"Graspability" of objects affects gaze patterns during perception and action tasks.

When grasping an object, our gaze marks key positions to which the fingertips are directed. In contrast, eye fixations during perceptual tasks are typically concentrated on an object's centre of mass (COM). However, previous studies have typically required subjects to either grasp the object at predetermined sites or just look at computer-generated shapes "as a whole". In the current study, we investigated gaze fixations during a reaching and grasping task to symmetrical objects and compared these fixations with those made during a perceptual size estimation task using real (Experiment 1) and computer-generated objects (Experiment 2). Our results demonstrated similar gaze patterns in both perception and action to real objects. Participants first fixated a location towards the top edge of the object, consistent with index finger location during grasping, followed by a subsequent fixation towards the object's COM. In contrast, during the perceptual task to computer-generated objects, an opposite pattern in fixation locations was observed, where first fixations were closer to the COM, followed by a subsequent fixation towards the top edge. Even though differential fixation patterns were observed between studies, the area in which these fixations occurred, between the centre of the object and top edge, was the same in all tasks. These results demonstrate for the first time consistencies in fixation locations across both perception and action tasks, particularly when the same type of information (e.g. object size) is important for the completion of both tasks, with fixation locations increasing relative to the object's COM with increases in block height.

Temporal integration limits of stereovision in reaching and grasping.

Even though there have been extensive investigations of the temporal integration limits of binocular vision in perceptual tasks, relatively little is known about temporal integration limits during the completion of visuomotor tasks. To assess the temporal integration limits of binocular disparity within the action domain, accuracy of reach kinematics in a reaching and grasping task under continuous binocular and monocular viewing conditions were compared with those obtained under alternating monocular viewing conditions with interocular delays ranging from 14 to 58 ms. Even the shortest of the interocular delays resulted in larger grip apertures than those in the continuous monocular and binocular viewing conditions. The short temporal integration interval of stereovision obtained in this study cannot be accounted for by differential visual feedback in the binocular and interocular delay conditions, nor is it likely to be a consequence of visual disruption due to the interocular delays. Our findings suggest that the visuomotor system has little tolerance to interocular delay.

The specificity of learned associations in visuomotor and perceptual processing.

Learned associations between object properties, such as weight and size, allow for quick and accurate manipulations of objects that we encounter repeatedly. This integration of learned sensory information reduces the overall computational load of our visuomotor system when interacting with familiar objects. In the laboratory, even novel associations can be quickly established after only brief training. Haffenden and Goodale in J Cogn Neurosci 12:950-964 (2000) found that learned associations between color and size affected grip scaling for manual estimations of size and visually guided grasping. But, how specific are these learned associations? In the current study, lighter-shaded "untrained" target objects were added to Haffenden and Goodale's color-size association paradigm to determine if the learned associations made by the perception and action systems are equally tolerant to within-category color changes. During perceptual estimations, training was generalized within color categories--manual estimations of size were influenced by both the trained and lighter-shaded untrained colors. In contrast, grasping was not influenced by the untrained colored blocks. These results demonstrate how the perception and action systems differ in their incorporation of learned perceptual information. In contrast to the object specific associations needed for grasping, our perceptual system is more categorical and uses generalized perceptual grouping strategies when relying on learned color information.

Spatial transformations for eye-hand coordination.

Eye-hand coordination is complex because it involves the visual guidance of both the eyes and hands, while simultaneously using eye movements to optimize vision. Since only hand motion directly affects the external world, eye movements are the slave in this system. This eye-hand visuomotor system incorporates closed-loop visual feedback but here we focus on early feedforward mechanisms that allow primates to make spatially accurate reaches. First, we consider how the parietal cortex might store and update gaze-centered representations of reach targets during a sequence of gaze shifts and fixations. Recent evidence suggests that such representations might be compared with hand position signals within this early gaze-centered frame. However, the resulting motor error commands cannot be treated independently of their frame of origin or the frame of their destined motor command. Behavioral experiments show that the brain deals with the nonlinear aspects of such reference frame transformations, and incorporates internal models of the complex linkage geometry of the eye-head-shoulder system. These transformations are modeled as a series of vector displacement commands, rotated by eye and head orientation, and implemented between parietal and frontal cortex through efficient parallel neuronal architectures. Finally, we consider how this reach system might interact with the visually guided grasp system through both parallel and coordinated neural algorithms.

Patient Schn: has Goldstein and Gelb's case withstood the test of time?

The current manuscript takes a critical look at the case of Goldstein and Gelb's patient, Schn, reported to be the first well-defined example of apperceptive visual agnosia. While doubts have been cast on the validity of the original investigations, we propose that perhaps the case of Schn should be reclassified as an example of integrative agnosia. Be that as it may, what is not in doubt is that the case of Schn has had a lasting impact on the development of neuropsychological theorem.

Kinematic rules for upper and lower arm contributions to grasp orientation.

The purpose of the current study was to investigate the contribution of upper and lower arm torsion to grasp orientation during a reaching and grasping movement. In particular, we examined how the visuomotor system deals with the conflicting demands of coordinating upper and lower arm torsion and maintaining Donders' Law of the upper arm (a behavioral restriction of the axes of arm rotation to a two-dimensional "surface"). In experiment 1, subjects reached out and grasped a target block that was presented in one of 19 orientations (5 degrees clockwise increments from horizontal to vertical) at one position in a vertical presentation board. In experiment 2, target blocks were presented in one of three orientations (horizontal, three-quarter, and vertical) at nine different positions in the presentation board. If reach and grasp commands control the proximal and distal arms separately, then one would only expect the lower arm to contribute to grasp orientations and that Donders' Law would hold for the upper arm-independent of grasp orientations. Instead, as the required grasp orientation increased from horizontal to vertical, there was a significant clockwise torsional rotation in the upper arm, which accounted for 9% of the final vertical grasp orientation, and the lower arm, which accounted for 42%. A linear relationship existed between the torsional rotations of the upper and lower arm, which indicates that the components of the arm rotate in coordination with one another. The location-dependent aspects of upper and lower arm torsion remained invariant, however, yielding consistently shaped Donders' "surfaces" (with different torsional offsets) for different grasp orientations. These observations suggest that the entire arm-hand system contributes to grasp orientation, and therefore, the reach/grasp distinction is not directly reflected in proximal-distal kinematics but is better reflected in the distinction between these coordinated orienting rules and the location-dependent kinematic rules for the upper arm that result in Donders' Law for one given grasp orientation.

Hemispatial neglect: its effects on visual perception and visually guided grasping.

Hemispatial neglect is a neurological disorder characterized by a failure to represent information appearing in the hemispace contralateral to a brain lesion. In addition to the perceptual consequences of hemispatial neglect, several authors have reported that hemispatial neglect impairs visually guided movements. Others have reported that the extent of the impairment depends on the type of visually guided task. Finally, in some cases, neglect has been shown to impair visual perception without affecting visuomotor control in relation to the very same stimuli. While neglect patients may be able to successfully pick up an object they have difficulty perceiving in its entirety, it does not mean that they are picking up the object in the same way that a neurologically intact individual would. In the current study, patients with hemispatial neglect were presented with irregularly shaped objects, directly in front of them, that lacked clear symmetry and required an analysis of their entire contour in order to calculate stable grasp points. In a perceptual discrimination task, the neglect patients had difficulty distinguishing one object from another on the basis of their shape. In a grasping task, the neglect patients showed more variance in the position of their grasp on the target objects than their control subjects, with an overall shift to the relative right side of the presented objects. The perceptual and visuomotor deficits seen in patients with hemispatial neglect deficits may be the result of an inability to form good structural representations of the entire object for use in visual perception and visuomotor control.

The effects of rotation and inversion on face processing in prosopagnosia.

The current study investigated the sensitivity of face recognition to two changes of the stimulus, a rotation in depth and an inversion, by comparing the performance of two prosopagnosic patients, RN and CR, with non-neurological control subjects on a face-matching task. The control subjects showed an effect of depth rotation, with errors and reaction times increasing systematically with rotation angle, and the traditional inversion effect, with errors and reaction times increasing under inverted conditions. In contrast, RN showed no effect of rotation or inversion on his error data but did show a less sensitively graded effect of rotation and the traditional inversion effect on reaction times. CR did not show a graded effect of rotation on his errors or reaction times. Although CR showed the traditional inversion effect on his error data, he displayed an inversion superiority effect on his reaction time data, which supports the claim that the damaged holistic processing systems continue to dominate face processing in prosopagnosia even though they are malfunctioning. These results suggest that the damage that occurs to the ventral temporal cortex in prosopagnosia may have forced the patients to rely on sources of information that are not dependent on the view of the face and, moreover, cannot be adapted to deal with rotated faces under both upright and inverted conditions.

A functional MRI study of face recognition in patients with prosopagnosia.

An fMRI investigation was conducted to determine whether patients with impaired face recognition, a deficit known as prosopagnosia, would show functional activation in the fusiform gyrus, the neural substrate for face processing, when viewing faces. While the patients did show activation in the fusiform gyrus, with significantly more voxels in posterior areas than their control subjects, this activation was not sufficient for face processing. In one of the patients, the posterior activation was particularly evident in the left hemisphere, which is thought to be involved in feature-based strategies of face perception. We conclude that an increased reliance on feature-based processing in prosopagnosia leads to a recruitment of neurons in posterior regions of the fusiform gyrus, regions that are not ideally suited for processing faces.

Role of familiar size in the control of grasping.

The present study examined whether the learned pictorial depth cue of "familiar size" could be used to plan a reaching and grasping movement in the absence of binocular vision. Sixteen right-handed subjects were presented with two different arrays, under monocular and binocular viewing conditions, in which a range of different "grasp-sized" spheres that were lit from within could be presented in an otherwise darkened environment. In the "familiar-size" presentation array, only one "standard" sized sphere was presented, which gave subjects an opportunity to learn the relationship between the standard sphere's retinal image size and its distance. In the "multiple" spheres presentation array, subjects could not learn such a relationship because on any one trial, one of four different sphere sizes could be present. In a second experiment, the effects of this paradigm on six subjects' perceptual reports of distance were examined by having subjects slide their index fingers apart along a horizontal rod to indicate the estimated distance of the spheres. When familiar size could not be used as a cue to distance, subjects produced more on-line corrections in their reaching and grasping movements to the standard-sized spheres--but only under monocular viewing conditions. It appears that subjects are able to exploit the learned relationship between an object's distance and its projected retinal image size to help program and control reaching and grasping movements when binocular vision is not available. Although the influence of familiar size on subjects' perceptual estimates is less clear, it is clear that subjects' perceptual estimates show poor absolute scaling for distance. This result further supports the notion that under normal viewing conditions the visuomotor system uses binocular information to program and control manual prehension, but is able to use pictorial information when binocular vision is denied.

The role of learned pictorial cues in the programming and control of grasping.

Binocular information has been shown to be important for the programming and control of reaching and grasping. Even without binocular vision, people are still able to reach out and pick up objects accurately - albeit less efficiently. It remains unclear, which of the many available monocular depth cues humans use to calibrate manual prehension when binocular information is not available. In the present experiment, we examined whether or not subjects could use a learned relationship between the elevation of a goal object in the visual scene and its distance to help program and control the required grasp. The elevation of the goal object was systematically varied with distance in some blocks of trials by presenting the object at different positions along a horizontal plane 35 cm below eye level. In other blocks of trials, elevation did not vary with distance because the objects were always presented along the subject's line of sight. When subjects viewed these two displays monocularly, they showed fewer on-line adjustments in the trajectory of the limb and the aperture of the fingers when the elevation of the target object in the visual scene could be used to help program the required movements. No such difference between performance on the two arrays was seen when subjects were allowed a full binocular view. This study confirms that subjects are indeed able to use a learned relationship between the elevation of an object and its distance as a cue for programming grasping movements when binocular information is not available. Together with evidence from work with neurological patients who have difficulty perceiving pictorial cues, these findings suggest that the visuomotor system might normally "prefer" to use binocular cues, but can fall back on learned pictorial information when binocular vision is denied.

Does a monocularly presented size-contrast illusion influence grip aperture?

The present study tested the idea that if subjects rely more on scene-based pictorial cues when binocular cues are not available, then both their perceptual judgements and their grasp might be influenced by pictorial illusions such as the Ebbinghaus (Titchener) Circles Illusion under monocular viewing conditions. Under binocular viewing conditions, subjects were always able to scale their grip accurately to the true size of the target disc and were unaffected by the illusion. Under monocular viewing, however, subjects appeared to be influenced by the illusion. Thus, when confronted with physically different target discs displayed on backgrounds that made them appear equivalent in size, subjects treated the two discs as equivalent--even when picking them up. These results, combined with earlier work from our laboratory suggests that binocular information plays a critical role in normal human prehension but when this information is not available the visuomotor system is able to "fall back" on the remaining monocular cues, which can cause the visuomotor system to be more susceptible to pictorial illusions.

The role of head movements in the control of manual prehension.

Binocular information has been shown to be important for the programming and control of reaching and grasping. But even without binocular vision, people are still able to reach out and pick up objects accurately - albeit less efficiently. As part of a continuing investigation into the role that monocular cues play in visuomotor control, we examined whether or not subjects could use retinal motion information, derived from movements of the head, to help program and control reaching and grasping movements when binocular vision is denied. Subjects reached out in the dark to an illuminated sphere presented at eye-level, under both monocular and binocular viewing conditions with their head either free to move or restrained. When subjects viewed the display monocularly, they showed fewer on-line corrections when they were allowed to move their head. No such difference in performance was seen when subjects were allowed a full binocular view. This study, combined with previous work with neurological patients, confirms that the visuomotor system "prefers" to use binocular vision but, when this information is not available, can fall back on other monocular depth cues, such as information produced by motion of the object (and the scene) on the retina, to help program and control manual prehension.

The removal of binocular cues disrupts the calibration of grasping in patients with visual form agnosia.

The present study tested the idea that the visuomotor systems mediating prehension do not have independent access to pictorial cues processed by perceptual mechanisms. Individuals with visual form agnosia, whose perceptual systems are compromised but who have intact visuomotor control, were examined to determine whether they could use pictorial scene cues to calibrate manual prehension when binocular information was removed. The removal of binocular cues produced considerable disruptions in size-constancy of grip aperture, which, combined with earlier observations in normal subjects, suggests that binocular cues are of primary importance in calibration of grasping. In the absence of binocular vision, normal subjects can use pictorial information, information that is severely compromised in individuals with visual form agnosia, to compute the distance (and thus the size) of the goal object. Thus, individuals with visual form agnosia must rely on a retinal image that remains uncalibrated, leading to inaccurate calibrations of grip aperture. The fact that these individuals scaled their grasp much less accurately under the monocular viewing condition, despite showing normal binocular grasping, suggests that pictorial cues to depth, which are presumably processed by mechanisms mediating our perception of objects and events in the world, can be accessed by visuomotor mechanisms only indirectly. These results, together with others, suggest that the visuomotor system 'prefers' to use binocular information and uses pictorial cues only as a last resort.

The development of adaptive head movements following enucleation.

Recent work in our laboratory has revealed that enucleated patients produce large lateral and vertical head movements during visually guided grasping. These movements may allow them to maximise the use of retinal motion cues in planning and controlling their grasp. The aim of the present study was to determine whether the tendency to produce these adaptive head movements increases as a function of time since enucleation. We tested a group of 12 enucleated patients in whom the time between surgery and testing varied from 2 weeks to 35 years (mean = 11.2 years). These patients were required to reach out and grasp oblong blocks of different sizes at different distances. Correlational tests revealed an increase in the proportion of self-generated lateral and vertical head movements versus forward head movements as a function of post-enucleation time (r(s)(12) = 0.68, p < 0.025 and r(s)(12) = 0.65, p < 0.025, respectively). This suggests that enucleated patients may be adapting to living with one eye by learning to increase the proportion of their lateral and vertical head movements during the performance of skilled motor acts.

Adapting to monocular vision: grasping with one eye.

The aim of the present study was to determine whether normal subjects with one eye covered and patients in whom one eye had been enucleated generate more head movements than subjects using binocular vision during the performance of a visually guided grasping movement. In experiment 1, 14 right-handed normal subjects were tested binocularly and monocularly in a task in which they were required to reach out and grasp oblong blocks of different sizes at different distances. Although the typical binocular advantage in reaching and grasping was observed, the overall head movement scores did not differ between these testing conditions. In experiment 2, seven right-handed enucleated patients were compared to seven age and sex-matched control subjects (tested under binocular and monocular viewing conditions), on the same task as used in experiment 1. While no differences were found in the kinematics of reaches produced by the enucleated patients and the control subjects, the patients did produce larger and faster resultant head movements, composed mainly of lateral and vertical movements. This suggests that enucleated patients may be generating more head movements in order to better utilize retinal motion cues to aid in manual prehension.

Os trigonum impingement in dancers.

Sixteen patients underwent surgical excision of an impinging ossicle through a posterior lateral approach. Twelve of these patients (15 ankles) were available for followup and were retrospectively surveyed at an average of 28 months after surgery. There were 9 women and 3 men. Nine were professional ballet dancers and 3 were students of advanced ballet schools. Preoperative symptoms included pain localized to the posterior ankle, limitation of motion, weakness, swelling, or neurologic changes associated with dance activities. All patients were severely hampered in their dance participation and had failed nonsurgical therapies. Postoperatively, all patients followed an aggressive rehabilitation protocol. All had improvement in their impingement symptoms; eight (67%) still had occasional discomfort. All professional dancers returned to unrestricted dance activity. The mean time to full activity was 3 months. One patient had a superficial wound infection requiring antibiotic treatment and another suffered a transient tibial nerve neurapraxia. Both of these complications resolved without sequelae. We conclude that posterior ankle impingement in ballet dancers, caused by an os trigonum and resistant to nonsurgical therapies, is effectively treated with simple excision of the offending structure.

Arteriovenous Fistula Development After Posterior Compartment Fasciotomy to Treat Shin Splints.

In brief: This case report presents an unusual complication in a distance runner who was treated for the shin splint syndrome. Following release of the deep posterior fascial compartment, he had mild erythema and swelling in the region of his incision. He could not bear full weight on his left leg because of pain. An arteriogram obtained approximately six months later showed an arteriovenous fistula, which was subsequently treated with resection and neurolysis of the saphenous nerve. The patient improved but did not reach his previous level of athletic performance. Recommendations for preventing this complication are outlined, and the use of the term shin splints is discussed.

Electron microscopy of CO2-laser-induced effects in human fibrocartilage.

Previous reports of effects of CO2 laser energy on human fibrocartilage suggest thermal injury extends to a depth of approximately 70 microns from the target surface with power settings of 35 W and exposure times of 0.5 seconds. The present study was undertaken to look for more subtle evidence of thermal alteration of human fibrocartilage treated with CO2 laser irradiation. Fifteen human menisci were irradiated at power settings of 10, 20, and 30 W with exposure times of 0.1 and 0.5 seconds. The specimens were immediately fixed and sectioned for electron microscopic examination. Loss of a normal cross banding, and marginal clarity of individual collagen fibers were observed in the extracellular matrix and were observed at distances up to 300 microns from the exposed tissue surface. In addition, cellular changes at similar tissue depth consisted of cell membrane invaginations, clumping of nuclear chromatin, breakdown of endoplasmic reticulum architecture, and loss of mitochondria and Golgi complexes from the cytoplasm were observed. This study demonstrates deeper penetration of a radiation that was previously appreciated by light microscopy in irradiated human fibrocartilage, although the implications with respect to contraside viability and healing potential of the tissue in vivo is not known.

Techniques of wrist arthroscopy.

Wrist arthroscopy is a promising new technique for the evaluation of wrist pain or dysfunction. Cadaveric wrist specimens were used to devise safe and advantageous entry portals for arthroscopy and to establish respective advantages for each portal. Thirty-five clinical cases were used to correlate the laboratory experience and to refine a reproducible surgical technique. Seven portals are useful: five in the radiocarpal interval, one in the midcarpal area, and one in the distal radioulnar joint space. Detailed wrist anatomy is reviewed in this paper and must be thoroughly understood to interpret arthroscopic views. Blunt subcutaneous dissection protects cutaneous nerve branches at the various portals. Intraoperative photographs illustrate the excellent perspectives achieved using these techniques.