Saccadic Eye Movements in Object-based Neglect

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Abstract
RR showed left-sided neglect of objects following a stroke; when asked to copy a drawing of a simple scene he drew only the right side of each individual object. When his eye movements were recorded, RR made left saccades to locate objects to the left of the midline of the display, but restricted his fixations to the right sides of individual objects. Detailed examination of RR's eye movements while he viewed chimaeric photographs (comprising left and right halves of two different stimuli) also showed a pronounced tendency to scan only the right side of each. However, RR did not have a left hemianopia, and when left half stimuli were presented in isolation in his left visual field, he made left saccades and recognised them correctly. When presented with chimaeric stimuli aligned to the right of initial fixation, RR's first saccades landed on the left side of the images and a fixation of some 200m sec was made in the region of the left eye. Thus the initial left-sided fixation of chimaerics was unrelated to his ability to recognise them. Instead, on trials where left sides of chimaerics were recognised, RR made later saccades back into the left side of the chimaeric. In contrast, when left half stimuli were tachistoscopically presented at 200m sec exposure, RR could recognise a good proportion of them. We propose that RR has a deficit in orienting his attention within an object-based frame of reference, while being less impaired at orienting between objects. This does not result from a primary deficit of eye movement control; instead, RR's defective eye movements are a consequence of his object-based neglect.

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