Influence of Background Appearance on Visual Accommodation

Many experimental and real-world viewing situations provide a context in which the target stimulus is displayed against a background set at a different but determinate distance. Conversely, other situations occur where the background distance is indeterminate, i.e., a textureless background. There has been evidence accumulating over the past two decades to suggest that the assumption of accurate visual accommodation will not be sustained under all these circumstances. Although earlier assumptions held that the centrally located stimulus would determine the level of accommodation, this experiment tests that assumption by varying the cues to background distance (well-textured, lighted, distant background and the same background unilluminated) and the distance to the target stimulus. Two groups of six participants observed targets (2 deg.) at six distances (0.9, 1.8, 3.7, 7.3, 14.6 and 29.3 m) and their visual accommodation was measured with a laser optometer. Results indicated that the group viewing the visible distant background evidenced a more distant accommodative response with the typical lag of accommodation. These results indicate that conditions of accommodation in the natural environment may have a profound effect on accommodative accuracy. In turn, this inaccuracy has been shown by others to correlate with inaccuracies in the perception of size and distance. Inaccurate accommodation has been found to delay target detection appreciably as well. Ameliorative approaches are discussed.