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Regional hemodynamic responses to visual stimulation in awake infants.
This study presents the first measurements using near infrared spectroscopy of changes in regional hemodynamics as a response to a visual stimulus in awake infants. Ten infants aged 3 d to 14 wk viewed a checkerboard with a 5-Hz pattern reversal. The emitter and detector (optodes) of a near infrared spectrophotometer were placed over the occipital region of the head. Changes in concentration of oxy- and deoxyhemoglobin (Hbo2 and Hb) were measured and compared during 10-s epochs of stimulus on and off. A control group of 10 infants aged 18 d to 13 wk were examined with the same setup, but with the optodes over the frontoparietal region. In the test group the total hemoglobin concentration (Hbo2 + Hb) increased while the stimulus was on by a mean (+/-SD) of 2.51 (+/-1.48) micromol x L(-1). Nine out of 10 infants showed an Hbo2 increase, and 9 out of 10 an Hb increase related to the stimulus. There was no significant change in any of these parameters in the control group. The results imply that there is increased cerebral blood flow due to stimulation that is specific to the visual cortex and that infants, unlike adults, show increased cerebral oxygen utilization during activation that outstrips this hemodynamic effect. The study demonstrates that near infrared spectroscopy can be used as a practical and noninvasive method of measuring visual functional activation and its hemodynamic correlates in the awake infant.
Visual function in full-term infants with hypoxic-ischaemic encephalopathy.
Thirty-one full term infants with hypoxic-ischaemic encephalopathy (HIE) were studied with a battery of tests designed to evaluate visual function in infancy and with serial MRI. Their age ranged between 5 and 31 months. The aim of the study was to evaluate whether the degree of HIE or the site and size of lesions on MRI could predict visual outcome. Twenty of the 31 infants studied showed abnormal results on at least one of the visual tests used. While visual function was generally normal in grade I HIE and severely impaired in grade III, visual outcome was variable in grade II. MRI findings were better predictors of visual impairment than the degree of HIE. Normal scans tended to be associated with normal visual function, irrespective of the severity of HIE. All but one of the infants with diffuse hemispheric involvement also showed multiple visual abnormalities. The concomitant involvement of basal ganglia was always associated with more severe visual outcome. These results suggest that infants with generalised lesions secondary to global insults are at high risk of severe visual impairment even in presence of normal acuity and require early assessment of various aspects of visual function.
Differential human brain activation by vertical and horizontal global visual textures.
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.
Visual function at school age in children with neonatal encephalopathy and low Apgar scores.
OBJECTIVE: To assess different aspects of visual function at school age in children who suffered from neonatal encephalopathy. METHOD: Thirty nine full term infants with neonatal encephalopathy, low Apgar scores, and early neonatal imaging were studied using a battery of tests assessing different aspects of visual function (crowding acuity, stereopsis, visual fields) at school age. The results were compared with brain magnetic resonance imaging (MRI) findings and, when possible, with the results of the assessment of visual function performed at 5 and 12 months, available in 24 of the 39 children examined at school age. RESULTS: Sixteen of the 39 children (41%) had abnormal results at school age in at least one of the visual tests used. Seven of these 16 were untestable on all tests. The remaining 23 children (59%) had normal results. CONCLUSIONS: The presence and severity of visual impairment was related to the severity of brain lesions. Moderate or severe basal ganglia lesions and severe white matter changes were always associated with abnormal visual function. Infants with normal MRI, minimal basal ganglia lesions, and minimal or moderate white matter involvement tended to have normal vision. It was also found that the assessment of visual function performed in the first year was a reliable indicator of visual function at school age. With two exceptions, the results on the 5 month visual assessment were predictive of visual outcome at school age. In the remaining two cases, a normal visual outcome at 5 years was associated with visual abnormalities at 5 months but these had already normalised by the age of 1 year.
Motion coherence thresholds in infants--different tasks identify at least two distinct motion systems.
Optokinetic nystagmus (OKN) can be demonstrated from birth, but behavioural discrimination tasks such as habituation and preferential looking do not reveal any sensitivity to motion direction until a few weeks of age. This study compared coherence threshold for motion direction for OKN and preferential looking responses using closely comparable stimuli, in infants between 6 and 27 weeks of age. Infants were tested with two random dot motion displays, a uniform area of moving dots for OKN responses and a display in which a region was segmented on one side by differential motion direction for preferential looking responses. Coherence thresholds for each response were determined by a staircase method. For OKN responses, mean coherence thresholds were between 20% and 25%, with no significant improvement in OKN performance throughout the age range. Preferential looking thresholds were significantly higher than OKN thresholds. Preferential looking thresholds improved significantly with age, but remained higher than OKN thresholds throughout the age range tested. Experiments varying direction reversal frequency and stimulus area indicated that these differences were not simply a consequence of the spatial and temporal non-uniformity of the preferential looking stimulus. The differences in sensitivity levels and age trends for OKN and preferential looking responses we have found suggest that different directional mechanisms are involved in the two responses. We discuss the possibility that, in early infancy, OKN and preferential looking reflect the performance of subcortical and cortical directional mechanisms respectively.
Directional performance in motion transparency.
Motion transparency provides a challenging test case for our understanding of how visual motion, and other attributes, are computed and represented in the brain. However, previous studies of visual transparency have used subjective criteria which do not confirm the existence of independent representations of the superimposed motions. We have developed measures of performance in motion transparency that require observers to extract information about two motions jointly, and therefore test the information that is simultaneously represented for each motion. Observers judged whether two motions were at 90 degrees to one another; the base direction was randomized so that neither motion taken alone was informative. The precision of performance was determined by the standard deviations (S.D.s) of probit functions fitted to the data. Observers also made judgments of orthogonal directions between a single motion stream and a line, for one of two transparent motions against a line and for two spatially segregated motions. The data show that direction judgments with transparency can be made with comparable accuracy to segregated (non-transparent) conditions, supporting the idea that transparency involves the equivalent representation of two global motions in the same region. The precision of this joint direction judgment is, however, 2-3 times poorer than that for a single motion stream. The precision in directional judgment for a single stream is reduced only by a factor of about 1.5 by superimposing a second stream. The major effect in performance, therefore, appears to be associated with the need to compute and compare two global representations of motion, rather than with interference between the dot streams per se. Experiment 2 tested the transparency of motions separated by a range of angles from 5 degrees to 180 degrees by requiring subjects to set a line matching the perceived direction of each motion. The S.D.s of these settings demonstrated that directions of transparent motions were represented independently for separations over 20 degrees. Increasing dot speeds from 1 to 10 deg/s improved directional performance but had no effect on transparency perception. Transparency was also unaffected by variations of density between 0.1 and 19 dots/deg(2)
Visual and visuospatial development in young children with Williams syndrome.
This study investigated the relation between sensory visual problems and the severity of visuospatial difficulties in a large group of young children with Williams' syndrome (WS). A questionnaire describing visual and associated problems was completed by the families of 108 children with WS and detailed follow-up assessments were conducted, including visual, spatial, motor, visuocognitive, and linguistic tests of 73 of these children (mean age 7 years 3 months; 40 males, 73 females). Children with WS showed a much higher incidence of common paediatric sensory vision problems (strabismus, visual acuity loss, amblyopia, reduced stereopsis) than normally developing children. It was found that delays with respect to age normative values increased with age on all tests. No significant correlation was found between the presence of a visual deficit and the severity of the visuospatial problems, suggesting that the difficulties children with WS have in understanding spatial arrangements are not simply a result of their earlier sensory visual problems. Results confirm the dissociation between visuospatial and language abilities in children with WS, and support the neurobiological model of a split between ventral and dorsal stream processing of visual information with a generalized deficit in dorsal stream processing in young children with WS.
Reduction of infant myopia: a longitudinal cycloplegic study.
Changes of cycloplegic retinoscopy refraction from 8.5 to 38.5 months of age were compared in two infant groups in the Cambridge population: "infant myopes", having at least one myopic axis (0 to -3.5 D inclusive), and a second, "control" group with low hyperopia (< or = +3.5 D). Cycloplegia eliminated the variable accommodation of infants. The myopic group showed a significant emmetropization of the mean spherical equivalent towards low hyperopia by 3 yr. There was no significant change in the control group's mean spherical equivalent power. Both groups showed a significant reduction in astigmatism with age. Analysis of the vertical and horizontal powers showed significant "emmetropization" of these meridians, in both groups, towards low hyperopia from 8.5 to 38.5 months. These meridional emmetropization changes were significant for both With-the-Rule and Against-the-Rule astigmatism.
Development of visual control in stepping down.
Stepping down at a change of height is a fundamental part of human locomotion. At a novel step, this requires the transformation of visual information about a depth change into a stepping movement of appropriate size. However, little is known about this process or its development. We studied adults, 3- and 4-year-old children stepping down a single stair of variable height. We assessed how well stepping down was scaled to stair height using several kinematic measures. Of these, 'kneedrop' and 'toedrop' describe how far the leg has descended by the time it begins to 'swing in' in preparation for landing; and 'toeheight (speedpeak)' describes where the toe begins to decelerate. If visually controlled, their values should scale to the height of the stair. Under normal visual conditions, children scaled these movements to stair height as well as adults. In a second condition, participants closed their eyes just before stepping down to remove visual feedback during the step. Adults' steps were barely affected. For 4-year olds, only toeheight (speedpeak) decreased. For 3-year olds, both toedrop and toeheight (speedpeak) scaled less well to stair height than normal. The results suggest that visuomotor processes for fine-tuned stepping control develop remarkably early, but are initially dependent on visual feedback.
Refractive errors in infancy predict reduced performance on the movement assessment battery for children at 3 1/2 and 5 1/2 years.
We have previously reported that significant hyperopia at 9 months predicts mild deficits on visuocognitive and visuomotor measures between 2 years and 5 years 6 months. Here we compare the motor skills of children who had been hyperopic in infancy (hyperopic group) with those who had been emmetropic (control group), using the Movement Assessment Battery for Children (Movement ABC). Children were tested at 3 years 6 months (hyperopic group: 47 males, 63 females, mean age 3 y 7 mo, SD 1.6 mo; control group: 61 males, 70 females, mean age 3 y 7 mo, SD 1.2 mo) and at 5 years 6 months (hyperopic group: 43 males, 56 females, mean age 5 y 4 mo, SD 1.7 mo; control group: 51 males, 62 females, mean age 5 y 3 mo, SD 1.6 mo). The hyperopic group performed significantly worse at both ages, overall and on at least one test from each category of motor skill (manual dexterity, balance, and ball skills). Distributions of scores showed that these differences were not due to poor performance by a minority but to a widespread mild deficit in the hyperopic group. This study also provides the first normative data on the Movement ABC for children below 4 years of age, and shows that it provides a useful measure of motor development at this young age.
'Preferential looking' for monocular and binocular acuity testing of infants.
A method is described for obtaining rapid and reliable estimates of acuity in infants, for both monocular and binocular viewing. The method depends on 'preferential looking', where the infant prefers to look at a striped pattern rather than a blank screen of matched mean luminance. A staircase procedure for testing is followed, with observations being recorded by a 'blind' observer (who does not know on which of the 2 screens the striped pattern is displayed). Monocular acuity estimates have been obtained for a group of infants 3 to 4 months old with normal refractions. Many of these infants show similar acuity values in the 2 eyes, with a few showing reliable differences between the eyes. To check reliability of the method a comparison of 2 independent interleaved staircase estimates of the same eye have been made. In general this check shows highly consistent estimates for a given eye of a given infant. Nearly all infants show slightly higher acuity estimates for binocular viewing than for monocular. The possible reasons for this difference are discussed. The clinical use of such a method is reported for a number of cases. The method has been found to be useful in a variety of clinical conditions where other available tests are not possible on young infants.
Latency measures of pattern-reversal VEP in adults and infants: different information from transient P1 response and steady-state phase.
PURPOSE: Temporal properties such as the peak latency of pattern-reversal (PR) visual evoked potentials (VEPs) have been found to be a sensitive indicator of visual development. Latency can be assessed from the slope of a plot of phase against temporal frequency (TF) for steady state VEP measurements as well as from the transient P1 peak. This study aimed to discover whether the two methods provide different information regarding early visual development. METHODS: Developmental changes of the transient peak latency were tracked using low TFs of one to four reversals per second (r/s) and a spatial frequency (SF) of 0.24 cycles per degree (cpd) in comparison with latencies calculated from the phase versus TF gradient in the range of 1 to 19 r/s. PR-VEP responses were recorded from 81 adults and 137 infants (ages 3.6-79 weeks). RESULTS: Values of the calculated and transient peak latencies were similar in adults, but the calculated latency was statistically longer than transient peak latency in younger infants. Moreover, while the transient peak latency asymptoted to an adult value of 104 ms at approximately 15 weeks of age, the calculated latency did not asymptote until after 30 weeks. CONCLUSIONS: In this study, the effectiveness of the phase-based method to calculate latency was confirmed. In infants, the rapid decrease of P1 latency may be due to the progressive maturation of conduction time in the afferent visual pathways, with the development of adult levels of phase-based calculated latency being due to the maturation of later cortical processing in infants.