The findings in Experiment 2, Chapter 3 for the SEAN cancellation task
demonstrated NPs had poor TIA in the FL region, identifying under a quarter of the target items there. NPs had far higher TIA in the Left Clock Task, nearly double the TIA obtained in the SEAN cancellation task. Although the current experiment contained two less NPs than Experiment 2, all the other NPs contributed to both sets of experimental findings. Task complexity and other possible reasons for poorer TIA in the SEAN cancellation task compared to the clock cancellation tasks are discussed next.
Unexpectedly, TIA in the FL region for the Right Clock Task was lower than in the Left Clock Task. This is the opposite effect to what was expected if NPs presented with allocentric, object-based, neglect and potential reasons for this are outlined in this section.
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There may be a number of reasons for sampling being more restricted in the SEAN task than the clock tasks. Firstly, the SEAN task required dual-target search (searching for two target letters), whereas for both the clock cancellation tasks, the participants were only required to search for clocks displaying one specific time during each task. This increases cognitive load (i.e. the number of tasks one is currently required to complete). It has been demonstrated that healthy control participants are poorer at identifying target items when searching for two items simultaneously compared to a single item (Menneer et al., 2007). This has been suggested to be due to difficulties experienced with holding two target templates in working memory simultaneously (or one template which is more global) to guide search during dual-target search, compared to only one, highly specific, template aiding search in single-target search (Menneer et al., 2007). Dual-target search was demonstrated to exacerbate neglect in Experiment 1 which investigated chronic neglect patient SS. These findings support those from other studies that have shown that more omissions are made by neglect patients in conjunction tasks than in feature detection tasks (e.g. Aglioti et al., 1997).
Importantly, poorer TIA in dual-target search compared to single-target search appeared to be limited to the left regions in NPs, with equivalent TIA for NPs in the right regions in the SEAN and clock cancellation tasks (NPs achieving between 70 and 80% TIA in the NR and FR regions of all tasks). This suggests that dual-target search only hindered search performance when NPs were searching for targets within a contralesional region; an area of space where attentional resources were already limited. Dual-target search may require more attentional resources being readily available and, thus, in a region of space where processing was deficient or delayed, extra cognitive processing required results in poorer performance. However, this was not the case for areas in which the NPs had adequate processing of information, i.e. in the ipsilesional regions. This provides further evidence in support of the hypothesis that deficient processing in contralesional regions contributes to neglect of information, as well as limited sampling of that information.
Another explanation for the differential TIA within contralesional regions for the SEAN and clock cancellation tasks is that the stimuli differed in visual properties and perceptual load may be increased in the SEAN task compared to the clock cancellation tasks. Generally, there were more items contained within the SEAN task than in the clock cancellation tasks. There were 80 items in each region in the letter cancellation task, compared to 18 in each region in the clock cancellation tasks. This difference may make it
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more difficult to conduct search due to the density of the visual information displayed in the SEAN cancellation task. Increased density of stimuli has been shown previously to exacerbate neglect (Ferber & Karnath, 2001) and this was suggested to be a factor contributing to SS’s poorer performance on the BIT letter cancellation task compared to the star and line cancellation tasks in Experiment 1. Together, the findings further support the premise that neglect is a dynamic disorder, with its presentation and manifestation being affected by a number of different factors. One of these factors, specifically, task demands, will be investigated and discussed in Chapter 5 in relation to its impact on neglect.
4.5 Conclusions
Neglect is a dynamic disorder, and with the level at which it operates (e.g. at an egocentric or a more allocentric level) within each individual, changes depending on a number of task-related factors. In the experiment reported in this chapter, the task
demands necessitated that each individual item that was presented within the stimulus had to be fixated, which could have resulted in the left side of each object being neglected. In the SEAN task the task demands did not require each letter to be fixated in order to
accurately identify targets, and so allocentric neglect was not exhibited. The findings from the current experiment indicated that the neglect shown was likely to be a result of
information to the left of a central fixation position being neglected within individual target items, i.e. egocentric neglect relating to eye position. Previously, this type of neglect behaviour has been interpreted to provide evidence for allocentric neglect.
More global egocentric neglect (neglect of targets within the left regions) was associated with patients failing to sample the FL region to the same extent as controls and NPs spending less time fixating that region overall. This, however, was not the sole reason for poor target identification on the left. On occasions NPs were fixating contralesional information, and spent time processing that information, but still failed to accurately identify the information there. This demonstrated that a processing deficit was also contributing to neglect of information and that neglect was not simply due to patients failing to fixate neglected information. This processing deficit may be a result of the information either being inadequately encoded or represented properly. The experiments reported up to this point have not directly addressed this question, even though it has been repeatedly demonstrated that a processing deficit was contributing to neglect as well as limited sampling of contralesional information. Experiment 4 aims to address this question
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and to determine how NPs encode and represent contralesional information in order to investigate whether either or both of these processes were deficient in neglect.
Furthermore, there was poorer performance in contralesional regions and less sampling of those regions for NPs in the SEAN task (reported in Chapter 3) than in the clock cancellation tasks reported here, which was likely to be a result of the differing visual properties of the stimuli (e.g. density of the display, number of target and distractor items) and cognitive factors (e.g. dual or single target search). These factors were
demonstrated to have more of an impact on search within contralesional regions, with accuracy in the right region being equivalent across the tasks, indicating that there was restricted processing of contralesional information. This, in conjunction with limited sampling of information, contributed to the neglect of target items within contralesional regions. These findings are important for developing effective rehabilitative methods for neglect, specifically, developing techniques that do not solely rely on attempting to increase sampling of the neglected region.
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