Eric Seemiller

Human infants can generate vergence responses to retinal disparity by 5 to 10 weeks of age

Vergence is defined as a binocular eye movement during which the two eyes move in opposite directions to align to a target in depth. In adults, fine vergence control is driven primarily by interocular retinal image disparity. Although infants have not typically been shown to respond to disparity until 3 to 5 months postpartum, they have been shown to align their eyes from hours after birth. It remains unclear what drives these responses in young infants. In this experiment, 5- to 10-week-old human infants were presented with a dynamic random noise stimulus oscillating in disparity at 0.1 Hz over an amplitude of 2° for 30 s. Fourier transforms of the horizontal eye movements revealed significant disparity-driven responses at the frequency of the stimulus in over half of the tested infants. Because the stimulus updated dynamically, this experiment precluded the possibility of independent monocular fixations to a sustained target. These data demonstrate cortical binocular function in humans by five weeks, the youngest age tested here, which is as much as two months younger than previously believed.

Cues For Accommodation and Vergence in Infancy and Early Childhood

PURPOSE Focused binocular fixation on a target relies on a combination of accommodation and vergence oculomotor responses. These require recalibration during early development, as a result of changes in refractive error and increases in inter-pupillary distance (IPD). This study tracked the roles of blur and disparity cues in these motor responses during this period. METHODS 103 infants were followed from 3 months until 3 years of age. Their accommodation and vergence responses to a movie with naturalistic spatial amplitude spectra were recorded simultaneously using eccentric photorefraction and Purkinje image tracking (PowerRefractor, MCS). The target was moved repeatedly between 90 and 33cm and recordings were made in i) full cue binocular conditions (Bn), ii) the absence of disparity cues while occluding one eye with an IR filter (Oc), and iii) a reduced blur cue condition (Bl) (the image was covered with a lowpass spatial filter and masked with a Difference of Gaussian). RESULTS Changes in vergence and accommodation between the two viewing distances were computed for each condition. The ratio of vergence to accommodation was compared across the three conditions. Relative to the full cue Bn baseline, the ratio in the absence of disparity (Oc) was reduced at all ages (F = 0.22, p=0.80), suggesting that accommodative convergence is consistently low in early childhood. In contrast, the ratio in the Bl condition was lower in infancy and childhood than in adults (F=4.78, p=0.02), suggesting convergence accommodation is higher during early development. CONCLUSIONS The results suggest that vergence driven by cues other than disparity remains low in early childhood, while accommodation driven by cues other than blur is increased. This relationship is theoretically beneficial during the developmental period when accommodative demand is typically high due to hyperopia and vergence demand is low due to the narrow IPD. Meeting abstract presented at VSS 2015.

Vergence Sensitivity in 5-10 Week-Old Infants.

Introduction Fine motor alignment of the two eyes is necessary for appropriate binocular experience, yet in adults it appears that binocular processes are responsible for fine motor alignment. There is substantial evidence that infants start to respond to binocular retinal disparity (a primary cue for motor alignment) at 3-5 months of age. However, rudimentary eye alignment is possible from birth and matures with age. How does eye alignment develop in the absence of adult-like binocular processes? Here we investigate the sensitivity of vergence eye movements in 5-10 week old humans to a target moving in depth, while addressing the possible influence of accommodation. Methods Infants (5-10 weeks) and adult controls viewed a naturalistic movie stimulus on a screen that moved sinusoidally in depth at three different amplitudes (1.0, 0.5 and 0.25 MA) at 0.1 Hz. A photorefractor measured horizontal eye position and refractive status at 25 Hz. FFTs provided the amplitude spectra of both vergence and accommodation responses for comparison with the stimulus spectrum. Signal plus noise : noise ratios (SNRs) were calculated by dividing the response at the stimulus frequency by the mean of the adjacent frequencies. Results Vergence SNRs were significantly different from 1 (signal + noise equals noise) at even the smallest amplitudes tested (mean SNR = 2.10; p =0.03). suggesting that infants of 5-10 weeks could generate a vergence response to a full cue stimulus moving in depth at 0.25 MA (~30 minutes of disparity) Accommodation responses were only significant for the two larger amplitudes (mean SNRs = 2.84, 1.47; p = 0.04, 0.03). Adult SNRs were all significant for accommodation and vergence (p < 0.002). Discussion Infants make vergence responses to 0.25 MA stimulus movements at least one month prior to the documented onset of disparity sensitivity. Implications for development of eye alignment will be discussed. Meeting abstract presented at VSS 2015.