Does gravity influence the visual line bisection task?

The visual line bisection task (LBT) is sensitive to perceptual biases of visuospatial attention, showing slight leftward (for horizontal lines) and upward (for vertical lines) errors in healthy subjects. It may be solved in an egocentric or allocentric reference frame and there is no obvious need for graviceptive input. However, for other visual line adjustments, such as the subjective visual vertical, otolith input is integrated. We hypothesized that graviceptive input is incorporated when performing the LBT and predicted reduced accuracy and precision when roll-tilted. Twenty healthy right-handed subjects repetitively bisected earth-horizontal and body-horizontal lines in darkness. Recordings were obtained before, during and after roll-tilt (+/-45deg, +/-90deg) for 5min each. Additionally, bisections of earth-vertical and oblique lines were obtained in 17 subjects. When roll-tilted +/-90deg ear-down, bisections of earth-horizontal (i.e., body-vertical) lines were shifted towards the direction of the head (p<0.001). However, after correcting for vertical line-bisection errors when upright, shifts disappeared. Bisecting body-horizontal lines while roll-tilted did not cause any shifts. The precision of earth-horizontal line bisections decreased (p<=0.006) when roll-tilted, while no such changes were observed for body-horizontal lines. Regardless of the trial condition and paradigm, the scanning direction of the bisecting cursor (leftward vs. rightward) significantly (p<=0.021) affected line bisections. Our findings reject our hypothesis, and suggest that gravity does not modulate the LBT. Roll-tilt dependent shifts are rather explained by the headward bias when bisecting lines oriented along a body-vertical axis. Increased variability when roll-tilted likely reflects larger variability when bisecting body-vertical than body-horizontal lines.

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