Biomedical subjects
Publications and source records attributed to O Braddick.
The brain can integrate local motion signals among noise to gain a global perception of visual motion. The scope and limits of this ability may reveal how the inputs to central motion systems are organized.
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.
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.
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.
Patients with damage to the striate cortex have a subjectively blind region of the visual field, but may still be able to detect and localize targets within this region. But the relative roles in this 'blindsight' of subcortical neural systems, and of pathways to extra-striate visual areas, have been uncertain. Here we report results on two infants in whom one cerebral hemisphere, including both striate and extra-striate visual cortex, needed surgical removal in their first year. Single conspicuous targets in the half-field contralateral to the lesion could elicit fixations, implying detection and orienting by a subcortical system. In contrast, binocular optokinetic nystagmus (OKN), for which a subcortical pathway has often been thought adequate, showed a marked asymmetry. In normal neonates, fixation shifts and OKN have both been taken to reflect subcortical control; our results are consistent with subcortical control for fixation but not for OKN.
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.
One-month-old infants were tested with a habituation-recovery paradigm to determine whether they could discriminate phase-shifting grating patterns that switched between two orientations, three or eight times a second, from grating patterns that only shifted in phase. The infants were found to discriminate patterns switching orientation at the lower temporal rate of 3 reversals s-1, but not 8 reversals s-1. This finding supports the idea that orientation-selective mechanisms improve in their temporal sensitivity during early infancy. Where they can be compared, the results from behavioural and electrophysiological studies agree as to the course of this development.
The abilities of 1-month-old and 3-month-old infants to shift their gaze from a central target to a peripheral target were compared in four experiments. In experiment 1 targets matched in mean luminance to the background were presented to infants in the periphery at varying levels of contrast. The contrast thresholds for target detection were found to be significantly different for 1-month-olds compared with 3-month-olds. With targets set close to these contrast thresholds, correct refixations and the latency for shifting attention were examined in experiment 2. Two conditions were used: a peripheral target was presented against a homogeneous background (noncompetition); and in the second condition, the patterned target appeared at one of two lighter peripheral windows set against a darker background (competition). Although there was no difference between the two age groups in the latency for shifting visual attention, 1-month-olds were found to make more directional errors in the competition condition. The competition effect of two potential targets on latencies was examined in experiment 3. In the competition condition, two identical peripheral patterned targets were presented to the infants. The 3-month-olds refixated more quickly to one of the double targets in the competition condition than to a single peripheral target, whereas 1-month-olds were slowed down by a double target display. Finally, in experiment 4 the ability of the infants to process and disengage from a central stimulus and to refixate towards a similar peripheral target was examined. This type of competition disrupted both the direction of the first eye movement and the latency to shift attention in both age groups. However, the effect was significantly greater for the 1-month-olds. Taken together, the results of these experiments demonstrate the greater disruption of fixation-shift behaviour in 1-month-olds compared with 3-month-olds when competing visual stimuli are used. This developmental change is explained in terms of maturation of executive cortical orienting systems over the first months of life.
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Isotropic photorefraction is a technique well suited for screening infants and young children for refractive errors. The photorefractive measurements have been empirically calibrated against retinoscopic refractions, so errors exceeding selected criteria can be identified in screening and followed up. Such a screening programme is in progress for the population of 6-9 month infants in the City of Cambridge. In 1096 infants screened 5% have been found to have large hypermetropic errors, 1.3% to show a refractive difference between the eyes (anisometropia) and less than 1% to have significant myopia or manifest strabismus. These findings were generally confirmed on retinoscopic examinations. In subsequent follow up of the large hypermetropic errors, most decline with age but a few show little or no change up to age 2 years and some show more change in one eye than the other leading to anisometropia. A trial is underway to examine whether early correction with spectacles can reduce the later incidence of strabismus and amblyopia in hypermetropic infants. Significant astigmatism is found in a large fraction of the infant population; the predominant axis of this astigmatism shows marked and unexplained variations between different locations in England.
The forced-choice preferential looking method (FPL) shows the development of acuity during the first year of life, and is applicable to clinical assessment. A tracking test using a narrow strip of grating yields a more sensitive measure for the later part of this age range, however. The development of acuity is dominated by neural rather than optical or accommodative factors. By age 3 years resolution acuity is very close to adult performance, but at 5 years 'crowding' effects may still impair performance on practical acuity tasks more than for the adult.
Binocular function in infancy can be assessed by means of the visual evoked potential (VEP) induced by a dynamic random dot pattern which alternates between a binocularly correlated and an anticorrelated state. A VEP time-locked to the alternation can only be produced by binocular interaction in the cortex. This interaction becomes detectable at around three months of age. Optokinetic nystagmus (OKN) in monocular viewing becomes symmetrical in nasal and temporal directions around the same age. Animal analogues and clinical cases suggest that this may also be an indicator of cortical binocularity. These techniques may be applicable to clinical assessment of binocular function in young infants.
Isotropic photorefraction is a technique which makes possible the rapid, economic, large scale screening of infants and young children for refractive errors. The relative size of blur circles reflected from the fundus in 3 flash photographs with different camera settings allows the size and direction of refractive errors to be estimated. The method has been validated against retinoscopic measures on a large group of infants. Findings of a high incidence of astigmatism in the first year are confirmed. Detection of hypermetropia in early infancy by this screening procedure may allow preventive treatment for accommodative strabismus.
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.
Visual evoked potentials (VEP) elicited by a dynamic random-dot correlogram were used to assess the development of cortical binocular function in infant subjects. In a group of newborn infants who showed a VEP for a comparable non-binocular stimulus, none showed evidence of binocular function. A further group of infants were tested longitudinally, starting between 35-50 days. The median age for the first evidence of binocular function in this group was 91 days, with individual variation from 54 to at least 105 days.