Ocular exploration of space as a function of neck proprioceptive and vestibular input ? observations in normal subjects and patients with spatial neglect after parietal lesions

Abstract

We recently argued that the specific compensation of spatial neglect by manipulating neck proprioceptive and vestibular input is due to a central "correction" of the disturbed neural transformation process converting the afferent input coordinates from the peripheral sensory organs into a central representation of egocentric space. Both types of stimulation were proposed to induce a reorientation of the deviated or distorted egocentric spatial reference frame. The aim of the present study was to observe this process of reorientation under a condition in which no visual stimulus can attract the subject's attention and thus influence exploration behaviour from outside. We recorded eye movements of normal subjects and of three patients with spatial neglect after right parietal lesions while searching for a non-existent target in complete darkness. It was assumed that the area of the outer space that subjects spontaneously explore under this condition is a direct function of the subject's representation of egocentric space. Ocular space exploration was biased and confined almost entirely to the right side of the midsagittal plane in patients with neglect. This spatial distribution of exploratory eye movements changed remarkably with left-sided neck muscle vibration as well as with left-sided vestibular stimulation using ice water calorics. The spatial area of exploration was significantly enlarged to the contralesional side and the exploration maximum shifted in the same direction. Whereas with both types of stimulation space exploration of patients with neglect was similar to that of normal subjects when not being stimulated, neck proprioceptive and vestibular stimulation in normal subjects induced a quasi neglect-like exploration pattern, i.e. a bias to one side of the objective midsagittal plane. If ocular space exploration was, however, related to the subjectively perceived position of the midsagittal plane in space, eye movements were symmetrically distributed and carried out to both sides of subjective "straight ahead" in all experimental conditions, in normal subjects as well as in patients with neglect. The present results support the above hypothesis and indicate that neck proprioceptive as well as vestibular input directly contribute to the computation of the subject's central representation of egocentric space used for localizing body orientation and for guiding motor behaviour in space.