The Value of Figure Copying Tasks in Revealing Frames of Reference Operating in

As discussed in the section entitled 1.2 The Behavioural Inattention Test, figure copying has been shown to be sensitive to the deficits experienced in neglect as accurate performance relies heavily on the visual information provided in the visual field to have been sufficiently encoded and represented by the participant (Halligan & Robertson, 1992). As the task provokes a strong engagement of focal attention, if an attentional deficit exists, then the task is likely to expose omissions of elements on the contralateral side (Ishiai, Seki, Koyama, & Yokota, 1996). Thus, many researchers have employed figure copying as a task to identify the spatial frames of reference that are operating in neglect and to investigate both egocentric and allocentric reference frames.

Copying tasks involve encoding of the object to be copied in terms of a variety of characteristics (Tchalenko & Miall, 2009), such as size, shape and spatial location, and therefore require direct processing of objects and activation of allocentric reference frames would be required by the task. Hillis, Rapp, Benzing, and Caramazza (1998) found that neglect patients failed to copy objects on the left side of the ‘Ogden scene’ (see Figure 7), but copied entire objects on their right (which were also on the right side of the scene). The results clearly demonstrated the influence of egocentric coordinates in neglect, even when object-based representations should be important, i.e. object based representations are activated due to the task requiring each object to be encoded, represented and stored in short-term memory during task completion.

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Figure 7. Classic ‘Ogden scene’ used by Hillis et al. (1998).

The dysfunctional spatial representations that do or do not exist in neglect can be investigated by manipulating the properties of the objects in a stimulus to be copied. This can allow investigation of whether the left side of the object is being neglected, or

information that falls on the individuals’ left is neglected. In a study that investigated reference frames operating during figure copying in neglect, Behrmann and Plaut (2001) report results from two neglect patients. The patients were required to copy a daisy, which was presented in four different orientations: upright, head of the daisy to the left, head to the right, or inverted. Both patients often failed to copy the left side of the daisy,

regardless of its orientation demonstrating object-centred neglect. So, even when the daisy was rotated 90o to the left (head of the daisy pointing towards the left), parts to the right of the egocentric reference frame midline, but on the left of the object, were not drawn. This suggests that allocentric neglect was occurring without the relative influence of egocentric neglect, as these two frames of reference in this task were placed out of alignment with one another by rotating the object.

However, as a daisy (like the circles of Ota et al., 2001) does not have canonical handedness, the left side is only defined as such due to the coding of spatial co-ordinates during its original, and usual, presentation form (i.e. that side is deemed the left side of the daisy as it fell to the left of the individual, and the egocentric midline, when presented upright). Thus, egocentric frames of reference may still influence whether this type of neglect is apparent, due to egocentric reference frames defining the allocentric left and right of the object initially. Behrmann and Plaut (2001) concluded that neither of the patients reported in the paper had exclusive allocentric neglect. Either patients presented exclusively with egocentric neglect or demonstrated a combination of allocentric and egocentric neglect. Thus, this is evidence that allocentric neglect often relies on the operation of egocentric reference frames in neglect.

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In the second experiment reported by Behrmann and Plaut (2001), the patients were required to copy a more complex image. This image was either two separate daisies that were not joined together (separate pots for each daisy) or two daisies that were joined at the stem (originating from one pot). If pure allocentric neglect was occurring, then one would expect that in the joined case the whole left daisy would be neglected as this would be the left side of the object. On the contrary, allocentric neglect in the non-joined case would manifest with both daisies being copied but with the left side of each being neglected. However, egocentric neglect would result in the left daisy being neglected in the joined and non-joined case. They found that all four patients neglected information to the left of their midline but also some information was missed to the left of each daisy, demonstrating that a combination of allocentric and egocentric reference frames were contributing to the neglect exhibited by the four patients examined in this experiment.

However, copying is a sequential task, thus each object would be the sole focus of attention for some period of time during the copying process. This may result in patients aligning their head and/or eye positions with the centre of the object currently being copied (thus aligning egocentric and allocentric frames of reference). Therefore, the left half of individual parts of the object may be neglected due to the left side of the currently fixated object falling on the left side of the participant (with regards to head and/or eye position), rather than reflecting neglect of the left side of the object. This results in a pattern of behaviour whereby elements on the left side of each daisy are not copied but the right sides are copied accurately, which has been provided as evidence for allocentric neglect.

However, this may be a result of the egocentric midpoint shifting during progression of the copying task (i.e. the egocentric midpoint [relating to the eyes/head] is aligned with the allocentric midpoint of the right daisy when this object is being copied but the egocentric midpoint shifts to the centre of the left daisy when this object is being copied).

Another explanation for the appearance of allocentric neglect in this task could be ‘relative egocentric neglect’ (Driver & Pouget, 2000). The left side of an object receiving less activation than the right side, as it lies more to the left relative to the egocentric midline, would also results in a pattern of behaviour whereby elements on the left side of each daisy are not copied but the right sides are copied accurately. For example, the left side of the daisy that is presented on the right side of the viewer receives relatively less activation than the right side of that daisy, and therefore this part of the figure is neglected when the patient is focusing on that object. The same principle (relative activation) also applied for the daisy presented on the left when the patient is copying that object. Why the

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task demands affect the spatial aspects, and therefore the effect of relative activation, of the visual information that is neglected is still not clear. This issue is the main focus of

Experiment 4.

In summary, the complexity of the disorder of hemispatial neglect is illustrated by the inconsistency in the literature regarding frames of reference. What is clear is that the definition and characterisation of hemispatial neglect cannot be accurately determined without considering reference frames underlying the deficit of attention. Even though the majority of studies investigating hemispatial neglect argue that neglect occurs on an egocentric level (e.g. Beschin, Cubelli, Della Sala, & Spinazzola, 1997; Driver & Pouget, 2000), neglect does appear to operate on an allocentric level under some circumstances (e.g. Caramazza & Hillis, 1990; Behrmann & Moscovitch, 1994; Ota et al., 2001; Walker et al, 1996). Even so, the extent to which allocentric neglect is a manifestation of

egocentric neglect and why allocentric neglect is demonstrated on some occasions have not been determined. Whether allocentric neglect is dependent upon egocentric frames of reference is still to be determined.

In order to minimise inconsistencies in the literature, future studies need to carefully consider the stimuli used in experimentation in order to differentiate allocentric neglect from that which is based on egocentric co-ordinates. To establish whether sequential eye fixations result in the apparent operation of allocentric reference frames, stimuli with canonical handedness that are presented at different orientations need to be contained within tasks employed. This is to ensure that the midlines of allocentric and egocentric reference frames are not aligned when the object is directly fixated during the course of the task progression. Furthermore, detailed eye movement analyses of neglect patients’ visual exploration whilst copying complex figures would be informative with regards to the information that is visually sampled and processed in neglect, which will be expanded on in the following section. Tracking eye movements during completion of the BIT, namely cancellation tasks and figure copying, has not been conducted before for neglect patients. This experimentation will be outlined in the empirical chapters of this thesis.

3. Mechanisms Underlying Neglect: The Value of Eye Movement Analyses