Optokinetic stimulation influences the disturbed perception of body orientation in spatial neglect

The effect of optokinetic stimulation on the disturbed perception of body orientation in three patients with right brain damage and spatial neglect was examined. The patients were asked to direct a laser point to the position which they felt lay exactly ''straight ahead'' of their bodies' orientation. Without stimulation they localised the body's sagittal midplane markedly to the right of the objective orientation. The patients' horizontal displacement of the sagittal midplane was reduced by a movement of the surround to the left and worsened by a movement to the right. The findings are consistent with those found in patients with spatial neglect using vestibular and neck proprioceptive stimulation. They show that visual input, together with vestibular and neck proprioceptive input, is used for computing a central representation of egocentric space. In spatial neglect this coordinate transformation works with a systematic error and deviation of the spatial reference frame to the ipsilesional side. The positive effect of optokinetic stimulation in patients with spatial neglect is interpreted with a ''correction'' of the neural coordinate transformation process by producing asymmetric input at the sensory organs of the contributing channels.