Age- and stereovision-dependent eye-hand coordination deficits in children with amblyopia and abnormal binocularity.

PURPOSE: To examine factors contributing to eye-hand coordination deficits in children with amblyopia and impaired stereovision. METHODS: Participants were 55 anisometropic or strabismic children aged 5.0 to 9.25 years with different degrees of amblyopia and abnormal binocularity, along with 28 age-matched visually-normal controls. Pilot data were obtained from four additional patients studied longitudinally at different treatment stages. Movements of the preferred hand were recorded using a 3D motion-capture system while subjects reached-to-precision grasp objects (two sizes, three locations) under binocular, dominant eye, and amblyopic/nonsighting eye conditions. Kinematic and "error" performance measures were quantified and compared by viewing condition and subject group using ANOVA, stepwise regression, and correlation analyses. RESULTS: Movements of the younger amblyopes (age 5-6 years; n = 30) were much slower, particularly in the final approach to the objects, and contained more spatial errors in reaching (∼×1.25-1.75) and grasping (∼×1.75-2.25) under all three views (P < 0.05) than their age-matched controls (n = 13). Amblyopia severity was the main contributor to their slower movements with absent stereovision a secondary factor and the unique determinant of their increased error-rates. Older amblyopes (age 7-9 years; n = 25) spent longer contacting the objects before lifting them (P = 0.015) compared with their matched controls (n = 15), with absence of stereovision still solely related to increases in reach and grasp errors, although these occurred less frequently than in younger patients. Pilot prospective data supported these findings by showing positive treatment-related associations between improved stereovision and reach-to-grasp performance. CONCLUSIONS: Strategies that children with amblyopia and abnormal binocularity use for reach-to-precision grasping change with age, from emphasis on visual feedback during the "in-flight" approach at ages 5 to 6 years to more reliance on tactile/kinesthetic feedback from object contact at ages 7 to 9 years. However, recovery of binocularity confers increasing benefits for eye-hand coordination speed and accuracy with age, and is a better predictor of these fundamental performance measures than the degree of visual acuity loss.

Getting a grip: different actions and visual guidance of the thumb and finger in precision grasping.

We manipulated the visual information available for grasping to examine what is visually guided when subjects get a precision grip on a common class of object (upright cylinders). In Experiment 1, objects (2 sizes) were placed at different eccentricities to vary the relative proximity to the participant's (n = 6) body of their thumb and finger contact positions in the final grip orientations, with vision available throughout or only for movement programming. Thumb trajectories were straighter and less variable than finger paths, and the thumb normally made initial contact with the objects at a relatively invariant landing site, but consistent thumb first-contacts were disrupted without visual guidance. Finger deviations were more affected by the object's properties and increased when vision was unavailable after movement onset. In Experiment 2, participants (n = 12) grasped 'glow-in-the-dark' objects wearing different luminous gloves in which the whole hand was visible or the thumb or the index finger was selectively occluded. Grip closure times were prolonged and thumb first-contacts disrupted when subjects could not see their thumb, whereas occluding the finger resulted in wider grips at contact because this digit remained distant from the object. Results were together consistent with visual feedback guiding the thumb in the period just prior to contacting the object, with the finger more involved in opening the grip and avoiding collision with the opposite contact surface. As people can overtly fixate only one object contact point at a time, we suggest that selecting one digit for online guidance represents an optimal strategy for initial grip placement. Other grasping tasks, in which the finger appears to be used for this purpose, are discussed.

Eye-hand coordination skills in children with and without amblyopia.

PURPOSE: To investigate whether binocular information provides benefits for programming and guidance of reach-to-grasp movements in normal children and whether these eye-hand coordination skills are impaired in children with amblyopia and abnormal binocularity. METHODS: Reach-to-grasp performance of the preferred hand in binocular versus monocular (dominant or nondominant eye occluded) conditions to different objects (two sizes, three locations, and two to three repetitions) was quantified by using a 3D motion-capture system. The participants were 36 children (age, 5-11 years) and 11 adults who were normally sighted and 21 children (age, 4-8 years) who had strabismus and/or anisometropia. Movement kinematics and error rates were compared for each viewing condition within and between subject groups. RESULTS: The youngest control subjects used a mainly programmed (ballistic) strategy and collided with the objects more often when viewing with only one eye, while older children progressively incorporated visual feedback to guide their reach and, eventually, their grasp, resulting in binocular advantages for both movement components resembling those of adult performance. Amblyopic children were the worst performers under all viewing conditions, even when using the dominant eye. They spent almost twice as long in the final approach to the objects and made many (1.5-3 times) more errors in reach direction and grip positioning than their normal counterparts, these impairments being most marked in those with the poorest binocularity, regardless of the severity or cause of their amblyopia. CONCLUSIONS: The importance of binocular vision for eye-hand coordination normally increases with age and use of online movement guidance. Restoring binocularity in children with amblyopia may improve their poor hand action control.

The Poggendorff illusion affects manual pointing as well as perceptual judgements.

Pointing movements made to a target defined by the imaginary intersection of a pointer with a distant landing line were examined in healthy human observers in order to determine whether such motor responses are susceptible to the Poggendorff effect. In this well-known geometric illusion observers make systematic extrapolation errors when the pointer abuts a second line (the inducer). The kinematics of extrapolation movements, in which no explicit target was present, where similar to those made in response to a rapid-onset (explicit) dot target. The results unambiguously demonstrate that motor (pointing) responses are susceptible to the illusion. In fact, raw motor biases were greater than for perceptual responses: in the absence of an inducer (and hence also the acute angle of the Poggendorff stimulus) perceptual responses were near-veridical, whilst motor responses retained a bias. Therefore, the full Poggendorff stimulus contained two biases: one mediated by the acute angle formed between the oblique pointer and the inducing line (the classic Poggendorff effect), which affected both motor and perceptual responses equally, and another bias, which was independent of the inducer and primarily affected motor responses. We conjecture that this additional motor bias is associated with an undershoot in the unknown direction of movement and provide evidence to justify this claim. In conclusion, both manual pointing and perceptual judgements are susceptible to the well-known Poggendorff effect, supporting the notion of a unitary representation of space for action and perception or else an early locus for the effect, prior to the divergence of processing streams.

Grasping deficits and adaptations in adults with stereo vision losses.

PURPOSE: To examine the effects of permanent versus brief reductions in binocular stereo vision on reaching and grasping (prehension) skills. METHODS: The first experiment compared prehension proficiency in 20 normal and 20 adults with long-term stereo-deficiency (10 with coarse and 10 with undetectable disparity sensitivities) when using binocular vision or just the dominant or nondominant eye. The second experiment examined effects of temporarily mimicking similar stereoacuity losses in normal adults, by placing defocusing low- or high-plus lenses over one eye, compared with their control (neutral lens) binocular performance. Kinematic and error measures of prehension planning and execution were quantified from movements of the subjects' preferred hand recorded while they reached, precision-grasped, and lifted cylindrical objects (two sizes, four locations) on 40 to 48 trials under each viewing condition. RESULTS: Performance was faster and more accurate with normal compared with reduced binocular vision and least accomplished under monocular conditions. Movement durations were extended (up to approximately 100 ms) whenever normal stereo vision was permanently (ANOVA P < 0.05) or briefly (ANOVA P < 0.001) reduced, with a doubling of error rates in executing the grasp (ANOVA P < 0.001). Binocular deficits in reaching occurred during its end phase (prolonged final approach, more velocity corrections, poorer coordination with object contact) and generally increased with the existing loss of disparity sensitivity. Binocular grasping was more uniformly impaired by stereoacuity loss and influenced by its duration. Adults with long-term stereo-deficiency showed increased variability in digit placement at initial object contact, and they adapted by prolonging (by approximately 25%) the time spent subsequently applying their grasp (ANOVA P < 0.001). Brief stereoreductions caused systematic shifts in initial digit placement and two to three times more postcontact adjustments in grip position (ANOVA P < 0.01). CONCLUSIONS: High-grade binocular stereo vision is essential for skilled precision grasping. Reduced disparity sensitivity results in inaccurate grasp-point selection and greater reliance on nonvisual (somesthetic) information from object contact to control grip stability.

Common cortical loci are activated during visuospatial interpolation and orientation discrimination judgements.

There is a wealth of literature on the role of short-range interactions between low-level orientation-tuned filters in the perception of discontinuous contours. However, little is known about how spatial information is integrated across more distant regions of the visual field in the absence of explicit local orientation cues, a process referred to here as visuospatial interpolation (VSI). To examine the neural correlates of VSI high field functional magnetic resonance imaging was used to study brain activity while observers either judged the alignment of three Gabor patches by a process of interpolation or discriminated the local orientation of the individual patches. Relative to a fixation baseline the two tasks activated a largely over-lapping network of regions within the occipito-temporal, occipito-parietal and frontal cortices. Activated clusters specific to the orientation task (orientation>interpolation) included the caudal intraparietal sulcus, an area whose role in orientation encoding per se has been hotly disputed. Surprisingly, there were few task-specific activations associated with visuospatial interpolation (VSI>orientation) suggesting that largely common cortical loci were activated by the two experimental tasks. These data are consistent with previous studies that suggest higher level grouping processes -putatively involved in VSI- are automatically engaged when the spatial properties of a stimulus (e.g. size, orientation or relative position) are used to make a judgement.

Biases and sensitivities in the Poggendorff effect when driven by subjective contours.

PURPOSE: A consensus in the existing literature suggests that the Poggendorff effect (a perceptual misalignment of two collinear transversal segments when separated by a pair of parallel contours) persists when the parallels are defined by Kanizsa-like subjective contours. However, previous studies have often been complicated by a lack of quantitative measures of effect size, statistical tests of significance, appropriate measures of baseline and control biases, or stringent definition of subjective contours. The aim of this study was thus to determine whether subjective contours are capable of driving the Poggendorff effect once other factors are accounted for. METHODS: Twenty participants were tested on a number of test and control figures incorporating first-order (luminance-defined) and subjective parallels using the method of adjustment. All figures were tested at two different orientations, and observer sensitivities and observer biases were assessed. RESULTS: A systematic response bias (in the direction of the classical effect) was found for Poggendorff figures that incorporated subjective parallels. The effect was highly significant and greater than for control figures. There was no concomitant change in judgment sensitivity (positional certainty). Finally, there was a positive correlation between the effect size for figures incorporating first-order and subjective parallels. CONCLUSIONS: The findings reported demonstrate conclusively that true Kanizsa-like subjective contours are capable of driving the Poggendorff effect. Further, the data are consistent with a growing body of evidence that suggests both first-order and subjective contours are processed at early loci in the visual pathways when position is encoded.

Dissociation between vergence and binocular disparity cues in the control of prehension.

Binocular vision provides important advantages for controlling reach-to-grasp movements. We examined the possible source(s) of these advantages by comparing prehension proficiency under four different binocular viewing conditions, created by randomly placing a neutral lens (control), an eight dioptre prism (Base In or Base Out) or a low-power (2.00-3.75 dioptre) Plus lens over the eye opposite the moving limb. The Base In versus Base Out prisms were intended to selectively alter vergence-specified distance (VSD) information, such that the targets appeared beyond or closer than their actual physical position, respectively. The Plus lens was individually tailored to reduce each subject's disparity sensitivity (to 400-800 arc s), while minimizing effects on distance estimation. In pre-testing, subjects pointed (without visual feedback) to mid-line targets at different distances, and produced the systematic directional errors expected of uncorrected movements programmed under each of the perturbed conditions. For the prehension tasks, subjects reached and precision grasped (with visual feedback available) cylindrical objects (two sizes and three locations), either following a 3 s preview in which to plan their actions or immediately after the object became visible. Viewing condition markedly affected performance, but the planning time allowed did not. Participants made the most errors suggesting premature collision with the object (shortest 'braking' times after peak deceleration; fastest velocity and widest grip at initial contact) under Base In prism viewing, consistent with over-reaching movements programmed to transport the hand beyond the actual target due to its 'further' VSD. Conversely, they produced the longest terminal reaches and grip closure times, with multiple corrections just before and after object contact, under the Plus lens (reduced disparity) condition. Base Out prism performance was intermediate between these two, with significant increases in additional forward movements during the transport end-phase, indicative of initial under-reaching in response to the target's 'nearer' VSD. Our findings suggest dissociations between the role of vergence and binocular disparity in natural prehension movements, with vergence contributing mainly to reach planning and high-grade disparity cues providing particular advantages for grasp-point selection during grip programming and application.

Prehension deficits in amblyopia.

PURPOSE: Visual defects associated with amblyopia have been extensively studied, but their impact on the performance of everyday visuomotor tasks is unclear. This study evaluates eye-hand coordination (prehension) skills in adult amblyopes compared with normal subjects. METHODS: Twenty amblyopes (10 strabismic, 10 nonstrabismic) with different degrees of visual acuity loss (mild, moderate, or severe) and stereodeficiency (reduced or undetectable) participated, along with 20 matched control subjects. Subjects reached, precision grasped, and lifted cylindrical household objects (two sizes, four locations) using binocular vision or just the dominant or amblyopic (nondominant) eye, while the actions of the preferred hand were recorded. Various indices of prehension planning and online control were quantified for all trials (n = 48) performed under each viewing condition. RESULTS: Initial reaching behavior and grip shaping before object contact, which result from movement programming, were relatively normal in the amblyopic subjects, despite their vision losses. By contrast, they exhibited a range of deficits under both binocular and nondominant eye conditions in their final approach to the object (terminal reach) and when closing and applying a grasp. These impairments included prolonged execution times and more errors compared with control subjects, the extents of which covaried with the existing depth of amblyopia, although not with its underlying cause. CONCLUSIONS: Visuomotor adaptations in amblyopes are relatively minor and limited to aspects of movement planning. Their deficits in movement execution should benefit, however, from treatments that restore spatial acuity and binocularity to progressively normal levels and so deserve more explicit consideration when assessing therapeutic outcomes.

Advantages of binocular vision for the control of reaching and grasping.

Theoretical considerations suggest that binocular information should provide advantages, compared to monocular viewing, for the planning and execution of natural reaching and grasping actions, but empirical support for this is quite equivocal. We have examined these predictions on a simple prehension task in which normal subjects reached, grasped and lifted isolated cylindrical household objects (two sizes, four locations) in a well-lit environment, using binocular vision or with one eye occluded. Various kinematic measures reflecting the programming and on-line control of the movements were quantified, in combination with analyses of different types of error occurring in the velocity, spatial path and grip aperture profiles of each trial. There was little consistent effect of viewing condition on the early phase of the reach, up to and including the peak deceleration, but all other aspects of performance were superior under binocular control. Subjects adopted a cautious approach when binocular information was unavailable: they extended the end phase of the reach and pre-shaped their hand with a wider grip aperture further away from the object. Despite these precautions, initial grip application was poorly coordinated with target contact and was inaccurately scaled to the objects' dimensions, with the subsequent post-contact phase of the grasp significantly more prolonged, error-prone and variable compared to binocular performance. These effects were obtained in two separate experiments in which the participants' performed the task under randomized or more predictable (blocked) viewing conditions. Our data suggest that binocular vision offers particular advantages for controlling the terminal reach and the grasp. We argue that these benefits derive from binocular disparity processing linked to changes in relative hand-target distance, and that this depth information is independently used to regulate the progress of the approaching hand and to guide the digits to the (pre-selected) contact points on the object, thereby ensuring that the grip is securely applied.

Scaling of letter size and contrast equalises perception across eccentricities and set sizes.

Double E(2)N(2) scaling, i.e. magnifying size and contrast, allows modelling of the deterioration of face recognition performance with increasing eccentricity (E) and the size (N) of the set from which a target face has to be identified. E(2) and N(2) values represent the eccentricities and set sizes at which stimulus size and contrast must double in order to keep performance unchanged, whilst parameter K represents the multiplicative interaction between E and N. In the current study we investigated whether double E(2)N(2) scaling can model performance deterioration with increasing eccentricity and set size in letter perception too. Contrast sensitivity for letter perception was investigated as a function of letter size at N=1-8 and E=0 degrees -10 degrees. The superimposition of contrast sensitivity functions produced two scaling surfaces, one for letter size and another for contrast, which allowed modelling of the changes in letter perception with increasing E and N. With increasing eccentricity/set size the change of scale was much faster for contrast than letter size. Thus, in letter perception, contrast scaling was more important than spatial scaling. When compared with face perception, the change of spatial scale with increasing eccentricity was slower for letters whereas the change of contrast scale was similar for both. With increasing set size the changes of both spatial and contrast scales are faster for faces. In spatial scaling the interaction between eccentricity and set size was similar for letters and faces whereas in contrast scaling letters showed no interaction. Thus, letter perception was less affected by eccentricity and set size than face perception.