Elsevier

Brain and Cognition

Volume 60, Issue 2, March 2006, Pages 156-165
Brain and Cognition

Understanding the effects of moving visual stimuli on unilateral neglect following stroke

https://doi.org/10.1016/j.bandc.2005.11.001Get rights and content

Abstract

Moving visual stimuli have been shown to reduce unilateral neglect (ULN), however, the mechanisms underlying these effects remain poorly understood. This study compared lateralised and non-lateralised moving visual stimuli to investigate whether the spatial characteristics or general alerting properties of moving visual stimuli are responsible for reducing neglect. Post-stroke left neglect patients as well as healthy and patient control subjects were tested on a computerised line bisection task under six visual stimulus conditions. The key finding was that, relative to the no stimulus condition, leftward moving and left-sided moving visual stimuli shifted neglect patients’ bisection errors leftward while the non-lateralised random moving visual stimuli did not reduce neglect patients’ rightward bisection errors. The results provide evidence that spatial characteristics rather than general alerting properties of moving visual stimuli reduce rightward bisection errors in ULN. Moreover, the pattern of findings strongly supports the notion that moving visual stimuli reduce neglect by capturing attention and drawing it to a spatial location rather than by activating the attentional system via superior collicular neurons.

Introduction

Unilateral neglect (ULN) is common following stroke and is characterised by difficulty shifting attention to the side of space opposite the brain lesion (Heilman, Watson, & Valenstein, 1993). A common method used to detect ULN is the line bisection test, which requires the person to place a mark in the centre of a horizontal line. Typically, patients with left ULN make rightward bisection errors, seemingly ignoring the left extent of the line.

Previous research has shown that a stationary visual stimulus, such as a letter or digit at the left end of the line, reduces the magnitude of line bisection error (Lin et al., 1996, Nichelli et al., 1989, Reuter-Lorenz and Posner, 1990, Riddoch and Humphreys, 1983). It was originally thought that left-sided cues may direct attention to the part of the line being neglected (Riddoch & Humphreys, 1983), however, it seems that a visual cue per se is not always mandatory for reducing bisection error. Simply having the person direct their attention to the left by pointing to the left end of the line, for example, can reduce rightward bisection errors (Lin et al., 1996, Mattingley et al., 1993). However, findings from manual line bisection tasks are confounded by the possible effects of moving the hand to the left endpoint of the line. Several studies have demonstrated that movement performed in the affected side of space reduces neglect (Halligan et al., 1991, Halligan et al., 1991, Halligan and Marshall, 1989a, Joanette et al., 1986, Lin et al., 1996, Robertson and North, 1994).

In spite of this limitation, previous research clearly shows that patients with ULN have the ability to shift attention to the neglected side, and that an overt shift of visual attention to the left end of the line at the beginning of the task can reduce rightward bisection errors. However, the difficulty remains that patients with ULN tend not to direct their attention to the affected side unless specifically instructed to do so (Riddoch & Humphreys, 1983).

Clinically, it could be argued that visual cues that rely upon instructions to direct the patient’s attention toward the neglected side are of limited practical value because they are dependent upon a therapist. In contrast, moving visual stimuli may capture attention automatically, thereby eliminating the need for a therapist to guide performance. Left-sided and leftward moving visual stimuli can assist in directing attention to the left (Butter et al., 1990, Mattingley et al., 1994). These effects appear to occur automatically, without any input from another person.

Mattingley et al. (1994) examined the effects of background motion on line bisection performance using a computerised line bisection task. The leftward moving background shifted the mean bisection errors of stroke patients leftward, relative to stationary and rightward moving background conditions. Another study involving a computerised line bisection task (Butter et al., 1990) showed that rightward bisection errors were reduced in the presence of a left-sided moving visual stimulus, relative to no stimulus and a left-sided stationary stimulus.

Other studies have also examined the effects of background motion on line bisection performance in patients with left ULN (e.g., Pizzamiglio, Frasca, Guariglia, Incoccia, & Antonucci, 1990). However, the velocity of the motion in this research was deliberately rapid (500 mm/s) so as to induce a horizontal optokinetic nystagmus. Although the exact mechanisms are unclear, it is thought that nystagmus toward the left may facilitate changes in the way spatial information is internally organised, allowing the patient to attend more to the left side (Pizzamiglio et al., 1990). This phenomenon cannot explain the effects of leftward motion in the study by Mattingley et al. (1994), as the background moving stimuli were far too slow (40 or 80 mm/s) to induce optokinetic nystagmus.

Rather, the effects of (slower) moving visual stimuli on ULN may occur because motion is powerful in capturing attention and directing it to a specific location in space (Dunai et al., 1999, Mattingley et al., 1994). Alternatively, the general alerting characteristics of moving stimuli may activate the brainstem component of the attentional system, thereby reducing neglect (Butter et al., 1990). Specifically, the deep neurons of the superior colliculus are thought to be activated by abrupt moving visual stimuli. To test this hypothesis, Butter et al. (1990) included a condition in which the moving stimulus was presented in the subjective centre of the line—determined by the subject’s mean response in the “no stimulus” trials—rather than at the left endpoint of the line. Relative to the no stimulus condition, there was no reduction in neglect when the moving stimulus was presented in the subjective centre. However, by the authors’ own admission, it is possible that the position of this stimulus focused the patients’ attention to the subjective midpoint, producing a tendency to set the marker at this point.

Butter and Kirsch (1995) used similar moving visual stimuli to investigate whether lateralised moving visual stimuli would enhance neglect patient’s search performance in a cancellation task presented on a computer screen. They found that when moving visual cues were presented on the left part of the screen, neglect patients had improved detection of targets on the left. By contrast, moving visual stimuli presented in the centre section of the screen had no effect on detection performance. Neither stimulus condition had an effect on search time relative to the no stimulus condition. Because the centre stimulus occupied the centre portion of the screen, it may have acted as spatial cue for the centre. To test more accurately the hypothesis that neglect is reduced by moving visual stimuli via activation of superior collicular neurons, it is necessary to compare spatial moving cues with non-spatial moving cues. Moreover, lateralised and non-lateralised moving visual stimuli have not been directly compared. These issues were addressed in the present study.

Specifically, we employed a computerised line bisection task under six visual stimulus conditions (no stimulus, left-sided stationary, left-sided abrupt moving, leftward abrupt moving, leftward continuous moving, and random abrupt moving; Fig. 1). Importantly, in the three abrupt moving conditions (left-sided abrupt, leftward abrupt, random abrupt), the visual stimuli were temporally identical, but differed in their spatial attributes. The left-sided abrupt stimulus was confined to the left endpoint of the line (i.e., spatial cue), the leftward abrupt stimulus moved across the entire screen from right to left (i.e., directional cue), and the random abrupt stimulus moved unpredictably across the width of the screen (i.e., neither spatial nor directional cue). If the spatial or directional attributes of moving visual stimuli contribute to reductions in neglect, then the left-sided (abrupt) and leftward moving (abrupt and continuous) conditions should reduce rightward bisection errors in neglect patients relative to no stimulus. If general alerting effects reduce neglect, then the random abrupt stimulus should reduce neglect relative to no stimulus. The left-sided stationary condition was included to allow comparison to previous research (Butter et al., 1990).

To avoid the possible effects of upper limb movement on line bisection performance, subjects made only a verbal response. Thus, it was assumed that this study investigated the effects of visual stimuli on visuospatial neglect. It was recognised, however, that the subjects were able to move their head and eyes during the task and that this may have influenced the results. Nonetheless, the present protocol was consistent with computerised line bisection tasks in previous research (Butter et al., 1990, Mattingley et al., 1994).

Section snippets

Subjects

The subjects in this study were 10 stroke patients with left ULN (neglect group), 10 stroke patients without ULN matched for age, sex, and side of lesion (stroke group), and 10 age and sex matched unimpaired adults (healthy group). Healthy control subjects had no prior history of stroke or other neurological disease. All patients had experienced a right hemisphere stroke.

Patients were included in the study if they had suffered a previous stroke provided there was no documented past history of

Results

Table 2 presents the mean bisection error scores as a function of group, side of marker start, and stimulus condition. The one-way ANOVA revealed a significant main effect for group [F (2, 18) = 5.795, p < .05]. The mean rightward bisection error of the neglect group in the no stimulus condition was significantly greater (5.09 mm) than both the stroke (−1.32 mm) and healthy (0.29 mm) groups, who did not differ significantly from each other.

The mean rightward bisection error of the neglect group in the

Spatial versus alerting properties of moving visual stimuli

The results of this investigation provide strong evidence that the spatial and directional characteristics of moving visual stimuli, rather than general alerting properties of moving stimuli, are important for reducing neglect on computerised line bisection tasks. This was demonstrated by the failure to find any significant difference between the no stimulus condition and the random abrupt condition for the neglect group. In contrast, the spatially/directionally biased conditions (left-sided

Acknowledgments

We would like to thank Jason Mattingley for most constructive comments on an earlier draft of this manuscript. We also acknowledge the support of the staff at the Austin & Repatriation Medical Centre, Bundoora Extended Care Centre, Kingston Centre, and Monash Medical Centre. We are grateful to T. Samiric for assistance with data collection and N. Berberovic for assistance with patient recruitment. We sincerely thank D. Chen for writing the software. We also express our thanks to the

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