Changes in waveform of congenital nystagmus associated with biofeedback treatment.

A study was made of parameters of congenital nystagmus which responded to auditory biofeedback treatment. The parameters studied included foveation time, amplitude, and frequency. The patient's right retina was observed with an infrared television fundus camera, and the fundus image was recorded on video tape. The position of the eye during nystagmus, observed via the fundus camera and recorded on video, was analysed at every 1/60 second intervals. The displacement in degrees between the fixation target, projected on to the retina, and the foveola was measured for each interval. Using biofeedback, the subjects could voluntarily suppress nystagmus and prolong foveation time. A damping of the nystagmus amplitude, intensity, and frequency was observed. On the average the intensity decreased by about 40%, and the foveation time was prolonged by about 190%. After completion of the training all the patients reported a subjective improvement in their vision when suppressing their nystagmus. Possibly biofeedback training acts to reduce the nystagmus and extend foveation time, thereby improving the ability to fixate.