Human infants' accommodation responses to dynamic stimuli.

Retinal image defocus results from the combination of an eye’s refractive error, the amount of accommodation exerted, and the distance of the object being viewed. The amount of defocus in an infant’s retinal image determines their visual experience and may affect experience-dependent processes such as the refinement of cortical synapses1 and emmetropization.2-4 The human neonate typically has a hyperopic refractive error5,6 and accommodates inaccurately to objects in their environment.7-13 Over time, emmetropization and increasing accommodative accuracy both act to improve the developing eye’s optical performance. The reduction in refractive error resulting from emmetropisation typically takes a number of months to years to occur, through a relatively slow process,3,6 and so it is the accommodative system that more routinely eliminates defocus in the infant eye by compensating for refractive error and viewing distance. A number of studies have documented that infants tend to overaccommodate to distant targets until approximately 2 months of age. They maintain their accommodation around approximately 30- to 50-cm viewing distances. During the next month, they start to adjust their accommodation to focus more accurately on nearer and farther targets.7-10,12 Most of the previous studies of infants’ accommodation have determined their steady state responses to stationary targets.7-10 The natural world, however, routinely consists of stimuli moving in depth, when objects are moved or the infant is carried through the environment. When tracking dynamic targets, the latencies and dynamics of infants’ accommodative responses have a large impact on their habitual retinal image defocus. Only one previous study has examined the dynamics of infants’ accommodation, to targets that underwent a step change from one distance to another.14 Howland et al.14 recorded responses at 2 Hz. A sample response from a 4.5-month-old infant was noted to be typical of the 21 responses they recorded from infants aged 4 to 9 months. This infant initiated an accommodative response within 1 second of the step stimulus and completed the response within another half second. No studies have been published to date that have recorded response characteristics while infants are presented with gradually moving ramp rather than with step targets. Howland et al.14 suggest there are no dramatic immaturities in infants’ motor ability to change their accommodation at 4 months and older, but no studies have been conducted of younger infants or as a function of stimulus velocity. As noted, short response latencies and tracking of a moving target are critical for maintaining a focused retinal image in a dynamic environment. The adult visual system is capable of tracking moving accommodative targets at relatively low stimulus velocities.15,16 As velocity increases, however, adults generate an increasing number of “catch-up” step responses and ultimately are not able to smoothly track at all.15 At high velocities, above approximately 4 D/s, the adult accommodative system merely generates a single step response related to the target’s final position.15 This combination of accommodative response strategies has been modeled as a dual-mode neural control system.15 Similar neural control models have been proposed to describe other oculomotor responses.17-20 The step responses are considered to be preprogrammed, whereas the slower, smooth tracking responses are under the influence of continuous feedback (see also Yamada and Ukai21). Young infants show evidence of immature step movements during a related form of tracking in that they exhibit a series of step saccades in pursuit eye movements at stimulus velocities that an adult can track smoothly.22,23 The goal of this study was to record infants’ responses to accommodative stimuli moving over a range of velocities to systematically determine their response latencies and whether they can adjust response velocity in a dynamic environment. We sought to determine whether young infants are able to discriminate and track different velocities and whether they demonstrate immaturities mimicking those found in their smooth pursuit responses.