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ASSESSMENT OF VISION IN INFANTS
A major difficulty in assessment of vision in infants is that they cannot be tested
with the standard tools that are used with adults. A second difficulty is that studies
have shown that even normal infant vision is greatly inferior to that of normal
adults. Thus, adult standards are not appropriate for use with infants. A third
difficulty in determining the visual status of infants is that their vision is not static;
it generally improves rapidly during the first postnatal year. In both normal and
visually at-risk infants, the time course of the measured improvement in vision
depends on both the assessment technique used and the aspect of vision that is
being assessed. Finally, assessment of vision in infants is complicated by the fact
that evidence of normal or abnormal visual status at one age is not necessarily
predictive of what the visual status will be at a later age. That is, visual
development during infancy is highly plastic and can be interrupted or modified by
either external or internal environmental factors.
Because of the immaturity of the infant's visual system and the dynamic nature of
visual development during the first postnatal months, any program for the
assessment of visual status in infants must recognize two important points. First,
the results of visual assessment must be compared with normative data from
infants of the same age, tested with the same assessment tool. Comparing results to
norms based on data from adults or older children or to infants tested with a
different procedure can lead to a misdiagnosis of visual impairment. Second,
results of visual assessments conducted during infancy are not necessarily
predictive of visual status later in life. An infant whose vision appears normal early
in life may later show visual impairment if the visual system fails to undergo the
considerable amount of development that normally occurs between infancy and
adulthood. Similarly, some infants who appear visually impaired early in life show
normal visual responses several weeks or months later.
VISUAL ACUITY
Visual acuity is a measure of the finest detail that can be resolved or recognized by
the visual system. Visual acuity can be reduced by the optical blur produced by
imperfect optics of the eye (refractive error), which can be corrected by spectacle
or contact lens correction, or it can be reduced by neural deficits, which cannot be
corrected optically. Because visual acuity deficits due to refractive error are
correctable and therefore do not result in a disability, visual acuity assessment
should be conducted with the individual wearing best optical correction. For adults,
best correction is typically evaluated by manifest refraction, in which the adult
judges which lenses produce optimal ability to read an eye chart. For infants, very
young children, and multiply handicapped individuals with who manifest
refraction cannot be performed, the estimate of best correction must be made using
objective techniques, such as autorefraction or retinoscopy.
Assessment methods:
Fixation and Following
In most clinical settings, the eye care practitioner makes a qualitative assessment of
an infant's vision, based on his or her ability to show steady fixation of a target and
to follow the target using smooth pursuit movements. However, the ability to fix
and follow does not necessarily indicate normal visual acuity, since many older
children with 20/200 or worse visual acuity fix and follow well (Day, 1990).
Similarly, failure to show normal fixation and following shortly after birth is not
necessarily predictive of a later visual deficit but may simply be an indicator of
delayed visual maturation (Fielder et al., 1985; Illingworth, 1961).
Visual Evoked Potential
The visual evoked potential (VEP, also called the visual evoked response or VER)
is an electrical signal generated by the occipital cortex of the brain in response to
visual stimulation. It is recorded through one or more electrodes placed on the
scalp over the visual cortex. Visual acuity can be estimated by recording VEP
responses to patterned stimuli, such as phase-alternating, black and white gratings,
in which the overall luminance of the target remains constant but the spatial
configuration of the pattern changes. Typically, as the size of the pattern elements
decreases, the amplitude of the VEP decreases, with the result that the visual acuity
threshold can be estimated as the finest grating or the smallest check size that
results in a measurable VEP (for details of recording and scoring techniques, see
Norcia, 1994). Normative data are available for VEP acuity for infants between
birth and age 1 year (McCulloch et al., 1999; Norcia & Tyler, 1985). However, use
of the VEP for measurement of visual acuity in individual infants has been limited
to a relatively small number of clinical sites, undoubtedly due to the expense of the
equipment and the technical expertise required conducting the test.
The advantages of using the pattern VEP for measurement of visual acuity in
infants are several:
(1) Measurements can be made quickly, within a time span over which most
infants will remain cooperative and will fixate on the stimulus.
(2) The procedure requires minimal response from the infant.
(3) The VEP can be a good indicator of macular function, since it is generated
primarily by the area of visual cortex that receives input from the macular region.
(4) Data on the distribution of acuity results in normal infants of different ages are
available, making it possible to interpret an infant's visual acuity score in terms of
number of standard deviations below normal.
There are limitations on the pattern VEP for assessment of visual acuity in infants:
(1) The testing equipment is expensive and not widely available
(2) Technical expertise is required for conducting the procedure and interpreting
the responses
(3) It can be difficult to obtain a measurable response from infants with such
oculomotor abnormalities as nystagmus and such neuromotor abnormalities as
cerebral palsy, which may cause muscle artifacts that obscure the visual signal
(4) Infants older than 9 months may resist having electrodes attached.
Forced-Choice Preferential Looking (FPL)
The basis of the forced-choice preferential looking procedure is that infants show
preferential fixation of a patterned stimulus in comparison to a homogeneous field.
Thus, visual acuity can be measured by observing an infant's eye movement
responses to black and white gratings paired with a gray stimulus matched to the
space-averaged luminance of the gratings.
The version of the procedure that is commercially available and is most widely
used to measure visual acuity in infants is the acuity card procedure (Teller et al.,
1986). In this procedure, the tester shows the infant a series of gray cards, each
containing a black and white grating on the left or the right of a central, small
peephole. Prior to testing, the cards are arranged in a stack, face-down, proceeding
from coarser to finer gratings. The tester presents each card to the infant several
times, usually rotating the card by 180° to change the left-right position of the
grating from presentation to presentation. The tester, who does not know the
location of the grating on each card, watches the infant's response through the
peephole and decides, based on the infant's eye movements and looking behavior
in response to repeated presentations of the card, whether the infant can resolve the
grating and, if so, the location (left-right position) of the grating. After this
decision has been made for a card, the tester is permitted to look at the card to
confirm the location of the grating.
An infant's visual acuity is scored as the finest grating that the tester judges that he
or she can resolve. Normative data have been published for the acuity card
procedure for both binocular and monocular testing of infants between birth and 1
year, as well for young children up to 3 to 4 years of age
The acuity card procedure has been used successfully in a wide range of clinical
settings to assess grating acuity in visually at-risk infants. In a multicenter study of
cryotherapy for retinopathy of prematurity (CRYO-ROP), the acuity card
procedure was used to measure acuity in more than 1,300 1-year-old infants with
birth weights less than 1,251 g, two-thirds of whom developed retinopathy of
prematurity in the perinatal period (Dobson et al., 1994). In the multicenter Ross
Pediatric Lipid Study, the acuity card procedure was used to test vision
longitudinally between ages 2 and 12 months in 197 infants, in order to evaluate
the effects of diet on visual function and growth (Auestad et al., 1997). There are
numerous single-center reports of the successful use of the acuity card procedure to
evaluate visual status in infants with ocular or neurodevelopmental abnormalities.
The advantages of using the acuity card procedure for measurement of visual
acuity in infants are that:
(1) Measurements can be made quickly, within a time span over which most
infants will remain cooperative and will fixate the stimulus.
(2) The procedure allows the tester to interact with the infant visually between card
presentations, which helps to maintain the infant's interest in the testing procedure.
(3) The procedure relies on the infant's natural eye movement responses to a
patterned stimulus.
(4) The procedure is easy to learn.
(5) The cost of the equipment is relatively low.
(6) The procedure can be used with infants of all ages, as well as with children
whose developmental age is that of an infant.
(7) With modifications in the positioning of the cards, the procedure can be used to
test infants with oculomotor abnormalities, such as nystagmus.
(8) Data are available on the distribution of acuity results in normal infants of
different ages, making it possible to interpret an infant's visual acuity score in
terms of number of standard deviations below normal.
There are limitations on the acuity card procedure for assessment of visual acuity
in infants:
(1) Results depend on the integrity of the tester in remaining masked to the
location of the gratings on the cards during their presentation.
(2) Cards must be kept free of dirt and smudges that could attract the infant's
attention away from the grating target.
(3) Grating acuity may underestimate recognition (letter) acuity loss in infants with
strabismic amblyopia or macular disease.
(4) Variability of acuity scores in normal infants is greater than that reported in
VEP studies of normal infants—approximately 0.2 log unit for acuity cards.
VISUAL FIELDS
The visual field is typically assessed using small spots of light that are illuminated
briefly at various peripheral locations (static perimetry) or are moved inward from
the periphery (kinetic perimetry) while the subject fixates on a central target.
However, standard static perimetry techniques are difficult to use with children
younger than about 8 years of age, and adult kinetic perimetry procedures typically
cannot be used with children younger than 5 or 6 years of age.
In children, as in adults, severely restricted visual fields can have a detrimental
effect on an individual's mobility, ability to read or benefit from visually presented
information, and ability to interact socially. There is a long history of using
perimetry and visual field testing to evaluate the status of peripheral vision in
adults in both clinical and research settings. Automated static perimetry is
available in the offices of most eye care practitioners, and the limitations of
restricted visual field extent and of nonseeing areas within the visual field have
been widely studied. For children who are old enough cognitively to be tested in a
standard adult perimeter, the results of testing can provide an accurate indicator of
visual field restrictions. Quantitative techniques for evaluating visual fields in
younger children and infants, however, are available only in a small number of
research and clinical settings. Thus, at the current time, quantitative evaluation of
visual fields in infants and young children is not a practical means of evaluating
disability in infants and preschool-age children.
Assessment methods:
Confrontation Techniques
Quantitative perimetry is not widely available for assessment of visual fields in
infants. Therefore, assessment of large visual field deficits in infants is usually
made using confrontation techniques. The examiner faces the infant and attracts his
or her attention centrally. Then an assistant introduces a toy or a light into the far
periphery, and the examiner watches to see if the infant makes a rapid eye or head
movement in the direction of the peripherally presented toy or light. A deficit that
can be detected by this method is likely to be functionally significant in the future
(Day, 1990).
White Sphere Kinetic Perimetry
Techniques for quantitative perimetry in infants are available, but their use has
been primarily in research settings. The most widely used is the white sphere
kinetic perimetry procedure (Mohn & van Hof-van Duin, 1986), in which an infant
is induced to fixate on a centrally located white sphere while an assistant moves a
second white sphere centrally from the far periphery along one of the arms of a
single- or double-arc black perimeter. An observer hidden behind a black curtain
watches to make sure that the infant is looking centrally at the beginning of each
trial and indicates when the infant makes an eye movement away from center. The
location of the peripheral white sphere when the infant makes an eye movement
toward that target is used as an estimate of visual field extent along that perimeter
arm. Normative data, available for infants between birth and 12 months of age,
indicate that a gradual enlargement of the measured visual field from
approximately 30° in each direction to nearly adult levels occurs during this time
period (Mohn & van Hof-van Duin, 1986; van Hof-van Duin et al., 1992).
Overall, the advantages of white sphere kinetic perimetry include the availability
of normative data against which to compare the results of visually at-risk infants,
the use of relatively simple equipment, the ease with which the procedure can be
used with any infant who has sufficient vision to fixate on a central target, and the
quantitative nature of the test results. Disadvantages include the lack of availability
of testing equipment in most clinical settings, the need for two adults (an observer
plus an assistant to present the peripheral target), the imprecision of the test results
due to the limited attention span of the infant for repeated presentations of the
peripheral target, and the continued presence of a central target, which may
interfere with some infants' ability to respond when the peripheral target is
presented.
Static Perimetry
Several research labs have conducted studies of infants using static perimetry, in
which the infant's eye movement responses are observed when a stationary
stimulus is presented at different locations in the peripheral field (Lewis & Maurer,
1992; Harvey et al., 1997c). The advantages of static perimetry include the ability to
extinguish the central fixation target during presentation of the peripheral stimulus,
as well as the ability to identify precisely the location of the peripheral target when
it was looked at by the infant. The major disadvantage of static perimetry is that
strategies have not yet been devised for eliciting enough trials from an individual
infant to quantify that infant's visual field status (Maurer & Lewis, 1991).
CONTRAST SENSITIVITY
In adults, contrast sensitivity is measured by determining the least amount of
contrast an individual needs to detect a difference in luminance between adjacent
parts of a pattern. Laboratory studies have used measurements of contrast
sensitivity in infants to produce a simulated view of what various patterns and
scenes look like to an infant (Banks & Salapatek, 1981; Teller, 1997). However, there are
no widely available, normed and validated tools for assessment of contrast
sensitivity in infants or preschool-age children.
While visual acuity provides a measure of the finest detail that an individual can
resolve, results of contrast sensitivity testing provide information on the
individual's ability to detect patterns of all sizes, and thus they provide a more
complete description of an individual's visual environment than can be obtained
from a visual acuity score. Because the world of the infant and young child is built
around global perceptions, rather than attention to fine detail as is required in
reading, it is likely that assessment of contrast sensitivity would provide a more
accurate estimate of an infant's or young child's ability to function visually than
would a measure of visual acuity. However, the development of techniques for
assessing contrast sensitivity in infants and young children has lagged far behind
the development of techniques for assessing visual acuity. Therefore, at this time,
visual acuity is the only aspect of spatial vision that can be assessed in a child too
young to be tested with adult measures of contrast sensitivity.
Currently, methods to assess contrast sensitivity in adults require the individual to
identify low contrast letters or to indicate, for a series of black and white gratings,
the lowest contrast at which each pattern is detectable. Use of letters in the first
type of test and the need for a large number of trials in the second type of test
prevent either from being useful in the assessment of preschool-age children and
infants.
Assessment method:
Although both normal infants (Adams et al., 1992) and infants with Down syndrome
(Courage et al., 1997) have been tested successfully with an acuity card type of
contrast sensitivity test and normative data are available for infants (Adams &
Courage, 1996), the test is not ready for widespread use, due to poor test-retest
reliability, long test times, and lack of commercial availability (Adams et al.,
2000).
Measurement of contrast sensitivity in infants is also possible using the pattern
VEP, and initial normative data are available (Norcia et al., 1990). However, it is
unlikely that this technique will achieve widespread use, due to the expense of the
equipment and the technical expertise required to interpret the results.

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assessment of vision in infants by optom faslu muhammed

  • 1. ASSESSMENT OF VISION IN INFANTS A major difficulty in assessment of vision in infants is that they cannot be tested with the standard tools that are used with adults. A second difficulty is that studies have shown that even normal infant vision is greatly inferior to that of normal adults. Thus, adult standards are not appropriate for use with infants. A third difficulty in determining the visual status of infants is that their vision is not static; it generally improves rapidly during the first postnatal year. In both normal and visually at-risk infants, the time course of the measured improvement in vision depends on both the assessment technique used and the aspect of vision that is being assessed. Finally, assessment of vision in infants is complicated by the fact that evidence of normal or abnormal visual status at one age is not necessarily predictive of what the visual status will be at a later age. That is, visual development during infancy is highly plastic and can be interrupted or modified by either external or internal environmental factors. Because of the immaturity of the infant's visual system and the dynamic nature of visual development during the first postnatal months, any program for the assessment of visual status in infants must recognize two important points. First, the results of visual assessment must be compared with normative data from infants of the same age, tested with the same assessment tool. Comparing results to norms based on data from adults or older children or to infants tested with a different procedure can lead to a misdiagnosis of visual impairment. Second, results of visual assessments conducted during infancy are not necessarily predictive of visual status later in life. An infant whose vision appears normal early in life may later show visual impairment if the visual system fails to undergo the considerable amount of development that normally occurs between infancy and
  • 2. adulthood. Similarly, some infants who appear visually impaired early in life show normal visual responses several weeks or months later. VISUAL ACUITY Visual acuity is a measure of the finest detail that can be resolved or recognized by the visual system. Visual acuity can be reduced by the optical blur produced by imperfect optics of the eye (refractive error), which can be corrected by spectacle or contact lens correction, or it can be reduced by neural deficits, which cannot be corrected optically. Because visual acuity deficits due to refractive error are correctable and therefore do not result in a disability, visual acuity assessment should be conducted with the individual wearing best optical correction. For adults, best correction is typically evaluated by manifest refraction, in which the adult judges which lenses produce optimal ability to read an eye chart. For infants, very young children, and multiply handicapped individuals with who manifest refraction cannot be performed, the estimate of best correction must be made using objective techniques, such as autorefraction or retinoscopy. Assessment methods: Fixation and Following In most clinical settings, the eye care practitioner makes a qualitative assessment of an infant's vision, based on his or her ability to show steady fixation of a target and to follow the target using smooth pursuit movements. However, the ability to fix and follow does not necessarily indicate normal visual acuity, since many older children with 20/200 or worse visual acuity fix and follow well (Day, 1990). Similarly, failure to show normal fixation and following shortly after birth is not necessarily predictive of a later visual deficit but may simply be an indicator of delayed visual maturation (Fielder et al., 1985; Illingworth, 1961). Visual Evoked Potential The visual evoked potential (VEP, also called the visual evoked response or VER) is an electrical signal generated by the occipital cortex of the brain in response to visual stimulation. It is recorded through one or more electrodes placed on the scalp over the visual cortex. Visual acuity can be estimated by recording VEP responses to patterned stimuli, such as phase-alternating, black and white gratings,
  • 3. in which the overall luminance of the target remains constant but the spatial configuration of the pattern changes. Typically, as the size of the pattern elements decreases, the amplitude of the VEP decreases, with the result that the visual acuity threshold can be estimated as the finest grating or the smallest check size that results in a measurable VEP (for details of recording and scoring techniques, see Norcia, 1994). Normative data are available for VEP acuity for infants between birth and age 1 year (McCulloch et al., 1999; Norcia & Tyler, 1985). However, use of the VEP for measurement of visual acuity in individual infants has been limited to a relatively small number of clinical sites, undoubtedly due to the expense of the equipment and the technical expertise required conducting the test. The advantages of using the pattern VEP for measurement of visual acuity in infants are several: (1) Measurements can be made quickly, within a time span over which most infants will remain cooperative and will fixate on the stimulus. (2) The procedure requires minimal response from the infant. (3) The VEP can be a good indicator of macular function, since it is generated primarily by the area of visual cortex that receives input from the macular region. (4) Data on the distribution of acuity results in normal infants of different ages are available, making it possible to interpret an infant's visual acuity score in terms of number of standard deviations below normal. There are limitations on the pattern VEP for assessment of visual acuity in infants: (1) The testing equipment is expensive and not widely available (2) Technical expertise is required for conducting the procedure and interpreting the responses (3) It can be difficult to obtain a measurable response from infants with such oculomotor abnormalities as nystagmus and such neuromotor abnormalities as cerebral palsy, which may cause muscle artifacts that obscure the visual signal (4) Infants older than 9 months may resist having electrodes attached. Forced-Choice Preferential Looking (FPL) The basis of the forced-choice preferential looking procedure is that infants show preferential fixation of a patterned stimulus in comparison to a homogeneous field. Thus, visual acuity can be measured by observing an infant's eye movement
  • 4. responses to black and white gratings paired with a gray stimulus matched to the space-averaged luminance of the gratings. The version of the procedure that is commercially available and is most widely used to measure visual acuity in infants is the acuity card procedure (Teller et al., 1986). In this procedure, the tester shows the infant a series of gray cards, each containing a black and white grating on the left or the right of a central, small peephole. Prior to testing, the cards are arranged in a stack, face-down, proceeding from coarser to finer gratings. The tester presents each card to the infant several times, usually rotating the card by 180° to change the left-right position of the grating from presentation to presentation. The tester, who does not know the location of the grating on each card, watches the infant's response through the peephole and decides, based on the infant's eye movements and looking behavior in response to repeated presentations of the card, whether the infant can resolve the grating and, if so, the location (left-right position) of the grating. After this decision has been made for a card, the tester is permitted to look at the card to confirm the location of the grating. An infant's visual acuity is scored as the finest grating that the tester judges that he or she can resolve. Normative data have been published for the acuity card procedure for both binocular and monocular testing of infants between birth and 1 year, as well for young children up to 3 to 4 years of age The acuity card procedure has been used successfully in a wide range of clinical settings to assess grating acuity in visually at-risk infants. In a multicenter study of cryotherapy for retinopathy of prematurity (CRYO-ROP), the acuity card procedure was used to measure acuity in more than 1,300 1-year-old infants with birth weights less than 1,251 g, two-thirds of whom developed retinopathy of prematurity in the perinatal period (Dobson et al., 1994). In the multicenter Ross Pediatric Lipid Study, the acuity card procedure was used to test vision longitudinally between ages 2 and 12 months in 197 infants, in order to evaluate the effects of diet on visual function and growth (Auestad et al., 1997). There are numerous single-center reports of the successful use of the acuity card procedure to evaluate visual status in infants with ocular or neurodevelopmental abnormalities. The advantages of using the acuity card procedure for measurement of visual acuity in infants are that: (1) Measurements can be made quickly, within a time span over which most infants will remain cooperative and will fixate the stimulus. (2) The procedure allows the tester to interact with the infant visually between card presentations, which helps to maintain the infant's interest in the testing procedure.
  • 5. (3) The procedure relies on the infant's natural eye movement responses to a patterned stimulus. (4) The procedure is easy to learn. (5) The cost of the equipment is relatively low. (6) The procedure can be used with infants of all ages, as well as with children whose developmental age is that of an infant. (7) With modifications in the positioning of the cards, the procedure can be used to test infants with oculomotor abnormalities, such as nystagmus. (8) Data are available on the distribution of acuity results in normal infants of different ages, making it possible to interpret an infant's visual acuity score in terms of number of standard deviations below normal. There are limitations on the acuity card procedure for assessment of visual acuity in infants: (1) Results depend on the integrity of the tester in remaining masked to the location of the gratings on the cards during their presentation. (2) Cards must be kept free of dirt and smudges that could attract the infant's attention away from the grating target. (3) Grating acuity may underestimate recognition (letter) acuity loss in infants with strabismic amblyopia or macular disease. (4) Variability of acuity scores in normal infants is greater than that reported in VEP studies of normal infants—approximately 0.2 log unit for acuity cards. VISUAL FIELDS The visual field is typically assessed using small spots of light that are illuminated briefly at various peripheral locations (static perimetry) or are moved inward from the periphery (kinetic perimetry) while the subject fixates on a central target. However, standard static perimetry techniques are difficult to use with children younger than about 8 years of age, and adult kinetic perimetry procedures typically cannot be used with children younger than 5 or 6 years of age. In children, as in adults, severely restricted visual fields can have a detrimental effect on an individual's mobility, ability to read or benefit from visually presented information, and ability to interact socially. There is a long history of using
  • 6. perimetry and visual field testing to evaluate the status of peripheral vision in adults in both clinical and research settings. Automated static perimetry is available in the offices of most eye care practitioners, and the limitations of restricted visual field extent and of nonseeing areas within the visual field have been widely studied. For children who are old enough cognitively to be tested in a standard adult perimeter, the results of testing can provide an accurate indicator of visual field restrictions. Quantitative techniques for evaluating visual fields in younger children and infants, however, are available only in a small number of research and clinical settings. Thus, at the current time, quantitative evaluation of visual fields in infants and young children is not a practical means of evaluating disability in infants and preschool-age children. Assessment methods: Confrontation Techniques Quantitative perimetry is not widely available for assessment of visual fields in infants. Therefore, assessment of large visual field deficits in infants is usually made using confrontation techniques. The examiner faces the infant and attracts his or her attention centrally. Then an assistant introduces a toy or a light into the far periphery, and the examiner watches to see if the infant makes a rapid eye or head movement in the direction of the peripherally presented toy or light. A deficit that can be detected by this method is likely to be functionally significant in the future (Day, 1990). White Sphere Kinetic Perimetry Techniques for quantitative perimetry in infants are available, but their use has been primarily in research settings. The most widely used is the white sphere kinetic perimetry procedure (Mohn & van Hof-van Duin, 1986), in which an infant is induced to fixate on a centrally located white sphere while an assistant moves a second white sphere centrally from the far periphery along one of the arms of a single- or double-arc black perimeter. An observer hidden behind a black curtain watches to make sure that the infant is looking centrally at the beginning of each trial and indicates when the infant makes an eye movement away from center. The location of the peripheral white sphere when the infant makes an eye movement toward that target is used as an estimate of visual field extent along that perimeter arm. Normative data, available for infants between birth and 12 months of age, indicate that a gradual enlargement of the measured visual field from
  • 7. approximately 30° in each direction to nearly adult levels occurs during this time period (Mohn & van Hof-van Duin, 1986; van Hof-van Duin et al., 1992). Overall, the advantages of white sphere kinetic perimetry include the availability of normative data against which to compare the results of visually at-risk infants, the use of relatively simple equipment, the ease with which the procedure can be used with any infant who has sufficient vision to fixate on a central target, and the quantitative nature of the test results. Disadvantages include the lack of availability of testing equipment in most clinical settings, the need for two adults (an observer plus an assistant to present the peripheral target), the imprecision of the test results due to the limited attention span of the infant for repeated presentations of the peripheral target, and the continued presence of a central target, which may interfere with some infants' ability to respond when the peripheral target is presented. Static Perimetry Several research labs have conducted studies of infants using static perimetry, in which the infant's eye movement responses are observed when a stationary stimulus is presented at different locations in the peripheral field (Lewis & Maurer, 1992; Harvey et al., 1997c). The advantages of static perimetry include the ability to extinguish the central fixation target during presentation of the peripheral stimulus, as well as the ability to identify precisely the location of the peripheral target when it was looked at by the infant. The major disadvantage of static perimetry is that strategies have not yet been devised for eliciting enough trials from an individual infant to quantify that infant's visual field status (Maurer & Lewis, 1991). CONTRAST SENSITIVITY In adults, contrast sensitivity is measured by determining the least amount of contrast an individual needs to detect a difference in luminance between adjacent parts of a pattern. Laboratory studies have used measurements of contrast sensitivity in infants to produce a simulated view of what various patterns and scenes look like to an infant (Banks & Salapatek, 1981; Teller, 1997). However, there are no widely available, normed and validated tools for assessment of contrast sensitivity in infants or preschool-age children. While visual acuity provides a measure of the finest detail that an individual can resolve, results of contrast sensitivity testing provide information on the individual's ability to detect patterns of all sizes, and thus they provide a more
  • 8. complete description of an individual's visual environment than can be obtained from a visual acuity score. Because the world of the infant and young child is built around global perceptions, rather than attention to fine detail as is required in reading, it is likely that assessment of contrast sensitivity would provide a more accurate estimate of an infant's or young child's ability to function visually than would a measure of visual acuity. However, the development of techniques for assessing contrast sensitivity in infants and young children has lagged far behind the development of techniques for assessing visual acuity. Therefore, at this time, visual acuity is the only aspect of spatial vision that can be assessed in a child too young to be tested with adult measures of contrast sensitivity. Currently, methods to assess contrast sensitivity in adults require the individual to identify low contrast letters or to indicate, for a series of black and white gratings, the lowest contrast at which each pattern is detectable. Use of letters in the first type of test and the need for a large number of trials in the second type of test prevent either from being useful in the assessment of preschool-age children and infants. Assessment method: Although both normal infants (Adams et al., 1992) and infants with Down syndrome (Courage et al., 1997) have been tested successfully with an acuity card type of contrast sensitivity test and normative data are available for infants (Adams & Courage, 1996), the test is not ready for widespread use, due to poor test-retest reliability, long test times, and lack of commercial availability (Adams et al., 2000). Measurement of contrast sensitivity in infants is also possible using the pattern VEP, and initial normative data are available (Norcia et al., 1990). However, it is unlikely that this technique will achieve widespread use, due to the expense of the equipment and the technical expertise required to interpret the results.