Accommodation and vergence latencies in human infants

Abstract

Purpose: Achieving simultaneous single and clear visual experience during postnatal development depends on the temporal relationship between accommodation and vergence, in addition to their accuracies. This study was designed to examine one component of the dynamic relationship, the latencies of the responses.

Methods: Infants and adults were tested in three conditions (i) binocular viewing of a target moving in depth at 5 cm/s (closed loop) (ii) monocular viewing of the same target (vergence open loop) (iii) binocular viewing of a low spatial frequency Difference of Gaussian target during a prism induced step change in retinal disparity (accommodation open loop).

Results: There was a significant correlation between accommodation and vergence latencies in binocular conditions for infants from 7 to 23 weeks of age. Some of the infants, as young as 7 or 8 weeks, generated adult-like latencies of less than 0.5 s. Latencies in the vergence open loop and accommodation open loop conditions tended to be shorter for the stimulated system than the open loop system in both cases, and all latencies were typically less than 2 s across the infant age range.

Conclusions: Many infants between 7 and 23 weeks of age were able to generate accommodation and vergence responses with latencies of less than a second in full binocular closed loop conditions. The correlation between the latencies in the two systems suggests that they are limited by related factors from the earliest ages tested.

Figure 1

The experimental apparatus. Panel A: For the CL and VOL ramp protocols, the stimulus, S, was moved by a motor, m, along a track in front of the stabilized subject. The stimulus position was recorded using a linear potentiometer, p, and the responses were recorded using a photorefractor, c. Panel B: In the AOL protocol, the subject viewed a DOG target via a beamsplitter, while responses were recorded with the photorefractor. Panel C: An IR filter was placed before one of the subject’s eyes in the VOL condition. The photorefractor could collect data binocularly, as shown in the top image, although the subject could not see through the filter. A prism was placed before the eye for the AOL condition, as shown in the bottom image.

Figure 2

The difference of Gaussian (DOG) target used to provide accommodation open-loop conditions.

Figure 3

Examples of the functions fit to the accommodation and vergence data collected in the CL (panel A, an 11 week-old) and AOL (panel B, an adult) conditions. The R² values for the CL fits were 0.46 for the accommodation data (48 data points) and 0.49 for the vergence data (82 data points). This individual’s CL data are presented as examples of fits that were restricted in length to reduce the impact of mismatches between the stimulus and response function shapes on the latency estimate.

Figure 4

Examples of raw data collected from an adult and an infant in the CL and VOL conditions. The infant’s accommodation data in the VOL, monocular, condition demonstrate that the accommodation is consensual at this age. The accommodation and vergence data are plotted relative to infinity, although the instrument was not calibrated for individual subjects (in terms of dioptric calibration, angle lambda or the Hirschberg ratio). The temporal structure of the response is therefore accurate, but the dioptric or angular response accuracy was not tested.

Figure 5

Examples of raw data collected from an adult and an infant in the AOL conditions. The infant data suggest a larger accommodative response than the adult, which would be consistent with the results of Bobier et al (2000), who found infants to have a larger CA/C ratio than adults.

Figure 6

Accommodative and vergence latencies of infants and adults for the CL, (binocular) conditions. Panel A: Relationship between the accommodation and vergence latencies of all the subjects in the far to near direction. Panel B: Relationship between the accommodation and vergence latencies of all the subjects in the near to far direction. Panel C: The data from the subjects who provided more than one set of latencies, in either the NF or FN condition. The ages of the infants in weeks are shown at the top of the graph.

Figure 7

Relationship between the accommodation and vergence latencies of all the subjects in the VOL (monocular) and the AOL (DOG) conditions.

Figure 8

A summary of the infant accommodation and vergence latency data across the different stimulus directions (NF or FN) and different viewing conditions. The numbers of subjects in each group are included at the top of the graph.