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Jason Forte, Mike Nicholls, Michele Hervatin; Radial line bisection biases in the periphery. Journal of Vision 2009;9(8):992. doi: 10.1167/9.8.992.
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© ARVO (1962-2015); The Authors (2016-present)
The perceived midpoint of a horizontal line is typically to the left of center. Line bisection biases also occur for centrally presented vertical lines. A number of studies have found that vertical and horizontal lines biases are uncorrelated, suggesting that bisection biases are based on independent mechanisms for vertical and horizontal lines. We measured line bisection biases in the near periphery to look for independent line bisection mechanisms beyond the central visual field. Lines with 0, 45, 90 or 135 degrees orientation were displayed at eight peripheral locations. Each trial displayed a pre-transected line for 200 msec and the observers (n=4) pressed a button to indicate which end the transector appeared nearest. The position of the transector was adjusted using a staircase procedure with the point of subjective equality calculated as the average of the last six staircase reversals. The line bisection bias at a given orientation and visual field location was based on the average point of subjective equality from eight to ten staircases. The results showed a consistent pattern of line bisection biases that appeared to be radial for three observers. Two observers showed a centripetal bias but one observer showed a centrifugal bias. The radial pattern of line bisection biases may indicate that vertical and horizontal biases are mediated by a single mechanism in the periphery. Furthermore, the individual differences in direction of bias may indicate that line bisection depends on a plastic representation of space across the visual field. The data can be explained by a metric of space that relies on the allocation of attention. Some observers may attend to the inner part of line segments, resulting in a centripetal line bisection bias, while others attend to the outer segment, resulting in a centrifugal line bisection bias.
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