Search

An interview with Janette Atkinson and Oliver Braddick, who study the development of visual processing in infants.

Visual disorders in childhood are conventionally classified as disorders of the eye (e.g., retinal dystrophy, cataract, refractive error) or of the visual pathways of the brain (cerebral visual impairment, specific impairments of dorsal, or ventral streams). However, the two must be considered together, since anomalous visual input affects the functional development of the visual brain (e.g., in strabismus and amblyopia), and some mechanisms of ocular development (e.g., refractive change) are actively controlled by central visual processing. In normal and disordered brain processing, the processes of visual perception, visual cognition, visuomotor control, and spatial attention depend on a network of areas and cannot readily be dissociated. The course of normal development of these processes is outlined along with methods for the assessment of functional vision at all levels in infants and young children, including those with multiple impairments.

Summary In the first day of life, infants respond specifically to the crude configuration of a human face. New research finds that this visual sensitivity is present even in the weeks before birth.

Although the contrast sensitivity function (CSF) changes markedly during infancy, there is no consensus regarding whether, how, and why it continues to develop in later childhood. Here, we analyzed previously published data (N = 1928 CSFs), and present new psychophysical findings from 98 children (4.7–14.8 years) and 50 adults (18.1–29.7 years), in order to answer the following questions: (1) Does the CSF change during childhood? (2) How large is the developmental effect size? (3) Are any changes uniform across the CSF, or frequency-specific? and (4) Can some or all of the changes be explained by “non-visual” (i.e. procedural/cognitive) factors, such as boredom or inattentiveness? The new data were collected using a four-alternative forced-choice (4AFC) Gabor-detection task, with two different psychophysical procedures (Weighted Staircase; QUEST+), and suprathreshold (false-negative) catch trials to quantify lapse rates. It is shown that from ages 4 to 18 years, the CSF improves (at an exponentially decaying rate) by approximately 0.3 log10 units (a doubling of contrast sensitivity [CS]), with 90% of this change complete by 12 years of age. The size of the effect was small relative to individual variability, with age alone explaining less than one sixth of variability (16%), and most children performing as well as some adults (i.e. falling within the 90% population limits for adults). Development was frequency-specific, with changes occurring primarily around or below the CSF peak (≤ 4 cpd). At least half — and potentially all — of the changes observed could be explained by non-visual factors (e.g. lapses in concentration), although possible biological mechanisms are discussed.

Purpose.: To validate a novel, automated test of infant resolution acuity based on remote eye-tracking.\ud \ud Methods.: Infants aged 2 to 12 months were tested binocularly using a new adaptive computerized test of infant vision using eye tracking (ACTIVE), and Keeler infant acuity cards (KIAC). The ACTIVE test ran automatically, using remote eye-tracking to assess whether the infant fixated a black-and-white grating of variable spatial frequency. Test-retest reliability was assessed by performing each test twice. Accuracy was assessed by comparing acuity measures across tests and with established age-norms, and by comparing low-contrast acuity estimates in adults with data reported previously.\ud \ud Results.: All infants completed the ACTIVE test at least once. Median test duration was 101 seconds. Measured visual acuity increased with age (P < 0.001), and 90% of mean acuity estimates were within previously published 90% tolerance limits (based on acuity-card age norms). Acuity estimates were also correlated, within-subjects, with results from the KIAC (P = 0.004). In terms of reliability, 86% of acuity estimates deviated by ≤1 octave, with no significant difference in test-retest reliability between the ACTIVE and KIAC procedures (P = 0.461). In adults, acuity estimates from the ACTIVE test did not differ significantly from values reported by previous authors (P > 0.183).\ud \ud Conclusions.: An adaptive computerized test of infant vision using eye-tracking provides a rapid, automated measure of resolution acuity in preverbal infants. The ACTIVE performed comparably to the current clinical gold standard (acuity cards) in terms of testability, reliability, and accuracy, and its principles can be extended to measure other visual functions.

Reduced global motion sensitivity, relative to global static form sensitivity, has been found in children with many neurodevelopmental disorders, leading to the “dorsal stream vulnerability” hypothesis (Braddick et al, Neuropsychologia, 2003). Individual differences in typically developing children’s global motion thresholds have been shown to be associated with variations in specific parietal cortical areas (Braddick et al, J Cog Neuro, 2016). Here, in 125 children aged 5–12 years, we relate individual differences in global motion and form coherence thresholds to fractional anisotropy (FA) in the superior longitudinal fasciculus (SLF), a major fiber tract communicating between parietal lobe and anterior cortical areas. We find a positive correlation between FA of the right SLF and individual children’s sensitivity to global motion coherence, while FA of the left SLF shows a negative correlation. Further analysis of parietal cortical area data shows that this is also asymmetrical, showing a stronger association with global motion sensitivity in the left hemisphere. None of these associations hold for an analogous measure of global form sensitivity. We conclude that a complex pattern of structural asymmetry, including the parietal lobe and the superior longitudinal fasciculus, is specifically linked to the development of sensitivity to global visual motion. This pattern suggests that individual differences in motion sensitivity are primarily linked to parietal brain areas interacting with frontal systems in making decisions on integrated motion signals, rather than in the extra-striate visual areas that perform the initial integration. The basis of motion processing deficits in neurodevelopmental disorders may depend on these same structures.

The ability to report the direction of apparent motion when an array of random dots is displaced fails when the displacement exceeds a limiting value (dmax). We find that dmax increases rapidly with retinal eccentricity, in a manner different from spatial measures such as acuity which are believed to depend on the "magnification factor" of projection to area 17. The minimum displacement giving detectable motion (dmin) shows a shallower increase with eccentricity which is more compatible with the variation of cortical magnification. The dependence of apparent motion on the timing variables (exposure duration, inter-stimulus interval) changes negligibly with eccentricity. Consequently the dynamic range and the upper limit of detectable velocities increases greatly with eccentricity. The increase of dmax with eccentricity means that the perception of apparent motion will show an approximate invariance with display scale, even though dmax has a locally fixed value depending on receptive field structure.

OBJECTIVES: To compare the age of onset of the pattern orientation reversal visual evoked potential (OR-VEP) in a group of very low birthweight (VLBW) premature infants with term infants matched for postconceptual age at testing. The OR-VEP measure is used as an indicator of visual cortical functioning because of the specificity of cortical neurones in showing sensitivity to changes of slant or orientation. DESIGN: Results are given for 24 VLBW infants, born at 24-32 weeks gestation weighing less than 1500 g, and 31 infants born at term. The steady state evoked potential to a grating pattern reversing in orientation (between 45 degrees and 135 degrees) at 4 reversals/second and 8 reversals/second was recorded. RESULTS: The VLBW infants with normal neonatal ultrasound results (and normal neurological outcome at 3 years of age) showed a significant OR-VEP with a similar postnatal time course to the term infants. Four premature infants, showing appreciable abnormalities on ultrasound examination, did not show normal onset of the OR-VEP, and all had an abnormal neurological outcome. CONCLUSIONS: This result can be taken to indicate that the onset of cortical function is similar in healthy preterm infants to term infants. The visual development of the premature infants was neither accelerated nor delayed as a result of their extra visual experience. The OR-VEP can be used as a prognostic indicator of early brain development alongside other neurological measures. It may also be a very early indicator of later neurological outcome.

Visual development is a key area for understanding and assessing early brain development. Different levels in the hierarchy of visual processing, from the initial response to flashes of light, through selective responses to contour orientation and motion in primary visual cortex (V1), to global processing in extrastriate of large-scale patterns of form and motion, can each be assessed using stimuli designed to isolate specific neural activity in visual event-related potentials (VERPs). This approach has been used to reveal the sequence of emergence of different visual cortical functions in the first 6 months of typical human development, and to provide early indicators of anomalies in brain development. Delayed or absent onset of orientation-reversal (OR-)VERPs, as a measure of cortical development, has been shown to be a sensitive indicator of perinatal brain damage in both term-born and prematurely born infants. Direction-reversal (DR-)VERPs appear a few weeks later than OR-VERPs in typical development, and are further delayed in even healthy children born preterm, reflecting possible early vulnerability of the motion (dorsal stream) system. High-density recordings of responses to global motion and global form patterns show that these extrastriate systems are typically functional by 5 months of age, but the topography of the activity distributions shows that the brain systems underlying these responses are radically reorganized between infancy and adulthood. In prematurely born infants whose structural brain MRI was evaluated at birth, the onset of the response is absent or delayed in those with severe brain injury, while in those with mild/moderate brain injury the response is present but its spatial organization is further from the adult pattern than those in controls. These findings are related to the development of distinct networks of brain areas in the dorsal and ventral cortical streams, and the apparent vulnerability of the dorsal-stream network in a wide range of both genetic and acquired developmental disorders.

Mid-level visual processes which integrate local orientation information for the detection of global structure can be investigated using global form stimuli of varying complexity. Several lines of evidence suggest that the identification of concentric and parallel organisations relies on different underlying neural substrates. The current study measured brain activation by concentric, horizontal parallel, and vertical parallel arrays of short line segments, compared to arrays of randomly oriented segments. Six subjects were scanned in a blocked design functional magnetic resonance imaging experiment. We compared percentage BOLD signal change during the concentric, horizontal and vertical blocks within early retinotopic areas, the fusiform face area and the lateral occipital complex. Unexpectedly, we found that vertical and horizontal parallel forms differentially activated visual cortical areas beyond V1, but in general, activations to concentric and parallel forms did not differ. Vertical patterns produced the highest percentage signal change overall and only area V3A showed a significant difference between concentric and parallel (horizontal) stimuli, with the former better activating this area. These data suggest that the difference in brain activation to vertical and horizontal forms arises at intermediate or global levels of visual representation since the differential activity was found in mid-level retinotopic areas V2 and V3 but not in V1. This may explain why earlier studies--using methods that emphasised responses to local orientation--did not discover this vertical-horizontal anisotropy.

Random dot kinematograms present a patch of random dots, uniformly displaced from one frame to the next. We have re-examined the question of whether the limiting displacement for this short-range apparent motion is determined by the retinal angle or by the number of pixels across which the patch is displaced; our data support the former hypothesis. The number of dots in the patch is not a confounding variable, and in fact has very little effect. Increasing the area of the patch does increase the displacement limit, apparently due to the invasion of greater retinal eccentricities.

Recent studies of motion perception show that, in the brain's internal representation of three-dimensional space, distances may be less important than relationships to surfaces.

Kenneth J.W. Craik (1914–1945), while discussing how the human visual system recognized objects, wrote: Now in mathematics it is legitimate to seek transformations through which certain quantities (such as the physical laws of nature and the velocity of light in relativity theory) remain invariant. In fact, the action of various physical devices which “recognize” or respond identically to certain simple objects can be treated in terms of such transformations. Thus the essential part of physical “recognizing” instruments is usually a filter—whether it be a mechanical sieve, an optical filter, or a tuned electrical circuit —which “passes” only quantities of the kind it is required to identify and rejects all others. Mathematically, the situation here is that, in a perfect filter, the transformation leaves the desired quantity unaltered, but reduces all others to zero.

We report clinical and magnetic resonance imaging findings in two young children, aged 2 years 4 months and 3 years, with Williams syndrome. Both showed a mild global delay, although their neurological examination was completely normal. Their magnetic resonance imaging, however, showed Chiari I malformation and some non-specific changes in the centrum semiovale and in the white matter posterior to the lateral ventricles. Cerebellar tonsils were displaced through the foramen magnum 8.5 and 7.5 mm respectively. Our results suggest that Chiari I malformation can also be a frequent feature in subjects with Williams syndrome even in the absence of overt neurological signs suggestive of it. Whether these children might develop acute signs later is not known at present. Further studies are needed not only to evaluate the incidence of these findings in the global population of subjects with Williams syndrome but also to identify the children who are at risk for developing acute neurological signs.

We have examined the occurrence of segregation in random dot kinematograms in which a central patch of dots, and the surrounding area, were each coherently displaced, either in the same or opposite directions (Fig. 1), by varying amounts. The limiting displacement for segregation to occur is determined primarily by the displacement of each region alone, rather than the relative displacement of neighbouring regions (Fig. 2). We conclude that the “correspondence problem” is solved by means of a short range motion detection process acting on each region separately; segregation is achieved by comparing the results of this process for adjacent regions.

The visual system has the task of computing global motions associated with objects and surfaces. This task strongly involves extrastriate brain areas, particularly V5/MT. Motion transparency provides a particular challenge for understanding how global motions are computed and represented in the brain. Psycho-physical experiments show that, for a single region, multiple motions can be quantitatively represented. However, at the most local scale, motion signals have a suppressive interaction so that only a single motion can be represented. Neurophysiological experiments show that this suppression is a property of MT, not of V1, reflecting a subunit structure within MT receptive fields and showing that transparency perception is related to MT rather than V1 activity. A full understanding of transparency perception and other global motion phenomena will require us to understand how perceived motions are related to the distribution of activity across a population of directionally selective neurones, and how the brain implements the representation of motions assigned to extended objects rather than to specific retinotopic locations.

The development of attention is critical for the young child's competence in dealing with the demands of everyday life. Here we review evidence from infants and preschool children regarding the development of three neural subsystems of attention: selective attention, sustained attention, and attentional (executive) control. These systems overlap with dorsal cortical visual streams and their disorders are related to the general hypothesis of 'dorsal stream vulnerability'. Infants' ability to control spatial selective attention can be measured using the 'Fixation Shift' task. From around 4 months of age, infants start to show cortical control in disengaging to switch between competing targets. Fixation shifts have proved to be an effective early indicator of attentional disorders associated with perinatal brain damage. Executive function emerges slowly, starting around 1 year of age. The new Early Childhood Attention Battery has identified the three attention subsystems as distinct before 5 years of age in typical development and allows assessment of individual attention profiles across these subsystems. The Early Childhood Attention Battery is now being used to identify specific profiles associated with developmental syndromes such as Williams, Down, and fragile X. These new methods offer the possibility of very early identification of attention disorders, raising the challenge of effective remediation and treatment at an early age.

We examined the development of visual cue integration in a desktop working-memory task using boxes with different visual action cues (opening actions) and perceptual surface cues (colours, monochromatic textures, or images of faces). Children had to recall which box held a hidden toy, based on (a) the action cue, (b) the surface cue, or (c) a conjunction of the two. Results from three experiments show a set of asymmetries in children's integration of action and surface cues. The 18-24-month-olds disregarded colour in conjunction judgements with action; 30-36-month-olds used colour but disregarded texture. Images of faces were not disregarded at either age. We suggest that 18-24-month-olds' disregard of colour, seen previously in reorientation tasks (Hermer and Spelke, 1994), may represent a general phenomenon, likened to uneven integration between the dorsal and ventral streams in early development.

Reliable motion perception requires processes that integrate visual motion signals from neighbouring locations in the visual field, which should have the effect of smoothing out spatial variations in velocity. However, we also require motion processing to be very sensitive to local velocity differences, so that moving objects appear sharply distinct from their background and specific differential properties of optic flow associated with the observer's motion can be detected. Perceptual experiments give evidence both for integrative processes, which lead to spreading of perceived motion, and for differential processes, which lead to motion contrast and segmentation. Current and future experiments might allow tests of theoretical schemes that employ adaptive networks and/or multiple representations in order to reconcile the conflicting demands of integration and segmentation.

Visual evoked potentials (VEPs) were recorded from a total of 97 1- to 10-day-old infants, with phase-reversing sinusoidal grating stimuli. The grating contrast or spatial frequency for which 50% of infants gave a statistically significant VEP was taken as a measure of threshold. This procedure yielded an estimate of neonatal acuity of 0.85 cycles/degree and an optimal contrast threshold of 50%. VEPs from an older infant showed good agreement with behavioral measures of sensitivity on the same individual. Comparison of the neonatal VEP results with behavioral data from 5-week-old infants, suggests little change in visual performance over the first month of life.

Analysis of the optics of photorefractively computed ray tracing shows that, for short camera-to-subject distances, the function relating image size to defocus of the eye is not symmetrical for errors of focus in front of and behind the camera. This asymmetry is exploited in the new method of isotropic photorefraction, in which the supplementary cylinder lenses of the original orthogonal photorefractors are replaced by defocusing of the camera lens itself. By comparing photographs taken with the camera focused in front of and behind the subject, the sign of the eyes' defocus (myopic or hyperopic relative to the camera) can be determined. The axis of any astigmatism is readily apparent as the direction in which the photorefractive images are elongated. The method is well adapted for the refractive screening of infants and young children.

Reorientation tasks, in which disoriented participants attempt to relocate objects using different visual cues, have previously been understood to depend on representing aspects of the global organisation of the space, for example its major axis for judgements based on geometry. Careful analysis of the visual information available for these tasks shows that successful performance could be based on the much simpler process of storing a visual 'snapshot' at the target location, and subsequently moving in order to match it. We tested 4-8-year olds on a new spatial reorientation task that could not be solved based on information directly contained in any retinal projection that they had been exposed to, but required participants to infer how the space is structured. Only 6-8-year olds showed flexible recall from novel viewpoints. Five-year olds were able to recall locations given movement information or a unique proximal landmark, but without these they could not do so, even when they were not disoriented or when the landmark was a familiar object. These results indicate that early developing spatial abilities based on view matching and self motion are supplemented by a later-developing process that takes into account the structure of spatial layouts and so enables flexible recall from arbitrary viewpoints.

PURPOSE: To report on two population screening programs designed to detect significant refractive errors in 8308 8- to 9-month-old infants, examine the sequelae of infant hyperopia, and test whether early partial spectacle correction improved visual outcome (strabismus and acuity). The second program also examined whether infant hyperopia was associated with developmental differences across various domains such as language, cognition, attention, and visuomotor competences up to age 7 years. Linked programs in six European countries assessed costs of infant refractive screening. METHOD: In the first program, screening included an orthoptic examination and isotropic photorefraction, with cycloplegia. In the second program we carried out the same screening procedure without cycloplegia. Hyperopic infants (> or = +4 D) were followed up alongside an emmetropic control group, with visual and developmental measures up to age 7 years, and entered a controlled trial of partial spectacle correction. RESULTS: The second program showed that accommodative lag during photorefraction with a target at 75 cm (focus > or = +1.5 D) was a marker for significant hyperopia. In each program, prevalence of significant hyperopia at 9 to 11 months was around 5%; manifest strabismus was 0.3% at 9 months and 1.5 to 2.0% by school age. Infant hyperopia was associated with increased strabismus and poor acuity at 4 years. Spectacle wear by infant hyperopes produced better visual outcome than in uncorrected infants, although an improvement in strabismus was found in the first program only. The corrections did not affect emmetropization to 3.5 years; however, both corrected and uncorrected groups remained more hyperopic than controls in the preschool years. The hyperopic group showed poorer overall performance than controls between 1 and 7 years on visuoperceptual, cognitive, motor, and attention tests, but showed no consistent differences in early language or phonological awareness. Relative cost estimates suggest that refractive screening programs can detect visual problems in infancy at lower overall cost than surveillance in primary care. CONCLUSIONS: Photo/videorefraction can successfully screen infants for refractive errors, with visual outcomes improved through early refractive correction. Infant hyperopia is associated with mild delays across many aspects of visuocognitive and visuomotor development. These studies raise the possibility that infant refractive screening can identify not only visual problems, but also potential developmental and learning difficulties.

Photorefraction of a sample of 93 infants of ages 1 day to 12 months showed that 63 percent of the subjects had astigmatism of 0.75 diopter or greater, and 12 percent greater than 2 diopters. Seventy percent of these astigmatisms were in the horizontal-vertical meridians. By comparison, only 8 percent of a sample of 26 adults tested by the same method showed astigmatism (all 0.75 to 1 diopter). The high incidence of infant astigmatism has implications for critical periods in human visual development and for infant acuity.

The infant's visual system contains orientation-sensitive mechanisms from the first weeks of life. Differences in texture orientation can serve as a basis for rapid preattentive localization and segmentation in adults. We tested whether infants could use their orientation-sensitive mechanisms in the same way, by forced-choice preferential looking, using displays of line segments oriented at 45 degrees in a rectangular patch and 135 degrees in the surrounding region. Performance was compared with that for displays of similar elements with uniform orientation but with the patch defined by luminance contrast. Infants of 14-18 weeks old showed consistent preference for both orientation- and contrast-defined patches, indicating the ability to segment the field by orientation. Infants of 8-12 weeks performed comparably to the older infants on contrast-based segmentation but did not show a statistically significant preference with orientation-based segmentation. In a second experiment, preference was also tested for a region of mixed orientation versus a region of uniform orientation. The 14-18-week-olds did not show this preference, suggesting that their preference for the discrepant texture patch genuinely reflected texture segmentation and not simply the presence of two different orientations on one side of the display. The results are discussed in terms of the possible maturation of intracortical connections subserving texture grouping and segmentation.

The full-text of this book chapter is not available in ORA. Citation: Atkinson, J. & Braddick, O. J. (1981) Development of optokinetic nystagmus in infants: an indicator of cortical binocularity. In: Fisher, D. F., Monty, R. A. & Senders, J. W. (eds.) Eye movements: cognition and visual perception, Hillsdale, N.J: Lawrence Erlbaum, pp. 35-64.

We report that motion coherence thresholds in children with autism are significantly higher than in matched controls. No corresponding difference in form coherence thresholds was found. We interpret this as a specific deficit in dorsal stream function in autism. To examine the possibility of a neural basis for the perceptual and motor related abnormalities frequently cited in autism we tested 23 children diagnosed with autistic disorder, on two tasks specific to dorsal and ventral cortical stream functions. The results provide evidence that autistic individuals have a specific impairment in dorsal stream functioning. We conclude that autism may have common features with other developmental disorders and with early stages of normal development, perhaps reflecting a greater vulnerability of the dorsal system.

The peak latency of pattern-reversal (PR)-VEP has been found to develop rapidly, reaching the adult level around 15 weeks of age. However, the development of orientation-reversal (OR)-VEP, reflecting the specific spatial organization of cortical receptive fields, still remains unknown. OR-VEP was tested in 81 adults at 1-12 reversals/sec (r/s) and 94 infants (age 4-79 weeks) at 2-8r/s. OR data at 4r/s from an additional 123 infants (age 4.0-20.3 weeks) studied previously were also analyzed. In addition to peak transient latencies at 1-4r/s, latency values derived from the gradient of phase against temporal frequency in steady-state recording were also calculated. For both adults and infants, no significant latency differences in the initial positive peaks were found among the low reversal rates. The calculated latency was statistically longer than the transient latency in both groups. While the transient latency asymptoted to adult value of 102 ms at around 50 weeks of age, the calculated latency, unlike that for PR-VEP, showed little variation across the age span. The data suggest a dominant effect of transmission delay on the initial peak in infancy, which reduces with age. However, the overall timing of the cortical response to orientation change remains slower than for pattern reversal in the fully developed visual cortex. Upon reaching maturity, the latencies of the initial positive peak in both pattern and orientation VEPs may arise from the same level of cortical processing in V1, but the overall time course reflected in the steady-state phase continues to show a much more prolonged response to orientation change than the transmission delay seen in the transient VEPs.

Coherence thresholds for concentric patterns of contour segments or dot trajectories have revealed differential development and vulnerability of extrastriate form and motion pathways respectively (Braddick et al, Neuropsychologia, 2003). Here we explore the properties of visual evoked potentials (VEPs) specific to form and motion coherence. Coherent contour or motion arrays alternate with incoherent arrays at 0.5-4 Hz. A 2nd harmonic VEP signal (F2) may arise from local changes occuring at each transition, but a first harmonic (F1) must reflect differential responses to coherence and incoherence, indicating global processing. F1 amplitude is approximately linear with coherence for both form and motion, confirming that F1 measures global processing. F2 is independent of coherence for form, and nearly so for motion, implying that it reflects local changes. Occipital F1 amplitudes for motion: form coherence are approximately 2:1. However, this does not reflect underlying sensitivity: psychophysical form and motion coherence thresholds are similar, and extrapolation of the linear region of F1 amplitude vs coherence reaches zero close to threshold coherence in each case. F1 amplitude for both displays increases gradually from 0.5-4 Hz. However, this is opposite to psychophysical coherence sensitivity, which falls over the same range. We conclude: (a) coherence-dependent VEPs provide measures of global form and motion processing, practical for infants and non-verbal (e.g. autistic) children; (b) this response is elicited efficiently at relatively high presentation rates; (c) the level tapped by the VEP does not provide the limiting factor on temporal integration in global perceptual processing.

Form and motion coherence was tested in children with dyspraxia and matched controls to assess their global spatial and global motion processing abilities. Thresholds for detecting form coherence patterns were significantly higher in the dyspraxic group than in the control group. No corresponding difference was found on the motion coherence task. We tested eight children with dyspraxic disorder (mean age 8.2 years) and 50 verbal-mental-age matched controls (mean age 8.4 years) to test for a neural basis to the perceptual abnormalities observed in dyspraxia. The results provide evidence that children with dyspraxia have a specific impairment in the global processing of spatial information. This finding contrasts with other developmental disorders such as Williams syndrome, autism and dyslexia where deficits have been found in global motion processing and not global form processing. We conclude that children with dyspraxia may have a specific occipitotemporal deficit and we argue that testing form and motion coherence thresholds might be a useful diagnostic tool for the often coexistent disorders of dyspraxia and dyslexia.

Inexperienced observers show a delay before experiencing the stereoscopic percept from a random-dot stereo pair. This perception time is progressively reduced with repeated exposures of the stereogram. We have investigated the specificity of this perceptual learning effect, using stereograms made up of short oblique line elements. Learning with a stereogram consisting of 45° line elements transferred completely to an uncorrelated pattern with the same element orientation, but there was a marked failure of transfer to a pattern whose elements had the opposed oblique orientation. Thus the stereoscopic skill that has been acquired may be specific to those orientation analysers that were stimulated during the training period.

The visual processing of complex motion is impaired in Alzheimer's disease (AD). However, it is unclear whether these impairments are biased toward the motion stream or part of a general disruption of global visual processing, given some reports of impaired static form processing in AD. Here, for the first time, we directly compared the relative preservation of motion and form systems in AD, mild cognitive impairment, and healthy aging, by measuring coherence thresholds for well-established global rotational motion and static form stimuli known to be of equivalent complexity. Our data confirm a marked motion-processing deficit specific to some AD patients, and greater than any form-processing deficit for this group. In parallel, we identified a more gradual decline in static form recognition, with thresholds raised in mild cognitive impairment patients and slightly further in the AD group compared with controls. We conclude that complex motion processing is more vulnerable to decline in dementia than complex form processing, perhaps owing to greater reliance on long-range neural connections heavily targeted by AD pathology.

While basic visual functions have been described in subjects with congenital achromatopsia (ACHM), little is known about their mid- or high-level cortical visual processing. We compared midlevel cortical visual processing in ACHM subjects (n = 11) and controls (n = 20).Abilities to detect global form, global motion, and biological motion embedded in noise were tested across a range of light levels, including scotopic, in which both ACHM subjects and controls must rely on rods. Contrast sensitivity functions (CSFs) were also measured.Achromatopsia subjects showed differential impairments across tests. In scotopic conditions, global form was most impaired, while biological motion was normal. In a subset of three ACHM subjects with normal scotopic CSFs, two of the three showed global form perception worse than controls; all showed global motion comparable to controls; and strikingly, two of the three showed biological motion perception superior to controls.The cone signal appears to play a crucial role in the development of perception of global form, as in ACHM this is impaired even in scotopic conditions, in which controls also have to rely on rods, and even in ACHM subjects with no scotopic spatial vision loss. In contrast, the rod signal appears sufficient for the development of normal (or even superior) extrastriate biological motion perception. These results suggest that ACHM leads to atypical development of cortical vision, highlighting the need to better understand the potential for further reorganization of cortical visual processing following new therapies aimed at restoring cone function.

Individuals with Williams syndrome (WS) have impairments in visuospatial tasks and in manual visuomotor control, consistent with parietal and cerebellar abnormalities. Here we examined whether individuals with WS also have difficulties in visually controlling whole-body movements. We investigated visual control of stepping down at a change of level in children with WS (5–16-year-olds), who descended a single step while their movement was kinematically recorded. On each trial step height was set unpredictably, so that visual information was necessary to perceive the step depth and position the legs appropriately before landing. Kinematic measures established that children with WS did not use visual information to slow the leg at an appropriate point during the step. This pattern contrasts with that observed in typically developing 3- and 4-year-old children, implying severe impairment in whole-body visuomotor control in WS. For children with WS, performance was not significantly predicted by low-level visual or balance problems, but improved significantly with verbal age. The results suggest some plasticity and development in WS whole-body control. These data clearly show that visuospatial and visuomotor deficits in WS extend to the locomotor domain. Taken together with evidence for parietal and cerebellar abnormalities in WS, these results also provide new evidence for the role of these circuits in the visual control of whole-body movement.