still show kinematic differences between grasping actions made toward real versus imagined objects. We found that their pantomimed actions in fact closely resembled real grasps when the object was visible (but displaced) (Experiment 1), but failed to do so when the object was absent (Experiment 2). CONCLUSIONS AND SIGNIFICANCE: We suggest that although the occipito-parietal visuomotor system in the dorsal stream is designed to guide goal-directed actions, prolonged practice may enable it to calibrate actions based on visual inputs displaced from the action.

Functional magnetic resonance imaging reveals the neural substrates of arm transport and grip formation in reach-to-grasp actions in humans.

Picking up a cup requires transporting the arm to the cup (transport component) and preshaping the hand appropriately to grasp the handle (grip component). Here, we used functional magnetic resonance imaging to examine the human neural substrates of the transport component and its relationship with the grip component. Participants were shown three-dimensional objects placed either at a near location, adjacent to the hand, or at a far location, within reach but not adjacent to the hand. Participants performed three tasks at each location as follows: (1) touching the object with the knuckles of the right hand; (2) grasping the object with the right hand; or (3) passively viewing the object. The transport component was manipulated by positioning the object in the far versus the near location. The grip component was manipulated by asking participants to grasp the object versus touching it. For the first time, we have identified the neural substrates of the transport component, which include the superior parieto-occipital cortex and the rostral superior parietal lobule. Consistent with past studies, we found specialization for the grip component in bilateral anterior intraparietal sulcus and left ventral premotor cortex; now, however, we also find activity for the grasp even when no transport is involved. In addition to finding areas specialized for the transport and grip components in parietal cortex, we found an integration of the two components in dorsal premotor cortex and supplementary motor areas, two regions that may be important for the coordination of reach and grasp.

Dissociable neural responses to hands and non-hand body parts in human left extrastriate visual cortex.

Accumulating evidence points to a map of visual regions encoding specific categories of objects. For example, a region in the human extrastriate visual cortex, the extrastriate body area (EBA), has been implicated in the visual processing of bodies and body parts. Although in the monkey, neurons selective for hands have been reported, in humans it is unclear whether areas selective for individual body parts such as the hand exist. Here, we conducted two functional MRI experiments to test for hand-preferring responses in the human extrastriate visual cortex. We found evidence for a hand-preferring region in left lateral occipitotemporal cortex in all 14 participants. This region, located in the lateral occipital sulcus, partially overlapped with left EBA, but could be functionally and anatomically dissociated from it. In experiment 2, we further investigated the functional profile of hand- and body-preferring regions by measuring responses to hands, fingers, feet, assorted body parts (arms, legs, torsos), and non-biological handlike stimuli such as robotic hands. The hand-preferring region responded most strongly to hands, followed by robotic hands, fingers, and feet, whereas its response to assorted body parts did not significantly differ from baseline. By contrast, EBA responded most strongly to body parts, followed by hands and feet, and did not significantly respond to robotic hands or fingers. Together, these results provide evidence for a representation of the hand in extrastriate visual cortex that is distinct from the representation of other body parts.

Impaired grasping in a patient with optic ataxia: primary visuomotor deficit or secondary consequence of misreaching?

Optic ataxia is defined as a spatial impairment of visually guided reaching, but it is typically accompanied by other visuomotor difficulties, notably a failure to scale the handgrip appropriately while reaching to grasp an object. This impaired grasping might reflect a primary visuomotor deficit, or it might be a secondary effect arising from the spatial uncertainty associated with poor reaching. To distinguish between these possibilities, we used a new paradigm to tease apart the proximal and distal components of prehension movements. In the "far" condition objects were placed 30 cm from the hand so that subjects had to make a reaching movement to grasp them, whereas in the "close" condition objects were placed adjacent to the hand, thereby removing the need for a reaching movement. Stimulus eccentricity was held constant. We tested a patient with optic ataxia (M.H.), whose misreaching affects only his right hand within the right visual hemifield. M.H. showed a clear impairment in grip scaling, but only when using his right hand to grasp objects in the right visual hemifield. Critically, this grip-scaling impairment was absent in M.H. in the "close" condition. These data suggest that M.H.'s grip scaling is impaired as a secondary consequence of making inaccurate reaching movements, and not because of any intrinsic visuomotor impairment of grasping. We suggest that primary misgrasping is not a core symptom of the optic ataxia syndrome, and that patients will show a primary deficit only when their lesion extends anteriorly within the intraparietal sulcus to include area aIPS.