Development of pattern visual evoked potentials: longitudinal measurements in human infants

Pediatric abusive head trauma: visual outcomes, evoked potentials, diffusion tensor imaging, and relationships to retinal hemorrhages

PURPOSE

Function and anatomy of the visual system were evaluated in children with abusive head trauma (AHT). The relationships between retinal hemorrhages at presentation were examined with outcome measures.

METHODS

Retrospective review of data in children with AHT for 1) visual acuity at last follow-up, 2) visual evoked potentials (VEP) after recovery, 3) diffusion metrics of white matter tracts and grey matter within the occipital lobe on diffusion tensor imaging (DTI), and 4) patterns of retinal hemorrhages at presentation. Visual acuity was converted into logarithm of minimum angle of resolution (logMAR) after correction for age. VEPs were also scored by objective signal-to-noise ratio (SNR).

RESULTS

Of 202 AHT victims reviewed, 45 met inclusion criteria. Median logMAR was reduced to 0.8 (approximately 20/125 Snellen equivalent), with 27% having no measurable vision. Thirty-two percent of subjects had no detectable VEP signal. VEPs were significantly reduced in subjects initially presenting with traumatic retinoschisis or hemorrhages involving the macula (p < 0.01). DTI tract volumes were decreased in AHT subjects compared to controls (p < 0.001). DTI metrics were most affected in AHT victims showing macular abnormalities on follow-up ocular examination. However, DTI metrics were not correlated with visual acuity or VEPS. There was large inter-subject variability within each grouping.

DISCUSSION

Mechanisms causing traumatic retinoschisis, or traumatic abnormalities of the macula, are associated with significant long-term visual pathway dysfunction. AHT associated abnormalities of the macula, and visual cortical pathways were more fully captured by VEPs than visual acuity or DTI metrics.

Cerebral Visual Impairment Characterized by Abnormal Visual Orienting Behavior With Preserved Visual Cortical Activation

Purpose

Children with cerebral visual impairment (CVI) often have abnormal visual orienting behaviors due to impaired or damaged visual cortex. Alternatively, visual-cortical function is intact but visual information is not transformed downstream into an appropriate oculomotor output (visuomotor dysfunction). We examined visual, anatomic, and oculomotor assessments to distinguish visuomotor dysfunction from CVI associated with severely reduced visual-cortical response.

Methods

We reviewed the medical records from children with CVI having abnormal visual orienting behaviors, normal ocular examinations, and born near term. Relevant data were visual evoked potentials (VEPs), Teller card acuity, eye movements recorded by video-oculography (VOG), and neuroimaging (magnetic resonance imaging [MRI]) including diffusion tensor imaging (DTI) tractography.

Results

Thirty subjects had visuomotor dysfunction based on a normal VEP; of these 33% had a normal MRI and 67% had white matter abnormalities associated with metabolic disease and/or decreased volume of brain parenchyma. VOG recordings showed smooth pursuit gains were uniformly reduced and saccades were dysmetric but followed the main sequence. Ten subjects had severe CVI based on VEPs at noise levels; visual acuities and MRI findings overlapped those of the visuomotor dysfunction group. Developmental delay, seizures, microcephaly, and hypotonia were common across all groups. All subjects with an abnormal conventional MRI had abnormal metrics on DTI tractography from the occipital lobe.

Conclusions

A subset of patients with CVI have abnormal visual orienting behaviors despite a normal VEP (visuomotor dysfunction). A majority have abnormal white matter metrics on tractography suggesting a downstream defect in sensorimotor transformation. Clinically, visuomotor dysfunction is indistinguishable from severe CVI.

Flash visual evoked potentials in preterm infants

OBJECTIVE

To describe the development of flash visual evoked potentials (FVEPs) in preterm infants from 1 to 18 months and to determine if the maturation of FVEPs is similar to that of term infants.

DESIGN

Longitudinal follow-up study.

PARTICIPANTS

Twenty very low birth weight (VLBW) preterm infants, 42 low birth weight (LBW) preterm infants, and 41 term infants underwent FVEP recordings and neurodevelopmental examinations at 1, 3, 6, 9, 12, and 18 months of corrected and chronological ages.

METHODS

The FVEP recordings were carried out with the VikingQuest-IV neuroelectrophysiological device (VikingQuest, Nicolet, WI), and neurodevelopmental assessments were made by the Development Screen Test and Bayley Scales of Infant Development, Second Edition.

MAIN OUTCOME MEASURES

At 1, 3, 6, and 9 months of age, neurodevelopment was measured with the Mental Index and Developmental Quotient. At 12 and 18 months, neurodevelopment was assessed using the Mental Developmental Index and Psychomotor Developmental Index. Two FVEP values were analyzed: the P2 amplitude (peak to peak from the preceding N2 wave) and the latency of the P2 wave.

RESULTS

There was no significant difference for age-dependent decreased pattern of FVEP P2 latency between preterm infants and the control group. This pattern consisted of a rapid decrease in the first 6 months of life, a gradual decline from 6 to 12 months of age, and a steady reduction from 12 to 18 months of age. The P2 latencies were prolonged significantly at all 6 recorded times in the VLBW group compared with the controls and showed a delay in the LBW group at 1 and 3 months of corrected age. The maturation of P2 latency in LBW infants is similar to that of the controls at 3 months of corrected age, but the maturation of P2 latency in VLBW children remained delayed when compared with the controls until 18 months of corrected age.

CONCLUSIONS

Although the FVEP development pattern of preterm infants was similar to that of healthy full-term infants, the former had deficits in visual electrophysiologic maturation, especially for VLBW children.

Morphology and microstructure of subcortical structures at birth: a large-scale Asian neonatal neuroimaging study

This paper presents the growth pattern and sexual dimorphism of the thalamus and basal ganglia in a large-scale Asian neonatal cohort using both T2-weighted magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). Our study observed a robust growth of the thalamus and basal ganglia (caudate, putamen, globus pallidus, and anterior limb of internal capsule) beyond the overall brain growth in the early postnatal period (36-43 weeks of the gestational age). Additionally, the microstructure of the two structures was integrated as reflected by an increase in fractional anisotropy (FA) and a decrease in axial and radial water diffusivities in the first few weeks of life. Sexual dimorphism was only observed in the whole brain growth and the left thalamic volume but not in the other volumes or DTI measures of the basal ganglia and thalamus at birth. Even though the pattern of sexual dimorphism in the total brain volume is present at birth and persists throughout postnatal brain development, sexual dimorphisms of the basal ganglia and thalamus differ from those found in later stages of brain development, indicating that regionally distinct patterns of postnatal brain development between males and females arise after birth.

Contrast adaptation reveals increased organizational complexity of chromatic processing in the visual evoked potential

Results from psychophysics and single-unit recordings suggest that color vision comprises multiple stages of processing. Postreceptoral channels appear to consist of both a stage of broadly tuned opponent channels that compare cone signals and a subsequent stage, which includes cells tuned to many different directions in color space. The chromatic visual evoked potential (crVEP) has demonstrated chromatic processing selective for cardinal axes of color space. However, crVEP evidence for higher-order color mechanisms is lacking. The present study aimed to assess the contribution of lower- and higher-order color mechanisms to the crVEP by using chromatic contrast adaptation. The results reveal the presence of mechanisms tuned to intermediate directions in color space in addition to those tuned to the fundamental cardinal axes.

Maturation of thalamic radiations between 34 and 41 weeks' gestation: a combined voxel-based study and probabilistic tractography with diffusion tensor imaging

BACKGROUND AND PURPOSE

This study aimed to investigate brain maturation along gestational age with diffusion tensor imaging in healthy preterm and term neonates. Therefore, a voxel-based study of fractional anisotropy (FA) and mean diffusivity (D(av)) was performed to reveal the brain regions experiencing microstructural changes with age. With tractography, the authors intended to identify which fiber tracts were included in these significant voxels.

MATERIALS AND METHODS

There were 22 healthy preterm and 6 healthy term infants who underwent MR imaging between 34 and 41 weeks of gestation. A statistical parametric approach was used to evidence the effect of age on regional distribution of FA and D(av) values. The fiber tracts suspected to be included in the significant clusters of voxels were identified with neuroanatomy and tractography atlases, reconstructed with probabilistic tractography, and superimposed on the parametric maps.

RESULTS

Parametric analysis showed that FA increases with age in the subcortical projections from the frontal (motor and premotor areas) and parietal cortices, the centrum semiovale, the anterior and posterior arms of the internal capsules, the optic radiations, the corpus callosum, and the thalami (P < .05, corrected). Superimposition of the parametric maps on tractography showed that the corticospinal tract (CST); the callosal radiations (CR); and the superior, anterior, and posterior thalamic radiations were included in the significant voxels. No statistically significant results were found for D(av) maps.

CONCLUSIONS

These results highlight that, besides the already-evidenced FA increase in the CST and CR, the thalami and the thalamic radiations experience microstructural changes in the early development of the human brain.

Microstructural correlates of infant functional development: example of the visual pathways

The development of cognitive functions during childhood relies on several neuroanatomical maturation processes. Among these processes is myelination of the white matter pathways, which speeds up electrical conduction. Quantitative indices of such structural processes can be obtained in vivo with diffusion tensor imaging (DTI), but their physiological significance remains uncertain. Here, we investigated the microstructural correlates of early functional development by combining DTI and visual event-related potentials (VEPs) in 15 one- to 4-month-old healthy infants. Interindividual variations of the apparent conduction speed, computed from the latency of the first positive VEP wave (P1), were significantly correlated with the infants' age and DTI indices measured in the optic radiations. This demonstrates that fractional anisotropy and transverse diffusivity are structural markers of functionally efficient myelination. Moreover, these indices computed along the optic radiations showed an early wave of maturation in the anterior region, with the posterior region catching up later in development, which suggests two asynchronous fronts of myelination in both the geniculocortical and corticogeniculate fibers. Thus, in addition to microstructural information, DTI provides noninvasive exquisite information on the functional development of the brain in human infants.

Abnormal spatial frequency processing in high-functioning children with pervasive developmental disorder (PDD)

OBJECTIVE

Basic abnormalities in visual information processing could be associated with the local visual bias often found in subjects with PDD. Therefore, the present study investigated the existence of deficits in spatial frequency processing at an early sensory level in children with PDD.

METHODS

Visual evoked potentials (VEPs) and VEP dipole sources elicited by high and low spatial frequency gratings were analyzed in high-functioning children with PDD and matched controls.

RESULTS

Around 80 ms (N80-latency) children with PDD did not show the same robust differences between high and low spatial frequencies in VEP amplitude and VEP brain sources as controls, because of atypical processing of high frequencies. Analyses at the P1-latency (130 ms) revealed that, although similar inferior-medial brain sources were activated for the processing of both spatial frequencies in the PDD and control group, source strength in response to both frequencies was weaker in the PDD compared to control group. Moreover, additional superior-lateral brain sources were activated during the processing of both frequencies in the PDD group.

CONCLUSIONS

Decreased specialized processing of high and low spatial frequencies might be a robust characteristic of PDD. Early in processing abnormalities in high spatial frequency processing seem to occur in PDD. At a later phase in processing there seems to be both atypical high and low spatial frequency processing. Considering that the processing of specific spatial frequencies plays an important role in the processing of global and local aspects of hierarchical stimuli and faces and of emotions, present data suggest that peculiarities in PDD subjects with respect to these stimuli might be related to an abnormality in more fundamental visual processes.

SIGNIFICANCE

A basic abnormality in visual frequency processing is established in children with PDD.

Pediatric clinical visual electrophysiology: a survey of actual practice

PURPOSE

Survey the actual clinical practice of pediatric visual electrophysiology. The electrophysiologists surveyed were members of the International Society for Clinical Electrophysiology of Vision (ISCEV).

METHODS

A self-administered questionnaire with 55 items about visual evoked potential (VEP) and electroretinogram (ERG) testing of pediatric patients was sent to ISCEV members. The survey queried personnel, facilities, referral patterns and conduct of tests.

RESULTS

Nearly all respondents (94%) had advanced scientific or clinical degrees or both, and most (96%) worked in academic or medical facilities. Of the 71 respondents, 68 tested patients 12 years or younger, and nearly all of those performed both VEPs and ERGs. However, fewer than a third did high volume (>10/month) testing of infants and young children (< or =6 years). Eye care professionals and neurologists made the majority (57%) of the referrals, with the most common reason for referral being suspected visual impairment. Conduct of a pediatric test session often required more than one practitioner. For both VEP and ERG, more than 70% of respondents required at least 30 min for each test. The majority indicated that they followed the ISCEV standards for stimuli and data acquisition. Almost all (94%) reported using the ISCEV recommended VEP electrode configuration. For ERG, most (88%) used ocular contact electrodes (including contact lens, thread, foil and HK loop), but 12% used skin electrodes exclusively and some (17%) used skin electrodes at times.

CONCLUSIONS

Pediatric ERG and VEP testing is a labor intensive endeavor of highly trained professionals. ISCEV technical standards are typically met or exceeded, indicating that high quality testing of infants and children is feasible. Revision of the ISCEV ERG standard is necessary to bring actual practice into accord with evidence-based recommendations for infant testing.

Differential aging of chromatic and achromatic visual pathways: behavior and electrophysiology

Aging of visual pathways was measured psychophysically and physiologically in subjects aged 20-89 years. Contrast thresholds for the chromatic pathways increased with age, but there were no significant changes for thresholds of the achromatic pathway at low spatial frequencies. For visual evoked potential (VEP) responses, again only the chromatic pathways were significantly affected by age. Age-related changes in chromatic responses were not apparent when stimuli were adjusted by age-related contrast thresholds. This suggests that the chromatic and achromatic visual pathways age differently and that the VEP accurately and objectively reflects behavioral changes with age.

[Compared progression of visual-evoked potentials in preterm and term newborns]

PURPOSE

To compare the development of the visual pathway in healthy preterm and full-term newborns using flash and patterned (both transient and steady-state) visual-evoked potentials (fVEP, pVEP-t, pVEP-ss).

PATIENTS AND METHODS

Twenty-nine preterm newborns (28-35 weeks gestational age) were evaluated at four different times: at 3 months postnatal and corrected age, at 8 months postnatal and corrected age. They were matched with 92 term newborns tested at 3 and 8 months of life.

RESULTS

The maximal perceived spatial frequencies in the groups were: 2 C/d at 3 months postnatal age, >2 C/d at 3 months corrected age and at 3 months at term, between 3 and 5C/d, >=5C/d at 8 months corrected age and at 8 months of life, respectively. The latencies of the P1 and P100 waves for the flash and the transient stimulation were, respectively, 192 ms and 207.9 ms at 3 months postnatal age, 144 ms and 137.7 ms at 3 months corrected age, 126.3 ms and 124.1 ms at 8 months postnatal age, 112.7 ms and 112.5 ms at 8 months corrected age, 137.3 and 110.1 ms at 3 months of life; and 122.3 and 100.5 ms at 8 months of life.

DISCUSSION

In the preterm infants of 3 gestational months, the latencies of the fVEP are comparable to those of the 3-month-old full-term infants, although the pVEP-t latencies are still delayed at 8 months. In the fVEP; however, we noted a shorter latency in the preterm infants at 8 months corrected age compared to those of the full-term infants. This tendency inversion was also evidenced at the steady-state stimulation.

CONCLUSIONS

As evidenced by our results, at 8 months of corrected age the preterm infants recovered the fVEP and pVEP-ss, but not the pVEP-t. Different results obtained with different visual-evoked stimulations suggest that similar neural pools are developed in not completely equal amounts of time.

Influence of the state of alertness on the pattern visual evoked potentials (PVEP) in very young infant

The aim of the study was to evaluate the pattern visual evoked potentials (PVEP) combined with an optimal state of vigilance, called liberated state (LS), in order to improve testing in very young infants. Transient PVEP were recorded in response to a checkerboard pattern of 120, 60 and 30 min of arc. in 56 fullterm newborns and 79 preterm infants from birth to 4 months of age. In the fullterms, 28 infants in each group were tested in LS or spontaneous alertness (SA) while in the preterms, 48 infants in each group were tested in LS or SA. No significant difference was found in the amplitude and/or peak time of the PVEP responses between subjects tested in LS compared to SA groups. However, the LS condition improved by approximately 25% the feasibility of prolonged PVEP testing in preterms aged less than 2 months (corrected age). This suggest that the LS condition enhances the clinical feasibility of PVEP testing in very young infants and should be used to optimize the evaluation of visual development in high risk infants in this age group.

Transient pattern Visual Evoked Potentials in children with Down's syndrome

Visual acuity and contrast sensitivity are lower in children with Down's syndrome than in those developing normally. In many cases, this difference might be accounted for by the relatively high incidence of ocular abnormalities (including refractive error and strabismus) in Down's syndrome. However, abnormal spatial vision persists in children with Down's syndrome in the absence of ocular abnormality, suggesting that abnormal retino-cortical visual processing explains reduced visual function in this group. The aim of the present study was to assess retino-cortical function in children with Down's syndrome by recording transient visual evoked potentials (VEPs) in response to pattern stimuli. Responses from children with Down's syndrome were compared with those recorded from children developing normally. Response latency is similar in the two groups, but morphology differs, with the N75 component being clearly present in the normal responses, but diminished or undetectable in responses from children with Down's syndrome. Our findings may suggest a cortical abnormality specific to the source of the N75 component of pattern-reversal achromatic VEPs.

Magnocellular and parvocellular developmental course in infants during the first year of life

The visual system undergoes major modifications during the first year of life. We wanted to examine whether the magnocellular (M) and parvocellular (P) pathways mature at the same rate or if they follow a different developmental course. A previous study carried out in our laboratory had shown that the N1 and P1 components of pattern visual evoked potentials (PVEPs) were preferentially related to the activity of P and M pathways, respectively. In the present study, PVEPs were recorded at Oz in 33 infants aged between 0 and 52 weeks, in response to two spatial frequencies (0.5 and 2.5 c deg(-1)) presented at four contrast levels (4, 12, 28 and 95%). Results indicate that the P1 component appeared before the N1 component in the periods tested and was unambiguously present at birth. The P1 component showed a rapid gain in amplitude in the following months, to reach a ceiling around 4-6 months. Conversely, the N1 component always appeared later and then gained in amplitude until the end of the first year without reaching a plateau. Latencies were also computed but no developmental dissociation was revealed. Results obtained on amplitude are interpreted as demonstrating a developmental dissociation between the underlying M and P pathways, suggesting that the former is functional earlier and matures faster than the latter during the first year of life.

Pattern ERG and VEP maturation in schoolchildren

OBJECTIVE

The maturation of the visual system has been studied with pattern electroretinograms (PERG) and pattern visual evoked potentials (PVEP) mostly in children under the age of 6 years. To address the question of maturation of the visual system in childhood and adolescence we investigated age-dependent PERG and PVEP changes in children aged 7-18 years.

METHODS

PERG were recorded with skin electrodes attached to the lower eyelid, and PVEP were recorded with 5 electrodes. Visual stimuli, consisting of pattern-reversal 50' checks to full-field and to half-field stimulation, were applied to obtain macular (N70, P100, N145) and paramacular waves (P80, N105, N135).

RESULTS

We found an age-dependent decrease (linear regression P<0.05) of PERG P50 amplitude and full-field PVEP P100 latency to monocular right and left eye stimulation, indicating central retinal and postretinal changes. In addition, waveform changes were found in responses to half-field stimulation. The paramacular wave N105 was typically enhanced in younger schoolchildren and diminished with age. The age-dependent decrease (linear regression P<0.01) of paramacular N105 amplitude indicated the increasing predominance of the macular structures of the visual system.

CONCLUSIONS

Our findings suggest that central retinal and postretinal electrophysiological maturation persists throughout childhood. Age-dependent PVEP changes seem to correlate with the morphological and metabolic findings that maturation of the visual cortex continues until puberty and even later.

Aging of the chromatic onset visual evoked potential

PURPOSE

To examine changes in the chromatic onset visual evoked potential (VEP) as a function of aging.

METHODS

VEP's were measured in response to chromatic sinusoidal gratings (1.0 and 0.5 cpd), selectively chosen to modulate the L-M channel and S - (L+M) channel and presented in onset-offset mode. Responses to achromatic gratings presented in a reversal mode were also measured. Twenty subjects were tested, ranging in age from 21 to 93 years.

RESULTS

Unlike changes observed earlier in life, the general shape of the chromatic onset wave-form changed little with age; however, latencies increased significantly as a function of age. Amplitude changes revealed a decreasing trend that was not statistically significant. There was little change in the achromatic responses with age.

CONCLUSIONS

Our results demonstrate a systematic slowing of the chromatic onset VEP with age. The gradual nature of the latency changes and the lack of dramatic and complex wave-form shape changes may allow development of age-based normative data for use in clinical settings.

Maturation of the pattern-reversal VEP in human infants: a theoretical framework

Visual evoked potentials to pattern reversal (PR-VEPs) are used to assess the integrity and maturation of the visual pathways in infants and young children. To establish normal ranges and to facilitate interpolation, we consider the maturation rate of PR-VEPs using published normative data. Curves based on the logistic function (a sigmoid model) are introduced and compared with three other models: (1) the power law function; (2) the sum of two decaying exponentials; and (3) a two-stage linear model. Although methods vary somewhat, remarkable consistency among laboratories is found for the maturation of the major positivity (P1) of PR-VEP. The P1 occurs at approximately 260 ms in neonates and is quite variable. It matures rapidly before 12-14 weeks of age and becomes much less variable. The logistic model provides a parsimonious description of P1 maturation with most rapid maturation at around 6 weeks of age for large patterns and around 9 weeks for small patterns. As inter-laboratory agreement is generally good, the normal ranges based on this model could be used in centres, which do not have their own normative databases for infant VEPs.

Development of the spatio-chromatic visual evoked potential (VEP): a longitudinal study

Most prior visual evoked potential (VEP) research on the development of color vision has employed pattern-reversing stimuli that are not optimal for producing chromatic responses. We measured infant VEPs using low spatial frequency, onset-offset stimuli, modulated along the three axes of a cone-based color space (Derrington et al. [J. Physiol 1984;357, 241-265.]). Three color-normal infants were tested in a longitudinal design over the first postnatal year. One red/green color-deficient infant was also tested at 197 days. We found that VEP responses to S-axis (tritan) stimuli have their initial onset later than responses to red/green (L-M) or achromatic stimuli, and that developmental changes in VEP waveforms are more complex and longer lasting for chromatic than for achromatic stimuli. Possible mechanisms underlying these changes are discussed.