If pure allocentric neglect was occurring in this experiment then NPs would have displayed poorer accuracy for identifying target clocks when the critical information for accurate identification was on the canonical left side of the clock, regardless of its orientation. Thus, NPs’ accuracy would have been poorer in the Left Clock Task than in the Right Clock Task. However, there was no significant group by target-type interaction for TIA in this experiment, suggesting NPs did not have poorer performance for one type of target compared to control participants. Furthermore, it was demonstrated that overall
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accuracy was significantly higher for left sided targets than right sided targets. This provides strong evidence that allocentric neglect of the canonical left side of a clock was not occurring for NPs and this was supported by individual analyses showing that none of the NPs had poorer left sided target performance compared to control participants. This is in line with the findings from Experiment 2 suggesting that allocentric neglect was not operating.
The lack of allocentric neglect in this experiment, as in Experiment 2, may be due to none of the patients included in this study presenting with allocentric neglect. However, there was evidence to suggest that fixation-based neglect was occurring in the clock
cancelation tasks, which may explain many findings of allocentric neglect. The NPs were significantly poorer at identifying target items when the critical information for target identification was to the left of a central fixation position. Target identification accuracy for upright left sided targets and inverted right sided targets (when the critical information for target identification was now to the left of a central fixation position) was significantly lower than target identification when the critical information was to the right of a central fixation position (i.e. upright right sided targets and inverted left sided targets). This was consistent with the view that neglect operates from an egocentric frame of reference based on eye position, with information falling in the LVF being neglected (e.g. Behrmann et al., 2002; Gainotti et al., 1986). This is distinct from object-centred allocentric neglect, where the canonical left side of the object is neglected, even if that information falls within the RVF (i.e. when the left sided target is inverted; refer back to Figure 4).
Despite the interpretation that this finding provided evidence for fixation-based neglect, caution has to be taken with drawing this conclusion as no information from the eye tracking data was provided as to the position of the fixation within the clocks during completion of the task. Participants may approach task by fixating the hands and the numbers of the clock, rather than by centrally fixating the object. If this were the case, then the target items that had the information to the left of a central fixation position may not have been identified due to a reluctance to fixate the left side of the object (object- based allocentric neglect) and not because the information fell to the left of a central fixation position. This was the first study to demonstrate that allocentric neglect may be a result of fixation-based neglect. Thus, when the task demands necessitate that each item to be fixated in order to successfully complete the task, fixation-based neglect (not allocentric neglect) is observed. Further studies need to be conducted with highly spatially accurate
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head mounted eye trackers in order to verify the position of the eyes within the objects in the task.
The finding that task demands can affect the level at which neglect operates
demonstrates further that neglect is a dynamic disorder that can be affected by a number of factors. Experiment 4 reported in Chapter 5 investigated in more detail the effect of task demands on the frame of reference that was operating in neglect. The finding of fixation- based neglect and that task demands affect the level at which neglect operates can explain the majority of effects that have been interpreted as support for allocentric neglect and may offer some explanation for inconsistencies in the literature with regard to the two different frames of reference. For example, patients failing to identify gaps on the left hand side of circles or apples when these are presented in the ipsilesional regions of the stimulus is likely to result from the task requiring each item in the stimulus to be directly fixated (Ota et al., 2001; Bickerton et al., 2011, respectively; refer back to section 2.1 Frames of Reference for the Coding of Spatial Information in Chapter 1) and may not be due to the operation of allocentric neglect.
Additionally, more global egocentric neglect was demonstrated by NPs in the clock cancellation task. Target identification accuracy was poorest in the FL region with TIA increasing towards the right. The existence of egocentric neglect is also far less controversial than allocentric neglect, and egocentric neglect has been evidenced in a number of studies investigating search behaviour in neglect (Behrmann et al., 2002; Behrmann & Moscovitch, 1994; Colby, & Goldberg, 1999; Hillis et al., 1998). One question that remains to be addressed relates to the factors that contribute to neglect of information to the left of a patient’s midline. Target identification accuracy may be poor due to patients failing to fixate the information in that region. However, poor accuracy may also reflect restricted processing within contralesional regions of space for NPs, perhaps due to deficits in either or both encoding and/or representation of that information. The previous experiments reported have indicated that neglect may reflect a combination of these factors.
4.3 To what Extent were Sampling and Processing Deficits Contributing to the