Janette Atkinson

Motion processing in autism: evidence for a dorsal stream deficiency

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.

Brain areas sensitive to coherent visual motion

Detection of coherent motion versus noise is widely used as a measure of global visual-motion processing. To localise the human brain mechanisms involved in this performance, functional magnetic resonance imaging (fMRI) was used to compare brain activation during viewing of coherently moving random dots with that during viewing spatially and temporally comparable dynamic noise. Rates of reversal of coherent motion and coherent-motion velocities (5 versus 20 deg s−1) were also compared. Differences in local activation between conditions were analysed by statistical parametric mapping. Greater activation by coherent motion compared to noise was found in V5 and putative V3A, but not in V1. In addition there were foci of activation on the occipital ventral surface, the intraparietal sulcus, and superior temporal sulcus. Thus, coherent-motion information has distinctive effects in a number of extrastriate visual brain areas. The rate of motion reversal showed only weak effects in motion-sensitive areas. V1 was better activated by noise than by coherent motion, possibly reflecting activation of neurons with a wider range of motion selectivities. This activation was at a more anterior location in the comparison of noise with the faster velocity, suggesting that 20 deg s−1 is beyond the velocity range of the V1 representation of central visual field. These results support the use of motion-coherence tests for extrastriate as opposed to V1 function. However, sensitivity to motion coherence is not confined to V5, and may extend beyond the classically defined dorsal stream.

Channels in Vision: Basic Aspects

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.

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) μmol·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.

Development of contrast sensitivity over the first 3 months of life in the human infant.

Abstract Contrast sensitivity functions have been obtained for infants aged 5–12 weeks. Sensitivity was derived from fixation preference for static or drifting sinusoidal gratings over a uniform field, assessed by two-alternative forced choice by a “blind” observer. In terms of acuity, extent of the low-frequency cut, and overall sensitivity, there were large differences between the 5-week group and older age groups. This difference in performance may reflect rapid neural development in the visual system over the first 3 months of life.

Two infant vision screening programmes: prediction and prevention of strabismus and amblyopia from photo- and videorefractive screening.

Two infant vision screening programmes on total populations in the Cambridge Health District have been designed to identify manifest strabismus and strabismogenic and amblyogenic refractive errors at 7–9 months of age. The first, completed, programme used the isotropic photorefractor with cycloplegia together with a standard orthoptic examination. The second, current, programme uses the VRP-1 isotropic videorefractor to identify infants with accommodative lags which are followed up by refraction under cycloplegia. Both programmes show good agreement between infants identified at screening and retinoscopic refractions at follow-up, showing that photo- and videorefraction (with or without cycloplegia) can be effective methods for screening for ametropia in infants and young children. In each programme 5–6% of infants showed abnormal levels of hyperopia (≥3.5 D in any meridian), less than 1% showed anisometropia ≥1.5 D; very few infants (0.25%) showed −3 D myopia or greater. Less than 1% showed manifest strabismus. Hyperopic and anisometropic children entered a randomised controlled trial of partial refractive correction. All children identified at screening, alongside appropriate control groups, are extensively followed up to age 4 years. The first programme has found that children who were hyperopic in infancy were 13 times more likely to become strabismic, and 6 times more likely to show measurable acuity deficits by 4 years, compared with controls. Wearing a partial spectacle correction reduced these risk ratios to 4:1 and 2.5:1 respectively. The impaired acuity can be attributed, in part, to meridional amblyopia resulting from persisting astigmatism. Both hyperopic and myopic infants showed refractive changes in the direction of emmetropia between 9 months and 4 years. Wearing a partial spectacle correction did not affect this process of emmetropisation, but does provide the possibility of reducing the incidence of common pre-school vision problems.

Dorsal and ventral stream sensitivity in normal development and hemiplegia

Form and motion coherence thresholds can provide comparable measures of global visual processing in the ventral and dorsal streams respectively. Normal development of thresholds was tested in 360 normally developing children aged 4–11 and in normal adults. The two tasks showed similar developmental trends, with some greater variability and a slight delay in motion coherence compared to form coherence performance, in reaching adult levels. To examine the proposal of dorsal stream vulnerability related to specific developmental disorders, we compared 24 children with hemiplegic cerebral palsy with the normally developing group. Hemiplegic children performed significantly worse than controls on the motion coherence task for their age, but not on the form coherence task; however, within this group no specific brain area was significantly associated with poor motion compared to form coherence performance. These results suggest that extrastriate mechanisms mediating these thresholds normally develop in parallel, but that the dorsal stream has a greater, general vulnerability to early neurological impairment.

The Magic Number 4 ± 0: A New Look at Visual Numerosity Judgements

Visual numerosity judgements were made for tachistoscopically presented linear arrays of dots or lines. The interelement interval (which could be specified in spatial frequency terms) was constant for a given array but varied across conditions. A clear limit in the accuracy of numerosity judgements was found to be set at 4 for regularly spaced elements with spatial frequencies below approximately 10 cycles/deg (element and interelement interval of 0·05 deg). This limit in terms of accuracy is accompanied by a fast and almost constant response time for arrays of 4 or less, compared to response times for arrays of more than 4 elements. The limit in accuracy falls to 2 elements rather than 4 for spacing narrower than 0·05 deg although with such spacing the elements are still easily resolved. The limit of 4 is found if the stimulus is a bright afterimage, lasting for approximately 60 s. This result suggests that the limit is independent of the time allowed for a single fixation and is a perceptual limit rather than a limit in some memory buffer. ‘Numerosity’ units are proposed to account for the results.

Disengaging visual attention in the infant and adult

The latency to make an eye movement towards a phase-reversing peripheral target was measured in 1.5-, 3-, and 6-month-old infants and adults using four interleaved conditions. In the short-gap condition, the central fixating stimulus (schematic face) disappeared 240 ms prior to the onset of the peripheral target. In the long-gap condition, the interval between the offset of the central stimulus and the target was 720 ms. In the no-overlap condition, the appearance of the peripheral target coincided with the offset of the central stimulus, and in the overlap condition, the central stimulus remained present for 2,120 ms following the onset of the peripheral target. In general, latencies decreased with age, though there was very little difference between the 3-and 6-month-olds. Gap conditions produced the fastest saccadic latencies compared to the no-overlap and overlap conditions in all groups. In line with previous studies, the overlap condition produced the slowest responses, even for adults who had been instructed to make an eye movement as quickly as possible. However, a manual response measure in the adult group using the same paradigm did not produce longer latencies for the overlap condition, suggesting that the effect may be specific to mechanisms controlling saccades and not motor responses per se. The short latencies for gap conditions and the longer saccadic latencies in the overlap condition are taken as evidence to support a cortical disengagement hypothesis of orienting eye movements.

Visual and visuocognitive development in children born very prematurely.

Preterm birth is a risk factor for deficits of neurological and cognitive development. Four cohort studies are reported investigating the effects of very premature birth (<32 weeks gestation) on visual, visuocognitive and visuomotor function between birth and 6-7 years of age. The first study used two measures of early visual cortical function, orientation reversal visual event-related potentials (OR-VERP) and fixation shifts under competition. Both these functional measures of visual development correlated with the severity of brain abnormality observed on structural MRI at and before term, and were sensitive predictors of neurodevelopmental outcome at 2 years. The second study compared VERP measures for orientation-reversal and direction-reversal (DR) stimuli, from 2 to 5 months post-term age, in healthy very premature infants compared to infants born at term. The groups did not differ on the development of OR-VERP responses, but the development of the DR-VERP motion responses was delayed in the premature group despite the absence of any brain damage visible on ultrasound, consistent with the developmental vulnerability we have identified in the dorsal cortical stream. The third study used the Atkinson Battery of Child Development for Examining Functional Vision (ABCDEFV) to assess sensory, perceptual, cognitive and spatial visual functions, together with preschool tests of attention and executive function. The premature group showed delays on these tests in line with severity of observed perinatal brain damage on structural MRI at term age. Deficits on certain spatial tasks (e.g. block-construction copying) and executive function tests (e.g. the detour box task) were apparent even in children with minimal damage apparent on MRI. The fourth study tested a large cohort of 6- to 7-year old children born before 32 weeks gestation, across a wide range of cognitive domains, including new tests of spatial cognition and memory. The premature group as a whole showed significant deficits on both auditory and visual tests of attention and attentional control from the TEA-Ch battery, on tests of location memory, block construction and on many visuocognitive and visuomotor tests. Development was generally relatively normal on language tests and on WPPSI scores. Factor analysis showed that while general cognitive ability accounted for the largest part of the variance, significant deficits, and a relationship to MRI results, were primarily in spatial, motor, attention and executive function tests. A model is proposed suggesting that the cluster of deficits seen in children born prematurely may be related to networks involving the cortical dorsal stream and its connections to parietal, frontal and hippocampal areas.