A new stereotest: the double two rod test.

A new stereoacuity test, the double two rod test, uses a two-alternative forced-choice (2AFC) paradigm. The subject's task is to state which pair of rods, the left or the right, contains depth information. The test gives repeatable results; comparison between the depth threshold measured binocularly and monocularly suggests that monocular cues, if present, provide only poor depth information, compared to the depth information arising from binocular disparity. As part of an investigation of stereoacuity and ageing, we tested stereoacuity as a function of viewing time and the number of comparisons made between the two pairs of rods. Stereoacuity was significantly reduced for viewing times less than 3 s while longer viewing times did not improve stereoacuity. The number of comparisons made between the two pairs of rods had no effect on stereoacuity.