A number of researchers have recognised the value of examining patterns of eye movements in neglect to reveal the impairment of space exploration (e.g. Barton et al., 1998; Behrmann et al., 1997; Forti et al., 2005; Walker, Findlay, Young, & Lincoln, 1996; Walker & Young, 1996). Eye movements have also been demonstrated to indicate the extent of neglect recovery (e.g. Olk, Harvey, & Gilchrist, 2002), which is important if pen- and-paper tasks’ sensitivity is low and no longer indicates that a deficit still exists. Given the advantages that would be gained by recording patterns of eye movements in neglect patients, there have been a surprisingly limited number of eye movement experiments conducted (e.g. on visual search, line bisection, chimeric faces; Behrmann, et al., 1997; Behrmann, Ebert, & Black, 2004; Forti et al., 2005; Fruhmann-Berger & Karnath, 2005; Walker & Findlay, 1996; Walker, Findlay, Young & Lincoln, 1996).
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3.1.1 Sampling and Processing of Contralesional Information in Visual Neglect An important question that has not been fully addressed by studies investigating neglect of visual information, is whether inattention to the left side of space results in patients failing to sample that information, which leads to the neglect that is manifest (e.g. a sampling deficit of the contralesional side of space; Chédru, Leblanc, & Lhermitte, 1973; Karnath, Neimeire, & Dichgans, 1998) or whether they fixate that information but fail to process it sufficiently in order to accurately respond to it (e.g. Forti et al., 2005; Walker et al., 1996). Eye movement methodology can reveal whether information is not reported due to the patients presenting with a sampling deficiency (i.e. they failed to fixate the area that was neglected) or a processing deficiency (i.e. they fixate the left side of space but failed to process information there adequately in order to report it accurately). This issue is critical in order to understand the underlying mechanisms that are deficient in neglect to better characterise the disorder, and in the future, to aid development of effective rehabilitative methods, which to date show limited success in ameliorating neglect (Manly, 2002).
Kinsbourne (1970; 1977; 1987) postulated that neglect was due to an imbalance in oculomotor mechanisms. Furthermore, Kinsbourne believed that increased arousal of the ipsilesional hemispace caused an exaggerated attention to that region, and consequential preferential responding to the right (Kinsbourne, 1977). This has been suggested to be due to the non-damaged (left) hemisphere having a higher activation overall compared to the damaged (right) hemisphere (Kinsbourne, 1970) and, therefore, there being increased inhibition of the damaged hemisphere. In this way, neglect of the left occurs due to over attention to the right, rather than inattention to the left (Ládavas, Petronio, Umiltá, 1990; Marshall & Halligan, 1989). The contribution of hyper-attention to the right in neglect can be examined by patterns of eye movements produced by neglect patients. Findings from studies that have investigated eye movements whilst neglect patients complete a variety of tasks have suggested that neglect patients saccade to the left less frequently, and spend more time fixating the right side of a stimulus compared to controls (e.g. Behrmann et al., 1997; Barton et al., 1998; Walker & Young, 1996).
Behrmann et al. (1997) tested nine left hemispatial neglect patients and two control groups (hemianopic and non-brain damaged controls) on a letter detection task.
Participants were asked to search for a target ‘A’ amongst an array of randomly distributed letters. Search for targets was performed whilst participants’ eye movements were
recorded, through the search coil technique. Participants were asked to report the number of letter ‘A’s that they found at the end of their search for which there was no time limit.
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Not only did Behrmann et al. find that the neglect patients performed significantly worse on the task than the hemianopic patients and the controls, they also found that neglect patients made significantly fewer fixations on the contralesional side of the stimulus and spent less time fixating that region. This demonstrated that there is evidence for a sampling deficit contributing to hemispatial neglect. Additionally, the neglect patients made more fixations on the ipsilesional side and had longer fixation durations in this region compared to the other two groups, and they predominantly started their search on the right side of the stimulus. These findings provided support that hyper-attention to the right side of space occurs in neglect. Hemianopic patients showed the reverse pattern of eye movements. Patients with hemianopia spent longer on the far left region than the controls and neglect patients (Behrmann et al., 1997). This is suggested to be due to a compensatory strategy hemianopic patients develop to bring information within their intact visual field that would otherwise fall within the blind LVF.
Since the neglect patients were able to fixate the left side on occasions (and they were able to do this during calibration of the eye tracker), Behrmann et al. (1997) concluded that there was no evidence of a fundamental oculomotor deficit underling the disorder. Therefore, neglect patients were able to fixate the usually neglected region of space but possessed an inherent bias to fixate the neglected side less frequently than controls, supporting the sampling deficit account of neglect. As the quantitative measure of neglect that the researchers obtained in Behrmann et al.’s (1997) study was the number of targets found overall, information with regard to accuracy within different regions of space was not accessible. This means that the eye movement measures (which were divided into areas of interest on the stimulus) could not be associated with accuracy within that region. Thus, the relationship between sampling and target identification accuracy in neglect could not be determined. Even though restricted sampling of the contralesional region of space has been associated with neglect, the extent to which this sampling deficit is linked with poor contralesional target identification is unknown, as is whether or not poor sampling of the left is a cause of neglect or a result of it. If the object has not been fixated, it is unlikely to be accurately identified. The value of using eye movements to investigate this theoretical issue is evident, with Behrmann et al. (1997) concluding that oculographic analysis is a ‘robust’ method for examining underlying deficits in visual neglect.
Evidence exists demonstrating that scanning training, encouraging sampling of the contralesional region in neglect, aids responding to information in the usually neglected
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region of space. This suggests that a sampling deficit may be a factor contributing to neglect, as if a sampling deficit is mitigated, so is neglect. Pizzamiglio et al. (1992) offered patients approximately 40 hours of scanning training via highly salient cues presented within the contralesional region and therapist encouragement to look to the left using a variety of tasks (including reading, naming objects in spatial arrays, following leftward moving lights projected over a wide area). The cues were progressively reduced in use as awareness of the left side of space increased in the neglect patients. As would be expected if a sampling deficit was contributing to the neglect exhibited, neglect patients’ performance on the tasks showed significant gains after scanning training. Bailey Riddoch, and Crome, (2002) also demonstrated that three of the five neglect patients who received scanning and cueing showed a reduction in neglect in one or more tests. Furthermore, this improvement was maintained during the treatment withdrawal phase. These studies
indicated that if visual sampling of the left is increased, then information can be reported in the usually neglected region. This suggests that a sampling deficit of contralesional
regions being present before treatment was likely to be contributing to targets not being identified within that area of space. Interestingly, scanning training has also been demonstrated to mitigate neglect in tasks that do not involve the visual modality. To be clear, this suggests that a sampling deficit of the left is strongly associated with visual neglect and contributes to target items not being identified in that area.
However, scanning training treatment tends to have only a short-lived effect on neglect and often fails to generalise to tasks outside that employed in the training situation. Thus, simply encouraging patients to sample the usually neglected information, does not necessarily result in that information being processed sufficiently in order for the patient to respond accurately. This is further demonstrated by a study investigating the effect of prismatic adaptation on neglect, which involves recoding visual-motor coordination. Ferber, Danckert, Joanisse, Goltz, and Goodale (2003) provided evidence that prismatic adaptation shifted neglect patients eye movements to the left. However, this increased sampling of the left following treatment failed to improve detection of contralesional information. Thus, these results appear to suggest that, in neglect, overt eye movements and covert attention may be decoupled (Benson, Ietswaart, & Milner, 2012). This is to say that, even when a neglect patient directly fixates a target or an object on the left, they still may fail to process it sufficiently in order to accurately identify that information. This same conclusion was reached by Benson et al. (2012) when investigating the eye
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dissociation between an intact ability to make appropriate reflexive eye movements to targets in the contralesional regions of space and the ability to accurately report targets presented in that area. Therefore, the tight coupling that is normally demonstrated between attention and eye movements appears to be disrupted for certain tasks in neglect (Benson et al. 2012; Liversedge & Findlay, 2000).
Emerging eye movement evidence suggests that limited sampling of contralesional information is not the only factor contributing to decreased awareness of contralesional stimuli. A number of studies have demonstrated that information on the neglected side is insufficiently processed by neglect patients or that more time is required to process the information (e.g. Forti et al., 2005; Walker & Young, 1996). Chimeric stimuli are useful to investigate neglect patients’ processing and perception of information. It has been shown that neglect patients often fail to notice that a chimeric face presented to them was composed of two different faces; one face on the left and another on the right, the faces either differing in identity or emotion (Walker et al., 1996). Neglect patients’ perceptions of a chimeric face are predominantly based on the information provided in the right part of the image (Walker et al., 1996). Walker et al. (1996) investigated the eye movements of one patient (RR) with visual neglect whilst he viewed and reported on chimeric stimuli. RR was required to either report the identity of the face was presented (which belonged to a famous person), which building was displayed (which was famous, e.g. the Eiffel Tower), or which two faces or buildings comprised the image when a chimeric face or building was shown.
When the stimuli were presented centrally (i.e. the midline of the stimulus lined up with the sagittal midplain of RR’s trunk), the participant failed to report the left side of the chimeric faces, whereas he was 100% accurate for the face on the right side. This
corresponded with a lack of visual sampling of the left side of the face, i.e. no saccades were made to the left side of the face. The findings were similar for the chimeric buildings, with only 2% of the total trial time spent fixating the neglected side on trials where the left side was not identified. In experiment two of this investigation into RR’s eye movements, faces and buildings were either presented centrally (as in experiment one) or to the right of the patient. Walker et al. (1996) found that when the chimeric face was presented on the right of the participant, accuracy in reporting the left side of the face improved (increasing by 21% compared to when it aligned with RR’s trunk midline). However, on occasions where the left side was not reported, RR still spent 26% of the total trial time fixating that side. This demonstrates that a significant proportion of fixation time was allocated to
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visual inspection of that area; however, this did not result in that information being reported. This provides clear evidence for a processing deficit associated with visual information that is fixated on the neglected side of space. These results imply that either the information on the left side was not encoded by RR during fixation, resulting in him failing to report it, or that the information was visually encoded but a problem associated with representing the information correctly was causing the processing deficit. This issue will be returned to in Experiment 4.
Forti et al. (2005) also demonstrated that a processing deficit may account for neglect in another neglect patient (MP). The authors presented a case study of MP’s eye movements whilst he searched for a target object on a table. The target was present amongst other distractor objects, which were equally distributed in a random order across the area of space. Despite MP directly fixating all objects presented within the
contralesional visual field, he still failed to report 28% of the targets on the left. This suggests that impaired scanning of the neglected area was not the primary cause of the evident neglect exhibited by this patient.
These studies appear to be suggesting that a processing deficit contributes to neglect. Further investigations are required to verify these findings in a group of neglect patients to establish whether this is an underlying deficit in the disorder of neglect and not just these individual patients that are experiencing these difficulties. The previous studies provided evidence that sampling and processing deficits contribute to visual neglect. However, neglect can occur in other modalities. It has been demonstrated that some neglect patients fail to respond tactically to information on the left, even when they have their eyes closed. Therefore, it is unlikely in this situation that the failure to sample the left side of space would be contributing to poor performance as no visual input is available. Bartolomeo (2002) investigated the relationship between visual neglect and neglect of visual mental images (imaginal neglect) by comparing performance of neglect patients on tasks which were visually mediated and those that were not (i.e. the patients’ eyes were closed). Performance in a task that required information to be visually encoded and represented was poorer than in the task where participants were asked to respond tactically to information they could not see. Bartolomeo concluded from this that there are different underlying mechanisms which are deficient in these types of neglect, suggesting there is not a common neural mechanism for visual perception and mental imagery (Halligan, Fink, Marshall, & Vallar, 2003). Furthermore, visual neglect may be exacerbated compared to other forms of neglect due to the reliance on visual encoding and representation of visual
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information, which have been suggested to be deficient in neglect (Denny-Brown, Meyer & Horenstein, 1952; Bisiach & Luzzatti, 1978). Given the evidence outlined above and the fact that not all patients with visual neglect experience neglect in other modalities (whether that be tactile or imaginal), it is expected that sampling and processing deficits would contribute to neglect of visual information and this will be one main focus of this thesis.
In conclusion, it appears that often there is a sampling deficit exhibited by patients with visual neglect, whereby they spend less time fixating the neglected side, make fewer eye movements to that region and spend more time fixating the ipsilesional side. This was not the case in hemianopia (Behrmann et al., 1997), confirming that the attentional deficit was the factor influencing inhibited visual sampling of contralesional space in neglect. A few empirical findings have indicated that even when contralesional information was directly fixated by the neglect patient, it was still not reported. This may indicate that a processing deficit of contralesional information contributes to neglect. This issue needs further investigation to determine whether a contralesional processing deficit occurs for a group of neglect patients as well as for those reported in the case studies described here. Furthermore, establishing whether a processing deficit occurs in a large group of neglect patients will reveal whether this is an underlying mechanism in the disorder of neglect and not simply limited to specific lesions in a sub-set of neglect patients. It is often assumed that the main factor causing neglect of information is that patients fail to fixate the left (as been demonstrated by Behrmann et al., 1997; Barton et al., 1998; Chédru, Leblanc, & Lhermitte, 1973; Karnath, Niemeier, & Dichgans, 1998; Walker & Young, 1996) and thus rehabilitative methods often focus on encouraging neglect patients to fixate the left side of space. If emerging evidence suggests that a sampling deficit is not the sole factor
contributing to neglect, this may provide insight into why those rehabilitative methods, involving encouraging increased sampling of the usually neglected region of space, have had limited success in aiding responding to the left in neglect (Manly, 2002).