The monochromatic optical aberrations of the eye degrade retinal image quality. Any significant aberrations during postnatal development could contribute to infants’ immature visual performance and provide signals for the control of eye growth. Aberrations of human infant eyes from 5 to 7 weeks old were compared with those of adult subjects using a model of an adultlike infant eye that accounted for differences in both eye and pupil size. Data were collected using the COAS Shack-Hartmann wavefront sensor. The results demonstrate that the higher order aberrations of the 5-to-7-week-old eye are less than a factor of 2 greater than predicted for an adultlike infant eye of this age. The data are discussed in the context of infants’ visual performance and the signals available for controlling growth of the eye.

^{2}. The illumination was made bright enough to provide a clear image of the eye for alignment purposes, but kept at the minimum usable value to maximize the subject’s pupil size. The subjects wore no optical correction.

^{st}Purkinje images generated by LEDs adjacent to the instrument’s viewing aperture (see Figure B1). Aberration data were collected only when the experimenter holding the infant and another observer operating the COAS were in agreement that the image of the eye was in focus and all Purkinje images fell within the infant’s entrance pupil (as demonstrated in Figure B1). This criterion led to an estimate of the deviation between the measurement and pupillary axes of less than 10 deg (see ). Assuming that the neonatal line of sight sits an average of 8 deg nasally from the pupillary axis, this limit would define an extreme range for the measurement axis from 2 deg nasally to 18 deg temporally from the line of sight (Slater & Findlay, 1972; Riddell, Hainline, & Abramov, 1994; Wick & London, 1980).

^{th}order and the combined RMS wavefront error were then calculated for each individual subject according to the Optical Society of America (OSA) recommended standards (Thibos, Applegate, Schwiegerling, & Webb, 2002). Thus the infant RMS data would be considered adultlike if they equaled two-thirds of the real adult values, and the infant PSFs would be considered adultlike if they had approximately the same angular size as the real adult data.

Pupil diameter (mm) | Infant | Adult |
---|---|---|

Minimum | 3.06 | 4.52 |

Maximum | 4.65 | 6.58 |

Mean | 3.89 | 6.65 |

^{nd}-order aberrations

^{nd}-order Zernike coefficients for individual infant and adult subjects is shown in Figure 2.

*t*tests indicated that there were no significant differences between the infant and adult means for either of the astigmatic terms (

*Z*

_{2}

^{−2}:

*p*= .429;

*Z*

_{2}

^{+2}:

*p*= .718). Overall, there was also no significant difference in the defocus term at this sample size (

*Z*

_{2}

^{0},

*p*=.152), even though some of the adults were myopic and we could not instruct the infants to accommodate accurately to the target. Infants in this age range are typically hyperopic (Cook & Glasscock, 1951; Mayer, Hansen, Moore, Kim, & Fulton, 2001), but overaccommodate for distant targets such as the one presented in the COAS (Banks, 1980). The

*Z*

_{2}

^{0}coefficients were converted to equivalent diopters giving a mean absolute magnitude of defocus of 1.79 D,

*SD*± 2.35, for the adults, and 1.62D,

*SD*± 0.71, for the infants (see , Equation 2).

^{rd}-, 4

^{th}-, and 5

^{th}-order aberrations

^{rd}-to-5

^{th}-order Zernike coefficient values is shown for the infant and adult groups in Figure 3, panel A. The mean values are all comparable and close to zero in the two groups. A Hotelling

*T*

^{2}test suggested that the vector of mean coefficients was not significantly different between groups,

*F*= 0.699, df1 = 22, df2 = 1,

*p*= .756. The difference between the groups for each individual Zernike component was also analyzed in a two-sample

*t*test (with no correction for multiple tests). The only component to reach a

*t*test

*p*value of < .05 for the difference between the groups was

*Z*

_{4}

^{+4}(

*p*= .014). This was not considered highly significant given the large number of

*t*tests being performed. The

*Z*

_{4}

^{0}spherical aberration term had a

*p*value of .091. One-sample

*t*tests were also performed to determine whether the individual 3

^{rd}-to-5

^{th}-order components differed significantly from a mean of zero. In the adult data, the components that reached a

*t*test

*p*value of < .05 were

*Z*

_{5}

^{−1}(

*p*= .015) and

*Z*

_{5}

^{−3}(

*p*=.031). The

*Z*

_{4}

^{0}term had a

*p*value of .051. In the infant data, the components that reached a

*t*test

*p*value of < .05 were

*Z*

_{4}

^{+2}(

*p*=.019) and

*Z*

_{4}

^{+4}(

*p*=.026). The

*Z*

_{4}

^{0}term for that group had a

*p*value of .93. These values < .05, again, were not considered highly significant due to the large number of tests being performed, although the adult data are consistent with the literature finding positive values of

*Z*

_{4}

^{0}(e.g., Thibos, Hong et al., 2002). Overall, these data suggest that there is no consistent trend in the sign of the coefficients within the populations, and that the infant distribution is not dramatically different from that of the adult.

*T*

^{2}test suggested no significant difference between the distribution of infant and adult absolute coefficient values,

*F*= 1.720, df1 = 22, df2 = 1,

*p*=.546, and the components that reached a

*p*value of < .05 in

*t*tests of the difference between groups for individual components were

*Z*

_{4}

^{+4}(

*p*=.014),

*Z*

_{3}

^{−1}(

*p*= .047), and

*Z*

_{4}

^{0}(

*p*= .021).

^{rd}to 6

^{th}order were combined to form RMS errors in Figure 4. The RMS wavefront error is shown for each subject and individual order, and then for each subject for the combined 3

^{rd}to 6

^{th}orders. The lowest

*p*value resulting from

*t*tests of the difference between infants and adults for each of these variables was

*p*= .337, which was considered insignificant.

*t*tests were performed as a function of scale factor applied to the adult data. The results indicated that the infant and adult combined RMS (3

^{rd}to 6

^{th}order) were most similar when the adult data were scaled by 0.87 (the

*p*value for the adult data scaled by the adultlike model of 0.67 was 0.076, for the adult data scaled by 0.8 was 0.580, scaled by 0.87 was 0.977, and scaled by 0.9 was 0.793). That is, when employing the same numerical aperture, infant eyes have an RMS that is 0.87 that of adult eyes, not the 0.67 predicted by the simple scaled eye model. Thus, the data suggest that the mean higher order aberrations of the infant eye are somewhat larger (by 20% of the mean adult value) than predicted by the adultlike model. This mean difference is small when compared with immaturities in infants’ visual performance at this age and the range of higher order aberrations seen in both adult and infant eyes.

^{rd}to 6

^{th}order. These data again suggest that the infants’ higher order optical quality is slightly worse than that of adults at 5 to 7 weeks after birth. For example, at 10 c/deg, the infant optics transfer a mean of 20% of the object contrast to the image, whereas adult optics transfer a mean of 33%. Interestingly, if we compare the contrast transferred at spatial frequencies scaled to the acuity limit, we find the reverse is true. For example, at 50% of the adult resolution limit (∼25 c/deg), only 11% is transferred, but at 50% of the infant resolution limit (∼1 c/deg), 93% is transferred. Thus, although the infant optics are inferior to those in the adult eye, they are more efficient at transferring a neurally detectable signal to the retinal image.

^{rd}-to-6

^{th}-order aberrations were calculated using Fourier optics. PSF width was quantified using the equivalent width metric (Thibos, Hong, Bradley, & Applegate, 2004), which gives the width of a uniform circular PSF with the same intensity as the peak of the actual PSF. The mean adult value was 1.57 arcmin (

*SD*± 0.49) and the mean infant value was 2.07 arcmin (

*SD*± 0.48). These distributions were significantly different (

*p*= .016), implying again that the effect of higher order aberrations is somewhat more disruptive to retinal image quality in infants than in adults. An adultlike infant eye was predicted to have the same angular PSF width as the real adults ().

*x*-axis represents infant subject number, and the

*y*-axis represents the correlation between that infant’s set of 3

^{rd}-to-6

^{th}-order coefficients and an adult’s. The black symbols show each infant’s correlation with their own parent, and the small gray symbols show the correlation with each of the other adults. The horizontal line shows the level at which the correlation becomes significant at the .01 level (one-tailed test, alpha level = 0.01, df = 10,

*r*= 0.658, with a null hypothesis of zero correlation and an alternative hypothesis that the correlation is positive, with no compensation for multiple tests). The graph demonstrates that the correlation between individual infants and their parents is inconsistent across infants, and also that the range of correlations with parents across the group approximates the range of correlations between each infant and any other adult. These correlations are typically insignificant, with only 4 (2 with parent and 2 with another adult) of the total 144 correlations reaching significance at the 0.01 level.

*x*-axis represents individual Zernike coefficients, and the

*y*-axis represents the correlation of that coefficient in the infant group with the adult group. The black symbols show the correlations when the infants are all aligned with their parents, and the small gray symbols show the correlations when the infants are compared with the other possible arrangements of the adult group (each infant matched with each adult only once). The horizontal line again shows the level at which the correlation becomes significant (one-tailed test, alpha level = 0.01, df = 20,

*r*= 0.492, with a null hypothesis of zero correlation and an alternative hypothesis that the correlation is positive, with no compensation for multiple tests). The aligned infant and parent correlations are very variable. The cases where the parent correlation is clearly greater than the correlation with other adults are

*Z*

_{3}

^{−3},

*Z*

_{4}

^{−2}, and

*Z*

_{4}

^{0}. The parent correlations for

*Z*

_{3}

^{−3}and

*Z*

_{4}

^{0}are also greater than the 0.01 significance level for the one-tailed test, although given the number of correlations calculated (264), this is not strong evidence of significance at least for this sample size.

^{nd}to 6

^{th}order excluding defocus) have both noted minimal change in adult higher order RMS for accommodative efforts from 0–3 D, but an increase in RMS of almost a factor of 2 between 3 D and 6 D of effort. If the infant eyes also exhibit this behavior, the relatively greater RMS in the infant eyes could result from their increased accommodative effort. If this were the case, the infant RMS may actually be closer to the adult prediction for a matched accommodative response (although any increase in aberrations associated with the myopic refractive errors of the adults could negate the increase due to the accommodation in the infants (Collins, Wildsoet, & Atchison,1995; Carkeet, Luo, Tong, Saw, & Tan, 2002; Llorente, Barbero, Cano, Dorronsoro, & Marcos, 2004; but see Cheng, Bradley, Hong, & Thibos, 2003).

*Z*

_{4}

^{0}) becomes less positive with increasing accommodation (He et al., 2000; Cheng, Barnett et al., 2004). If this is also true in infants, the spherical aberration data (Figure 3) are also consistent with the infants exerting a greater accommodative effort than the adults. The mean

*Z*

_{4}

^{0}coefficient is less positive in the infants than in the adults. An alternative interpretation of these data is that the positive increase in mean

*Z*

_{4}

^{0}with age is consistent with the same trend seen across the adult age range (McLellan, Marcos, & Burns, 2001; Glasser & Campbell, 1998).

^{rd}-to-6

^{th}-order RMS of adult and infant eyes are equivalent to 0.22 D (

*SD*± 0.09) and 0.43D (

*SD*± 0.13), respectively, and thus the difference between them is equivalent to 0.21 D of defocus.

*Z*

_{4}

^{0}spherical aberration (even though it had one of the larger differences between adult and infant mean amplitudes in Figure 3). There is some evidence of a correlation between the amount of spherical aberration and refractive error in adults (Collins et al.,1995; Carkeet et al., 2002; Llorente et al., 2004; but see Cheng, Bradley et al., 2003), and so it could be interesting to explore more closely this coefficient and its relationship with current and future refractive error in infants. Overall, these correlations might obviously be increased if the infant alignment were controlled.

^{th}to 27

^{th}week of gestation (Mund et al, 1972), but the absolute density of pigment has not been plotted as a function of age. However, of relevance to the current study, Friedman and Ts’o’s (1968) study of donor tissue notes that the gradient of pigment density typically found in adults (greatest density in the macula region and least in the periphery) was reversed in their fetal and neonatal eyes. Streeten (1969, p. 393) also notes a delay in RPE pigmentation in the macula until well into infancy. Her data document a reduced RPE cell density in the posterior pole in the neonatal period with a later cell migration into the macular area. An adultlike or higher RPE cell density was found in the neonate only at eccentricities in the mid-periphery or beyond. Robb (1985) also notes a postnatal migration of RPE cells toward the macular area during the first 6 postnatal months. These data suggest that the infant RPE in the posterior pole does not contain a higher pigment density than found at the same location in the adult.

^{st}Purkinje images generated by the LEDs around the instrument’s viewing aperture all fell within the entrance pupil (a typical situation is shown in Figure B1). This inclusion criterion limited the angular deviation of the pupillary axis from the instrument axis, and thus the deviation of the line of sight from the instrument axis. For example, if the Purkinje image ring had the same radius as the pupil, this criterion would force the data to be collected on the pupillary axis. The radii of the pupil and the Purkinje image ring (which depends on the individual’s corneal curvature) were not equal, however, and were not constant across observers. The fixation error tolerance therefore varied across observers.

^{rd}dimension will reduce the amplitude of the optical path difference (OPD) by the scaling factor at all points (Figure C1B). Thus a 3D-scaled version of the adult eye is predicted to have a total RMS that is reduced from the true adult value by the scaling factor. Beyond its simplicity, this compression model is attractive in that it results in the same numerical aperture for the adult and scaled eyes and therefore light being collected over the same angle at the retina (Figure C1C).

*b*, can be approximated using the following equation: where

*P*is the pupil diameter in meters, and

*D*is defocus in diopters (Smith, 1982).

*D*, using the following equation: where

*r*is the pupil radius in mm,

*RMS*is the root mean square wavefront error in microns and

*k*is a constant (Thibos, Hong et al., 2002).

*D*, will therefore increase by the scaling factor, as a result of the

*r*

^{2}term in the denominator.