Descartes's blind man revisited: bimanual triangulation of distance using static hand-held rods

Analogous to binocular convergence, we tested Descartes's proposal that a blind person could triangulate distances using crossed hand-held rods by sensing hand separation and wrist angles. In Experiment 1, 22 participants judged distances to loci at 4 distances, 18-36 in. (45.7-91.4 cm), at 5 eccentricities, 0-12 in. (0-30.5 cm) left and right of center. Participants reliably differentiated and scaled distances. The median individual multiple correlation (actual distances and eccentricities vs. judged distances; nonlinear with eccentricity) was .91 (range, .75 to .95; group R = .98). In Experiment 2, 2 experienced observers judged 7 distances, 30-66 in. (76.2-167.6 cm) at 7 eccentricities, 0-18 in. (0-45.7 cm) left and right. Results replicated those of Experiment 1, with nonlinear multiple Rs of .97 and .99 for the 2 participants. Bimanual triangulation supported reliable distance judgments, comparable to binocular distance perception. No evidence of perceptual learning was found.