Purpose: To examine the waveform characteristics of 37 subjects with manifest latent nystagmus (MLN) and determine the manner in which visual feedback influences the nature of the waveform.
Methods: Binocular recordings of the eye movements of all subjects were undertaken using an infrared tracking system. Subjects viewed the target binocularly and monocularly in primary gaze. The effect of visual feedback on the nature of the MLN waveform was examined by either removing the fixation target or by progressively stabilizing the target in relation to the retina. This progressive stabilization was achieved by feeding back the eye movement signal to move an otherwise stationary target.
Results: Four types of MLN were distinguished on the basis of the fixation characteristics seen during binocular and monocular viewing. First, under binocular viewing conditions, subjects could theoretically exhibit stable fixation (type 1 MLN). In addition, three other MLN types were recorded during binocular fixation: conjugate horizontal square-wave jerks (type 2 MLN), conjugate torsional nystagmus (type 3 MLN) and conjugate horizontal jerk MLN waveforms (type 4 MLN). Monocular viewing always gave rise to a conjugate horizontal jerk MLN waveform for each of the four types of MLN. More than 80% of the subjects exhibited either type 3 or type 4 MLN, both of which conform with previous classic descriptions of MLN. Much less common was type 2 MLN. Type 1 MLN (conventionally referred to as a latent nystagmus) appeared to be a rare occurrence. In addition to the two classic linear and decelerating MLN slow phases, four additional slow-phase shapes with either saccadic or pendular elements were recorded and described. Removing visual feedback generally reduced the mean slow-phase velocity and the number of fast phases. For each subject some variability of the slow-phase class was documented from session to session.
Conclusions: Four types of MLN have been described. Their differences are based on their binocular oculomotor behavior, and it is proposed that type 1 MLN and type 4 MLN represent the absolute states and types 2 and 3 the intermediate levels of the MLN spectrum. All types of MLN appear to be strongly visually driven and are largely dependent on the attentional state of the subject and the target conditions. Six different classes of slow phase were found among the four MLN types. The introduction of visual feedback had an immediate effect on the subsequent slow phase or fast phase. It is likely that adaptation mechanisms are in play after a period of visual feedback.