Abstract
Purpose: :
Clear and single vision requires coordinated accommodation and vergence motor responses. The accommodative demand of infants is typically greater than that of adults due to hyperopia, while their vergence demand is smaller due to reduced inter-pupillary distance. The relationship between these demands also changes with growth of the eye and head, requiring some form of flexibility in the interaction between accommodation and vergence to maintain clear and single vision. Here, we assessed the capacity of the developing visual system to overcome short-term experimentally induced conflicts between accommodation and vergence demands.
Methods: :
Accommodative and vergence responses were measured using a PowerRefractor (25Hz). 101 subjects (2.0months to 40.1years) experienced binocular step changes in either blur (-2D lenses) or disparity (2MA base-out prisms) while they watched a high contrast cartoon movie at 80cms. 14 and 12 subjects also experienced 1D or MA and 3D or MA step changes in blur and disparity demands, respectively. The strengths of accommodation-vergence cross-coupling (AC/A and CA/C ratios) was also measured in 38 and 20 subjects respectively (3.2months to 11.1years) under conditions of open-loop disparity and blur cues.
Results: :
The frequency of response to both lenses and prisms increased significantly with age (p<0.001), with no significant difference between lenses and prisms (p=0.38). The direct response gain (i.e. accommodation to lenses and vergence to prisms) increased significantly with age (p<0.001), with a slightly but significantly larger response to prisms (Mean+/-SEM: 0.71+/-0.02) than to lenses (0.58+/-0.02) at all ages (p<0.001). Across all ages (p=0.52), the coupled response gain (i.e. vergence to lenses and accommodation to prisms) was much smaller for both lenses (0.10+/-0.01) and prisms (0.17+/-0.03) and they were significantly smaller than (p<0.001) and poorly correlated with the subject’s AC/A (r=0.25) and CA/C (r=0.21) ratios. Unlike response frequency (p=0.14) and coupled response gain (p=0.73), the direct response gain decreased significantly with increased stimulus demand for both lenses and prisms (both p<0.001).
Conclusions: :
The typically developing visual system can, at least partially, compensate for conflicts in blur and disparity demands by independently modulating its accommodative and vergence responses, suggesting short-term flexibility in the interaction between the two motor systems.
Keywords: accomodation • vergence • development