Mammalian navigation depends both on visual landmarks and on self-generated (e.g., vestibular and proprioceptive) cues that signal the organism's own movement [1-5]. When these conflict, landmarks can either reset estimates of self-motion or be integrated with them [6-9]. We asked how humans combine these information sources and whether children, who use both from a young age [10-12], combine them as adults do. Participants attempted to return an object to its original place in an arena when given either visual landmarks only, nonvisual self-motion information only, or both. Adults, but not 4- to 5-year-olds or 7- to 8-year-olds, reduced their response variance when both information sources were available. In an additional "conflict" condition that measured relative reliance on landmarks and self-motion, we predicted behavior under two models: integration (weighted averaging) of the cues and alternation between them. Adults' behavior was predicted by integration, in which the cues were weighted nearly optimally to reduce variance, whereas children's behavior was predicted by alternation. These results suggest that development of individual spatial-representational systems precedes development of the capacity to combine these within a common reference frame. Humans can integrate spatial cues nearly optimally to navigate, but this ability depends on an extended developmental process.

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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.

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.

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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.

This study presents the first measurements using near infrared spectroscopy of changes in regional hemodynamics as a response to a visual stimulus in awake infants. Ten infants aged 3 d to 14 wk viewed a checkerboard with a 5-Hz pattern reversal. The emitter and detector (optodes) of a near infrared spectrophotometer were placed over the occipital region of the head. Changes in concentration of oxy- and deoxyhemoglobin (Hbo2 and Hb) were measured and compared during 10-s epochs of stimulus on and off. A control group of 10 infants aged 18 d to 13 wk were examined with the same setup, but with the optodes over the frontoparietal region. In the test group the total hemoglobin concentration (Hbo2 + Hb) increased while the stimulus was on by a mean (+/-SD) of 2.51 (+/-1.48) micromol x L(-1). Nine out of 10 infants showed an Hbo2 increase, and 9 out of 10 an Hb increase related to the stimulus. There was no significant change in any of these parameters in the control group. The results imply that there is increased cerebral blood flow due to stimulation that is specific to the visual cortex and that infants, unlike adults, show increased cerebral oxygen utilization during activation that outstrips this hemodynamic effect. The study demonstrates that near infrared spectroscopy can be used as a practical and noninvasive method of measuring visual functional activation and its hemodynamic correlates in the awake infant.