Forced-choice Preferential Looking Research Articles

The Assessment of Vision in Children with Severe Learning Difficulties: A Systematic Review

The Assessment of Vision in Children with Severe Learning Difficulties: A Systematic Review

Binocular vision in infancy: Responsiveness to uncrossed horizontal disparity

This research explored the onset of stereopsis, the ability to perceive depth from the different views provided by the two eyes. In a longitudinal study, infants were tested weekly from 6 to 20 weeks of age. The primary goal of the study was to establish the onset and the early development of sensitivity to uncrossed horizontal disparity. The infant participants were shown dynamic random dot stereograms displaying two squares, one with uncrossed horizontal disparity (0.5°) and one with vertical disparity (0.5°). The stimuli were presented on an autostereoscopic monitor. We used two methods, the forced-choice preferential looking (FPL) method and the classical natural preference (CNP) method, to measure whether the infants preferred the uncrossed over the vertical disparity display. According to the FPL data, the mean relative preferences for horizontal over vertical disparity were significantly greater than chance probability (0.50) from 13 weeks of age onward. With the CNP method we found significant preferences for uncrossed horizontal disparity from 15 weeks onward. The FPL method was hence more sensitive than the CNP method as it indicated an earlier onset of responsiveness to stereoscopic information.

The onset of sensitivity to horizontal disparity in infancy: A short-term longitudinal study

In this short-term longitudinal study, infants were examined for their natural preference of a square defined by crossed horizontal disparity (either 1° or 0.5°) over a square defined by a vertical disparity (either 1° or 0.5°). The square targets were embedded in a dynamic random dot stereogram. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. The infants were tested weekly between 6 and 16 weeks of age. Four experiments were conducted. In two experiments, the infants were examined with the forced-choice preferential looking (FPL) method for their ability to perceive either 1° or 0.5° horizontal disparity. In the remaining two experiments, the classical natural preference (CNP) method (measurement of looking times) was applied. According to the results of the FPL experiments, mean relative preference for the horizontal disparity square became significant at 8 weeks of age. The CNP data indicated an onset of stereopsis at 12–15 weeks. The mean relative preferences for horizontal disparity indicated by the CNP method were smaller than those found in the FPL experiments. Thus, the FPL method was more sensitive than the CNP method in the measurement of infant responsiveness to crossed horizontal disparity.

Infants’ responsiveness to rivalrous gratings

The study investigated the early development of responsiveness to rivalrous gratings. Infants were tested weekly between 6 and 16weeks of age for their ability to discriminate between interocularly identical (fusible) lines and interocularly orthogonal (unfusible, rivalrous) lines. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. Two psychophysical techniques, the forced-choice preferential looking (FPL) method and measurement of looking times, were employed. Contrary to earlier findings, infants at all ages avoided looking at the rivalrous gratings instead of showing a developmental shift from a relative preference for unfusible, rivalrous gratings to a relative preference for fusible gratings. Avoidance of the rivalrous gratings became significant at 8–9weeks of age, suggesting that infants clearly exhibit binocular rivalry from that age onwards. Control experiments secured that the infants’ preference for the fusible gratings was not governed by a natural preference for less over more complex line patterns.

The effect of contrast on vertical motion processing asymmetries in 11-week-old infants.

Recent forced-choice preferential looking (FPL) experiments with random-dot patterns [Wattam-Bell, 1998 Investigative Ophthalmology & Visual Science 39(4) S885] found evidence for a perceptual asymmetry of vertical-motion processing in young infants: a preference for patterns that move downwards. This asymmetry was in the opposite direction to the asymmetry of vertical optokinetic nystagmus, which was biased towards upwards motion. However, the FPL bias was weak, and the object of present experiments was to explore the possibility that it could be enhanced by reducing stimulus contrast. In experiment 1, contrast thresholds for gratings moving upwards and downwards were compared, and no directional asymmetry at threshold was found. In experiment 2, the effect of contrast on infants' preference between simultaneously displayed upwards-drifting and downwards-drifting gratings was examined. Infants showed no preference at 5% contrast, a marked preference for downwards motion at intermediate contrasts (10% and 20%), and a similar but smaller preference at 40% contrast. These results suggest that the vertical-motion asymmetry is a result of differences in the gains of directionally selective mechanisms for upwards and downwards motion.

A randomized controlled trial of early dietary supply of long‐chain polyunsaturated fatty acids and mental development in term infants

The effects of dietary docosahexaenoic acid (DHA) supply during infancy on later cognitive development of healthy term infants were evaluated in a randomized clinical trial of infant formula milk supplemented with 0.35% DHA or with 0.36% DHA and 0.72% arachidonic acid (AA), or control formula which provided no DHA or AA. Fifty‐six 18‐month‐old children (26 male, 30 female) who were enrolled in the trial within the first 5 days of life and fed the assigned diet to 17 weeks of age were tested using the Bayley Scales of Infant Development, 2nd edition (BSID‐II) (Bayley 1993) at the Retina Foundation of the Southwest, Dallas, TX. These children had also been assessed at 4 months and 12 months of age for blood fatty‐acid composition, sweep visual evoked potential (VEP) acuity, and forced‐choice preferential looking (FPL) acuity (Birch et al. 1998). Supplementation of infant formula with DHA+AA was associated with a mean increase of 7 points on the Mental Development Index (MDI) of the BSID‐II. Both the cognitive and motor subscales of the MDI showed a significant developmental age advantage for DHA‐ and DHA+AA‐supplemented groups over the control group. While a similar trend was found for the language subscale, it did not reach statistical significance. Neither the Psychomotor Development Index nor the Behavior Rating Scale of the BSID‐II showed significant differences among diet groups, consistent with a specific advantage of DHA supplementation on mental development. Significant correlations between plasma and RBC‐DHA at 4 months of age but not at 12 months of age and MDI at 18 months of age suggest that early dietary supply of DHA was a major dietary determinant of improved performance on the MDI.

FPL and VEP measures of fusion, stereopsis and stereoacuity in normal infants

Dynamic random dot fusion, stereopsis and stereoacuity were evaluated in 149 healthy, fullterm infants, using both forced-choice preferential looking (FPL) and steady-state visual evoked potential (VEP) protocols. Few infants aged 2–3 months demonstrated fusion or stereopsis in either the FPL or VEP protocol; most infants aged 5 months and older demonstrated fusion and stereopsis in both protocols. Both FPL and VEP stereoacuity approached adult-level (<60 sec) by 6–7 months of age. Both infants and adults exhibited non-monotomic VEP amplitude vs disparity functions with a step change in phase at an intermediate disparity, consistent with separate fine and coarse disparity mechanisms.

Infant contrast detectors are selective for direction of motion

In order to investigate the presence of directionally selective mechanisms in 3-month-old infants, we employed a summation-near-threshold paradigm previously developed for studies of adult vision (Levinson & Sekuler, 1975 Journal of Physiology (London), 250, 347–366); Watson, Thompson, Murphy & Nachmias, 1980 Vision Research, 20, 341–347). The degree of contrast summation occurring between two sinusoidal gratings moving in opposite directions was determined by comparing the contrast threshold for a compound stimulus (a counterphase-reversing grating) with the contrast threshold for one of its components (a single moving grating). Using the forced-choice preferential looking (FPL) technique, contrast thresholds were obtained for both counterphase and single moving gratings within individual infant subjects. Data were collected at several speeds, ranging from 2.8 to 66.8°/sec (temporal frequency range: 0.7–16.7 Hz). At slow speeds, infants' thresholds were approximately equal for counterphase and moving gratings, indicating that non-directional mechanisms were responsible for detection. At an intermediate speed (22.3°/sec), thresholds were nearly twice as high for counterphase gratings as for single moving gratings, indicating the existence of directionally selective mechanisms at detection threshold for this speed. For faster speeds, relative thresholds for the two types of stimuli fell between the two extremes; a model incorporating probability summation between directionally selective mechanisms was sufficient to account for the data. These results demonstrate that, at speeds greater than or equal to 22.3°/sec (t.f. = 5.6 Hz), 3-month-old infants possess directionally selective mechanisms at threshold.

The robustness of infants' detection of visual motion

Fourteen-week-olds were tested for detection of the movement of a small vertical bar with varying levels of visual complexity using a speeded version of the forced-choice preferential looking (FPL) paradigm. The target bar horizontally oscillating at temporal frequencies of 0.6, 1.2 or 2.4 Hz could appear amidst 1, 5, or 13 other static bars. Detection accuracy was (a) generally unaffected by the complexity of the visual field, (b) affected by the distance of the moving bar from the center of the screen, and (c) affected directly by temporal frequency. Response times depended inversely on temporal frequency. When the similarity of the static elements was decreased by making them chromatically heterogeneous, the FPL observer's average response time increased, although detection accuracy remained the same. The FPL observer's response times actually decreased slightly as more heterogeneous distractors were added. Adults tested using an FPL observer also showed this same effect. Both infants and adults showed disproportionately long response times on incorrect trials when only 1 other distractor was present in the visual field. The number of static elements in the visual field and their similarity affected primarily the speed with which infants at this age responded to movement.

Intrinsic contrast noise and infant visual contrast discrimination

In these experiments, the amounts of intrinsic contrast noise and contrast-dependent noise were independently estimated from optokinetic nystagmus (OKN) contrast discrimination thresholds in 7-week-old human infants and adults. This report shows that the poor contrast sensitivity of infants occurs mostly because there is about 106 times more intrinsic contrast noise in the infant visual system than in the adult visual system. The intrinsic contrast noise is combined nearly linearly with the visual signal in infancy, whereas noise and visual signal are combined in a highly non-linear way for adults. The standard deviation of the visual signal differs between infants and adults by a factor of about 2 for OKN data and a factor of 13 for behavioral data. The large amount of intrinsic contrast noise found by this analysis is quantitatively consistent with data obtained by forced-choice preferential looking (FPL), and visually evoked potential (VEP) contrast sensitivity is correctly predicted to be near adult values in infancy.

Contrast Sensitivity in 24- and 36-Month-Olds as Assessed with the Contrast Sensitivity Card Procedure

We used a new card procedure to evaluate binocular contrast sensitivity (CS) in 24- and 36-month-old children. The test consists of 40 large (50 by 28 cm) matteboard cards, each of which contains a sine wave grafting with 1 of 5 spatial frequencies (0.4, 0.8, 1.6, 3.2, and 4.8 cpd at 80 cm) and with 1 of 8 contrast levels [from 33% (CS = 3) to 0.4% (CS = 260)]. Estimates of subjects' CS to each spatial frequency were obtained with a modified forced-choice preferential looking (FPL) technique similar to that developed for the Teller Acuity Cards (TAC). All 36-month-olds and 74% of the 24-month-olds completed the entire procedure in an average of 12 min per child. For both groups, the shape of the mean constant sensitivity functions (CSF's) resembled the characteristic inverted U shown by normal adults, except that the toddlers' functions were shifted to lower contrasts and spatial frequencies. Combined with previous results, it appears that the CS cards hold promise as a time-efficient means of assessing CS in infants and young children, and may provide the prototype for a clinical tool to help detect early visual and neurological dysfunction.

The effects of luminance on FPL and VEP acuity in human infants

Grating acuity was measured in 16-week-old human infants. Three measurement techniques were used: forced-choice preferential-looking (FPL), and two visual-evoked-potential (VEP) techniques. The stimuli were counterphase flickering sinewave gratings with a space-average luminance of -1.0 or 2.0 log cd/m2. Slightly different luminance-dependent changes occur between FPL and VEP acuities, suggesting that some factor influences the two methods differently as stimulus luminance varies. A comparison between FPL acuities and VEP acuities within infants suggests a quantitative relationship between techniques. Infant's acuity for sinewave gratings with a space-average luminance of -2.0, -1.0, 0.0, 1.0 and 2.0 log cd/m2 was also measured using a single VEP paradigm. The results are compared to the same measurements in adults and to infant and adult ideal observers. VEP acuity in this group of infants improves by about 0.5 log units between -2.0 and 0.0 log cd/m2 and remains asymptotic between 0.0 and 2.0 log cd/m2. This result suggests that luminance-dependent changes in infant acuity cannot be fully accounted for by immaturities in the optics and photoreceptor spacing and efficiency.

Visual acuity from birth to 5 months as measured by habituation: A comparison to forced-choice preferential looking

Newborns, 1-, 3-, and 5-month-olds were habituated to a homogeneous gray square and then tested for recovery to square-wave gratings of varying spatial frequency. The finest grating that elicited recovery was 0.43 c/deg for newborns, 0.63 c/deg for 1-month-olds, 2.73 c/deg for 3-month-olds, and 3.75 c/deg for 5-month-olds. These results, similar to those obtained with forced-choice preferential looking (FPL), are indicative of rapid development of visual acuity in the first 5 months.

Validation of the Acuity Card Procedure for Assessment of Infants with Ocular Disorders

The acuity card procedure has been shown to be a rapid method for the assessment of monocular and binocular grating acuity in normal infants from birth through 36 months of age. The current study seeks to validate the procedure further by using the acuity cards to assess 20 2- to 8-month-old infant patients with ocular disorders, including aphakia, strabismus, ptosis, and orbital hemangioma. Assessments were made with the acuity cards by two different observers, both blind to the infant's diagnosis, and by a third observer using a traditional forced-choice preferential looking (FPL) procedure. One hundred percent of the infant patients completed both binocular and monocular acuity card testing in an average time of 8 minutes per test. Interobserver agreement between acuity card observers and inter-technique agreement were high, and were sustained in individual cases in which the infant's acuity was not predictable from its visible signs. These results help to establish the potential clinical utility of the acuity card procedure for the assessment of infant patients.

A microprocessor-driven one-operator forced-choice preferential looking apparatus.

Forced-choice preferential looking (FPL) is a behavioural technique for the estimation of visual acuity in preverbal children. We have constructed an electromechanical FPL apparatus controlled by a microprocessor requiring just one operator. Sixty-three babies aged between six weeks and eleven months were evaluated with an overall success rate of 83% in obtaining an acuity measurement. FPL is a practical method for measurement of infant visual acuity in the ophthalmic clinic setting.

Visual Acuity of Infants and Young Children: Forced-Choice Preferential Looking Procedures

Traditional forced-choice preferential looking (FPL) procedures have been useful laboratory tools in the documentation of normal acuity development in infants and young children. FPL procedures have also been used in several clinical settings to assess acuity in infants and young children with a variety of visual disorders. Nevertheless, FPL procedures have limitations that may hinder clinical utility. The most serious problem is a statistical limitation that requires a large number of trials to produce a reasonably accurate estimate of acuity. Because of this limitation we developed the acuity card technique, a procedure that relies on an observer's subjective judgment of the highest spatial frequency grating an infant can detect during presentation of FPL-type stimuli. Results of initial studies indicate that the procedure is faster than and as accurate as FPL procedures in the laboratory setting. Tests of the clinical usefulness of the acuity card procedure are in progress.

Infant color vision: The effect of test field size on rayleigh discriminations

The capacity of 1- and 3-month-old infants to discriminate 589 nm and 650 nm test fields from a 589 nm surround was tested using the forced-choice preferential looking (FPL) technique. The size of the test field ranged from 1 to 8°. Test field size strongly influenced the infants' performance. One-month-olds discriminated 8 and 4° (but not 2°) 650 nm fields from the 589 nm surround; 3-month-olds discriminated 4 and 2° (but not 1°) 650 nm fields from the 589 nm surround. The dependence of performance on field size suggests that infants' discrimination failures with small fields are due to immaturities of spatial processing or postreceptoral chromatic mechanisms, rather than to any absence or anomaly of receptor types. In addition, adult subjects rated the hue, brightness, and salience of the test stimuli at 0, 26, 52, and 78° eccentricity. The analogy often made between infant vision and adult peripheral vision is discussed in relation to these data.

Area-threshold relations at controlled retinal locations in 1-month-old infants

A fixation-and-flash technique has been developed to provide control over the retinal eccentricities of stimuli presented to infant subjects, to within a few degrees of visual angle. The technique is a variant of forced-choice preferential looking (FPL). An adult observer triggers presentation of the test stimulus when she judges that the infant is fixating a centrally located fixation target. The stimuli are short in relation to the infant's refixation latency. Auxiliary experiments confirmed that on most trials the stimuli fell within ± 4° of the designated eccentricity. Test fields of two sizes, 3.1 and 17°, were presented to 1-month-old infants at one of four retinal locations, 9, 18, 27 and 36° eccentric. The infants' data show a perfect area-intensity tradeoff at all four locations. Adult control subjects showed summation over areas of only 1–2°. The results are discussed in relation to other evidence of coarse spatial processing in human infants and other immature mammalian systems.

Effect of focus on visual acuity of human infants

Recent theoretical arguments (Green et al. (1980) Viston Res. 20, 827–835) predict that young human infants should have large depths of focus compared to adults. If so, optical blur should have relatively little effect on the resolving power of infant subjects. We tested this hypothesis by measuring the influence of optical blur on acuity thresholds in 6-week old infants and adults. Using the forcedchoice preferential looking (FPL) technique, we obtained acuity thresholds for each subject with five different lens powers (piano, −14 D, −3D, +6 D and +14 D). Acuity was differentially affected by lens power in all subjects, with the best acuity found with low power lenses and the worst with the highest power lenses. However, the amount of reduction in acuity produced by high power lenses, both plus and minus, was considerably less for infants than for adults. Although the effects of negative lenses are difficult to interpret without knowing the infants' exact accommodative state, the results with positive lenses support the theoretical predictions.

Development of critical flicker frequency in human infants

The forced-choice preferential looking (FPL) technique was used to trace the development of critical flicker frequency (CFF) in human infants. CFF was found to improve markedly between 4 and 8 weeks of age and to a lesser extent between 8 and 12 weeks. The CFF of the 12-week-olds was not significantly different from the adult CFF measured under similar conditions. A decrease in luminance level resulted in a decrease in the CFF of 8-week-old infants that was qualitatively similar to the decrease found for adults.