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

We recently argued that the specific compensation of spatial neglect b y manipulating neck proprioceptive and vestibular input is due to a ce ntral ''correction'' of the disturbed neural transformation process co nverting the afferent input coordinates from the peripheral sensory or gans 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 i n which no visual stimulus can attract the subject's attention and thu s influence exploration behaviour from outside. We recorded eye moveme nts of normal subjects and of three patients with spatial neglect afte r right parietal lesions while searching for a non-existent target in complete darkness. It was assumed that the area of the outer space tha t subjects spontaneously explore under this condition is a direct func tion 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 distr ibution of exploratory eye movements changed remarkably with left-side d neck muscle vibration as well as with left-sided vestibular stimulat ion using ice water calorics. The spatial area of exploration was sign ificantly enlarged to the contralesional side and the exploration maxi mum shifted in the same direction. Whereas with both types of stimulat ion space exploration of patients with neglect was similar to that of normal subjects when not being stimulated, neck proprioceptive and ves tibular stimulation in normal subjects induced a quasi neglect-like ex ploration pattern, i.e. a bias to one side of the objective midsagitta l plane. If ocular space exploration was, however, related to the subj ectively perceived position of the midsagittal plane in space, eye mov ements were symmetrically distributed and carried out to both sides of subjective ''straight ahead'' in all experimental conditions, in norm al subjects as well as in patients with neglect. The present results s upport the above hypothesis and indicate that neck proprioceptive as w ell as vestibular input directly contribute to the computation of the subject's central representation of egocentric space used for localizi ng body orientation and for guiding motor behaviour in space.