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Several studies have shown that growth of the primate eye responds in a compensatory direction to both positive and negative spectacle lenses--eyes grow more slowly and become hyperopic in response to positive lenses, and eyes grow more rapidly and become myopic in response to negative lenses. On the other hand, extended wear soft contact lenses, whether positively or negatively powered, induce hyperopia (Hung & Smith, 1996. Extended-wear, soft, contact lenses produce hyperopia in young monkeys. Optometry & Vision Science 73, 579-584.). We investigated whether responses in a compensatory direction occurred to soft contact lenses worn on a daily wear basis (8 h per day on an 8:16 h light:dark cycle). Ten infant marmosets (8-13 weeks of age) wore a soft contact lens, in one eye only, for 5-9 weeks. Lens powers used were zero (n = 2), +2 D (n = 1), +2 D followed after 5 weeks of lens wear by +4 D (n = 1) for 4 weeks, +4 D (n = 2), -2 D followed after 5 weeks of lens wear by -4 D (n = 2) for 4 weeks, -4 D (n = 2). At the end of the lens-wear period the positive lens-wearing eyes were more hyperopic relative to the fellow untreated eyes [mean +2.39 +/- 0.24 D (SE)] and the negative lens-wearing eyes were more myopic than the fellow untreated eyes [mean -2.48 +/- 0.91 D (SE)]. Fellow eyes were unaffected by lens wear [mean final refraction +0.45 +/- 0.09 D (SE)]. Plano lenses did not affect eye growth in either marmoset fitted with plano contact lenses.

Infant marmosets were fitted with zero-powered (plano) soft contact lenses from 4 to 8 weeks of age worn either continuously (24 h per day) (n = 4), for 12 h (n = 4), or for 8 h (n = 3) per day to determine whether limiting the daily duration of lens-wear could significantly reduce or eliminate the effects of continuous lens-wear on ocular growth and refractive state. As in macaques (Hung, L. F., & Smith, E. L. (1996). Extended-wear, soft, contact lenses produce hyperopia in young monkeys. Optometry and Vision Science, 73, 579-584), eyes fitted with contact lenses worn continuously developed more hyperopic refractions (mean +3.22 +/- 1.49 D SE) compared to their fellow untreated eyes, inconsistent changes in vitreous chamber depth (-0.02 +/- 0.09 mm SE) and flatter corneas (mean decrease in corneal power 4.22 +/- 0.39 D SE). Eyes wearing lenses for only 12 h per day showed similar but reduced effects compared to the 24-h group. Most importantly, ocular growth, corneal power and refraction were unaffected in the 8-h group. Future studies using contact lenses in infant primates should employ a reduced daily duration of lens-wear to eliminate the undesirable effect of contact lens-wear per se on ocular development.

Refractive state and ocular dimensions were studied longitudinally in nine normal marmosets. Animals were anaesthetised and examined (with some exceptions) at 4, 6, 7, 8, 10, 15, 24 and 39 weeks of age. Cycloplegic retinoscopy showed that hyperopia early in life rapidly diminished. Refraction corrected for the artefact of retinoscopy stabilised by 8 weeks of age, but at a slightly myopic value, rather than at emmetropia. The ocular components continued to change throughout the period studied. Corneal radius, measured by photokeratometry, increased slightly during development. Anterior segment depth and vitreous chamber depth (VCD), measured by A-scan ultrasonography, increased throughout development while lens thickness initially increased and then decreased. Data from the eyes of these normal animals were compared with that from the contralateral eyes of animals which received short periods of monocular deprivation early in life (Troilo, D., & Judge S.J. (1993). Ocular development and visual deprivation myopia in the common marmoset (Callithrix jacchus jacchus). Vision Research, 33, 1311-24); eyes which viewed through no lens or a plano lens (Graham, B. & Judge, S.J. (1999)). The effects of spectacle wear in infancy on eye growth and refractive error in the marmoset (Callithrix jacchus). Vision Research, 39, 189-206), and eyes of normal animals in another colony. There were no significant differences between the first two groups and the normal animals in our colony while age-matched animals from the other colony were slightly but significantly less myopic than our animals.

We made a comprehensive study, involving observations on 45 marmosets, of the effects on ocular growth and refraction of wearing spectacles from the ages of 4-8 weeks. This period was within the period early in life when the eye grows rapidly and refraction changes from hyperopia to its adult value of modest myopia. In one series of experiments we studied the effect of lenses of powers -8, -4, +4 and +8D fitted monocularly. In another series of experiments we studied the effect of lenses of equal and opposite powers fitted binocularly, with the two eyes alternately occluded, so as to give an incentive to use both eyes, and in particular to accommodate, for at least part of each day, through the negative lens. The vitreous chamber of eyes that wore negative lenses of -4D or -8D, combined with alternate occlusion, elongated more rapidly than that of the fellow eye (negative lens eye-positive lens eye, 0.21 +/- 0.03 mm (S.E.M.), P < 0.01 and 0.25 +/- 0.06 mm, P < 0.05, respectively) and became relatively more myopic (2.8 +/- 0.26D, P < 0.01 and 2.4 +/- 0.61D, P < 0.05 respectively). Eyes that wore -4D lenses monocularly elongated more rapidly and became myopic than fellow eyes. Eyes that wore +4D or +8D lenses were less strongly affected: animals that wore +8D lenses monocularly (without alternate occlusion) developed a slight relative hyperopia (0.99 +/- 0.21D, P < 0.01), with the more hyperopic eyes also slightly shorter (0.09 +/- 0.05 mm) than their fellow eyes, but eyes wearing +4D lenses were not significantly different from their fellow eyes. Animals that wore -8D lenses monocularly (without alternate occlusion) developed a slight relative hyperopia after three weeks of lens-wear (0.85 +/- 0.26D, P < 0.05). These were the only eyes that responded in a non-compensatory direction to the optical challenge of spectacle wear, and we interpret this effect as one due to visual deprivation. After the removal of lenses, the degree of anisometropia slowly diminished in those groups of animals in which it had been induced, but in the three groups in which the largest effects had been produced by lens-wear the overall mean anisometropia (0.68 +/- 0.24D, P < 0.01) and vitreous chamber depth (VCD) discrepancy (0.09 +/- 0.03 mm, P < 0.01) were still significant at the end of the experiments, when the animals were 273 days old. The reduction of anisometropia in these groups was associated with an increase in the rate of elongation of the vitreous chamber in the eyes that had previously grown normally i.e. the less myopic eyes grew more rapidly than their fellow eyes: in the seven weeks following lens-wear these eyes became more myopic and longer than normal eyes (refraction P < 0.001; VCD P < 0.001). Control experiments showed that occlusion of one eye for 50% of the day had no effect on eye growth and refraction, and therefore that alternate occlusion itself had no effect.

The common marmoset (Callithrix jacchus) is a small, diurnal, New World monkey amenable to vision research. In this paper we describe the visual optics and cone photoreceptor topography of the normal adult marmoset. Paraxial optical ray-tracing shows that the marmoset eye is well represented as a scaled-down version of the human eye. The density of foveal and perifoveal cone photoreceptors in the marmoset is as high, and in peripheral retina higher, than those reported in humans and macaques. The foveal acuity predicted by the Nyquist limits set by the cone mosaic (30 c/deg) is in agreement with behavioral measures of visual acuity. Foveal depth of focus is remarkably small (< 0.2 D) for an eye of this size (axial length about 11 mm). Estimates of the amplitude of accommodation using infrared photorefraction indicate that the marmoset is capable of more than 20 D of accommodation.

The normal postnatal ocular development of the common marmoset (Callithrix jacchus) and the effects of visual deprivation on eye growth and refractive state are described. The marmoset normally undergoes a developmental process of emmetropization from high hyperopia at birth. This emmetropization is easily disrupted by visual deprivation produced by lid-suture. Myopia and axial elongation of the vitreous chamber are induced by visual deprivations of 12, 5, and 3 weeks duration. The development of axial myopia after 3 weeks of visual deprivation differs from longer duration deprivations in that the experimental eyes are initially shorter than normal and hyperopic at the end of the visual deprivation period, but subsequently become longer than normal and myopic. Visual deprivation myopia in the marmoset persists even after the deprivation is discontinued and a visual signal is restored. In all experimental groups, the development of the eye in response to the cessation of visual deprivation shows no slowing of vitreous chamber enlargement; the axial enlargement relative to the control eye is either maintained or increases and produces significantly greater myopia. These results suggest that the visual control of postnatal eye growth in the marmoset may be unidirectional in its response to visual experience and able only to increase the growth rate of the vitreous chamber, possibly after an initial delay.