Abstract
Previous studies showed that healthy individuals bisect radial lines oriented along the midsagittal plane farther than the true center (distal bisection bias). It was proposed that the distal bisection bias depended on the presence of an attention bias directed toward far space (distal attention bias) and that this bias is related to the activity of the occipitotemporal visual processing stream. Other studies have also suggested that a similar distal attention bias is linked to the activity of the right hemisphere. In the present experiment we investigated whether distal bisection bias increased when radial lines were placed in the left hemispace. Furthermore, we also examined whether the bisection bias was enhanced by the use of the left hand, as left hand movements are mainly controlled by the right hemisphere. Right-handed participants were asked to bisect radial lines presented below eye level along the midsagittal plane (central lines), or laterally and parallel to the midsagittal plane, in the left or right hemispace (left and right lines, respectively). Participants used their right or left hand. The results showed that participants consistently bisected left and central radial lines farther than (i) the true center and (ii) the subjective midpoint of right radial lines. Conversely, they bisected accurately right radial lines. The hand did not influence bisection error. The present study suggests that the distal bisection bias found in the bisection of left radial lines might depend on the presence of a distal attention bias related to right hemisphere activity. The relative contribution of retinotopic and spatiotopic factors in producing the distal bisection bias is discussed.
Highlights
Line bisection is a perceptual-motor task in which participants are asked to localize and mark with a pencil the center of a line drawn on a sheet of paper
When the radial line was located along the midsagittal plane, the gaze plane crossed the line perpendicularly so that (i) the image of the distal portion was projected onto the inferior retina; (ii) the image of the proximal portion was projected onto the superior retina (Figure 5)
More precisely: (1) for left radial lines, the image of the distal portion was projected onto the inferior left retinal quadrants; the image of the proximal portion onto the superior right retinal quadrants (Figure 6); (2) for right radial lines, the image of the distal portion was projected onto the inferior right retinal quadrants; the image of the proximal portion was projected onto the superior left retinal quadrants
Summary
Line bisection is a perceptual-motor task in which participants are asked to localize and mark with a pencil the center of a line drawn on a sheet of paper. When patients with hemispatial neglect bisect horizontal lines, they place the subjective midpoint toward the ipsilesional. Drain and Reuter-Lorenz (1996) hypothesized that the occipitotemporal (ventral) stream shifts attention toward far/upper space, the occipitoparietal (dorsal) stream toward the near/lower space. Patients bisect radial and vertical lines, respectively, farther and more above the true center; in the second case, nearer and more below the true center. They suggested that the two streams are in mutually inhibitory control of attention orienting. Damage to occipitoparietal (occipitotemporal) regions would lead to a concomitant disinhibition in occipitotemporal (occipitoparietal) activity and a far/upward (near/downward) orienting bias (Drain and Reuter-Lorenz, 1996)
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