Spectral sensitivity of the 2-month infant as measured by the visually evoked cortical potential

Chromatic visual evoked potentials: A review of physiology, methods and clinical applications

Objective assessment of the visual system can be performed electrophysiologically using the visual evoked potential (VEP). In many clinical circumstances, this is performed using high contrast achromatic patterns or diffuse flash stimuli. These methods are clinically valuable but they may only assess a subset of possible physiological circuitries within the visual system, particularly those involved in achromatic (luminance) processing. The use of chromatic VEPs (cVEPs) in addition to standard VEPs can inform us of the function or dysfunction of chromatic pathways. The chromatic VEP has been well studied in human health and disease. Yet, to date our knowledge of their underlying mechanisms and applications remains limited. This likely reflects a heterogeneity in the methodology, analysis and conclusions of different works, which leads to ambiguity in their clinical use. This review sought to identify the primary methodologies employed for recording cVEPs. Furthermore cVEP maturation and application in understanding the function of the chromatic system under healthy and diseased conditions are reviewed. We first briefly describe the physiology of normal colour vision, before describing the methodologies and historical developments which have led to our understanding of cVEPs. We thereafter describe the expected maturation of the cVEP, followed by reviewing their application in several disorders: congenital colour vision deficiencies, retinal disease, glaucoma, optic nerve and neurological disorders, diabetes, amblyopia and dyslexia. We finalise the review with recommendations for testing and future directions.

Frequency tagging with infants: The visual oddball paradigm

Combining frequency tagging with electroencephalography (EEG) provides excellent opportunities for developmental research and is increasingly employed as a powerful tool in cognitive neuroscience within the last decade. In particular, the visual oddball paradigm has been employed to elucidate face and object categorization and intermodal influences on visual perception. Still, EEG research with infants poses special challenges that require consideration and adaptations of analyses. These challenges include limits to attentional capacity, variation in looking times, and presence of artefacts in the EEG signal. Moreover, potential differences between age-groups must be carefully evaluated. This manuscript evaluates challenges theoretically and empirically by (1) a systematic review of frequency tagging studies employing the oddball paradigm and (2) combining and re-analyzing data from seven-month-old infants (N = 124, 59 females) collected in a categorization task with artifical, unfamiliar stimuli. Specifically, different criteria for sequence retention and selection of harmonics, the influence of bins considered for baseline correction and the relation between fast periodic visual stimulation (FPVS) responses and looking time are analyzed. Overall, evidence indicates that analysis decisions should be tailored based on age-group to optimally capture the observed signal. Recommendations for infant frequency tagging studies are developed to aid researchers in selecting appropriate stimulation and analysis strategies in future work.

Pediatric Perimeter-A Novel Device to Measure Visual Fields in Infants and Patients with Special Needs

PURPOSE

There are no commercially available devices to measure visual fields in infants. We developed a device, "Pediatric Perimeter," that quantifies visual field extent (VFE) for infants. We describe the construction, validation, and use of this device.

METHODS

A hemispherical dome with light emitting diodes (LEDs) was constructed. The LEDs were controlled using a computer program to measure reaction time (RT) to gross visual fields (GVF) and the VFE. Participants were tested in supine position in a dark room. Eye or head movement towards the stimuli was monitored with an infrared (IR) camera. Validation was done on 10 adults (mean age: 24.4 ± 5 years) with tunnel vision simulator.

RESULTS

Perimetry was performed on 19 infants (age: 2.3-12 months), five infants with normal milestones. GVF and VFE were estimated in 17 and 7 infants, respectively. Median RT of infants with developmental delay was 663 ms and 380 ms for healthy infants. Also, 14 children (age: 14 months-6 years) with developmental delay and five patients with cognitive impairment were tested.

CONCLUSION

Visual field isopter and RT can be examined with the Pediatric Perimeter device on infants and patients with special needs. Further testing on infants will need to assess the repeatability. A large-scale study will be needed to compare typically developing infants and infants with delayed milestones with this device.

TRANSLATIONAL RELEVANCE

Quantifiable parameters obtained with this device can be used as outcome measures in clinical examination of infants and patients with special needs. This device can be used in pediatric, neurology, and ophthalmology clinics.

Cortical networks for face perception in two-month-old infants

Newborns have an innate system for preferentially looking at an upright human face. This face preference behaviour disappears at approximately one month of age and reappears a few months later. However, the neural mechanisms underlying this U-shaped behavioural change remain unclear. Here, we isolate the functional development of the cortical visual pathway for face processing using S-cone-isolating stimulation, which blinds the subcortical visual pathway. Using luminance stimuli, which are conveyed by both the subcortical and cortical visual pathways, the preference for upright faces was not observed in two-month-old infants, but it was observed in four- and six-month-old infants, confirming the recovery phase of the U-shaped development. By contrast, using S-cone stimuli, two-month-old infants already showed a preference for upright faces, as did four- and six-month-old infants, demonstrating that the cortical visual pathway for face processing is already functioning at the bottom of the U-shape at two months of age. The present results suggest that the transient functional deterioration stems from a conflict between the subcortical and cortical functional pathways, and that the recovery thereafter involves establishing a level of coordination between the two pathways.

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.

Latitude-of-birth and season-of-birth effects on human color vision in the Arctic

Extreme natural ambient light reduction, in both energy and range of wavelength spectrum, occurs during the winter season at very high latitudes (above the Arctic Circle or 66 degrees 32' North) that in turn results in increased exposure to artificial lighting. In contrast, during the summer months, the sun remains above the horizon and there is no darkness or night. Little is known about these extreme changes in light exposure on human visual perception. Measuring color discriminations with the FM100 Test revealed that Norwegians born above the Arctic Circle were less sensitive to yellow-green, green, and green-blue spectrum differences whereas they were more sensitive to hue variations in the purple range than individuals born below the Arctic Circle. Additionally, it was found that the Norwegian individuals born above the Arctic Circle and during autumn showed an overall decrease in color sensitivity, whereas those born in the summer showed a relative increase. All participants were adults and their color vision was tested in the same location (i.e., in Tromsø at 69.7 degrees North). These findings are consistent with the idea that there is a measurable impact on colour vision as adults of the photic environment that individuals born above the Arctic Circle and in the autumn experienced during infancy, namely a reduction in exposure to direct sunlight and an increase in exposure to twilight and artificial lighting.

Visão de cores no primeiro ano de vida

Procedimentos comportamentais que permitem inferir capacidades sensoriais ou perceptuais em bebês e crianças baseiam-se, na sua maior parte, na descoberta seminal feita no fim da década de 1950 pelo psicólogo Robert Fantz. Desde o nascimento, há uma preferência do bebê por dirigir o olhar para estímulos complexos a fixá-lo em cenas monótonas. Baseado no novo conhecimento, o grupo liderado por Davida Teller na Universidade de Washington desenvolveu uma metodologia de avaliação psicofísica da visão em bebês, o que possibilitou grande avanço no estudo do desenvolvimento e da maturação de várias funções visuais básicas, como acuidade visual, visão de cores, visão de contrastes, estereopsia e visão de movimento. A presente revisão examina essa literatura, mostrando como é avaliada a visão em bebês e o que se conhece até o momento sobre a capacidade dos recém-nascidos de ver cores.

Infant color vision: prediction of infants' spontaneous color preferences

Infants show spontaneous looking preferences among isoluminant chromatic stimuli [Adams, R. J. (1987). An evaluation of color preferences in early infancy. Infant Behavior and Development, 10, 143-150; Bornstein, M. H. (1975). Qualities of color vision in infancy. Journal of Experimental Social Psychology, 19 (3), 401-419.]. These differences in preference have often been called "hue" or "color" preferences, and attributed to differences in hue, but there are alternative explanations. Spontaneous preference variations remain after stimuli are equated for adult brightness, and thus cannot be attributed to adult-like brightness differences [Teller, D. Y., Civan, A., & Bronson-Castain, K. (2004). Infants' spontaneous color preferences are not due to adult-like brightness variations. Visual Neuroscience, 21 (3), 397-401]. In the present paper, we address three more alternative explanations: colorimetric purity; infant detection thresholds; and adult-like variations in saturation. Three experiments were conducted. In Experiment 1 we measured infants' spontaneous preferences for each of 22 different chromatic stimuli of varying dominant wavelength and colorimetric purity, each paired against the same white standard. In Experiment 2, we measured infants' chromatic detection thresholds. In Experiment 3, adult subjects made saturation matches between a blue-green standard and each of five other chromatic stimuli. Infant detection thresholds accounted for 34% of the variance in infant "hue" preferences, much more than colorimetric purity (2.4%) or adult saturation judgments (3%), but none of the three variables accounted for the majority of the variance. In our view, the most likely remaining option is that infants' spontaneous "hue" preferences indeed arise from preferences for the hues of stimuli that adults see as blue, purple and red.

Infant photometry: are mean adult isoluminance values a sufficient approximation to individual infant values?

Individual differences in isoluminance values were studied in infants and adults using a motion nulling paradigm. Two luminance-modulated sinusoidal grating components (spatial frequency=0.25 cpd, temporal frequency=5.6 Hz, speed=22.4 deg/s) were superimposed and moved in opposite directions across a color video screen. The contrasts of the two components were traded off to determine motion nulls. Two conditions were used: red/black vs. green/black, and red/black vs. blue/black grating components. An eye movement based response measure was used for infant subjects, and an average of 308 trials per infant were obtained. As observed in earlier studies, the mean motion null values for infants and adults were highly similar in each condition. The standard errors of motion null values for individual subjects were very small. Individual differences among infants were also small, and were clearly measurable only in the red/black vs. blue/black condition. The close similarity of mean null values, combined with the small individual differences among infants, supports the idea that under the right circumstances mean adult isoluminance values can be used as a sufficient approximation to individual infant isoluminance values in studies of infant color vision. These circumstances are discussed and evaluated in detail.

The transition from scotopic to photopic vision in 3-month-old infants and adults: an evaluation of the rod dominance hypothesis

The scotopic to photopic transition was tested in adults and 12-week-old infants using a large field motion nulling technique at a series of luminances between -3.57 and 2.70 log cd m(-2). The stimuli were composed of 0.25 cyc deg(-1), 5.6 Hz blue/black and yellow/black sinusoidal grating components, superimposed and moving in opposite directions. The contrasts of the two components were traded off to determine motion nulls at each luminance level. An eye movement based response measure was used for infant subjects, whereas self-report was used in adults. In both age groups, the motion null values approached a scotopic asymptote consistent with V'(lambda) at the lowest luminance levels, and a photopic asymptote consistent with V(10)(lambda) at the highest luminance levels. The scotopic to photopic transition was gradual and occurred over about 3 log units between about -2 and 1 log cd m(-2) in both groups. The null values for infants and adults were highly similar at each luminance level, and the shapes of the transition curves were virtually identical at the two ages. These data suggest that at each different luminance level, the balance between rod-initiated and cone-initiated signals in the extrafoveal luminance channel is similar or identical in 12-week-old infants and adults.

Variation of chromatic sensitivity across the life span

Thresholds were measured along three directions in color space for detecting an equiluminant color change of a set of bars embedded in a larger field of spatio-temporal achromatic noise for observers ranging in age from 3 months to 86 years. Pre-verbal observers were assessed with a forced-choice preferential-looking technique while older observers responded orally or manually. Over the life span, thresholds could be described along each color axis tested by a curve with two trends. Thresholds decreased with each doubling of age by nearly a factor of two until adolescence. Thereafter, thresholds increased by a factor of 1.4-2 with each doubling of age. Sensitivity to chromatic differences varied similarly along all three axes tested, suggesting uniformity in the sensitivity of chromatic mechanisms across the life span.

M- and L-cones in early infancy: I. VEP responses to receptor-isolating stimuli at 4- and 8-weeks of age

A silent-substitution technique combined with measures of the visually-evoked potential (VEP) was used to determine whether M- and L-cones are functional in early infancy. Data were successfully collected from twenty six infants in response to three receptor-isolation conditions (rod, M- and L-cone isolation) and a luminance-modulation condition. The efficacy of the receptor-isolation conditions was first verified by measuring VEP responses from both dichromatic and color-normal adults to each of the receptor-isolation conditions. Both 4- and 8-week-old infants demonstrated VEP responses to the M- and L-cone isolating stimuli, though the amplitude of the the responses at 4-weeks were reduced compared to those at 8-weeks. These data suggest that the functioning of M- and L-cones can be differentiated as early as 4-weeks of age.

Spectral efficiency measured by heterochromatic flicker photometry is similar in human infants and adults

Spectral efficiency functions based on heterochromatic flicker photometry (HFP) were measured for three adults and 42 infants using a rapid visually-evoked potential (VEP) method. A 5 degrees-diameter, broadband standard (0.6 cd/m2) was presented in square-wave counterphase (15 Hz) with one of 13 monochromatic lights (420-660 nm; 20 nm steps). The intensity of the monochromatic light was continuously varied while extracting the phase-locked VEP amplitude of the fundamental component. HFP functions measured psychophysically by the method of adjustment were also obtained for the adults. Adult HFP functions from the two methods were found to be essentially the same. Both of these functions were compared to Vos'-modified 2 degrees V(lambda) function and the 10 degrees CIEV(lambda) function. The mean adult data were slightly better fit to the 2 degrees V(lambda) function than to the 10 degrees CIEV(lambda) function, although there was an elevation in sensitivity at 420 and 440 nm. Infant HFP functions were similar to Vos' modified V(lambda) except for an elevation in efficiency at short wavelengths. The mean infant HFP function agreed better with the 10 degrees CIEV(lambda) function than Vos'-modified V(lambda) function, but infant sensitivity was elevated by 0.4 log units at 420 nm compared to the 10 degrees CIE observer. The elevation found at short wavelengths for both adults and infants is attributed to individual and age-related variation in the density of the ocular media, and to reduced macular pigment screening resulting from use of a 5 degrees field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Infant visual development: an overview of studies using visual evoked potential measures from Harter to the present

Studies of sensory and perceptual abilities in infants require creative, innovative techniques. Although the young infant's response repertoire may appear limited to the naive individual, a number of highly refined procedures have been developed and implemented with these non-verbal humans over the last twenty years. The most successful protocols for evaluating visual development rely either on behavioral responses or on electrophysiological recordings. The first published report using visual evoked potentials to study the development of pattern vision in human infants was presented by M. Russell Harter. This work provided the impetus for a wealth of studies exploring issues of visual information processing abilities in early infancy. The available range of data and experimental techniques are now sufficiently refined that many clinical issues are currently being addressed. The purpose of this review is to document the evolution of scientific studies since Harter's seminal work. The selection of protocols presented focuses on those with either current clinical applications or those which hold promise for future applications in the evaluation and treatment issues of abnormal visual development.

Systematic measurement of human neonatal color vision

We used a new time-efficient method to evaluate chromatic-achromatic discrimination in newborn (n = 36) and 1-month-old (n = 34) human infants. Results showed that 74% of newborns discriminated a 10.5 x 17.5 deg broadband red patch from all relative luminances of an achromatic background, but only 14% of newborns did so with a blue, 36% with a green, and 25% with a yellow patch. Most infants who "failed" did so at relative luminances very close to the respective photopic luminance match. At 1 month, performance improved somewhat although infants still show clear evidence of discriminating only the red patch. These results, the first to be obtained from individual newborns with a method incorporating a systematic variation of luminance, imply that early color vision is very limited. Possible photoreceptoral and neural bases for these immaturities are discussed.

Visual electrophysiology to achromatic and chromatic stimuli in premature and full term infants

We have previously reported on the development of black/white pattern reversal VEPs in premature babies of more than 30 weeks post-menstrual age (PMA). Unlike the flash VEP, the pattern reversal VEP shows a similar morphology to that of the full term infant and the major positive component (P1) decreased in latency with increasing PMA. The N1 and N2 components were more likely to be present with increasing maturity. In our present study we are examining the development of the transient chromatic pattern VEP. In order to produce a purely chromatic stimulus it is necessary to remove luminance cues. Based on forced choice preferential looking we developed a method of determining the isoluminant point for infants. Preference was tested for a flickering sinusoidal red and green grating over the uniform field. As sensitivity for chromatic flicker is much poorer than for luminance flicker, sensitivity is expected to be least when the residual luminance variation in the stimulus is at a minimum. The red/green luminance ratio at which this occurs represents the isoluminant point. From this method we found the subjective isoluminant point for infants of 2-3 months of age to be very close to the objective measure of isoluminance. Using this information, pattern reversal VEPs to 20 chromatic red/green and achromatic checks were studied and it would appear that pattern reversal VEPs cannot be obtained to isoluminant stimuli before 7 weeks chronological age.

Development of contrast sensitivity and acuity of the infant colour system

We have monitored the development of infant colour vision by measuring chromatic contrast sensitivity and acuity in eight young infants over a period of 6 months. Steady-state visual evoked potentials (VEPS) were recorded in response to both chromatic (red-green) and luminance (red-black or green-black) patterns that were reversed in contrast over time. For most infants, no response could be obtained to chromatic stimuli of any size or contrast before 5 weeks of age, although luminance stimuli of 20% contrast gave reliable responses at that age. When responses to chromatic stimuli first appeared, they could be obtained only with stimuli of very low spatial frequency, 20 times lower than the acuity for luminance stimuli. Both contrast sensitivity and acuity for chromatic stimuli increased steadily, more rapidly than for luminance stimuli. As the spectral selectivities of infant cones are similar to those of adults, the difference in rate of development of luminance and chromatic contrast sensitivity and acuity stimuli probably reflects neural development of the infant colour system.

Development of visual sensitivity to light and color vision in human infants: a critical review

The recent literature on test threshold and color vision in human infants is critically reviewed. Test thresholds are higher in infants than in adults at the absolute threshold and at all adapting luminances, but approach adult values rapidly over the first six months of life. The spectral luminous efficiency function of infants is similar to V(lambda) above 1.0 log phot. cd/m2. For lights below about 1.0 log scot. cd/m2 and wavelengths shorter than 590 nm, the luminous efficiency function is similar to V'(lambda). The luminous efficiency of any given stimulus may differ markedly between infants and adults, especially when the adult data depend on the temporal and spatial parameters of the stimuli. Color vision improves greatly over the first three postnatal months, and most normal 3-month olds have at least some color vision. The overall insensitivity of infants to contrast is likely to provide a satisfactory explanation of the poor color vision of infants. The critical immaturity primarily responsible for the high thresholds and poor color vision of infants is probably after the site of visual adaptation, although lower-level factors may also play a role.

Visual acuity of human infants at scotopic, mesopic and photopic luminances

Visual acuity of 2-month-old infants and adults was measured between -2.6 and +2.7 log10 cd/m2 using standard psychophysical techniques. The acuity-vs-luminance curve was similar for infants and adults: acuity improved with increasing luminance until 0.0 log cd/m2, and was constant above that luminance. However, the infant curve was slightly shallower than the adult curve, and infant acuity was over 3.5 octaves poorer than adult acuity at all luminances. The infant acuity-vs-luminance curve was compared with adult curves measured 21 degrees and 51 degrees from fixation, and at 21 degrees with +8, +/- 3.25 and 0.0 D of experimental refractive error. The results indicated that infants' poor acuity cannot be fully explained by: refractive error, foveal immaturity, scotopic detection of stimuli, functional similarity between infant vision and adult vision in the periphery, or by an overall reduction in visual sensitivity.

Isolation of short-wavelength-sensitive cone photoreceptors in 4-6-week-old human infants

Spectral sensitivity was measured for nine infants, 4-6 weeks of age, and three adults under conditions of chromatic adaptation chosen to reveal the presence of short-wavelength-sensitive cones. Monochromatic test stimuli (400-550 nm) were presented at 2 Hz superimposed on a broadband, yellow background. Following 4 min of adaptation to the background, test stimuli were presented while recording the steady-state, visually-evoked cortical potential (VECP). Response averages were obtained for several radiance levels at each test wavelength, and the amplitude of the fundamental frequency was extracted from the digitized response with a fast-Fourier transform. These data were used to construct response vs intensity functions for each wavelength. A fixed criterion response was chosen from the latter family of functions to generate individual spectral sensitivity curves. These VECP spectral sensitivity functions matched the psychophysically-determined functions of adults, measured by the method of adjustment and with the same stimulus configuration. Peak sensitivity for infants and adults under these conditions occurred at about 440 nm, and the main lobe of the curve (400-500 nm) was well fitted by the Vos-Walraven short-wavelength cone fundamental. The only major difference between the infant and adult data was in the relative sensitivity of the secondary mode of the curves (above 500 nm). These results demonstrate the presence of short-wavelength-sensitive cones and a functional pathway to the visual cortex by 4-6 weeks of age.

Newborns' discrimination of chromatic from achromatic stimuli

Two experiments assessed newborns' ability to discriminate chromatic from achromatic stimuli. In Experiment 1, newborns differentiated gray from green, from yellow, and from red: For each of these hues they preferred chromatic-and-gray checkerboards over gray squares matched in mean luminance, even though the luminance of the gray checks was varied systematically over a wide range so as to minimize nonchromatic cues. However, newborns showed no evidence of differentiating gray from blue: At some luminances they showed no preference for a blue-and-gray checkerboard over a gray square. In Experiment 2, newborns differentiated red from gray but appeared not to differentiate blue from gray: Following habituation to a series of gray squares of varying luminance, they looked longer at a red square than at a gray square of novel luminance but showed no such pattern of recovery to a blue square. The results imply that newborns have some, albeit limited, ability to discriminate chromatic from achromatic stimuli and hence, that they are at least dichromats.

Sensitivity of the human neonate to short- and long-wavelength stimuli

Increment thresholds for long- and short-wavelength stimuli were determined under both white and orange adaptation. The forced-choice preferential looking technique (FPL) was used to test neonates shortly after birth and again at 2 months postnatal. Results were compared to an adult control group. Neonates, 2-month-olds, and adults showed a change in relative sensitivity to the 617 and 453 nm test stimuli with a change in chromatic adaptation, indicating the existence of at least 2 separately adaptable chromatic mechanisms at birth. These results are tentatively discussed in terms of the functioning of rods and short-, middle- and long-wavelength cones. A decrease in all thresholds with age is discussed with regard to attentional differences and postnatal neural development in the visual system.

Tritan discriminations by 1- and 2-month-old human infants

The capacity of 1- and 2-month-old infants to make a tritan discrimination between a 4 degree, 416 nm test field and a 547 nm surround was tested by means of the forced-choice preferential looking technique. Most of the 2-month-olds and the other 1-month-olds made the tritan discrimination and must therefore have functional SWS cones. Most of the youngest 1-month-olds failed to make the tritan discrimination and therefore either do not encode or do not preserve the information ordinarily encoded by SWS cones. The implications of these data and the prior data of Hamer et al. [Vision Res. 22, 575-587 (1982)] are discussed in relation to color theory.

Rayleigh discriminations in young human infants

The capacity of young infants to discriminate 3 x 3 degrees broadband red or 550 nm green squares from a 589 nm yellow surround was tested by means of the forced-choice preferential looking technique. All 3-month olds, about 3/4 of the 2-month olds, and just under half of the one-month-olds could make at least one of these discriminations. Taken together with other known properties of infant color vision, the failures of discrimination shown by the younger infants are more readily modeled as immaturities of neural processing than as an absence of anomaly of LWS or MWS cones.

Scotopic vision, color vision, and stereopsis in infants

Infants as young as 1 month postnatal have scotopic spectral sensitivity closely resembling the CIE standard curve and thus appear to have a functional, rhodopsin-based rod system. Neutrally adapted photopic spectral sensitivities are broad and roughly equal to those of adults tested under similar conditions, although infants may show an elevated sensitivity in the short-wavelength region of the spectrum. Spectral sensitivity has been shown to change with changes in chromatic adaptation. Some wavelength discriminations are possible under conditions in which brightness artifacts can be ruled out. All of these factors support the conclusion that young infants are at least dichromats. However, 2-month-olds to date have shown some unpredicted failures of chromatic discrimination (for example, yellow-greens from white), and the percentage of infants who demonstrate discriminations (for example, among wavelengths greater than or equal to 550 nm) improves with age in early infancy. Thus the status of young infants as trichromats is not yet clearly established. Stereopsis develops rapidly during months 3 to 7, and stereoacuities as good as 1' have been reported by 5 months postnatal. In summary, a variety of visual capabilities can now be measured in infants. Infants' visual capacities are rather surprisingly well established shortly after birth and undergo clear improvements during the first few postnatal months. The interpretation of developmental data and their bearing on visual theory are discussed.

Visually evoked potentials: theory, techniques and clinical applications

The visually evoked potential (VEP), the recording of which has recently been made possible by the development of computer averaging techniques, is a gross electrical signal generated by the occipital region of the cortex in response to visual stimulation. It is more specific than the electroencephalogram (EEG) and more sensitive to changes in the visual stimulus; thus, it can provide ophthalmologists and vision researchers with information about the human visual system that is unavailable by other methods. Clinically, the VEP is of special value in the areas of refraction, infant acuity, diseases of the optic nerve, color blindness, amblyopia and field defects. Theory, techniques and instrumentation are described, and applications of the VEP to clinical situations and to vision research are discussed.