Abstract
This study explores the processing of mental number lines and physical lines in five patients with left unilateral neglect. Three tasks were used: mental number bisection (‘report the middle number between two numbers’), physical line bisection (‘mark the middle of a line’), and a landmark task (‘is the mark on the line to the left/right or higher/lower than the middle of the line?’). We manipulated the number line orientation purely by task instruction: neglect patients were told that the number-pairs represented either houses on a street (horizontal condition) or floors in a building (vertical condition). We also manipulated physical line orientation for comparison. All five neglect patients showed a rightward bias for horizontally oriented physical and number lines (e.g. saying ‘five’ is the middle house number between ‘two’ and ‘six’). Only three of these patients also showed an upward bias for vertically oriented number lines. The remaining two patients did not show any bias in processing vertical lines. Our results suggest that: (1) horizontal and vertical neglect can associate or dissociate among different patients; (2) bisecting number lines operates on internal horizontal and vertical representations possibly analogous to horizontal and vertical physical lines; (3) at least partially independent mechanisms may be involved in processing horizontal and vertical number lines.
Highlights
Unilateral spatial neglect is characterized by the failure to perceive or respond to stimuli located on the side of space opposite to a focal brain lesion (e.g. Driver & Vuilleumier, 2001)
All five patients were tested on bisection of mental number line with both horizontal and vertical lines (Task 1); three out of these five patients were tested on bisection of horizontal and vertical physical lines (Task 2), whereas the remaining two were only tested on bisection of horizontal physical lines
Summary All patients showed a bias in bisecting mental number lines symbolically oriented horizontally
Summary
Unilateral spatial neglect is characterized by the failure to perceive or respond to stimuli located on the side of space opposite to a focal brain lesion (e.g. Driver & Vuilleumier, 2001). Altitudinal neglect has been documented as: a upward bias in bisecting vertical lines; a tendency to omit or respond slowly to stimuli presented in the lower part of the space, or slower neglect recovery of the lower quadrant Ergun-Marterer et al, 2001; Pitzalis et al, 1997; Rapcsak et al, 1988; Shelton et al, 1990), whereas in others vertical lines were better preserved than horizontal lines (Milner & Harvey, 1995; Shelton et al, 1990). Investigations into neglect have often used visuo-spatial stimuli providing insight into the way the brain represents space. A few recent neglect studies have used numbers as stimuli, allowing insight into the way the brain may represent numbers spatially
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