The effect of blur on the perception of up.

PURPOSE Knowing one's orientation relative to the environment is important for many aspects of vision including object recognition, action planning, and balance. Here we assess how inadequate optical correction for typical refractive errors might influence this. We measured the effect of blur on the perception of orientation as measured by the subjective visual vertical (SVV) and the perceptual upright (PU). METHODS The SVV and the PU were determined using a tilted line (was the line tilted left or right of vertical?) and the Oriented CHAracter Recognition Test (OCHART; was a character a "p" or a "d"?), respectively, in the presence of tilted visual backgrounds that were blurred using Gaussian blur with a radius of from 0 to 91 arc min. This is approximately equivalent to between 0 and 13 diopters of refractive error. RESULTS Blur reduced the influence of vision on both the SVV and PU by one just noticeable difference (84%) when vision was blurred by 11 to 13 arc min. That is, visual cues to self-orientation remain effective until vision is degraded to about 20/240 - roughly equivalent of taking off a pair of 2 diopter prescription glasses. CONCLUSIONS This reduction in the effectiveness of vision for determining orientation has important implications for the visually impaired and the elderly. Attempting tasks that require balance in the presence of uncorrected refractive errors may be more hazardous than expected. The effect of not optically correcting peripheral vision may also be consequential owing to the role of the far periphery in balance control.