Vestibulo-ocular and optokinetic impairments in left unilateral neglect
Right brain damaged patients affected by left unilateral neglect (N+) typically fail to explore the contralesional space. For the first time, this study investigates the dynamic and spatial features of the horizontal vestibular-ocular response (VOR), the optokinetic response (OKR) and the VOR-OKR interaction in six N+ and in five right brain damaged patients without neglect (N-). No lateral asymmetry of the gain (i.e. eye velocity to head velocity ratio) of VOR slow phases was found in either group. In the VOR, N+ had higher frequency of slow-rightward/fast-leftward phases and higher contralesional shift of the beating field (i.e. orbital position of fast phases). In the VOR-OKR, there was an increase of gain in both lateral directions and in both groups even though in N-, there was a lower phase shift between eye and head velocity. In contrast to the VOR, in the VOR-OKR, N+ had higher frequency of slow-leftward/fast-rightward phases. The VOR-OKR interaction also introduced an ipsilesional shift of the beating field in both N+ and N-. In the OKR, N+ showed a drop in the velocity, amplitude and frequency of slow-rightward/fast-leftward phases. These findings potentially suggest that each hemisphere modulates VOR with contralaterally directed slow phases and OKR with ipsilaterally directed slow phases. This organisation could facilitate maintenance or fast recovery of combined VOR + OKR after unilateral brain damage. The same findings suggest that by inducing slow-leftward phases, vestibular and optokinetic stimulation improve left side neglect through the activation of different hemispheric pathways. No ipsilesional deviation of the subjective "straight ahead" was found in N+. These results show that chronic unilateral neglect can be dissociated both from deficits of ipsilesionally directed VOR and from ipsilesional deviation of the subjective midsagittal plane of the body.