Scanning direction and line bisection: a study of normal subjects and unilateral neglect patients with opposite reading habits

Abstract

Sixty normal dextrals (30 left-to-right and 30 right-to-left readers) and two left unilateral neglect patient with opposite reading habits performed a passive line bisection task. In order to study the effect of scanning direction on performance, subjects had to stop a mark moving on the to-be-bisected line either from the left to the right or in the opposite direction. Results showed that the position of the subjective middle was dependent upon the scanning direction of the line for all the subjects. A leftward deviation appeared for left to right scanning, whereas a rightward shift occurred when the mark moved from the right to the left. These results emphasize the role of scanning direction in space organization and are discussed with respect to the explanatory hypotheses of unilateral neglect.

Introduction

Unilateral neglect is a neurological disorder clinically characterized by the inability to perceive or orient to stimuli presented to one side of space (usually the left), despite the absence of significant sensory or motor deficit 14, 21. Typically, when asked to indicate the middle of a visually perceived line (i.e., visuo-motor bisection task), right brain-damaged patients with left neglect tend to transect the line to the right of the geometric midpoint [34]. Certain studies have emphasized the role of exploration strategies on left neglect 29, 36, 37. Weintraub and Mesulam [36], suggest that damage to the right hemisphere (but not to the left), impairs search strategies and this may contribute to the severity of the visual inattention observed in patients with this kind of lesion. Their study revealed that patients with left sided lesions like normal control subjects initiated the search on the left side of the page and spontaneously adopted a systematic search pattern even with an unstructured array. In contrast, patients with right cerebral lesions typically began on the right side of the page and searched erratically, particularly when the stimulus array was unstructured (see also Refs. 2, 16). The `right bias' observed in bisection tasks among left-neglect patients, may be reduced by visual 30, 33or spatio-motor cueing 17, 31but also by manipulating scanning direction.

Reuter-Lorenz and Posner [29], showed that the rightward transection observed among right-damaged patients with neglect decreased in a passive visuo-motor bisection task, when the experimentor moved a pen along the line from left to right as opposed to the right-to-left condition, which increased the amount of rightward shift. This effect of scanning direction on passive line bisection was replicated by Mattingley et al. [24]among normal subjects and left neglect patients.

In the same way, the study of Halligan and Marshall [20]revealed an effect of the starting position and in this way of the scanning direction on line bisection performed with computerised visual display unit.

In fact, Halligan et al. [18]maintain that bisection is dependent upon the `attentional' direction to approach the midpoint; they hypothesize that left neglect patients approach the midpoint from the right and stop short of the true midpoint by a value corresponding to their (pathologically increased) Weber fraction.

The use of bisection protocols with normal subjects has made obvious an asymmetric perception of space, with a tendency to place the subjective middle to the left of the objective middle when the subject is asked to estimate the centre of a line 6, 7, 23, 26or a rod 5, 6, 7, 11, 32. The position of the subjective middle during visuo-motor or tactilo-kinesthetic bisection tasks seems sensitive to various parameters, including the laterality of the subject [32], the hand used 11, 32, gravitational coordinates [6]and the direction of gaze [11].

Two recent studies 10, 12demonstrated that the estimation of the subjective middle during a classical visuo-motor task may depend on the subject's reading habits. Left-to-right readers (French normal subjects) deviated to the left of the objective middle, whereas right-to-left readers (Israeli subjects) deviated to the right. This effect of reading habits on normal subjects' performance was also found in a line extension task, where the subject had to construct either the right half of a line from the left given half line or the reverse [9]. Whereas left-to-right readers significantly underconstructed the left half from the right given one, right-to-left readers exhibited no significant bias during this task. Interestingly, when two left neglect patients with opposite reading habits were studied with the same paradigm, they did not show a different pattern from each other, both underconstructing the right half from the left given half and overconstructing the left half from the right given one [9], thus confirming previous results in that they reproduced the usual rightward bias in the line extension task 3, 4. Note also that, in a straight-ahead pointing task, the position of the subjective straight ahead was dependent upon the motor direction in both normal and neglect subjects 8, 13.

Thus, these findings supported the idea that acquired directional trends like reading habits may affect subjects' performance in visuo-spatial tasks [1].

The aim of the present study was to investigate how an imposed scanning direction can play a role on space perception among normal dextrals and neglect patients with opposite reading habits. In other words, is it possible to change the perception of the subjective middle of a normal subject by changing the scanning direction of the line? We employed a passive visuo-motor bisection task, with exploration of the line from left to right or from right to left in 60 normal dextrals with opposite reading habits (30 left-to-right readers and 30 right-to-left readers) and in two left unilateral neglect patients with opposite reading habits.

On the basis of the previously reviewed findings, we expected that an imposed scanning direction from left to right or from right to left may have had an effect on the perception of the subjective middle by normal and neglect subjects.