Accommodation, vergence and the accommodative-vergence cross link in infancy

Clear and single percepts of visual objects depend on the coordination of accommodation and vergence, which are tightly coordinated in adults with each being triggered by the other. Although clinical abnormalities in these systems can lead to blur, diplopia, and permanent disruption of visual development in infancy, little is known about normal development of the relationship between the two systems. This study had two goals; (i) to determine whether infants can respond to naturalistic dynamic accommodative stimuli; and (ii) to investigate the development of the link between accommodation and vergence and its dynamics.;Accommodation and vergence estimates at 25 Hz were collected using eccentric photorefraction (PowerRefractor, Multichannel Systems). Three studies were conducted: (i) Dynamic stimuli of different velocities (0.5, 0.2 & 0.05 m/s) were presented in binocular conditions. (ii) One dynamic stimulus velocity was presented under binocular and open-loop vergence (monocular) conditions. (iii) A 10Δ prism was used to stimulate vergence while a very low spatial frequency target was used to create open-loop accommodation conditions. Data were collected from 91 infants (6 to 23 weeks old) and 15 young adults.;The results show that: (i) infants older than 8 weeks of age can generate tracking accommodative responses to the different velocities. The infants however did have longer latencies than the adults' (0.6s vs. 0.2s). (ii) A tight link between accommodation and vergence was shown for both open-loop accommodation and vergence conditions. Monocularly driven vergence was demonstrated at least from 11 weeks with similar AC/A ratios to adults. Convergence accommodation was demonstrated at least from 7 weeks with higher CA/C ratios than in adults. Infants again exhibited longer latencies than adults for all responses although the difference between accommodation and vergence was similar across groups. This possibly suggests mature control of the latency of the link. This analysis of the link between accommodation and vergence in infancy demonstrates the need for early clinical assessment of the relationship between these mechanisms to prevent clinical abnormalities.