SUBJECTIVE BODY ORIENTATION IN NEGLECT AND THE INTERACTIVE CONTRIBUTION OF NECK MUSCLE PROPRIOCEPTION AND VESTIBULAR STIMULATION

Three patients with a right, predominantly parietal lesion and marked left-sided neglect without visual field defects were asked to direct a laser point to the position which they felt to lie exactly 'straight ahead' of their bodies orientation Whereas in both light and darkness, the subjective body orientation was close to the objective body posit ion in the control groups, the three neglect patients localized the bo dy's sagittal midplane similar to 15 degrees to the right of the objec tive orientation. No relevant differences of 'straight ahead' were fou nd between the neglect patients and controls in the vertical plane. Th e neglect patients' horizontal displacement of sagittal midplane to th e right could be compensated for either by neck muscle vibration or by caloric vestibular stimulation on the left side. When vestibular stim ulation was combined with neck muscle vibration, the horizontal deviat ion linearly combined by adding or neutralizing the effects observed w hen both types of stimulation were applied exclusively in the control groups as well as in the neglect patients. Moreover; data analysis rev ealed that the neglect patients' ipsilesionally displaced subjective b ody orientation does not result fi om a disturbed primary perception o r disturbed transmission of the vestibular or proprioceptive input fro m the periphery. The present results support the hypothesis that the e ssential aspect leading to neglect in brain-damaged patients is a dist urbance of those cortical structures that are crucial for transforming the sensory input coordinates from the peripheral sensory organs-here the retina, neck muscle spindles and cuprlae-into an egocentric, body -centred coordinate frame of reference. In neglect patients the coordi nate transformation seems to work with a systematic error and deviatio n of the spatial reference frame to the ipsilesional side leading to a corresponding displacement of subjective localization of body orienta tion. It can be concluded further that neck muscle proprioception and vestibular stimulation directly interact in contributing to the subjec t's mental representation of space. The data suggest that the afferent information from these different input channels is used simultaneousl y for computing egocentric, body-centred coordinates that allow us to determine our body position in space.