Cortex
Volume 47, Issue 1 , Pages 47-52, January 2011

Line bisection in unilateral homonymous visual field defects

  • Susanne Schuett

      Affiliations

    • Department of Psychology, University of Durham, UK
    • Corresponding Author InformationCorresponding author. Department of Psychology, University of Durham, Science Laboratories, South Road, Durham City DH1 3LE, UK.
    • ,
  • Ruth Dauner

      Affiliations

    • Department of Psychology, Neuropsychology, Ludwig-Maximilians-Universität Munich, Germany
    • ,
  • Josef Zihl

      Affiliations

    • Department of Psychology, Neuropsychology, Ludwig-Maximilians-Universität Munich, Germany
    • Max-Planck-Institute of Psychiatry, Neuropsychology, Munich, Germany

Received 20 July 2009; received in revised form 15 September 2009 and 27 November 2009; accepted 16 January 2010. published online 22 February 2010.

Action editor Giuseppe Vallar

Article Outline

Abstract 

The contralesional line bisection error in unilateral homonymous hemianopia is a frequent but neglected clinical phenomenon. Our knowledge about this bisection error is based on small samples of hemianopic patients. Moreover, horizontal line bisection has never been investigated in other unilateral visual field defects. The present study is the first to examine line bisection in a large, representative sample of patients with unilateral homonymous visual field defects. We investigated horizontal line bisection in 129 patients with left- or right-sided homonymous hemianopia (60.5%), upper and lower quadranopia (24.8%), and paracentral scotoma (14.7%), and determined the magnitude and direction of line bisection error. The contralesional horizontal line bisection error was present not only in patients with hemianopia but also in those with upper or lower quadranopia or paracentral scotoma. Neither the type nor the severity of the visual field defect was found to determine the bisection error. Only the side of the field defect seemed to determine the horizontal direction of the bisection error (left-/rightward). The contralesional bisection error is not a specifically “hemianopic” phenomenon. It is frequently associated with any unilateral homonymous visual field defect, i.e., hemianopia, upper/lower quadranopia, paracentral scotoma. Moreover, our results further support the recent finding that the contralesional bisection error is not a direct consequence of the visual field defect. Yet, they also suggest that, although the visual field defect does not seem to be the primary cause of the contralesional bisection error, it may nevertheless contribute to it.

Keywords: Line bisection, Visual field defect, Visual field, Visual-spatial perception

 

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1. Introduction 

Unilateral homonymous visual field defects are one of the most frequent functional sequelae of acquired brain injury (Suchoff et al., 2008). They are caused by unilateral injury to the postchiasmatic visual pathway, which is frequently accompanied by extrastriate lesions, and lead to the loss of vision in corresponding parts of both monocular hemifields contralateral to the side of brain injury. Hemianopia is the most common field defect (loss of both monocular hemifields), followed by quadranopia (vision loss in the upper or lower quadrant), and paracentral scotoma (small island-like parafoveal field defect) (Zihl, 2000, Zhang et al., 2006).

It is well-known that these patients show severe impairments of reading (Schuett et al., 2008) and visual exploration (Zihl, 2000). It is not well-known, however, that patients with unilateral homonymous hemianopia also frequently suffer from a persistent spatial distortion that is characterized by a reliable contralesional deviation in manual bisection of horizontal lines towards the side of their affected hemifield (Barton and Black, 1998, Barton et al., 1998, Zihl, 2000, Hausmann et al., 2003, Doricchi et al., 2005). This error is significantly larger than that of normal observers, who typically bisect horizontal lines with a slight leftward error (i.e., pseudoneglect) (Jewell and McCourt, 2000).

The contralesional bisection error may indicate an underlying visual–spatial disorder that affects the horizontal egocentric visual midline and causes a systematic, contralesional shift of the visual midline or subjective straight-ahead direction in visual–spatial judgments as well as spatial orientation problems (Ferber and Karnath, 1999, Kerkhoff, 1999, Zihl, 2000, Zihl et al., 2009). Since such a visual–spatial disorder is unexpected in a pure visual–perceptual deficit such as hemianopia, it is not surprising that unfortunately, and despite a much longer history, the contralesional line bisection error is also less well-known than the ipsilesional bisection error associated with visual–spatial neglect (Kerkhoff and Bucher, 2008).

Only few studies have dealt with line bisection in visual field loss since the first reports on the hemianopic contralesional line bisection error (Axenfeld, 1894, Liepmann and Kalmus, 1900, Best, 1910a, Best, 1910b). Further, most of our knowledge is based on relatively small samples of patients with unilateral homonymous hemianopia (Barton and Black, 1998, Barton et al., 1998, Zihl, 2000, Hausmann et al., 2003, Doricchi et al., 2005). Although a recent study examined the contralesional bisection error and determined its origin in a large sample of hemianopic patients (Zihl et al., 2009), line bisection has never been investigated in other homonymous visual field defects [with the exception of a single report that studied line bisection in six patients with altitudinal visual field defects (Kerkhoff, 1993)]. Thus, it is still unknown whether the contralesional bisection error is a specifically “hemianopic” phenomenon, or whether it is also present in other types of unilateral visual field defects.

We therefore investigated line bisection in a large, representative sample of 129 patients with left- or right-sided unilateral homonymous hemianopia, upper or lower quadranopia, or paracentral scotoma, and determined the magnitude and direction of line bisection error for each subgroup. We also wished to examine the effects of the severity and side of the visual field defect on line bisection performance in order to test whether the contralesional bisection error is a consequence of the visual field defect itself. If the bisection error is a direct consequence of hemianopia, the error should be found in all patients with visual field loss and its magnitude should be negatively correlated with the severity of the visual defect, i.e., the smaller the visual field sparing, the larger the bisection error.

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2. Methods 

2.1. Subjects 

Ethical approval for this study was in accordance with the ethical standards of the Max-Planck-Institute of Psychiatry Munich, and written consent was obtained from all participants. 129 patients with homonymous unilateral visual field defects (visual field sparing≤10°) participated in this study. None of the patients had received any treatment for their visual field defect. Only right-handed patients [laterality quotient of >+80 in the Edinburgh Handedness Inventory (Oldfield, 1971)] were included to eliminate the effects of handedness, which is a significant factor affecting line bisection performance (Jewell and McCourt, 2000), and all patients had adequate motor performance of the right hand. Patients with reduced visual acuity (<.90 for near and far binocular vision), impaired spatial contrast sensitivity as assessed with the Vistech contrast sensitivity test, disturbances of the anterior visual pathways or the oculomotor system (according to ophthalmologic examination), visual neglect, alexia, impaired verbal comprehension, or paresis of the upper extremities were excluded. For excluding patients with any signs of visual neglect, we administered tests similar to those described in the Behavioural Inattention Test (Halligan et al., 1991) (letter and star cancellation, figure and shape copying, and drawing from memory); none of our patients omitted targets in the contralesional hemifield, and copying and drawing from memory were entirely normal. All participants had at least 5 years of education. Demographic and clinical details of the patients are presented in Table 1. Homonymous hemianopia was the most frequent type of visual field loss (60.5%), followed by quadranopia (24.8%), and paracentral scotoma (14.7%). 53.5% of patients showed a left-sided visual field defect, 46.5% a right-sided field defect. Mean visual field sparing was 3.4° (range: 1–10°). Occipital stroke was the most common etiology of brain injury (109 patients, 84.5%); 11 patients (8.5%) suffered from closed head trauma, and 9 patients (7%) underwent surgical removal of an occipital tumor. Time since brain injury was on average 26.1 weeks, and varied between 2 weeks and 8 years.

Table 1. Demographic and clinical data for patients [mean (SD; range)].
Side and type of field defectnField sparing (°)Sex (f/m)Age (years)Time since injury (weeks)
Hemianopia
Left-sided423.3 (2.3; 1–10)11/3157.8 (13.8; 30–85)24.5 (34.5; 2–192)
Right-sided363.6 (2.2; 1–10)15/2147.2 (17.6; 12–79)32.6 (62.7; 5–384)

Upper quadranopia
Left-sided114.6 (2.0; 2–8)1/1048.1 (11.5; 23–59)24.6 (32.6; 4–120)
Right-sided82.9 (2.5; 1–8)2/656.5 (17.7; 19–77)18.4 (19.9; 6–65)

Lower quadranopia
Left-sided84.8 (2.3; 3–10)3/555.6 (12.4; 42–75)10.8 (7.5; 3–23)
Right-sided53.4 (1.8; 1–6)1/452.2 (8.7; 43–63)24.4 (19.1; 9–53)

Paracentral scotoma
Left-sided82.1 (1.1; 1–4)1/757.6 (19.0; 21–80)32.4 (33.9; 6–113)
Right-sided112.1 (1.3; 1–5)4/763.0 (11.3; 45–76)23.7 (22.6; 6–84)

Total left-sided field loss693.5 (2.2; 1–10)16/5356.0 (14.1; 21–85)23.8 (32.1; 2–192)
Total right-sided field loss603.2 (2.1; 1–10)22/3851.7 (17.0; 12–79)28.6 (50.9; 3–384)

Total patient group1293.4 (2.2; 1–10)38/9154.0 (15.6; 12–85)26.1 (41.8; 2–384)

2.2. Visual field testing 

Monocular and binocular visual fields were assessed using kinetic perimetry with a Tübingen perimeter. Target diameter was 69min of arc of visual angle, its luminance was 102cd/m2; background luminance was 3.2cd/m2. The target was moved with a speed of ∼2°/sec from the periphery towards the perimeter's centre. Patients were instructed to fixate a small red spot of light (diameter: 30min of arc) in the centre of the sphere and to press a response button as soon as they detected the target. Fixation accuracy was monitored through a telescope. The visual field border was determined along 16 meridians. Visual field sparing was defined as the extent of visual field in degrees (°) between the fovea and the visual field border along the left or right horizontal axes in cases with hemianopia or paracentral scotoma, or along the main meridians in the right upper (45°), left upper (135°), left lower (225°), or right lower (315°) meridians in cases with upper and lower quadranopia, respectively. Visual field sparing indicates the severity of the visual field defect, i.e., the smaller the field sparing the more severe is the visual field defect.

2.3. Assessment of line bisection 

We assessed line bisection performance using the conventional paper-and-pencil bisection task, which is typically used with hemianopic patients and has been found to be a valid test for assessing line bisection performance in visual field defects (Fischer, 2001). Short (10cm, 18.4° of visual angle) and long (20cm, 33.7°) horizontal lines (width: .2cm) were presented in the centre of separate white paper sheets (one at a time), which were aligned with the patients' midsagittal plane. Five lines of each length were presented in randomized sequence at a viewing distance of 30cm. Patients were instructed to mark the centre of each line, i.e., the position they perceived to be its centre, as accurately as possible by using a fine pencil. Viewing time was unlimited, and eye and head movements were not restricted. To eliminate the effects of hand use on line bisection performance (Jewell and McCourt, 2000), patients were asked to use their right hand. To control for the effects of oculomotor scanning direction (Jewell and McCourt, 2000) and to ensure that patients have seen the entire line, patients were also asked to search for and mark the left end and then the right end of the line before eventually marking the centre of the line (Liepmann and Kalmus, 1900). After marking the subjective line centre, the test sheet was immediately exchanged and the next line presented. Patients were allowed to correct their bisection once, and they never received any feedback to eliminate practice effects and to ensure that subsequent bisections were not biased. The paper-and-pencil line bisection task was performed under normal daylight conditions. For assessing line bisection performance we used the bisection mark and calculated its deviation from the left or right of the objective line centre; measurement accuracy was .5mm. We determined the direction of the bisection error (left- or rightward) and report the mean bisection error (absolute value in mm) of 5 trials as a measure of error magnitude for each line length. Comparison data were available from a sample of 60 control participants (30 females, 30 males) (Zihl et al., 2009), who were tested under the same conditions and whose mean age [52.9 years (standard deviation – SD: 11.7, range: 22–77)] did not significantly differ from the patient group (t(149.8)=.55, p>.05).

2.4. Data analysis 

For analyzing the differences in line bisection performance between the total patient group and the control group as well as between the two total groups of patients with left- and right-sided visual field defects (LVFD, RVFD), and the control group, a planned independent t-test (two-tailed) or an analysis of variance was performed. The differences between patients with hemianopia, upper or lower quadranopia, or paracentral scotoma, and the control group were analyzed with Mann–Whitney-U-tests. We examined the effects of the side and severity of visual field loss on line bisection performance by conducting an analysis of covariance. As multiple tests were carried out, the significance level was adjusted using a Bonferroni correction to an alpha-level of .05 for multiple comparisons (corrected alpha<.05).

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3. Results 

Fig. 1 presents the line bisection performance, i.e., the magnitude and direction of the bisection error, for the different subgroups of patients as well as for the control group. In the control group, line bisection performance is very accurate; normal observers show a small leftward error for the short line (.85mm, SD=.74, range: 0–4) and a slightly larger error for the long line (1.79mm, SD=1.33, range: 0–7) (Zihl et al., 2009). In the patient group, we found that all patients showed the typical contralesional bisection error, i.e., patients with LVFD showed leftward, patients with RVFD rightward errors. This error was also larger for the long than for the short line (see Fig. 1). Only 7 of 69 patients with LVFD (10.1%), and 7 of 60 patients with RVFD (11.7%) bisected short lines within average bisection error of the control group, and only 3 patients with LVFD (4.3%) and 2 with RVFD (3.3%) bisected long lines within the normal bisection error. Thus, at least 90% of all patients showed the contralesional bisection error.

  • View full-size image.
  • Fig. 1 

    Line bisection performance in normal subjects and patient groups. Mean bisection error (in mm, ±1 SD) for 10cm- (white bars) and 20cm-lines (black bars). N1, N2: normal subjects with left- (10cm-line: n=22, 20cm-line: n=37) and rightward error (10cm-line: n=31, 20cm-line: n=21); LH, RH: patients with left- (n=42) or right-sided (n=36) hemianopia; LQu, RQu: patients with left- (n=11) or right-sided (n=8) upper quadranopia; LQl, RQl: patients with left- (n=8) or right-sided (n=5) lower quadranopia; LPCS, RPCS: patients with left- (n=8) or right-sided (n=11) paracentral scotoma.

The magnitude of the contralesional bisection error was significantly larger than the bisection error of the control group for both line lengths (smaller t(181.92)=12.74, p<.001). This difference was also significant when comparing the control group with the LVFD- and RVFD-groups for both line lengths (smaller F(2,106.47)=81.19, p<.001); the magnitude of the bisection error was significantly greater for patients with LVFD or RVFD than that of the control group (LVFD: smaller t(92.02)=10.25, p<.001; RVFD: smaller t(78.81)=9.11, p<.001). Comparing the error magnitude between the control group and patients with hemianopia, upper or lower quadranopia, or paracentral scotoma also revealed the same significant differences (smallest z=−2.83, p=.02).

We then investigated the effects of the side and severity of the visual field defect (i.e., field sparing) on line bisection performance for both line lengths. Our analyses revealed that neither the side nor the severity of field loss had a significant effect on the error magnitude (non-significant main- or interaction effects: largest F(1,125)=1.36, p=.50). This result was substantiated by the finding that the magnitude of the contralesional bisection error for both line lengths did not significantly differ between LVFD and RVFD (larger t(127)=1.31, p=.19). The side of the visual field defect only determined the direction of the bisection error: all patients with LVFD showed leftward, patients with RVFD rightward errors.

Subsequently, we investigated whether the magnitude of the contralesional bisection error differed between different types of visual field defects. There were no significant differences between the subgroups of patients with hemianopia, upper/lower quadranopia, and paracentral scotoma (largest z=−2.28, p=.29). The only exception was that patients with left-sided hemianopia showed significantly larger bisection errors than those with left-sided upper quadranopia (for both line lengths; smaller z=−2.95, p=.02) and left-sided lower quadranopia (marginal significance for the long line: z=−2.70, p=.06) (see Fig. 1).

It is noteworthy that the error magnitude for both line lengths in the total patient group was neither significantly associated with age or sex nor with time since brain injury (Kendall's taub: largest taub=.66, p>.05), which is consistent with our previous study where the effect of these variables was examined in more detail; in this regard, we have also shown that line bisection errors did not differ between patients in the acute phase and the more chronic patients (Zihl et al., 2009).

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4. Discussion 

Although the contralesional line bisection error in unilateral homonymous hemianopia is a frequent clinical phenomenon, only few studies have dealt with line bisection in homonymous visual field loss. Moreover, the investigated patient samples were relatively small and included only hemianopic patients (Barton and Black, 1998, Barton et al., 1998, Zihl, 2000, Hausmann et al., 2003, Doricchi et al., 2005). This is the first study to investigate line bisection in a large, representative sample of patients with unilateral homonymous visual field defects.

We found that the contralesional horizontal bisection error was present in patients with unilateral homonymous hemianopia, upper or lower quadranopia, as well as in those with a paracentral scotoma. This error was significantly larger than in normal observers and cannot be explained by differences in age or sex. Thus, the contralesional horizontal bisection error is not a specifically “hemianopic” phenomenon. The reliable contralesional deviation in manual bisection of horizontal lines towards the side of their affected hemifield is not only frequently associated with unilateral homonymous hemianopia but also with upper and lower quadranopia and paracentral scotoma, which has not been reported hitherto.

Moreover, our results show that the side of the visual field defect only predicts the direction (left-/rightward) but not the magnitude of the horizontal contralesional bisection error, which has only been demonstrated for (real or simulated) unilateral homonymous hemianopia (Schuett et al., 2009, Zihl et al., 2009) and altitudinal visual field defects thus far (Kerkhoff, 1993). Our results also show that neither the severity of the visual field defect (field sparing up to 10°) nor time since injury, age or sex determines the contralesional bisection error, which has also only been demonstrated for patients with hemianopia (Zihl et al., 2009); it is important to note, however, that patients with visual field defects in the very acute stage may not show the contralesional bisection error at all (Machner et al., 2009). Yet, more importantly, we demonstrated that the type of unilateral homonymous visual field loss (hemianopia, quadranopia, paracentral scotoma) does also not determine the direction or magnitude of the horizontal contralesional bisection error.

These results confirm the recent finding, obtained from patients with unilateral homonymous hemianopia (Zihl et al., 2009) as well as from healthy observers with simulated hemianopia (Schuett et al., 2009), that the horizontal contralesional line bisection error is not a direct consequence of the visual field defect itself [i.e., a natural consequence of the fact that in visual field loss (particularly in hemianopia) the line is viewed in only one hemifield (Barton and Black, 1998, Barton et al., 1998)]. Neither the type nor the severity of the visual field loss or its side was found to determine the bisection error in our patients; the visual field loss was causative only in so far as its side was found to determine the direction of the error. Moreover, our observation of accurate line bisection performance despite visual field loss in a small group of patients is consistent with reports of dissociations between hemianopia and the contralesional bisection error (Best, 1919, Zihl, 2000) and also supports the view that the visual field defect is not the primary cause of the contralesional horizontal bisection error (Schuett et al., 2009, Zihl et al., 2009).

Our finding of larger bisection errors in patients with left-sided hemianopia than patients with upper or lower quadranopia or paracentral scotoma suggests, however, that the visual field defect may nevertheless contribute to the line bisection error, particularly since differences in visual field sparing, age, sex or time since injury could not account for this finding. Postchiasmatic injury causes a bottom-up restriction of the visual field which may impair accurate line bisection since lines are only partly visible (Schuett et al., 2009). Hemianopia causes a greater bottom-up restriction of the visual field than quadranopia or paracentral scotoma and may therefore be associated with larger bisection errors. Yet, we obtained this finding only in patients with left-sided visual field loss. Thus, the type of visual field loss seems to influence line bisection performance only if the right hemisphere is affected by brain injury, which is consistent with the right-hemispheric dominance in visual–spatial tasks (Mennemeier et al., 1997, Fink et al., 2000). The effect of the type of visual field loss on line bisection can therefore not be regarded as purely visual.

In conclusion, our study demonstrated for the first time that the contralesional horizontal line bisection error is not only frequently associated with unilateral homonymous hemianopia but also with upper and lower quadranopia and paracentral scotoma. The contralesional bisection error is a frequent clinical phenomenon in patients with unilateral homonymous visual field loss of any type, side and severity. Yet, although the visual field defect does not seem to be the primary cause of the contralesional bisection error, it may nevertheless contribute to it. Moreover, our results support the view that contralesional horizontal bisection error is not a direct consequence of the visual field defect (Zihl et al., 2009).

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Funding source 

Susanne Schuett was funded by the German Academic Exchange Service (DAAD). Ruth Dauner is funded by the German Federal Ministry of Education and Research (BMBF; Grant 01GW0762). Josef Zihl receives research funding from the German Federal Ministry of Education and Research (BMBF; Grant 01GW0762). These sponsors were not involved in any aspect of the research presented in this article.

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Conflict of interest 

The authors declare no conflict of interest.

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PII: S0010-9452(10)00032-8

doi:10.1016/j.cortex.2010.01.008

Cortex
Volume 47, Issue 1 , Pages 47-52, January 2011

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