3.5.1 Behavioural measure: Target Identification Accuracy (TIA)
In contrast to the behavioural results from the letter and star cancellation tasks, the line cancellation task assessment indicated that in all sessions SS scored above the cut-off value for clinical inattention. Only in one session was a target missed (97.7% accuracy in Session 2) and this target was in the centre of the stimulus (recall that there are three regions of interest for this stimulus: left, centre and right). This demonstrates that this task was not as sensitive as the letter and star cancellation tasks in revealing the presence of neglect, as has been demonstrated previously (Ferber & Karnath, 2001). As has been explained previously, it was not necessary to distinguish targets from distractors in the line cancellation task (as every item that was present in the task was required to be cancelled). Furthermore, the line cancellation stimulus had a less dense array of target items than the star and letter cancellation tasks, which may also make it less cognitively demanding to complete. Fewer cognitive demands required to complete the line cancellation task (for example, participants not being required to distinguish between targets and distractors) may have enabled allocation of attention to the usually neglected side of space for SS.
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3.5.2 Eye movement measures: Proportion of total gazes and total trial time spent fixating each region on the stimulus
SS had poor TIA in the letter and star cancellation tasks. These tasks, therefore, allowed investigation of factors that were contributing to neglect in the task, specifically whether sampling and/or processing deficits were causing information to be neglected. The line cancellation task, due to SS’s high TIA, was informative as it enabled
determination of potential patterns of eye movements that were required for optimal task performance. Once again, the eye movement patterns that control participants produced will be outlined first. As in the letter and star cancellation tasks, control participants made more gazes on the central region (see Table 3). It is suggested that this is likely to result from the central region being the largest region in the line cancellation task and contained slightly more target items (two more than the left and right regions). Furthermore, the proportion of time spent in each region for the controls was greatest for the central region, which was in accordance with the size of the region and number of targets present.
As shown in Table 3, in Sessions 1 and 2, SS appeared to exhibit reduced sampling of the left region compared to the right, with smaller proportions of the total gazes being made to that region. However, SS’s sampling of the left, represented in proportion of gazes or fixation time, was not significantly lower than that of the controls for Session 1, ts(4) < 1.86, p > .05, although there was a trend towards fewer gazes and less time being spent by SS on the left region in Session 2, t (4) = 1.862, p = .068; t (4) = 1.580, p = .095. This trend suggests that SS’s reduced proportion of gazes on the left did not represent a defect in sampling in Session 1. In Session 2, it appears that there was a reduction in proportion of gazes and time on the left and that may have been due to SS hyper-attending to the right. This is to say that, the proportion of time spent fixating the left was
significantly reduced in Session 2 due to a tendency for SS to spend a disproportionate amount of time fixating the right region.
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Target Identification Accuracy (TIA; % of Targets Found), Proportion of Gazes (Gazes), Proportion of Trial Time spent Fixating (Time) and Average Gaze Durations (AGD; ms) on the Three Regions of the Line Cancellation Task Stimulus for the Three Regions (Left, Centre, Right) across Three Sessions for SS (presented in bold), SCs and OACs.
Measure Group Session 1 Session 2 Session 3
Left Centre Right Left Centre Right Left Centre Right
Gazes SS .10 .50 .40 .07 .47 .47 .24 .47 .29 SCs .23 .50 .28 .26 .47 .28 .33 .50 .17 OACs .23 .49 .28 .19 .50 .31 .25 .50 .25 Time SS .29 .42 .29 .26 .40 .34 .31 .40 .29 SCs .33 .39 .29 .31 .41 .29 .34 .37 .30 OACs .32 .40 .29 .29 .41 .30 .33 .38 .29 AGD SS 9570 2878 2401 9834 2140 1862 2401 1572 1841 SCs 3732 2074 3075 2588 1709 2007 3003 2090 4373 OACs 3443 1991 2930 3826 2007 2550 2544 1474 2884
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In Session 2, SS made a significantly higher proportion of gazes on the right than the controls, t(4) = 3.22, p = .016. This is particularly noteworthy considering that the proportion of gazes on the right was equivalent to that of the centre, which was a larger region (and one would expect it to receive more gazes overall on the task). Re-visits made to the right (and central) regions were unlikely to be required to complete the task, as the ease of the task meant that often all the targets within one region were often cancelled during the first gaze. This demonstrates that SS was more likely to make unnecessary gazes on the right region, providing support for that region capturing neglect patients’ attention and that hyper-attention to the ipsilesional side was occurring. Hyper-attention to the right, however, did not result in neglect of targets on the left in the line cancellation task. This suggests that hyper-attention, on its own, may not be a factor contributing to neglect of information on the left, and therefore neglect is likely to be a result of hypo- attention to the left, in conjunction with hyper-attention to the right occurring.
In Session 3, there was no significant difference between the proportion of gazes SS spent on the left region compared to the controls, t(4) = .57, p = .301. The hyper- attention that was present in Sessions 1 and 2 (with a similar proportion of gazes being made to the right region as the larger central region) had completely dissipated, with SS not having significantly higher proportion of gazes and time on the right than the controls, ts(4) < 1.02, p > .05. Additionally, as can be seen in Table 3, SS spent a slightly higher proportion of the total trial time fixating the left compared to the right. This demonstrates that there was a reduction in hyper-attention to the right in this session which may be a result of the treatment.
An additional measure is reported for this task due to SS’s high TIA. Analyses of average gaze durations for each region were completed to investigate whether there was a delay in processing contralesional targets by SS when accurate identification was achieved. It appears that more time was required in order for SS to process information on the left side. As shown in Table 3, average gaze durations were inflated on the left region, indicating processing difficulty. In Session 1, SS spent 3.3 times longer, on average, in a gaze made on the left region than the right region and had significantly longer gaze
durations than the control participants on the left, t(4) = 2.86, p = .023. In Session 2, AGD were also increased. Patient SS’s AGD were 4.2 times longer on the left compared to the region with next highest average gaze duration (the centre) and was significantly longer than the control participants spent fixating during a gaze on that region, t(4) = 5.51, p = .003. Therefore, a delay in processing contralesional information was apparent when all
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targets items were identified within that region. This suggests that eye movements may be a more sensitive measure of neglect when offline behavioural measures indicate normal performance. Interestingly, average gaze durations on the left were no longer inflated in Session 3, t(3) = .341, p > .05, indicating there was no longer a delay in processing contralesional targets. It appears that the treatment may not only have reduced hyper- attention to the right for SS in the line cancellation task (as indicated by the reduction in the proportion of the total trial time spent fixating the right), but also appeared to aid the processing of contralesional information, as was apparent in Session 3 of the star
cancellation task.
In summary, the line cancellation task was less sensitive than the letter and star cancellation tasks, failing to reveal that SS exhibited neglect in any of the sessions, as has been demonstrated previously (Ferber & Karnath, 2001). Due to the high accuracy in this task, there was an opportunity to investigate the pattern of eye movements exhibited when SS performed optimally. The eye movement measures demonstrated that SS fixated the left to the same extent as the control participants and spent as much time overall exploring that region. The proportion of gazes on the right region was similar to that made on the central region, which was 10% larger in size. These results provide evidence that SS exhibited hyper-attention to the ipsilesional side. However, importantly, during each gaze made on the left, SS required more processing time in order to accurately identify the targets within the contralesional side. This indicates that eye movements may be a more sensitive measure of neglect than behavioural measures for less complex search tasks included in the BIT. Importantly, inflated processing time and hyper-attention to the right mitigated after treatment, suggesting that limb stimulation may aid attention to, and processing of, contralesional information during completion of more simple tasks.
4. General Discussion