Neglect and Extinction

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only encountered when the cued and the target position are in opposite hemifields, or is it more specifically related to the direction in which attention has to be moved? This was tested by Posner et al. 1987, in a study where cues and targets could appear in one of six boxes on the left or right side, respectively. Generally, the RTs of neglect patients to targets presented in the neglected (contralesional) visual field were delayed, but this delay depended upon whether attention had to be moved to the ipsilesional or contralesional side. When cue and target were presented in the neglected hemifield, RTs were massively delayed when an attentional shift to the contralesional side was required. Posner et al. concluded that the direction of attention shifts is critical for some of the deficits observed in neglect; neglect patients encounter specific problems when attention is to be moved to the contralesional side. Similarly, extinction can also be found when both stimuli are presented in the unaffected hemifield. Here, the stimulus located nearer to fixation (towards the affected side) will often be undetected.

Location-centred and object-centred reference frames in neglect

While neglect is often described as a disorder related to co-ordinates of external space, there is growing evidence that neglect may sometimes be based on object-centred co-ordinates. Driver et al. 1992, asked a left-neglect patient to match the edges within figures to a subsequent probe edge. Performance was better when the relevant edge was on the right side of the figure, even when the figure was presented on the left (neglected) side. Figure-ground segmentation was obviously possible in the neglected visual field, and neglect was relative to a figure-centred reference frame. Driver & Halligan 1991, asked neglect patients whether two shapes were the same of different. When the part of the shape that contained the difference was presented in the neglected hemifield, patients were unable to do this task (as expected). When the stimuli were moved, so that the relevant part was in the good visual field, they still neglected the left side of the object, suggesting that neglect is related to the contralesional side of the object’s principal axis, rather than to contralesional visual space.

A compelling demonstration of the role of object-based frames of reference in neglect comes from Tipper & Behrmann 1996. They presented ’objects’ (barbells consisting of two circles connected with a horizontal bar, and right-hemisphere patients had to detect the onset of a target in the left or right circle. In some blocks, the barbell was rotated through 180o prior to target presentation, in other blocks, the display was static. If neglect was entirely location-based, the rotation of the object should have no effect on performance. If neglect was object-centred, performance following rotation should be the reverse of performance in the static condition, because the left of the object is moved to the right, and vice versa. This pattern was indeed found: After rotation, RTs to contralesional targets were faster than to ipsilesional targets, and performance benefits for contralesional space and costs for ipsilesional space were found relative to the static condition. Although this is an impressive demonstration of object-centred neglect, it should be noted that not all patients showed this pattern, and that a main effect of target side remained (delayed RTs for targets on the left). Tipper & Behrmann therefore suggested that neglect might be based on object-centred as well as location-based frames of reference.Similar findings have been reported for extinction: Extinction can be dramatically reduced by inserting connecting lines between simultaneously presented targets.

Unconscious perception in neglect?

How much processing do features within neglected locations or objects receive? It has been argued that semantic aspects of neglected information can affect performance. Marshall & Halligan 1988 showed neglect patients two drawings of a house that were identical except for bright red flames appearing from the window on the left side of one house. Although the patients denied any difference between the two, the tended to choose the one not on fire when asked which house they preferred to live in. Volpe et al. 1979 showed pairs of line drawings in the left and right visual field. Patients were able to name only the right one, but their performance was above chance when they were asked to make forced-choice same/different judgements. In these cases, there were always between-object differences in low-level perceptual features, so they do not unequivocally support unconscious semantic processing! More convincing evidence was provided by McGlinchley-Berroth et al. 1994, who showed patients line drawings (primes) on the left or right side together with scrambled figures on the opposite side and found priming effects of semantically related figures on a subsequent lexical decision task both for left-side and right-side primes. In contrast, when the same patients matched left primes to subsequent probe displays, performance was at chance level.

Conclusion

Many different phenomena have been subsumed under ’the neglect syndrome’. Neglect can affect different locations in space, can be related to different co-ordinate systems, and is observed with anatomically distinct brain lesions. It is likely that ’neglect’ is not a unitary, well-defined entity, but that it represents various, partially dissociable impairments in spatial cognition.

Overall, it can be said that because Unilateral Neglect causes patients to behave as if the contralesional half of their world has become unimportant, it makes the capacity of reacting to certain stimuli selectively when several occur simultaneously. This makes it difficult for the patient to