HEREDITARY CONGENITAL NYSTAGMUS AND GAZE-HOLDING FAILURE - THE ROLE OF THE NEURAL INTEGRATOR

Congenital nystagmus (CN) may be due to an instability of the neural i ntegrator responsible for gaze holding. This longitudinal study tests that hypothesis, investigates the saccadic instabilities of relatives, and assesses the effects of afferent stimulation on both the CN and t he coexisting gaze-holding failure. We recorded four siblings who had CN and gaze-holding failure while fixating in primary position and lat eral gaze. In lateral gaze, the CN waveforms were superimposed on the centripetal drift caused by the gaze-holding failure; the drift time c onstants ranged from 300 to 1,450 msec. CN waveforms lacked extended f oveation periods. Saccadic instabilities were present in the father an d two clinically unaffected siblings; the mother's eye movements were normal. We conclude that CN in the subjects of this study, and in othe rs with idiopathic CN, is not due to gaze-holding abnormalities, and w e speculate that development of the fixation reflexes that produce CN foveation periods requires some minimal foveation interval during whic h the target image is in the foveal area with low retinal slip velocit y and acceleration.