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.

The effect of contrast on vertical motion processing asymmetries in 11-week-old infants.

Recent forced-choice preferential looking (FPL) experiments with random-dot patterns [Wattam-Bell, 1998 Investigative Ophthalmology & Visual Science 39(4) S885] found evidence for a perceptual asymmetry of vertical-motion processing in young infants: a preference for patterns that move downwards. This asymmetry was in the opposite direction to the asymmetry of vertical optokinetic nystagmus, which was biased towards upwards motion. However, the FPL bias was weak, and the object of present experiments was to explore the possibility that it could be enhanced by reducing stimulus contrast. In experiment 1, contrast thresholds for gratings moving upwards and downwards were compared, and no directional asymmetry at threshold was found. In experiment 2, the effect of contrast on infants' preference between simultaneously displayed upwards-drifting and downwards-drifting gratings was examined. Infants showed no preference at 5% contrast, a marked preference for downwards motion at intermediate contrasts (10% and 20%), and a similar but smaller preference at 40% contrast. These results suggest that the vertical-motion asymmetry is a result of differences in the gains of directionally selective mechanisms for upwards and downwards motion.

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.

Directional motion asymmetry in infant VEPs--which direction?

Monocular viewing during early infancy reveals asymmetries in optokinetic nystagmus (OKN) and visual evoked potentials (VEPs). This study investigates the VEP asymmetry to see if it is consistent in direction with the OKN asymmetry. Steady-state VEPs were recorded from infants (5-21 weeks) viewing gratings that underwent successive displacements in the same direction, leftward or rightward. In addition, transient VEPs were recorded to the two directions of an oscillating stimulus. Both tests produced larger VEP amplitudes for nasal-to-temporal compared to temporal-to-nasal movement. Horizontal eye movements were monitored by EOG while viewing these stimuli to test whether the asymmetry was a consequence of eye movements. No difference in eye movements as a function of the stimulus was found, excluding differences in retinal slip as an explanation of the asymmetry. The stronger neural response for nasal-to-temporal displacements is opposite to the asymmetry of OKN. Oculomotor and VEP asymmetries may be related; however this relationship is not simply that the stronger neural response, indicated by the VEP, leads to a stronger optokinetic response.

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.

Form and motion coherence activate independent, but not dorsal/ventral segregated, networks in the human brain.

There is much evidence in primates' visual processing for distinct mechanisms involved in object recognition and encoding object position and motion, which have been identified with 'ventral' and 'dorsal' streams, respectively, of the extra-striate visual areas [1] [2] [3]. This distinction may yield insights into normal human perception, its development and pathology. Motion coherence sensitivity has been taken as a test of global processing in the dorsal stream [4] [5]. We have proposed an analogous 'form coherence' measure of global processing in the ventral stream [6]. In a functional magnetic resonance imaging (fMRI) experiment, we found that the cortical regions activated by form coherence did not overlap with those activated by motion coherence in the same individuals. Areas differentially activated by form coherence included regions in the middle occipital gyrus, the ventral occipital surface, the intraparietal sulcus, and the temporal lobe. Motion coherence activated areas consistent with those previously identified as V5 and V3a, the ventral occipital surface, the intraparietal sulcus, and temporal structures. Neither form nor motion coherence activated area V1 differentially. Form and motion foci in occipital, parietal, and temporal areas were nearby but showed almost no overlap. These results support the idea that form and motion coherence test distinct functional brain systems, but that these do not necessarily correspond to a gross anatomical separation of dorsal and ventral processing streams.

Development of illusory-contour perception in infants.

We investigated whether infants from 8-22 weeks of age were sensitive to the illusory contour created by aligned line terminators. Previous reports of illusory-contour detection in infants under 4 months old could be due to infants' preference for the presence of terminators rather than their configuration. We generated preferential-looking stimuli containing sinusoidal lines whose oscillating, abutting terminators give a strong illusory contour in adult perception. Our experiments demonstrated a preference in infants 8 weeks old and above for an oscillating illusory contour compared with a stimulus containing equal terminator density and movement. Control experiments excluded local line density, or attention to alignment in general, as the basis for this result. In the youngest age group (8-10 weeks) stimulus velocity appears to be critical in determining the visibility of illusory contours, which is consistent with other data on motion processing at this age. We conclude that, by 2 months of age, the infant's visual system contains the nonlinear mechanisms necessary to extract an illusory contour from aligned terminators.

Orientation-reversal and phase-reversal visual evoked potentials in full-term infants with brain lesions: a longitudinal study.

The onset and maturation of visual cortical mechanisms can be recorded by using steady-state visual evoked potentials. The aim of this study was to evaluate and compare orientation-reversal (OR) and phase-reversal (PH) VEP as indicators of the maturation of cortical function in a population of fullterm infants with brain lesions on neonatal MRI. Forty-six infants with brain lesions on neonatal MRI were tested on both PH and OR VEP at 8 reversals/second at the age of 5 months and, if the responses were not significant, at a lower temporal frequency (4 reversals/second). Children whose VEPs were not significant at 5 months were tested longitudinally at 6, 9, 12 and 18 months. The results showed that 23 of the 46 infants (50%) did not show significant responses at 5 months and that while in 7 of the 23 (14% of the whole cohort) the responses became significant between 5 and 12 months, in the other 16 infants (34%) the VEP responses were persistently abnormal. Children with focal lesions, such as focal infarction or haemorrhages, tended to show normal or only mildly delayed VEP while more generalised lesions, such as the ones seen in infants with hypoxic-ischaemic encephalopathy grade 2 and 3, tended to be associated with abnormal VEP responses. The involvement of the optic radiations and occipital cortex was not always associated with abnormal VEP responses but the concomitant involvement of the basal ganglia was always associated with abnormal VEP. We were also able to demonstrate that VEP can be also used as a prognostic indicator: while normal OR VEP are reliably associated with a normal visual and neurodevelopmental outcome, abnormal 4 OR or 8 PH at 5 months are consistently associated with abnormal outcome.

Measurement of astigmatism by automated infrared photoretinoscopy.

BACKGROUND: There are basically two possibilities to measure cylindrical refractive errors by eccentric photorefraction. The first is to determine the size and the tilt of the light crescent in the subject's pupil. Sphere, cylinder, and axis can be obtained from two pictures with the knife edge at two different orientations by using equations derived by Wesemann et al. In natural eyes, the procedure has limitations because undetermined factors (not considered in the theory) affect size, shape, and intensity of the light crescent. A second possibility is to perform eccentric photorefraction separately in at least three different meridians. METHODS: We have tested the power of the second possibility. The three critical parameters (sphere, cylinder, and axis) were calculated from Euler's law, which describes curvatures (or refractions) at any given angle. The procedure relied only on empirical calibration and not on a theoretical treatment of the optics. Therefore, it was not necessary to identify all factors that determine the path of light. RESULTS: The procedure compared favorably with subjective refractive (first population: students, age 26-30 years, N = 7 (14 eyes); correlations: sphere, r = 0.983; cylinder, r = 0.867; axis, r = 0.935) and with a Canon R-1 Autorefractor (second population: children, age 4-14 years, N = 48 (96 eyes); correlations: sphere, r = 0.955; cylinder, r = 0.600; axis, r = 0.846). CONCLUSIONS: Because it is fast, the technique may be suitable for screening in children. The refractions in the different meridians are performed in real time (25 to 30 Hz) and a single reading (the average from 4-6 refractions in each of the 6 meridians) is obtained in 1-2 s. It constitutes a major improvement to commercially available videorefractors which use measurements only in two meridians in conjunction with the formula by Wesemann et al., although it is still not precise enough to permit spectacle prescription.

Visual motion processing in one-month-old infants: preferential looking experiments.

The ability of infants to discriminate between opposite directions of motion was assessed using forced-choice preferential looking between a random-dot pattern which was segregated into regions which moved in opposite directions, and a uniform pattern in which all the dots moved in the same direction. The first experiment measured velocity thresholds (Vmin and Vmax) for direction discrimination; between 10 and 13 weeks Vmin decreased, while at the same time Vmax increased. The second experiment explored possible implications of this expanding velocity range for direction discrimination by younger infants. One-month-olds showed no evidence for direction discrimination at any of a number of test velocities in the range 1-43 deg/sec. The 1-month-olds were also tested with two additional conditions: they could discriminate between moving and static patterns at velocities of 10 deg/sec or above, and they could also discriminate between coherent and incoherent motion at velocities of 21 deg/sec or below. Neither of these discriminations depends on sensitivity to the direction of the coherent motion. The results suggest that 1-month-olds may not be sensitive to the direction of visual motion.

Visual motion processing in one-month-old infants: habituation experiments.

The ability of infants to discriminate between opposite directions of motion was examined in infant control habituation experiments. A group of 3-5-week-olds showed no evidence of discrimination between a random-dot pattern which was segregated into regions that moved in opposite directions, and a uniform pattern in which the dots all moved in the same direction. However, they did discriminate between segregated and uniform patterns in two additional conditions, neither of which required sensitivity to direction: in the first of these, segregation was based on the contrast between coherently moving and stationary dots, while in the second the contrast was between coherently and incoherently moving dots. Unlike the younger infants, a slightly older group of 6-8-week-olds proved capable of discriminating between the segregated and uniform patterns when the contrast was between opposite directions of motion. These results confirm and extend the results from preferential looking [Wattam-Bell, J. (1996). Motion processing in one-month-old infants: Preferential looking experiments. Vision Research, 36, 1671-1677]; they suggest that direction discrimination may not emerge until around 6-8 weeks of age. The apparent lack of direction discrimination before 6 weeks may reflect an inability to use directional cues to visually segment the segregated pattern, rather than an insensitivity to direction as such. To examine this, a further set of infants was tested for absolute direction discrimination-i.e., between leftwards- and rightwards-moving uniform patterns. However, in this case neither 3-5- nor 6-8-week-olds showed any evidence of discrimination, which suggests that direction discrimination may first emerge for relative motion. Possible reasons for this are discussed.

Coherence thresholds for discrimination of motion direction in infants.

The sensitivity of 3-month-old infants to direction of motion in random-dot patterns was assessed by measuring coherence thresholds for the discrimination of a pattern, in which opposite directions were segregated into alternate horizontal strips, from an unsegregated pattern. The coherently moving dots had a displacement size of 0.16 deg (velocity 8 deg/sec), and their direction of motion reversed periodically. For both infants and an adult subject coherence thresholds decreased with increasing height of the segregated strips, and with increasing duration of the interval between direction reversals. However the infants required larger minimum heights and longer minimum durations in order to extract motion direction. Even under the best conditions infants were markedly less sensitive, with coherence thresholds of around 50%, compared with 5-7% for the adult. In addition, within the group of infants coherence thresholds were negatively correlated with age. This developmental increase in motion sensitivity at an intermediate velocity suggests that a large part of the improvement in upper and lower velocity thresholds during development is a result of a uniform increase in sensitivity across all velocities, though the results do not rule out additional specific improvements in sensitivity at the extremes of the velocity range.

The development of maximum displacement limits for discrimination of motion direction in infancy.

The development of visual motion mechanisms has been studied in infants by using forced-choice preferential looking to measure maximum displacement limits (dmax) for the detection of coherent motion in random-dot patterns. The motion consisted of a sequence of coherent displacements at intervals of 20 msec. Between 8 and 15 weeks, dmax for discrimination of coherent from incoherent motion, and for discrimination of opposite directions of coherent motion, increased with age; in both conditions, dmax for the oldest infants was less than a third of the value found in adults. For 10-11-week-old infants, and for an adult subject, reducing stimulus contrast from 88 to 48% had no effect on dmax for direction discrimination, which indicates that the rise in dmax with age is not simply a result of improving contrast sensitivity. When the displacement interval was increased from 20 to 40 msec, dmax increased significantly in 8-11-week-olds, but remained unchanged in 12-15-week-olds. These results show that while directional mechanisms are present in the visual system by 8 weeks, they operate over a restricted velocity range. The upper limit of this velocity range increases with age. After about 12 weeks, the increase is mainly due to changes in the spatial properties of motion mechanisms; however, in younger infants changes in their temporal properties are also important.

Orientation selectivity in infancy: behavioural evidence for temporal sensitivity.

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.

Changes in infants' ability to switch visual attention in the first three months of life.

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.

Development of motion-specific cortical responses in infancy.

The development of visual motion mechanisms has been studied in infants with a visual evoked potential (VEP) technique which isolates responses from directionally-selective mechanisms. In adults, the amplitude of this directional VEP increased with velocity up to a maximum at 15-20 deg/sec, and then declined with further increases in velocity. In a group of infants tested longitudinally, directional responses were first found at a median age of 74 days with a stimulus velocity of 5 deg/sec, and 90 days with a velocity of 20 deg/sec; this age difference was statistically significant. Initially, VEP amplitudes were significantly greater at 5 deg/sec than at 20 deg/sec. By the end of the longitudinal study, there was no significant difference in amplitudes at the two velocities. In a second group of infants, simultaneous recording of VEPs and electrooculograms indicated that eye movements tracking the stimulus were not a significant factor in the development of the directional VEP. It is concluded that the development of directional selectivity starts at low velocities, and extends to higher velocities with age.

Development of orientation discrimination in infancy.

It has previously been found by us, with a visual evoked potential (VEP) measure, that orientation discrimination of dynamic patterns in infants can be demonstrated from around 6 weeks after birth. Experiments are reported in which orientation discrimination was measured behaviourally, in two infant control habituation procedures, with both dynamic and static patterns. When dynamic patterns identical to those in our previous VEP studies were used, the first positive evidence of orientation discrimination was found at around 6 weeks postnatally. The time course of both the VEP and the behavioural measures was similar. However, with static patterns, evidence of orientation discrimination by newborns was found if the infants were allowed to compare the habituated and novel orientations in a paired simultaneous comparison after habituation, but was not found when the habituated and novel stimulus were presented sequentially. The positive evidence of orientation discrimination in newborns supports the hypothesis that some form of orientationally tuned detectors can be used for discrimination of static patterns at birth. However, some developmental change over several weeks seems to be required before a positive electrophysiological VEP response can be measured for dynamic patterns changing in orientation.

Orientation-specific cortical responses develop in early infancy.

Neurones in the visual cortex of higher mammals differ from those elsewhere in the visual pathway in that the majority respond selectively to particular edge or bar orientations in the stimulus. We have developed a visually evoked potential (VEP) technique which isolates the response of orientation-selective mechanisms from that of cortical or sub-cortical neurones which lack orientation selectivity. We are unable to find such orientation-selective responses in newborn human infants within the sensitivity of our method, but repeated longitudinal testing of individual infants shows that measurable responses emerge around 6 weeks of age. This result is consistent with the idea that human cortical visual function is very immature at birth, but develops rapidly in the first two postnatal months.

Development of the discrimination of spatial phase in infancy.

The ability to discriminate grating patterns, containing the same spatial frequency components but in different phase relationships, has been studied in infants by comparing looking times following habituation to one pattern. The performance of 1-month-olds was compared with that of 2/3-month-old infants. Both age groups could discriminate a set of components in square-wave-phase (fundamental 0.18 c/deg) from components of the same amplitude combined in random phase. However, these compounds differ in peak-to-trough contrast, which infants of both ages could discriminate even for a constant waveform. When contrast was randomized from presentation to presentation, the older group still demonstrated discrimination, implying that they were sensitive to the pattern differences, but the younger group did not. The younger group also failed to demonstrate discrimination between the two waveforms when they were of fixed, matched, peak-to-trough contrast, indicating that the previous absence of discrimination was not simply due to distraction by the contrast variations. We conclude that 1-month-olds are insensitive to the configuration of these compound grating patterns even when they are capable of detecting their components. This loss of spatial information has some analogies with adult peripheral and amblyopic vision. Like other aspects of vision, it shows striking development between 1 and 3 months of age.

The onset of binocular function in human infants.

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.