PMC

Note. (A) An illustration of the mean bisection errors of the line and object bisection tasks. We observed a significant difference between the direction of the mean bisection errors associated with lines and objects (all line lengths***, shortest lines*). In addition, object bisection errors were bisected significantly to the right of the objects’ center*. (B) An illustration of the significant association between greater line lengths and increasingly leftward bisection errors*. (C) An illustration of the significant** relationship between the object asymmetry and bisection errors. Objects with right > left displayed more rightward bisection errors. Conversely, objects with left > right displayed more leftward bisection errors. Negative bisection errors depict leftward bisection errors, and positive bisection errors depict rightward bisection errors. * p < .05. ** p < .01. *** p < .001.

Bisection with linearly spaced probe trials. A) Log ratio of the distance between the bisection point, B, and the geometric mean, GM, and the bisection point and the arithmetic mean, AM, as a function of spread (ratio of the reference durations). Values above zero are closer to the arithmetic mean, values below zero are closer to the geometric mean. B) Bisection normalized by the geometric mean of the reference durations as a function of spread. Regions marked by the gray box enlarged in the insert. Note that bisection is sub-geometric below log₂(spread) of 1. C) Bisection normalized by the arithmetic mean of the reference durations as a function of spread. Note that bisection becomes more sub-arithmetic with increasing spread. D) Bisection normalized by the arithmetic mean of the reference durations as a function of the arithmetic mean of the reference durations. Only experiments with a spread of 4 were used here. Bisection shows no correlation with absolute durations.

Note. (A) We observed shape bisection errors that were not significantly different from center or all line lengths but were significantly rightward of lines of a similar length to the shapes*. (B) An illustration of the significant*** relationship between object asymmetry and bisection errors. Shapes with right > left displayed more rightward bisection errors. Conversely, shapes with left > right displayed more leftward bisection errors. Negative bisection errors depict leftward bisection errors, and positive bisection errors depict rightward bisection errors. * p < .05. *** p < .001.

Each sub-plot shows the position of scanpath fixations (black dots) and point of line bisection (red asterisk) under various healthy and lesion conditions when the bisection bias weight was systematically raised, being set to the following values in order left to right: 3, 4, 4.5, 5. Axes refer to pixel position in the scene. a. Intact model. Raising the weight of the bisection task bias has the effect of concentrating the scanpath at the line centre. Bisection was consistently at line centre, position 500 (zero error). b. V1 lesion model. Raising the weight of the bisection task bias causes more fixations to be clustered around the location, offset contralesionally to line centre, which will later become the bisection point. As the task bias weight increases, less fixations are placed at the contralesional line end, but this is still where the majority of fixations are made. Bisection points were always contralesionally offset, in these simulations, from left to right, being: 455 (−4.1° offset, 9.1% of line length error), 468 (−2.9° offset, 6.5% error), 488 (−1.1° offset, 2.4% error), 482 (−1.6° offset, 3.6% error). c. Parietal lesion model using a step-function. If the weight of the bisection task bias is weak, the scanpath does not stay on the line. Bisection points were always highly ipsilesionally offset, in these simulations, from left to right, being: 688 (17.1° offset, 38.0% of line length error), 680 (16.4° offset, 36.4% error), 674 (15.8° offset, 35.2% error), 681 (16.5° offset, 36.6% error). d. Parietal lesion model using a gradient. Bisection points again were highly ipsilesionally offset, in these simulations, from left to right, being: 611 (10.1° offset, 22.4% of line length error), 669 (15.4° offset, 34.1% error), 652 (13.8° offset, 30.7% error), 644 (13.1° offset, 29.1% error).