Vertical and bisection bias in active touch

We investigated the conditions that underlie the vertical and bisection ill usion in touch, in order to understand the basis of their similarity to vis ual illusions, and the means of reducing the biases in length perception by active touch. Movement, speed, and spatial reference cues were tested. Mov ements in scanning L-shapes in ipsilateral and contralateral (across the bo dy midline) table-top space produced significant underestimation of the ver tical line with the right hand, but not with the left hand. Right-handed sc anning of L-shapes showed no significant bias when the Vertical line in the figure was aligned to the body midline, suggesting that spatial cues were involved. The vertical line was overestimated in inverted T-shapes, but und erestimated in rotated T-shapes, implicating line bisection. Holding scanni ng latencies constant reduced the vertical error for inverted T-shapes, but could not explain the bisection bias. Sectioning biases were predicted by the location of junctions on sectioned lines, showing that junction points act as misleading anchor cues for movement extents. The illusion was signif icantly reduced when reference information was added by instructing subject s to relate two-handed scanning of the figure to an external frame and to b ody-centred cues. It is argued that disparities in spatial reference (ancho r) cues for movement extents are involved in vertical and bisection biases in active touch. The hypothesis that length illusions depend on disparities in spatial reference information can also account for the similarity of th e tactile to the visual horizontal-vertical illusion.