An investigation of viewer-, stimulus-, and object-centred neglect in the visual and tactile modalities

Abstract

This thesis is an investigation of viewer-, stimulus-, and object-centred neglect in both the visual and tactile modalities. The main aim was to provide experimental evidence of neglect in an object-centred frame of reference. It has been proposed that mental rotation of stimuli to upright may underlie previous findings of object-centred neglect, thus the mental rotation hypothesis of object-centred neglect was also investigated. The visual discriminative-cancellation task originally designed by Ota and colleagues (2001), and the tactile version of this task used by Marsh and Hillis (2008), were replicated with Norwegian patients with neglect. These tasks disambiguate viewer- from stimulus-centred neglect. Object-centred neglect is more difficult to disentangle since it involves neglect of the intrinsic left side of an object independently of the left side of the viewer or stimulus. Taking into account Behrmann and Moscovitch's (1994) criterion of utilising stimuli with an intrinsic left and right side to demonstrate object{u00AD}centred neglect, we introduced three new tasks aimed specifically to distinguish between stimulus- and object-centred neglect. All tasks required Norwegian patients with neglect to discriminate the Norwegian letter '{u00C6} (with an intrinsic left side 'A' and a right side 'E') from the letters 'A' and/or 'E'. To disambiguate viewer-, stimulus-, and object-centred neglect, the letters were presented in normal and backward parity and in eight angles of orientation. Discrimination errors were interpreted as either stimulus- or object-centred neglect, whereas omissions on the left side of the page in the letter{u00AD} cancellation version of the task were interpreted as viewer-centred neglect. Vision was precluded in the tactile task, and patients identified tactually each of the letters ('E', 'A', '{u00C6}') presented individually on a small rectangular board. The results provide evidence of neglect in viewer-, stimulus-, and object-centred frames of reference in the visual modality, and they indicate that visual neglect may occur in all three frames of reference simultaneously. The results also demonstrate that neglect may operate in an object{u00AD} centred frame of reference in the tactile modality. Numerous studies have investigated object-centred neglect in the visual modality, but no previous study has provided evidence of object-centred neglect in the tactile modality. A computerised version of the Norwegian letter '{u00C6}' task was used to answer a long-standing question in the literature about the role of mental rotation in previous findings of stimulus- and object-centred neglect. The absence of a mental rotation curve in the '{u00C6}' letter-identification reaction times demonstrates that the current findings cannot be explained by mental rotation of the stimuli to upright and re-alignment of all three reference frames. The high accuracy rates demonstrated by neurologically healthy participants in all tasks also indicates that the results cannot be attributed to 'normal' biases in the identification of misoriented letters.