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Is left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction
~
Pergamon
Neuropsychologia, Vol. 34, No. 5, pp. 467M70, 1996 Copyright ~) 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0028-3932 /96 $15.00 + 0.00
0028 3932(95)00130-1
Note Is left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction ERIC E. BRODIE*
and LAURAE.
L.
PETTIGREW
Department of Psychology, Glasgow Caledonian University, Glasgow G4 0BA, Scotland, U.K.
(Received 5 April 1995; accepted 4 August 1995)
Abstract--Directional deviations in visual line bisection were investigated using normal subjects. Significant main effects were found for hand and initial scan direction resulting from significantly greater deviations to the left by the left hand compared with the right hand and by a scan from the left compared with a scan from the right. These results suggest that the amelioration of neglect can only be inferred from the left hand deviations of neglect patients if they are significantly leftwards of the objective middle and that the degree of leftward deviation in normal subjects results from an interaction between right hemispheric activation and unilateral allocation of attention. Key Words: hand; hemispace; scan strategy; visual line bisection.
attempt to control which scanning strategy a subject will adopt whilst performing visual line bisection. Unidirectional deviations regardless of hand and hemispace should result if the sole reason for the directional deviation present in visual line bisection is a consistently applied scan strategy. Although only a few studies have measured both left and right hands performing a standard visual line bisection task inconsistent results were reported. In a single case study the left hand was found to deviate to the left and the right hand to the right [12]. In group studies deviations to the left were found for the right hand [1,28, 29] and for the left hand [28, 29] and no deviation in either direction was found for the left hand [1], though in two of these studies the leftward deviations of the left hand were found to be significantly different from zero [28, 29]. Hemispatial mechanisms have been implicated as the source of the directional deviations present in line bisection. For example, significant directional deviations for tactile line bisection have only been found at the midline position suggesting differential scaling of left and right egocentric space [7]. Studies of visual line bisection report inconsistent findings for standard administration conditions (no cue and free scanning) with deviations in right hemispace found to be non-significantly leftwards [6,21] and rightwards [20] and deviations in left hemispace to be non-significantly rightwards [21] and significantly leftwards [6, 20, 29]. However, as the right hand was only tested in these studies and as the scan strategy was not controlled it is difficult to be certain that the significant leftward deviations resulted simply from hemispatial mechanisms. The directional deviations displayed by neglect patients performing visual line bisection can be modified if unilateral cueing is introduced [21,24]. Two studies of normal subjects have reported significant displacement of directional deviations
Introduction Studies investigating visual line bisection in normal subjects have not consistently reported 'pseudoneglect' [3], the placing of the subjective middle of a line to the left of the objective middle. Out of 15 approximately equivalent studies investigating visual line bisection (normal subjects using their preferred hand to bisect lines presented at the midline) only four reported significant leftwards deviations [4, 6, 20, 28]. A further three studies reported non-significant leftwards deviations [9, 18, 19]. Two studies reported no significant deviations in either direction [23, 32] and three reported that approximately half the subjects deviated to the right and half to the left [11, 12, 17]. Finally three studies reported non-significant rightwards deviations [14,21,29]. A similar discrepancy between comparable studies investigating tactile line bisection was found [3, 13, 26, 27] and has been explained in part by subjects adopting different strategies when performing the task [7, 16]. Scanning strategies have been implicated as the source of directional deviations in visual line bisection with a left-to-right scan thought to result in a deviation to the left and a right-toleft scan in a deviation to the right [17, 18]. Although a left-toright scanning strategy may be 'innate' [15] it is more likely to result from over-learned habits acquired during learning to read as left-to-right readers have been found to deviate to the left and right-to-left readers to deviate to the right in visual line bisection [8]. Nevertheless, in most studies there is normally no
* To whom correspondence should be addressed; fax: 0141331-3636. 467
468 towards the side of cueing when the letter cues were presented beyond the end of the line [20, 21] though one study has reported no cueing effects when cues were presented above the end of the line [19]. It is not clear whether a unilateral cue merely initiates a directional scan from the ipsilateral side of the line or whether it orients attention to the ipsilateral side of the line. The aim of this study is to disentangle the influences of hand, spatial position and initial scan strategy upon the directional deviations in visual line bisection.
Method
Subjects. Eighteen subjects (nine female, nine male) participated as unpaid volunteers in the experiment. All were righthanded and were assessed by the administration of the Edinburgh Handedness Inventory [22]. Materials. One of five 2 mm thick stimulus lines was printed in black upon an A4 sheet of white paper at one of four spatial positions. The lines were 8, 10, 12, 14 and 16 cm in length and were printed with their midpoints either 9, 10, 11 or 12 cm from the edge of the page. The position on the page of each line was varied so that the middle of the stimulus line was offset relative to the middle of the page edge to prevent subjects utilising the page edge as a cue for the bisection task. Five line lengths at four page positions resulted in 20 pages that were bound into a booklet in a randomised order. Procedure. The experimenter administered the Handedness Inventory at the start of the test session and ensured its correct completion. The Inventory was scored to yield a Hand Preference Quotient (HPQ) between - 1 0 0 (strong left) and + 100 (strong right) by the formula R - L / R + L × 100, where R was the number of crosses for the right hand and L was the number of crosses for the left. Subjects were seated at the experimental table opposite the experimenter, who ensured that the subject's body position remained constant for the duration of the experiment. Three booklets were placed before each subject at left, mid and right spatial positions. Before each trial the middle of the stimulus line to be bisected by the subject was horizontally aligned either 25 cm to the left of the subject, in front of the subject's midline or 25 cm to the right by the experimenter. Subjects were instructed by the experimenter to bisect the line centrally as accurately as possible after scanning either from the left hand side of the line or from the right hand side of the line. Before each trial the experimenter informed the subjects of the spatial position of the line to be bisected and of the hand and of the side from which to scan. Subjects were aware that the task was not timed but were encouraged to perform the task quickly. After each trial the experimenter turned to the next page in the booklet. Each session consisted of a total of 60 trials (plus two practice trials) with ~" randomised sequence generated for spatial position, hand and scanning direction. Directional deviations were measured to the nearest millimeter with errors to the left of the midpoint given a negative value and errors to the right a positive value.
Note Table 1. Directional deviations (in mm) for visual line bisection as a function of hand and scan direction Hand Scan
Left
Right
from left
from right
from left
from right
Subject 1 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17 18
-2.20 -2.73 - 2.40 -2.27 - 1.47 - 1.40 - 1.33 - 1.67 - 1.73 -4.00 -0.93 - 1.47 -1.40 -0.67 - 1.73 -2.73 -3.67 -2.20
-1.27 -0.13 0.00 - 1.00 -0.33 -0.47 -0.87 -2.47 -2.33 -3.40 +0.67 - 1.67 -1.33 + 1.80 -0.47 -1.73 -4.67 -0.73
-3.80 -0.53 - 0.47 - 1.93 +0.73 +2.07 - 1.40 - 1.47 + 1.20 -3.07 + 1.43 -0.47 +0.07 +2.00 - 1.40 -2.80 -3.20 -2.27
-1.53 +1.07 + 1.07 +2.33 -0.40 +3.20 +0.60 -0.93 + 1.87 -2.53 + 1.93 - 1.40 +1.00 -0.93 -0.67 -0.27 -0.33 + 1.67
Mean S.D.
-2.00 0.88
- 1.13 1.48
-0.85 1.83
+0.32 1.55
- Signifies a deviation to the left; + signifies a deviation to the right.
right) hand (left, right) and initial scanning direction (from the left, from the right), with repeated measures on the last three factors. The analysis revealed significant main effects for hand [F(I, 16) = 22.70, P < 0.001] due to use of the left hand producing significantly greater leftwards deviations compared to the right hand [left hand, mean ( S . D . ) = - 1 . 5 7 mm (I.09), right hand, mean (S.D.) = - 0 . 2 6 mm (1.45)] and for scanning direction [F(1, 1 6 ) = 18.48, P < 0.005] due to scanning from the left producing significantly greater leftwards deviations than scanning from the right [scan from the left, mean (S.D.)=-1.43 mm (1.27); scan from the right, mean (S.D.) = - 0 . 4 1 mm (1.23)]. There were no significant interactions. T-tests revealed that the leftwards deviations of the left hand were significantly different from zero for both scanning from the left (t = - 9 . 6 7 8 , d f l 7 , P < 0.0005) and scanning from the right (t = - 3 . 2 3 8 , df 17, P < 0.005). Deviations of the right hand from zero approached significance for scanning from the left (t = - 1.972, df 17, P < 0.07) but were not significantly different from zero for scanning from the right (t = 0.876, df 17, P > 0.05). The absolute errors were submitted to the A N O V A by sex (male, female), spatial position (left, midline, right), hand (left, right) and scanning direction (from the left, from the right), with repeated measures on the last three factors. No main effects or interactions reached significance.
Results Discussion
The mean H P Q of the subjects was + 9 7 with the values ranging from + 7 9 to + 1 0 0 indicating a strong right hand preference. The constant errors were submitted to an analysis of variance ( A N O V A ) by sex (male~ female), spatial position (left, midline,
This study confirms that for right-handed subjects visual line bisection performed with the left hand results in significantly greater deviations to the left than when performed with the right hand. Additionally the initial scan direction was found
Note to have a significant influence because scanning from the left resulted in significantly greater deviations to the left than scanning from the right. Thus the lack of consistent deviations in the visual line bisection literature may be explained in part by subjects adopting inconsistent scanning strategies. However, neither the effects of hand nor scan strategy was found to be sufficient to overcome the overall tendency to bisect to the left of the objective midpoint. The leftward deviations of the left hand were found to be significantly different from zero for both of the scan strategies, the leftward deviations of the right hand after scanning from the left were almost significantly different from zero and only the right hand was found to have a very small (non-significant)rightward deviation when subjects initially scanned from the right. These findings cast doubt upon the idea that a right-to-left scan path [8, 12] or an overestimation of extent resulting from the allocation of attention to the right [21] can induce significant rightward directional deviations in normal subjects but reveals that such factors can significantly modify leftwards deviations. It may be argued that instructing a subject to scan initially from the left or from the right is simply a form of unilateral cueing. For brain damaged subjects a cue beyond the left has been shown to reduce directional deviations to the right [19, 21,24] though only one of these studies found that a cue to the right increased deviations to the right [24]. In normal subjects when attention was cued beyond the end of the line by a letter a significant main effect for cue has been found, indicating that the directional deviations were drawn towards the side of the cue [20,21]. However, the suggestion that a visible cue beyond the end of the line alters the perceptual equilibrium of the visual display towards the side of cue cannot explain the deviations in this study as no visible cues were present nor can it explain the reduction in left neglect when the cue was placed at the left end of the line [10]. Also the empirical evidence in support of the lack of significant directional deviations once the cue is not placed beyond the endpoint of the line may have been compromised by small subject numbers [19]. Inspection of the data reveals that 67% of subjects deviated to the left in the two left cue conditions and to the right in the two right cue conditions, suggesting that a significant cueing effect may emerge with larger subject numbers. Nevertheless it is possible that the scanning strategy used in this study and unilateral cueing may invoke similar attentional mechanisms, as recent research suggests that attention is not allocated immediately to a target but moves along a path towards it [30]. Thus a rapid attentional shift or saccade from a unilateral cue to the midpoint of a line is likely to involve a unidirectional attentional scan of part of the line. In this study not only were the leftward deviations of the left hand significantly greater than those of the right hand but also significantly to the left of the objective middle, a finding in keeping with previous studies [5, 28, 29]. This has an important implication for the interpretation of the directional deviations displayed by neglect patients. When visual line bisection is performed by a neglect patient using the left hand it is therefore only correct to conclude that neglect is reduced if significant leftwards deviations from the objective middle remain present. A number of studies have reported the absence of significant deviations in either direction for the left hand with the presence of significant rightwards directional deviations for the right hand [9, 12]. The amelioration of neglect has not been demonstrated in these cases as the directional deviations of both left and right hands has been displaced rightwards when compared to the directional deviations of normal subjects reported in this and in other studies [5, 28, 29]. The significant leftward deviations of the left hand for both scan strategies provides further evidence that both left hemisphere activation and attentional allocation interact to produce the bias present in visual line bisection [20]. However, paying
469 more attention to the left side of a line and less to the right may have resulted either from the additional attention required to use the non-preferred hand or from the activation of the right hemisphere by the left hand. The latter explanation is preferred as significant leftwards deviations for the left hand of lefthanded subjects have been reported [28] and a significant leftward deviation for the left hand with a right-to-left scan strategy was found in this study. In conclusion, the bias displayed by normal right-handed subjects when bisecting a visually presented line was found to be a function of the hand and of the initial scan strategy used to perform the task. Using the left hand or initially scanning from the left will result in significant leftward deviations whereas initially scanning from the right with the right hand will normally result in no significant deviations from the objective midpoint. Although the hand effect implicates right hemispheric activation and the scan effect implicates attentional factors as possible sources of the bias the presence of an underlying perceptual [20] or representational [2] distortion of a horizontal line cannot be ruled out. This is because the visual analysis of spatial extent is likely to be tailored to action rather than to an objective spatial metric [31] and because the initiation of a movement automatically results in the recruitment of attention [251.
References 1. Bisiach, E., Capitani, E., Sterzi, R. and Vallar, G. Halving a horizontal segment: A study on hemisphere-damaged patients with cerebral focal lesions. Arch. Suisses Neurol. Neurochir. Psyehiat. 118, 199-206, 1976. 2. Bisiach, E., Rusconi, M. L., Peretti, V. A. and Vallar, G. Challenging current accounts of unilateral neglect. Neuropsychologia 32, 1431-1434, 1994. 3. Bowers, D. and Heilman, K. Pseudoneglect: Effects of hemispace on a tactile line bisection task. Neuropsyehologia 18, 491-498, 1980. 4. Bradshaw, J. L., Nettleton, N. C., Nathan, G. and Wilson, L. E. Bisecting rods and lines: Effects of horizontal and vertical posture on left-side underestimation by normal subjects. Neuropsycholoqia 23, 421-425, 1985. 5. Bradshaw, J. L., Bradshaw, J. A., Nathan, G., Nettleton, N. C. and Wilson, L. E. Leftward errors in bisecting the gap between two points: Stimulus quality and hand effects. Neuropsyeholoqia 24, 849 855, 1986. 6. Bradshaw, J. L., Nathan, G., Nettleton, N. C., Wilson, L. E. and Pierson, J. M. Why is there a left side underestimation in rod bisection? Neuropsycholoqia 25, 735 738, 1987. 7. Brodie, E. E. and Pettigrew, L. E. L. Spatial field advantages for tactile line bisection as a function of hemispheric specialization inferred from dichotic listening. Neuropsycholoyia 33, 53-61, 1995. 8. Chokron, S. and Imbert, M. Influence of reading habits on line bisection. Cognit. Brain Res. 1,219-222, 1993. 9. Halligan, P. W. and Marshall, J. C. Line bisection in visuospatial neglect: Disproof of a conjecture. Cortex 25, 503515, 1989. 10. Halligan, P. W., Donegan, C. and Marshall, J. C. When is a cue not a cue? On the intractability of visuo-spatial neglect. Neuropo,ch. Rehabil. 2, 283 293, 1992. 11. Halligan, P. W., Manning, L. and Marshall, J. C. Individual variation in line bisection: A study of four patients with right hemispheric damage and normal controls. NeuropsTchologia 28, 1043-1051, 1990. 12. Halligan, P. W., Manning, L. and Marshall, J. C. Hemi-
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Hemi-inattention and Hemispheric Specialization, E. A. 16. 17.
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Weinstein and R. P. Freidland (Editors). Raven Press, New York, 1977. Levander, M., Tegn6r, R. and Caneman, A. Tactile line bisection in normal subjects. Percept. Motor Skills 76, 123 131, 1993. Manning, L., Halligan, P. W. and Marshall, J. C. Individual variation in line bisection: A study of normal subjects with application to the interpretation of visual neglect. Neuropsychologia 28, 647-655, 1990. Marshall, J. C. and Halligan, P. W. When right goes to left: An investigation of line bisection in a case of visual neglect. Cortex 25, 503-515, 1989. Mattingley, J. B., Pierson, J. M., Bradshaw, J. L., Phillips, J. G. and Bradshaw, J. To see or not to see: The effects of visible and invisible cues on line bisection judgements and unilateral neglect. Neuropsychologia 31, 1201--1215, 1993. Milner, A. D., Brechmann, M. and Pagliarini, L. To halve and to halve not: An analysis of line bisection judgements in normal subjects. Neuropsychologia 30, 515-526, 1992. Nichelli, P., Rinaldi, M. and Cubelli, R. Selective spatial attention and length representation in normal subjects and
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Pergamon
Neuropsychologia, Vol. 34, No. 5, pp. 467M70, 1996 Copyright ~) 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0028-3932 /96 $15.00 + 0.00
0028 3932(95)00130-1
Note Is left always right? Directional deviations in visual line bisection as a function of hand and initial scanning direction ERIC E. BRODIE*
and LAURAE.
L.
PETTIGREW
Department of Psychology, Glasgow Caledonian University, Glasgow G4 0BA, Scotland, U.K.
(Received 5 April 1995; accepted 4 August 1995)
Abstract--Directional deviations in visual line bisection were investigated using normal subjects. Significant main effects were found for hand and initial scan direction resulting from significantly greater deviations to the left by the left hand compared with the right hand and by a scan from the left compared with a scan from the right. These results suggest that the amelioration of neglect can only be inferred from the left hand deviations of neglect patients if they are significantly leftwards of the objective middle and that the degree of leftward deviation in normal subjects results from an interaction between right hemispheric activation and unilateral allocation of attention. Key Words: hand; hemispace; scan strategy; visual line bisection.
attempt to control which scanning strategy a subject will adopt whilst performing visual line bisection. Unidirectional deviations regardless of hand and hemispace should result if the sole reason for the directional deviation present in visual line bisection is a consistently applied scan strategy. Although only a few studies have measured both left and right hands performing a standard visual line bisection task inconsistent results were reported. In a single case study the left hand was found to deviate to the left and the right hand to the right [12]. In group studies deviations to the left were found for the right hand [1,28, 29] and for the left hand [28, 29] and no deviation in either direction was found for the left hand [1], though in two of these studies the leftward deviations of the left hand were found to be significantly different from zero [28, 29]. Hemispatial mechanisms have been implicated as the source of the directional deviations present in line bisection. For example, significant directional deviations for tactile line bisection have only been found at the midline position suggesting differential scaling of left and right egocentric space [7]. Studies of visual line bisection report inconsistent findings for standard administration conditions (no cue and free scanning) with deviations in right hemispace found to be non-significantly leftwards [6,21] and rightwards [20] and deviations in left hemispace to be non-significantly rightwards [21] and significantly leftwards [6, 20, 29]. However, as the right hand was only tested in these studies and as the scan strategy was not controlled it is difficult to be certain that the significant leftward deviations resulted simply from hemispatial mechanisms. The directional deviations displayed by neglect patients performing visual line bisection can be modified if unilateral cueing is introduced [21,24]. Two studies of normal subjects have reported significant displacement of directional deviations
Introduction Studies investigating visual line bisection in normal subjects have not consistently reported 'pseudoneglect' [3], the placing of the subjective middle of a line to the left of the objective middle. Out of 15 approximately equivalent studies investigating visual line bisection (normal subjects using their preferred hand to bisect lines presented at the midline) only four reported significant leftwards deviations [4, 6, 20, 28]. A further three studies reported non-significant leftwards deviations [9, 18, 19]. Two studies reported no significant deviations in either direction [23, 32] and three reported that approximately half the subjects deviated to the right and half to the left [11, 12, 17]. Finally three studies reported non-significant rightwards deviations [14,21,29]. A similar discrepancy between comparable studies investigating tactile line bisection was found [3, 13, 26, 27] and has been explained in part by subjects adopting different strategies when performing the task [7, 16]. Scanning strategies have been implicated as the source of directional deviations in visual line bisection with a left-to-right scan thought to result in a deviation to the left and a right-toleft scan in a deviation to the right [17, 18]. Although a left-toright scanning strategy may be 'innate' [15] it is more likely to result from over-learned habits acquired during learning to read as left-to-right readers have been found to deviate to the left and right-to-left readers to deviate to the right in visual line bisection [8]. Nevertheless, in most studies there is normally no
* To whom correspondence should be addressed; fax: 0141331-3636. 467
468 towards the side of cueing when the letter cues were presented beyond the end of the line [20, 21] though one study has reported no cueing effects when cues were presented above the end of the line [19]. It is not clear whether a unilateral cue merely initiates a directional scan from the ipsilateral side of the line or whether it orients attention to the ipsilateral side of the line. The aim of this study is to disentangle the influences of hand, spatial position and initial scan strategy upon the directional deviations in visual line bisection.
Method
Subjects. Eighteen subjects (nine female, nine male) participated as unpaid volunteers in the experiment. All were righthanded and were assessed by the administration of the Edinburgh Handedness Inventory [22]. Materials. One of five 2 mm thick stimulus lines was printed in black upon an A4 sheet of white paper at one of four spatial positions. The lines were 8, 10, 12, 14 and 16 cm in length and were printed with their midpoints either 9, 10, 11 or 12 cm from the edge of the page. The position on the page of each line was varied so that the middle of the stimulus line was offset relative to the middle of the page edge to prevent subjects utilising the page edge as a cue for the bisection task. Five line lengths at four page positions resulted in 20 pages that were bound into a booklet in a randomised order. Procedure. The experimenter administered the Handedness Inventory at the start of the test session and ensured its correct completion. The Inventory was scored to yield a Hand Preference Quotient (HPQ) between - 1 0 0 (strong left) and + 100 (strong right) by the formula R - L / R + L × 100, where R was the number of crosses for the right hand and L was the number of crosses for the left. Subjects were seated at the experimental table opposite the experimenter, who ensured that the subject's body position remained constant for the duration of the experiment. Three booklets were placed before each subject at left, mid and right spatial positions. Before each trial the middle of the stimulus line to be bisected by the subject was horizontally aligned either 25 cm to the left of the subject, in front of the subject's midline or 25 cm to the right by the experimenter. Subjects were instructed by the experimenter to bisect the line centrally as accurately as possible after scanning either from the left hand side of the line or from the right hand side of the line. Before each trial the experimenter informed the subjects of the spatial position of the line to be bisected and of the hand and of the side from which to scan. Subjects were aware that the task was not timed but were encouraged to perform the task quickly. After each trial the experimenter turned to the next page in the booklet. Each session consisted of a total of 60 trials (plus two practice trials) with ~" randomised sequence generated for spatial position, hand and scanning direction. Directional deviations were measured to the nearest millimeter with errors to the left of the midpoint given a negative value and errors to the right a positive value.
Note Table 1. Directional deviations (in mm) for visual line bisection as a function of hand and scan direction Hand Scan
Left
Right
from left
from right
from left
from right
Subject 1 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17 18
-2.20 -2.73 - 2.40 -2.27 - 1.47 - 1.40 - 1.33 - 1.67 - 1.73 -4.00 -0.93 - 1.47 -1.40 -0.67 - 1.73 -2.73 -3.67 -2.20
-1.27 -0.13 0.00 - 1.00 -0.33 -0.47 -0.87 -2.47 -2.33 -3.40 +0.67 - 1.67 -1.33 + 1.80 -0.47 -1.73 -4.67 -0.73
-3.80 -0.53 - 0.47 - 1.93 +0.73 +2.07 - 1.40 - 1.47 + 1.20 -3.07 + 1.43 -0.47 +0.07 +2.00 - 1.40 -2.80 -3.20 -2.27
-1.53 +1.07 + 1.07 +2.33 -0.40 +3.20 +0.60 -0.93 + 1.87 -2.53 + 1.93 - 1.40 +1.00 -0.93 -0.67 -0.27 -0.33 + 1.67
Mean S.D.
-2.00 0.88
- 1.13 1.48
-0.85 1.83
+0.32 1.55
- Signifies a deviation to the left; + signifies a deviation to the right.
right) hand (left, right) and initial scanning direction (from the left, from the right), with repeated measures on the last three factors. The analysis revealed significant main effects for hand [F(I, 16) = 22.70, P < 0.001] due to use of the left hand producing significantly greater leftwards deviations compared to the right hand [left hand, mean ( S . D . ) = - 1 . 5 7 mm (I.09), right hand, mean (S.D.) = - 0 . 2 6 mm (1.45)] and for scanning direction [F(1, 1 6 ) = 18.48, P < 0.005] due to scanning from the left producing significantly greater leftwards deviations than scanning from the right [scan from the left, mean (S.D.)=-1.43 mm (1.27); scan from the right, mean (S.D.) = - 0 . 4 1 mm (1.23)]. There were no significant interactions. T-tests revealed that the leftwards deviations of the left hand were significantly different from zero for both scanning from the left (t = - 9 . 6 7 8 , d f l 7 , P < 0.0005) and scanning from the right (t = - 3 . 2 3 8 , df 17, P < 0.005). Deviations of the right hand from zero approached significance for scanning from the left (t = - 1.972, df 17, P < 0.07) but were not significantly different from zero for scanning from the right (t = 0.876, df 17, P > 0.05). The absolute errors were submitted to the A N O V A by sex (male, female), spatial position (left, midline, right), hand (left, right) and scanning direction (from the left, from the right), with repeated measures on the last three factors. No main effects or interactions reached significance.
Results Discussion
The mean H P Q of the subjects was + 9 7 with the values ranging from + 7 9 to + 1 0 0 indicating a strong right hand preference. The constant errors were submitted to an analysis of variance ( A N O V A ) by sex (male~ female), spatial position (left, midline,
This study confirms that for right-handed subjects visual line bisection performed with the left hand results in significantly greater deviations to the left than when performed with the right hand. Additionally the initial scan direction was found
Note to have a significant influence because scanning from the left resulted in significantly greater deviations to the left than scanning from the right. Thus the lack of consistent deviations in the visual line bisection literature may be explained in part by subjects adopting inconsistent scanning strategies. However, neither the effects of hand nor scan strategy was found to be sufficient to overcome the overall tendency to bisect to the left of the objective midpoint. The leftward deviations of the left hand were found to be significantly different from zero for both of the scan strategies, the leftward deviations of the right hand after scanning from the left were almost significantly different from zero and only the right hand was found to have a very small (non-significant)rightward deviation when subjects initially scanned from the right. These findings cast doubt upon the idea that a right-to-left scan path [8, 12] or an overestimation of extent resulting from the allocation of attention to the right [21] can induce significant rightward directional deviations in normal subjects but reveals that such factors can significantly modify leftwards deviations. It may be argued that instructing a subject to scan initially from the left or from the right is simply a form of unilateral cueing. For brain damaged subjects a cue beyond the left has been shown to reduce directional deviations to the right [19, 21,24] though only one of these studies found that a cue to the right increased deviations to the right [24]. In normal subjects when attention was cued beyond the end of the line by a letter a significant main effect for cue has been found, indicating that the directional deviations were drawn towards the side of the cue [20,21]. However, the suggestion that a visible cue beyond the end of the line alters the perceptual equilibrium of the visual display towards the side of cue cannot explain the deviations in this study as no visible cues were present nor can it explain the reduction in left neglect when the cue was placed at the left end of the line [10]. Also the empirical evidence in support of the lack of significant directional deviations once the cue is not placed beyond the endpoint of the line may have been compromised by small subject numbers [19]. Inspection of the data reveals that 67% of subjects deviated to the left in the two left cue conditions and to the right in the two right cue conditions, suggesting that a significant cueing effect may emerge with larger subject numbers. Nevertheless it is possible that the scanning strategy used in this study and unilateral cueing may invoke similar attentional mechanisms, as recent research suggests that attention is not allocated immediately to a target but moves along a path towards it [30]. Thus a rapid attentional shift or saccade from a unilateral cue to the midpoint of a line is likely to involve a unidirectional attentional scan of part of the line. In this study not only were the leftward deviations of the left hand significantly greater than those of the right hand but also significantly to the left of the objective middle, a finding in keeping with previous studies [5, 28, 29]. This has an important implication for the interpretation of the directional deviations displayed by neglect patients. When visual line bisection is performed by a neglect patient using the left hand it is therefore only correct to conclude that neglect is reduced if significant leftwards deviations from the objective middle remain present. A number of studies have reported the absence of significant deviations in either direction for the left hand with the presence of significant rightwards directional deviations for the right hand [9, 12]. The amelioration of neglect has not been demonstrated in these cases as the directional deviations of both left and right hands has been displaced rightwards when compared to the directional deviations of normal subjects reported in this and in other studies [5, 28, 29]. The significant leftward deviations of the left hand for both scan strategies provides further evidence that both left hemisphere activation and attentional allocation interact to produce the bias present in visual line bisection [20]. However, paying
469 more attention to the left side of a line and less to the right may have resulted either from the additional attention required to use the non-preferred hand or from the activation of the right hemisphere by the left hand. The latter explanation is preferred as significant leftwards deviations for the left hand of lefthanded subjects have been reported [28] and a significant leftward deviation for the left hand with a right-to-left scan strategy was found in this study. In conclusion, the bias displayed by normal right-handed subjects when bisecting a visually presented line was found to be a function of the hand and of the initial scan strategy used to perform the task. Using the left hand or initially scanning from the left will result in significant leftward deviations whereas initially scanning from the right with the right hand will normally result in no significant deviations from the objective midpoint. Although the hand effect implicates right hemispheric activation and the scan effect implicates attentional factors as possible sources of the bias the presence of an underlying perceptual [20] or representational [2] distortion of a horizontal line cannot be ruled out. This is because the visual analysis of spatial extent is likely to be tailored to action rather than to an objective spatial metric [31] and because the initiation of a movement automatically results in the recruitment of attention [251.
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