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

Postnatal Development of Visual Cortical Function in the Mammalian Brain

This review aims to discuss (1) the refinement of mammalian visual cortical circuits and the maturation of visual functions they subserve in primary visual cortex (V1) and other visual cortical areas, and (2) existing evidence supporting the notion of differential rates of maturation of visual functions in different species. It is well known that different visual functions and their underlying circuitry mature and attain adultlike characteristics at different stages in postnatal development with varying growth rates. The developmental timecourse and duration of refinement varies significantly both in V1 of various species and among different visual cortical areas; while basic visual functions like spatial acuity mature earlier requiring less time, higher form perception such as contour integration is more complex and requires longer postnatal time to refine. This review will highlight the importance of systematic comparative analysis of the differential rates of refinement of visual circuitry and function as that may help reveal underlying key mechanisms necessary for healthy visual development during infancy and adulthood. This type of approach will help future studies to establish direct links between various developmental aspects of different visual cortical areas in both human and animal models; thus enhancing our understanding of vision related neurological disorders and their potential therapeutic remedies.

Development of Visual-Spatial Attention

This chapter reviews literature on development of visual-spatial attention. A brief overview of brain mechanisms of visual perception is provided, followed by discussion of neural maturation in the prenatal period, infancy, and childhood. This is followed by sections on gaze control, eye movement systems, and orienting. The chapter concludes with consideration of development of space, objects, and scenes. Visual-spatial attention reflects an intricate set of motor, perceptual, and cognitive systems that work jointly and all develop in tandem.

A survey on stimuli for visual cortical function assessment in infants

Visual processing, as a significant and complex functionality of the human brain, changes during the life span with the most developmental changes in the infancy. Different types of visual stimuli are needed for evaluating different functionalities of the infants' visual system. Selecting appropriate visual stimuli is an important issue in evaluating visual cortical functions in infants. Properties of stimulation influence responses of visual system and must be adjusted according to the age and specific function which is going to be investigated. In this review, the most commonly used stimuli to elicit visual evoked potentials (VEPs) are evaluated and characteristics of VEPs extracted by these stimulations are studied. Furthermore, various studies investigating different functionalities such as selectivity for orientation and directional motion are presented. Valuable results regarding emerging and maturation times of different functions and normative data for clinical diagnosis are provided by these studies.

The Davida Teller Award Lecture, 2016: Visual Brain Development: A review of "Dorsal Stream Vulnerability"-motion, mathematics, amblyopia, actions, and attention

Research in the Visual Development Unit on "dorsal stream vulnerability' (DSV) arose from research in two somewhat different areas. In the first, using cortical milestones for local and global processing from our neurobiological model, we identified cerebral visual impairment in infants in the first year of life. In the second, using photo/videorefraction in population refractive screening programs, we showed that infant spectacle wear could reduce the incidence of strabismus and amblyopia, but many preschool children, who had been significantly hyperopic earlier, showed visuo-motor and attentional deficits. This led us to compare developing dorsal and ventral streams, using sensitivity to global motion and form as signatures, finding deficits in motion sensitivity relative to form in children with Williams syndrome, or perinatal brain injury in hemiplegia or preterm birth. Later research showed that this "DSV" was common across many disorders, both genetic and acquired, from autism to amblyopia. Here, we extend DSV to be a cluster of problems, common to many disorders, including poor motion sensitivity, visuo-motor spatial integration for planning actions, attention, and number skills. In current research, we find that individual differences in motion coherence sensitivity in typically developing children are correlated with MRI measures of area variations in parietal lobe, fractional anisotropy (from TBSS) of the superior longitudinal fasciculus, and performance on tasks of mathematics and visuo-motor integration. These findings suggest that individual differences in motion sensitivity reflect decision making and attentional control rather than integration in MT/V5 or V3A. Its neural underpinnings may be related to Duncan's "multiple-demand" (MD) system.

The Perception of Moving Subjective Contours by 4-Month-Old Infants

We investigated whether 4-month-old infants are capable of perceiving illusory contours produced by the Kanizsa-square display, first introduced by Prazdny (1983, Perception & Psychophysics34 403–404), which tests whether a viewer perceives the illusory contour in the absence of brightness contrast (illusory brightness). Because the illusory square appears to move across the computer screen and infants are attracted to motion, this display holds their interest. In experiment 1, 4-month-old infants were tested for their ability to distinguish between a continuously moving illusory square and a continuously moving control display in which the pacman elements were rotated so that the perception of subjective contours did not occur. Data analysis revealed a significant preference for the subjective contour display. In experiment 2, habituation-dishabituation was used with 4-month-old infants. They were tested for their ability to discriminate between the illusory Kanizsa square that continuously moved back and forth and an illusory square which changed positions randomly. Although the infants did not show differences in dishabituation as a function of the habituation display, they looked significantly longer at the continuously moving display.

The effect of blur on cortical responses to global form and motion

Global form and motion sensitivity undergo long development in childhood with motion sensitivity rather than form being impaired in a number of childhood disorders and both impaired in adult clinical populations. This suggests extended development and vulnerability of extrastriate cortical areas associated with global processing. However, in some developmental and clinical populations, it remains unclear to what extent impairments might reflect deficits at earlier stages of visual processing, such as reduced visual acuity and contrast sensitivity. To address this, we investigated the impact of degraded spatial vision on cortical global form and motion processing in healthy adults. Loss of high spatial frequencies was simulated using a diffuser to blur the stimuli. Participants completed behavioral and EEG tests of global form and motion perception under three levels of blur. For the behavioral tests, participants' form and motion coherence thresholds were measured using a two-alternative, forced-choice procedure. Steady-state visual evoked potentials were used to measure cortical responses to changes in the coherence of global form and motion stimuli. Both global form and global motion perception were impaired with increasing blur as measured by elevated behavioral thresholds and reduced cortical responses. However, form thresholds showed greater impairment in both behavioral and EEG measures than motion thresholds at the highest levels of blur. The results suggest that high spatial frequencies play an important role in the perception of both global form and motion but are especially significant for global form. Overall, the results reveal complex interactions between low-level factors and global visual processing, highlighting the importance of taking these factors into account when investigating extrastriate function in low vision populations.

Development of Three-Dimensional Perception in Human Infants

The play of light on the retina contains multiple sources of information about the three-dimensional (3D) structure of the world. Some of the best information is derived from differencing operations that act on the images that result from the two eyes’ laterally displaced vantage points. Other information is available in systematic retinal patterns of local texture and motion cues. This article describes what is currently known about the development of sensitivity to these binocular and monocular cues for depth in human infants, and it places the results in the context of what is known about the underlying neural mechanisms from work in nonhuman primates and human neuroimaging studies.

Are visual peripheries forever young?

The paper presents a concept of lifelong plasticity of peripheral vision. Central vision processing is accepted as critical and irreplaceable for normal perception in humans. While peripheral processing chiefly carries information about motion stimuli features and redirects foveal attention to new objects, it can also take over functions typical for central vision. Here I review the data showing the plasticity of peripheral vision found in functional, developmental, and comparative studies. Even though it is well established that afferent projections from central and peripheral retinal regions are not established simultaneously during early postnatal life, central vision is commonly used as a general model of development of the visual system. Based on clinical studies and visually deprived animal models, I describe how central and peripheral visual field representations separately rely on early visual experience. Peripheral visual processing (motion) is more affected by binocular visual deprivation than central visual processing (spatial resolution). In addition, our own experimental findings show the possible recruitment of coarse peripheral vision for fine spatial analysis. Accordingly, I hypothesize that the balance between central and peripheral visual processing, established in the course of development, is susceptible to plastic adaptations during the entire life span, with peripheral vision capable of taking over central processing.

Implied motion perception from a still image in infancy

Visual motion perception can arise from non-directional visual stimuli, such as still images (implied motion, cf. Kourtzi, Trends Cogn Sci 8:47-49, 2004). We tested 5- to 8-month-old infants' implied motion perception with two experiments using the forced-choice preferential looking method. Our results indicated that a still image of a person running toward either the left or right side significantly enhanced infants' visual preference for a visual target that consistently appeared on the same side as the running direction (the run condition in Experiment 1). Such enhanced visual preference disappeared in response to an image of the same person standing and facing the left/right side (the stand condition in Experiment 1), an image of the running figure covered with a set of opaque rectangles (the block condition in Experiment 2) (Gervais et al. in Atten Percept Psychophys 72:1437-1443, 2010), and an image of the inverted running figure (the inversion condition in Experiment 3). These results suggest that only the figure that implied dynamic body motion shifted the infants' visual preference to the same direction as the implied running action. These findings demonstrate that even infants as young as 5 to 8 months old are sensitive to the implied motion of static figures.

Developmental changes in attention to faces and bodies in static and dynamic scenes

Typically developing individuals show a strong visual preference for faces and face-like stimuli; however, this may come at the expense of attending to bodies or to other aspects of a scene. The primary goal of the present study was to provide additional insight into the development of attentional mechanisms that underlie perception of real people in naturalistic scenes. We examined the looking behaviors of typical children, adolescents, and young adults as they viewed static and dynamic scenes depicting one or more people. Overall, participants showed a bias to attend to faces more than on other parts of the scenes. Adding motion cues led to a reduction in the number, but an increase in the average duration of face fixations in single-character scenes. When multiple characters appeared in a scene, motion-related effects were attenuated and participants shifted their gaze from faces to bodies, or made off-screen glances. Children showed the largest effects related to the introduction of motion cues or additional characters, suggesting that they find dynamic faces difficult to process, and are especially prone to look away from faces when viewing complex social scenes-a strategy that could reduce the cognitive and the affective load imposed by having to divide one's attention between multiple faces. Our findings provide new insights into the typical development of social attention during natural scene viewing, and lay the foundation for future work examining gaze behaviors in typical and atypical development.

Inferences about infants’ visual brain mechanisms

We discuss hypotheses that link the measurements we can make with infants to inferences about their developing neural mechanisms. First, we examine evidence from the sensitivity to visual stimulus properties seen in infants’ responses, using both electrophysiological measures (transient and steady-state recordings of visual evoked potentials/visual event-related potentials) and behavioral measures and compare this with the sensitivity of brain processes, known from data on mammalian neurophysiology and human neuroimaging. The evidence for multiple behavioral systems with different patterns of visual sensitivity is discussed. Second, we consider the analogies which can be made between infants’ behavior and that of adults with identified brain damage, and extend these links to hypothesize about the brain basis of visual deficits in infants and children with developmental disorders. Last, we consider how these lines of data might allow us to form “inverse linking hypotheses” about infants’ visual experience.

Fast development of global motion processing in human infants

Although global motion processing is thought to emerge early in infancy, there is debate regarding the age at which it matures to an adult-like level. In the current study, we address the possibility that the apparent age-related improvement in global motion processing might be secondary to age-related increases in the sensitivity of mechanisms (i.e., local motion detectors) that provide input to global motion mechanisms. To address this, we measured global motion processing by obtaining motion coherence thresholds using stimuli that were equally detectable in terms of contrast across all individuals and ages (3-, 4-, 5-, 6-, and 7-month-olds and adults). For infants, we employed a directional eye movement (DEM) technique. For adults, we employed both DEM and a self-report method. First, contrast sensitivity was obtained for a local task, using a stochastic motion display in which all the dots moved coherently. Contrast sensitivity increased significantly between 3 and 7 months, and between infancy and adulthood. Each subject was then tested on the global motion task with the contrast of the dots set to 2.5 × each individual's contrast threshold. Coherence thresholds were obtained by varying the percentage of coherently moving "signal" versus "noise" dots in the stochastic motion display. Results revealed remarkably stable global motion sensitivity between 3 and 7 months of age, as well as between infancy and adulthood. These results suggest that the mechanisms underlying global motion processing develop to an adult-like state very quickly.

Binocular pattern deprivation with delayed onset has impact on motion perception in adulthood

The quality of motion perception depends on visual input during early development. Even 1month of binocular deprivation (BD) from birth impairs motion coherence thresholds when tested in kittens; conversely BD with a 1-month delayed onset does not impair it (Mitchell et al., 2009). We showed that 6months of BD applied from birth induces a selective impairment in a Global Motion Detection task, but not in global form perception, when tested in adulthood (Burnat et al., 2002, 2005). In these animals cell counts of the retinal motion-sensitive alpha ganglion revealed a life-long increase in OFF-type ganglion cell (Burnat et al., 2012). Here we examined in adult cats the effect of BD on global motion perception using an array of tasks with gradually increasing perceptual difficulty. Two conditions of BD were applied: from birth, lasting for 1, 2, 4 or 6months, and with a delayed onset with first 2months of normal vision followed by 2months of BD. Cats deprived from birth for a 6-month period had Global Motion Detection impaired, as compared to the normal group. Velocity and low contrast-defined motion processing was impaired when BD was applied exclusively in months 3-4 of life. The cats deprived from birth for 1 or 2months were not impaired in any of the tested motion tasks. Motion coherence thresholds, when tested at the end of a long motion training were not affected by BD and did not differ from those obtained for the normal group. Impaired extraction of low contrast-defined motion signal was found in cats deprived solely in months 3-4 of life. Surprisingly, binocular pattern deprivation during the first 2months of life did not weaken motion sensitivity, revealing the occurrence of a critical period for motion perception later in development than previously suggested.

The effect of dot speed and density on the development of global motion perception

The purpose of this study was to investigate the effect of dot speed and dot density on the development of global motion perception by comparing the performance of adults and children (5-6years old) on a direction-discrimination task. Motion coherence thresholds were measured at two dot speeds (1 and 4deg/s) and three dot densities (1, 15, 30dots/deg(2)). Adult coherence thresholds were constant at approximately 9%, regardless of speed or density. Child coherence thresholds were significantly higher across conditions, and were most immature at the slow speed and at the sparse density. Thus, the development of global motion perception depends heavily on stimulus parameters. This finding can account for some of the discrepancy in the current developmental literature. Our results, however, caution against making general claims about motion deficits in clinical populations based on only a single measurement at a specific combination of speed and density.

Keep your eyes on development: the behavioral and neurophysiological development of visual mechanisms underlying form processing

Visual form perception is essential for correct interpretation of, and interaction with, our environment. Form perception depends on visual acuity and processing of specific form characteristics, such as luminance contrast, spatial frequency, color, orientation, depth, and even motion information. As other cognitive processes, form perception matures with age. This paper aims at providing a concise overview of our current understanding of the typical development, from birth to adulthood, of form-characteristic processing, as measured both behaviorally and neurophysiologically. Two main conclusions can be drawn. First, the current literature conveys that for most reviewed characteristics a developmental pattern is apparent. These trajectories are discussed in relation to the organization of the visual system. The second conclusion is that significant gaps in the literature exist for several age-ranges. To complete our understanding of the typical and, by consequence, atypical development of visual mechanisms underlying form processing, future research should uncover these missing segments.

Effects of speed, age, and amblyopia on the perception of motion-defined form

We determined the effect of dot speed on the typical and atypical development of motion-defined form perception. Monocular motion coherence thresholds for orientation discrimination of motion-defined rectangles were determined at slow (0.1 deg/s), medium (0.9 deg/s) and fast (5.0 deg/s) dot speeds. First we examined typical development from age 4 to 31 years. We found that performance was most immature at the slow speed and in the youngest group of children (4-6 years). Next we measured motion-defined form perception in the amblyopic and fellow eyes of patients with amblyopia. Deficits were found in both eyes and were most pronounced at the slow speed. These results demonstrate the importance of dot speed to the development of motion-defined form perception. Implications regarding sensitive periods and the neural correlates of motion-defined form perception are discussed.

Sensitivity to first- and second-order drifting gratings in 3-month-old infants

In two experiments, we investigated 3-month-old infants' sensitivity to first- and second-order drifting gratings. In Experiment 1 we used forced-choice preferential looking with drifting versus stationary gratings to estimate depth modulation thresholds for 3-month-old infants and a similar task for a comparison group of adults. Thresholds for infants were more adult-like for second-order than first-order gratings. In Experiment 2, 3-month-olds dishabituated to a change in first-order orientation, but not to a change in direction of first- or second-order motion. Hence, results from Experiment 1 were likely driven by the perception of flicker rather than motion. Thus, infants' sensitivity to uniform motion is slow to develop and appears to be driven initially by flicker-sensitive mechanisms. The underlying mechanisms have more mature tuning for second-order than for first-order information.

Development of human visual function

By 1985 newly devised behavioral and electrophysiological techniques had been used to track development of infants' acuity, contrast sensitivity and binocularity, and for clinical evaluation of developing visual function. This review focus on advances in the development and assessment of infant vision in the following 25 years. Infants' visual cortical function has been studied through selectivity for orientation, directional motion and binocular disparity, and the control of subcortical oculomotor mechanisms in fixation shifts and optokinetic nystagmus, leading to a model of increasing cortical dominance over subcortical pathways. Neonatal face processing remains a challenge for this model. Recent research has focused on development of integrative processing (hyperacuity, texture segmentation, and sensitivity to global form and motion coherence) in extra-striate visual areas, including signatures of dorsal and ventral stream processing. Asynchronies in development of these two streams may be related to their differential vulnerability in both acquired and genetic disorders. New methods and approaches to clinical disorders are reviewed, in particular the increasing focus on paediatric neurology as well as ophthalmology. Visual measures in early infancy in high-risk children are allowing measures not only of existing deficits in infancy but prediction of later visual and cognitive outcome. Work with early cataract and later recovery from blinding disorders has thrown new light on the plasticity of the visual system and its limitations. The review concludes with a forward look to future opportunities provided by studies of development post infancy, new imaging and eye tracking methods, and sampling infants' visual ecology.

Perception of Surface Glossiness by Infants Aged 5 to 8 Months

We examined glossiness perception in infants using a preferential looking paradigm. In experiment 1, the images of two doll-shaped objects with matte and glossy surfaces were presented to infants aged 5 to 6 and 7 to 8 months. The results showed that the 7 to 8 month olds, but not the 5 to 6 month olds, looked significantly longer at the glossy object than at the matte object. In experiment 2, we additionally employed an object that was matte and covered with textures of white paint splashes, whose luminance histogram was almost identical to that of the glossy object. The results showed that the 7 to 8 month old infants could discriminate between the glossy object and the textured object even though both had similar luminance histograms. Qualitatively similar results were obtained for simple spheres that did not contain facial features. Therefore, the results of experiments 1 and 2 were not due to differences in the visibility of the dolls' facial features. These findings suggest that 7 to 8 month old infants perceive difference between glossy objects and matte objects on the basis of surface representations.

Development of visual perception

Processes of visual development that yield a view of the world as coherent and stable begin well before birth and extend over the first several years after the onset of visual experience. Infants are born capable of seeing and with specific preferences that guide the point of gaze to relevant portions of the visual scene to support learning about objects and faces. Visual development after birth is characterized by critical periods in many notable visual functions, and by extensive learning from experience and increasing control over eye movement systems. WIREs Cogni Sci 2011 2 515-528 DOI: 10.1002/wcs.128 For further resources related to this article, please visit the WIREs website.

Asymmetrical cortical processing of radial expansion/contraction in infants and adults

We report asymmetrical cortical responses (steady-state visual evoked potentials) to radial expansion and contraction in human infants and adults. Forty-four infants (22 3-month-olds and 22 4-month-olds) and nine adults viewed dynamic dot patterns which cyclically (2.1 Hz) alternate between radial expansion (or contraction) and random directional motion. The first harmonic (F1) response in the steady-state VEP response must arise from mechanisms sensitive to the global radial motion structure. We compared F1 amplitudes between expansion-random and contraction-random motion alternations. F1 amplitudes for contraction were significantly larger than those for expansion for the older infants and adults but not for the younger infants. These results suggest that the human cortical motion mechanisms have asymmetrical sensitivity for radial expansion vs. contraction, which develops at around 4 months of age. The relation between development of sensitivity to radial motion and cortical motion mechanisms is discussed.

Normal development of pattern motion sensitivity in macaque monkeys

We studied the development of sensitivity to complex motion using plaid patterns. We hypothesized, based on neurophysiological data showing a dearth of pattern direction-selective (PDS) cells in area medial temporal (MT) of infant macaques, that sensitivity to pattern motion would develop later than other forms of global motion sensitivity. We tested 10 macaque monkeys (Macaca nemestrina) ranging in age from 7 weeks to 109-160 weeks (adult). The monkeys discriminated horizontal from vertical pattern motion; sensitivity for one-dimensional (1D) direction discrimination and detection were tested as control tasks. The results show that pattern motion discrimination ability develops relatively late, between 10 and 18 weeks, while performance on the 1D control tasks was excellent at the earliest test ages. Plaid discrimination performance depends on both the speed and spatial scale of the underlying patterns. However, development is not limited by contrast sensitivity. These results support the idea that pattern motion perception depends on a different mechanism than other forms of global motion perception and are consistent with the idea that the representation of PDS neurons in MT may limit the development of complex motion perception.

Development of perceptual completion originates in information acquisition

Adults have little difficulty perceiving objects as complete despite occlusion, but newborn infants perceive moving partly occluded objects solely in terms of visible surfaces. The developmental mechanisms leading to perceptual completion have never been adequately explained. Here, the authors examine the potential contributions of oculomotor behavior and motion sensitivity to perceptual completion performance in individual infants. Young infants were presented with a center-occluded rod, moving back and forth against a textured background, to assess perceptual completion. Infants also participated in tasks to assess oculomotor scanning patterns and motion direction discrimination. Individual differences in perceptual completion performance were strongly correlated with scanning patterns but were unrelated to motion direction discrimination. The authors present a new model of development of perceptual completion that posits a critical role for targeted visual scanning, an early developing oculomotor action system.

Sensitivity to rotational motion in early infancy

Sensitivity to rotational motion, one of the fundamental components of optic flow, was tested in infants aged 2 and 3 months. The infants in both groups showed significant sensitivity to rotational motion only in the high-speed condition (10.62 degrees/s). There was no significant increase in motion sensitivity between 2 and 3 months of age, indicating that there is not a significant developmental change during this period. A comparison of our results with previous findings that showed a significant increase in radial motion sensitivity between 2 and 3 months suggests that different motion sensitivities have different developmental time courses.

The role of kinetic information in newborns' perception of illusory contours

Previous research, in which static figures were used, showed that the ability to perceive illusory contours emerges around 7 months of age. However, recently, evidence has suggested that 2-3-month-old infants are able to perceive illusory contours when motion information is available (Johnson & Mason, 2002; Otsuka & Yamaguchi, 2003). The present study was aimed at investigating whether even newborns might perceive kinetic illusory contours when a motion easily detected by the immature newborn's visual system (i.e. stroboscopic motion) is used. In Experiment 1, using a preference looking technique, newborns' perception of kinetic illusory contours was explored using a Kanizsa figure in a static and in a kinetic display. The results showed that newborns manifest a preference for the illusory contours only in the kinetic, but not in the static, condition. In Experiment 2, using an habituation technique, newborns were habituated to a moving shape that was matched with the background in terms of random-texture-surface; thus the recovery of the shape was possible relying only on kinetic information. The results showed that infants manifested a novelty preference when presented with luminance-defined familiar and novel shapes. Altogether these findings provide evidence that motion enhances (Experiment 1) and sometimes is sufficient (Experiment 2) to induce newborns' perception of illusory contours.

Perceptual Completion in Newborn Human Infants

Despite decades of studies of human infants, a still open question concerns the role of visual experience in the development of the ability to perceive complete shapes over partial occlusion. Previous studies show that newborns fail to manifest this ability, either because they lack the visual experience required for perceptual completion or because they fail to detect the pattern of motion. To distinguish these possibilities, newborns' perception of a center-occluded object was tested, using stroboscopic motion. Infants (mean age of 72 hr) perceived the object as a connected unit, providing the first evidence that the newborn is capable of filling in gaps in the visible surface layout when the relevant visual information can be detected by his or her immature visual system.

Maturation of luminance- and motion-defined form perception beyond adolescence: a combined ERP and fMRI study

Abilities to discriminate forms defined by motion continue to develop throughout childhood. To investigate late development of the visual motion system, we measured brain activity with event-related EEG potentials (ERPs) and functional magnetic resonance imaging (fMRI) in groups of adolescents (15-17 years) and adults (20-30 years) during a visual form discrimination task--with forms being either defined by motion or luminance contrast. We further explored whether possible developmental changes varied with the degree of motion coherence reflecting maturation specific to global motion processing. Both the fMRI activation patterns and ERP topographies were very similar between adolescents and adults, suggesting that the basic visual networks for processing motion and form are established by the age of 15-17. The ERP response to luminance- and motion-defined forms was dominated by a posterior negativity (N1: 120-270 ms). The N1 of the motion contrast was delayed in adolescents, whereas the N1 of the static condition did not differ between groups. Since the motion-evoked N1 is thought to arise in the middle temporal area MT/V5, our results indicate that visual motion processing in MT continues to get faster, becoming still more efficient during late development. Neither the ERP nor the fMRI results revealed maturation effects specific to motion coherence. This indicates that the specific mechanisms to process global dot motion are already mature in adolescence. The present findings support the view that static perception matures earlier than dynamic perception, and that these visual systems have different developmental courses.

Motion- and orientation-specific cortical responses in infancy

During the first 3 months, infants develop visual evoked potential (VEP) responses that are signatures of cortical orientation-selectivity and directional motion selectivity. Orientation-specific cortical responses develop in early infancy. This study compared these responses directly in the same infants, to investigate whether the later appearance of direction selectivity was intrinsic, or a function of the spatio-temporal characteristics of the stimuli used. Steady-state orientation-reversal (OR-) VEPs and direction-reversal (DR-) VEPs were recorded in infants aged 4-18 weeks. DR-VEPs were elicited with random pixel patterns and with gratings spatially similar to those used for OR-VEPs, at velocities of 5.5 and 11 deg/s, and reversal rates of 2 and 4 reversals/s. Infants throughout the age range showed significant responses to orientation-reversal. Direction-reversal responses appeared in less than 25% of infants under 7 weeks of age, rising to 80% or more at 11-13 weeks, whether tested with dots or gratings and for both speeds and reversal rates. However, 2 reversals/s elicits the DR-VEP on average about 2 weeks earlier than 4 reversal/s stimulation. We conclude that human cortical direction selectivity develops separately from orientation-selectivity and emerges at a later age, even with tests that are designed to optimise the former.

Motion detection in normal infants and young patients with infantile esotropia

The purpose of this study was to investigate asymmetries in detection of horizontal motion in normal infants and children and in patients with infantile esotropia. Motion detection thresholds (% motion signal) were measured in 75 normal infants and in 36 eyes of 27 infants with infantile esotropia (ET), using a forced-choice preferential looking paradigm with random-dot patterns. Absolute motion detection sensitivity and asymmetries in sensitivity for nasalward (N) vs. temporalward (T) directions of motion were compared in normal and patient populations, ranging in age from 1 month to 5 years. In normal infants, N and T thresholds were equivalent under 2.5 months of age, whereas a superiority for monocular detection of N motion was observed between 3.5 and 6.5 months of age. The nasalward advantage gradually diminished to symmetrical T:N performance by 8 months of age, matching that of adults. No asymmetry was observed in 15 normal infants who performed the task binocularly, hence, the asymmetry was not a leftward/rightward bias. In the youngest infantile ET patients tested, at 5 months of age, a nasalward superiority in motion detection was observed and was equivalent to that of same-age normal infants. However, unlike normals, this asymmetry persists in older patients. This greater asymmetry in infantile ET represents worse detection of T than N motion. This is the first report of an asymmetry in motion detection in normal infants across a wide age range. Initially, motion detection is normal in infants with infantile esotropia. Cumulative abnormal binocular experience in these patients may disrupt motion mechanisms.

Normal and anomalous development of visual motion processing: motion coherence and 'dorsal-stream vulnerability'

Directional motion processing is a pervasive and functionally important feature of the visual system. Behavioural and VEP studies indicate that it appears as a cortical function after about 7 weeks of age, with global processing, motion based segmentation, and the use of motion in complex perceptual tasks emerging shortly afterwards. A distinct, subcortical motion system controls optokinetic nystagmus (OKN) from birth, showing characteristic monocular asymmetries which disappear as binocular cortical function takes over in normal development. Asymmetries in cortical responses are linked to this interaction in a way that is not yet fully understood. Beyond infancy, a range of developmental disorders show a deficit of global motion compared to global form processing which we argue reflects a general 'dorsal-stream vulnerability'.

Motion processing asymmetries and stereopsis in infants

These experiments used forced-choice preferential looking to test infants for preferences between pairs of random-dot patterns that moved in opposite directions. With monocularly-viewed horizontally moving patterns, 6-12-week-old infants showed a preference for nasalwards motion. With binocularly-viewed vertical motion, there was no overall preference, but the results did show a significant correlation between upwards bias of OKN and preference for downwards motion. In a longitudinal experiment, the nasalwards preference first appeared at 7-8 weeks, and thereafter persisted until the end of testing (23-25 weeks). In this experiment the infants were also tested for stereopsis, under conditions that were as nearly as possible identical to the direction preference test. There was no evidence that the onset of stereopsis had any effect on the directional asymmetry. The directional asymmetries revealed by these experiments appear to be distinct from the asymmetries of OKN and motion VEPs. It is possible that they reflect asymmetrical directional responses in extrastriate visual cortex (e.g. area V5/MT).

Infant direction discrimination thresholds

Although adults can detect direction differences as small as 1 arc degree, the ability of infants to discriminate direction of motion is less clear. This study measures the precision with which 6-, 12-, and 18-week-old infants discriminate direction of motion. Infants viewed random dot kinematograms in which a direction difference between the target and background dots defined a circular target. The target was then placed into continuous motion. An FPL paradigm was used to assess infants' preference for the target as a function of the direction difference between the target and background dots. Direction discrimination thresholds with a moving target were indeterminate at 6 weeks of age, 22 degrees at 12 weeks of age and 17 degrees at 18 weeks of age. This precision was maintained across different testing conditions. However, performance dropped markedly when dot motion was presented within a flickering stationary target. It was concluded that infants can make relatively fine discriminations of motion direction if given an engaging stimulus.

Perception of three-dimensional shape specified by optic flow by 8-week-old infants

Sensitivity of 8-week-old infants to optical flow specifying the shape of a three-dimensional object was assessed. Infants viewed kinetic random-dot displays that specified three-dimensional cubes. The cubes were identical except for the presence or absence of an interior corner. Half of the infants viewed the full display. The other half viewed the central region of the displays, where the flow specifying the presence or absence of the corner differed. Infants in the full-view condition looked significantly longer to a novel cube than to the familiar cube following habituation. In contrast, infants in the partial-view condition looked equally to the novel and familiar cubes, ruling out the possibility that infants who viewed the full displays merely discriminated differences in motion in the central region of the two displays. These findings suggest that infants as young as 8 weeks perceive three-dimensional object shape from optic flow.

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.