Why was there no Evidence of Allocentric Neglect Observed in this Experiment?

experiment were postulated. Firstly, a number of studies have demonstrated that

allocentric neglect is far less common that egocentric neglect (e.g. Bickerton et al., 2011; Hillis et al., 2005). Hillis et al. (2005) found that only 25% of 16 neglect patients

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Therefore, it was possible that, by chance, none of the 13 neglect patients included in this study presented with allocentric neglect. Furthermore, their performance on the Ogden scene screening task indicated that none of the patients demonstrated allocentric neglect. It may be that a larger sample of NPs need to be investigated to determine whether any patients presented with allocentric neglect on this task. However, the existence of allocentric neglect is still highly contentious (e.g. Driver & Pouget, 2000).

Secondly, some researchers have postulated that allocentric neglect is a

manifestation of egocentric neglect, for example, ‘relative egocentric neglect’ (Driver & Pouget, 2000). Relative egocentric neglect is the notion that the left part of an object is neglected because the neural response to that side of the object is relatively lower compared to that of the right side. So, when an object is presented upright, both the absolute egocentric position and the relative egocentric position of parts within the visual field are important in the spatial coding of information, and, therefore, which spatial information is neglected by NPs. Similarly, ‘fixation-based neglect’, which may result in what appears to be allocentric neglect, relies on the operation of egocentric frames of reference. This would result when the task requires the neglect participant to make sequential eye fixations to the centre of each object in order to obtain detailed information from it by bringing it under the high acuity area of the fovea.

Targeting of objects’ centres during search tasks has been demonstrated.

Henderson (1993) found that the initial landing position (LP) of the eyes on an object was found to be normally distributed around the centre of the object, with the modal LP at the centre. Thus, when a fixation is made on, or near, an object’s centre in neglect, the left part of the object may be neglected due to that part now falling to the left of the point of fixation and, therefore, left in relation to an egocentric reference frame (the point of fixation, which has been shown to be an important egocentric reference frame; e.g.

Behrmann et al., 2002). Evidence from a number of different paradigms has demonstrated that information that falls to the left of fixation (i.e. within the LVF) is neglected by NPs (e.g. Behrmann et al., 2002; Colby & Goldberg, 1999; Karnath & Hartje, 1987). Thus, disruption does not arise as a result of the left part of the object being neglected, but instead because the information to the left of any current fixation position is neglected. This, in turn, suggests that allocentric neglect may in fact be a form of egocentric neglect. Allocentric neglect that does not rely on egocentric frames of reference would result in a failure for the left side of an object to be perceived, even when allocentric and egocentric frames of reference were placed out of alignment. Such a situation could be easily

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achieved by rotating an object 180o so that the canonical left side of the object would fall on the right side of an egocentric reference frame (e.g. to the right of the point of fixation).

The second explanation just outlined is based on the premise that allocentric

neglect is in fact a result of ‘fixation-based neglect’. Fixation-based neglect is the idea that information falling to the left of a central fixation position was neglected. Thus, a pattern consistent with allocentric neglect would be apparent if the centre of each object was fixated during the task, which is in fact neglect operating at an egocentric level based on the position of gaze. Fixation-based neglect at the letter level may not have occurred

during the SEAN task as each individual letter included may not have been directly fixated; therefore patterns of allocentric neglect were not evident. Even though information

regarding precise fixation location information within the SEAN task could not be obtained due to the accuracy of the eye tracker, numerous letters were included in the task and, therefore, it was unlikely to be efficient to fixate each letter during search for targets in this task. Letters being presented in close proximity to one another in the task may have

resulted in low-level featural information being extracted from letters that were not directly under the fovea (Treisman & Gelade, 1980; Rayner, 1998). Treisman & Gelade (1980) explained findings of display size increasing but processing time not, as being due to the larger displays having an increase in density of items within the array. This increase in density meant that more stimuli would, on average, have fallen within foveal vision during each fixation, allowing the number of items that could be processed in parallel to increase with display size. As the SEAN task was a dense array of small letters, each of the items may not have required a fixation. It has been shown that during normal reading, people can extract low-level featural information from letters 7-9 characters to the right of the current fixation position (Rayner, 1998).

A sequential search task including only letters, developed by Trukenbrod and Engbert (2007), demonstrated that the same oculomotor principles guided eye movements during the search task as those that operate during reading. Trukenbrod and Engbert provided evidence that low-level information can be extracted from letters that are not currently being fixated, indicating that parallel processing of letters can occur in this type of search task. Therefore, it would appear that each letter in the SEAN task may not have received individual central fixations, as has been demonstrated by a number of studies (e.g. Shiffrin & Gardner, 1979; Itti & Koch, 2000; Treisman & Gelade, 1980; Trukenbrod & Engbert, 2007; Rayner, 1998). Fixation-based neglect may be more likely to occur for larger objects presented in the visual field, where it is more likely that each object is

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directly fixated as in tasks employed by Henderson (1993). This would suggest that

physical properties of the stimulus and the task demands, influence the nature and extent of neglect exhibited. In order to confirm that during this task, and similar tasks, that NPs did not fixate the centre of each letter, resulting in no fixation-based neglect occurring, and therefore no evidence being provided for allocentric neglect, a head-mounted eye tracker with higher spatial resolution would need to be developed and employed to record participants’ landing positions within each letter.

Determining whether allocentric neglect is a result of fixation-based neglect will be directly addressed in the next experiment. Upright and inverted objects with canonical handedness were employed in Experiment 3 in order to investigate whether the left side of the object was neglected (i.e. its canonical left side) or, as predicted, information to the left of a central-fixation position was being neglected. This is a critical question, as evidence for fixation-based neglect may explain why allocentric neglect can be exhibited on occasions but has not been consistently demonstrated previously (due to the nature of the stimulus employed).

Thirdly, another reason for the lack of allocentric neglect presented in this task by the NPs is that the task may not have been sensitive to this type of neglect if it was present. As the target items were letters, it may be that each letter is not coded as an object, which would mean that the left side of the letter would not be neglected even if allocentric neglect was occurring. It may be more likely that a number of letters are grouped together to form an object (as discussed previously) and then the letters that fell on the left side of that group letters (i.e., that object) would be neglected, not the left side of each letter.

Therefore, letters may not be an appropriate stimulus to employ to investigate allocentric (object-centred) neglect and so other targets and stimuli will be employed in the following experiments. Despite this, there is some suggestion that the intrinsic left side is neglected of letters that have been rotated and are presented in isolation (e.g., Behrmann &

Moscovitch, 1994) so the premise that the left side of a letter may be neglected cannot be dismissed completely.

4.2 Did a Sampling or Processing Deficit of Contralesional Space Occur?