Binocular summation of detection and resolution thresholds in the central visual field using parallel-line targets

PURPOSE. To investigate the effect of thresholds on binocular summation for detection and resolution of parallel-line targets of different widths. METHODS. The automatic perimeter Octopus 201 (Haag-Streit International, Koniz, Switzerland) combined with a modified haploscope device and targets consisting of parallel lines that were 10' (10' target), 2.5' (2.5' target), or 1.43' (1.43' target) of visual angle in width and apart were used to measure the thresholds for monocular - binocular detection and resolution of the targets under the same binocular fusion stimulation conditions in seven young adults with normal vision. The custom program in the automatic perimeter was used to test 27 points. Seventeen of the 27 points were located in the central 6 degrees of visual field and 10 on the horizontal meridian subtending visual angles of 8 degrees, 10 degrees, 12 degrees, 16 degrees, and 20 degrees. RESULTS. The resolution threshold was significantly higher than the detection threshold for the 2.5' and 1.43' targets ( P < 0.01 by Bonferroni/Dunn test). Furthermore, as the target width decreased to 2.5' or 1.43', the binocular summation ratio for the resolution threshold increased significantly over that for the detection threshold, with increasing distance from the fovea ( P < 0.01 by Wilcoxon signed ranks test). CONCLUSIONS. Binocular summation for detection and resolution thresholds varies as a function of the width of a parallel-line target. The difference between binocular summation ratios for detection and resolution thresholds increases with decrease in target width and increase in eccentricity from the fovea. Binocular interaction plays an important role in the task of recognizing a high-resolution target in the parafoveal area.