The sensitivity of the 2- to 4-month-old human infant accommodation system

Abstract

Purpose: The goal of this study was to compare objectively the sensitivity of the accommodation system in human infants and adults under binocular and monocular viewing conditions.

Methods: Full-term infants from 2 to 4 months of age and pre-presbyopic adults were presented with a high-contrast cartoon stimulus moving sinusoidally in diopters around a mean position of 2 D (50 cm). Three stimulus amplitudes were used in one trial (0.25, 0.50, and 0.75 D), with unpredictable stimulus motion during each amplitude change. Eccentric photorefraction was used to record accommodative responses at 25 Hz. The stimulus was made monocular by placing an infrared filter over the right eye, to block visible light but pass the near-infrared wavelength of the photorefractor and allow responses to be recorded from both eyes.

Results: Fourier analysis was used to determine the accommodative response at the frequency of the stimulus. Significant signal-to-noise ratios indicated that, on average, the 2- to 4-month-old infants generated an accommodative response to at least the 0.75 D amplitude monocular stimulus and the 0.75 and 0.50 D binocular stimuli. Adults responded to the 0.25 D amplitude both binocularly and monocularly.

Conclusions: In infants 2 to 4 months of age, the developing visual system compensates for small changes in defocus relative to the typical amounts of hyperopic refractive error found at that age.

Figure 1.

The photorefractor camera was mounted 1 m from the eyes of the subject. An LCD screen and infrared transmitting beamsplitter was moved by a motor system over a range of dioptric distances from 1.1 to 5 D. The target on the LCD screen was presented to the subject by reflection at the beamsplitter.

Figure 2.

The accommodative stimulus as a function of time. Three cycles each of three quasisinusoidal amplitudes (0.25, 0.5, and 0.75 D) were presented at 0.1 Hz in each trial. Aperiodic sections were inserted between the different amplitudes to make the stimulus unpredictable and to confirm that the subjects were maintaining active tracking. The stimulus duration at each amplitude was 30 seconds, and the total duration was approximately 135 seconds.

Figure 3.

Data collected from a 13-week-old infant in the binocular viewing condition. The stimulus amplitude was 0.75 D and its frequency was 0.1 Hz. (A) The 0.75 D stimulus and accommodative response of the left eye. The corresponding amplitude spectra are plotted in (B). The stimulus amplitude and the response amplitude of the left eye are represented.

Figure 4.

The mean amplitudes at 0.1 Hz from the FFTs of the individual raw infant and adult data collected in binocular viewing conditions. Solid line: a response that equals the stimulus, an accurate response. The symbols plotted at a stimulus amplitude of 0 show the data collected in the static target control condition (experiment 1A). Error bars, SEM.

Figure 5.

Pooled infant raw data at (A) 0.25, (B) 0.50, and (C) 0.75 D from the left eyes of all the infant subjects (n = 33) for the binocular stimulus presented at 0.1 Hz. (D–F) The corresponding amplitude spectra from the FFT analyses. The stimulus is plotted in each case for comparison. The SNRs were 1.3, 3.5, and 10.5, respectively, for the 0.25, 0.5, and 0.75 D stimuli.

Figure 6.

Pooled adult raw data at (A) 0.25 (B) 0.50, (C) 0.75 D from the left eyes of all the adult subjects (n = 10) for the binocular stimulus presented at 0.1 Hz. (D–F) Corresponding amplitude spectra from the FFT analysis. The stimulus is plotted in each case for comparison. The SNRs were 11.9, 19.9, and 21.0, respectively, for the 0.25, 0.5, and 0.75 D stimuli.

Figure 7.

Mean FFT amplitudes at 0.1 Hz for the infant and adult individual data in monocular viewing conditions. Solid line: a response that equals the stimulus. The symbols plotted at a stimulus amplitude of 0 show the data collected when the target was static in the control experiment in binocular viewing (experiment 1A). Error bars, SEM.

Figure 8.

Pooled raw data for the monocular viewing condition at (A) 0.25, (B) 0.50, and (C) 0.75 D from the left eyes of the infant subjects (n = 26), compared with the stimulus. (D–F) Corresponding amplitude spectra from the FFT analyses compared with the stimuli. The SNRs were 0.9, 1.2, and 7.3, respectively, for 0.25, 0.5, and 0.75 D stimulus amplitudes.