Abnormal saccadic substitution during small-amplitude pursuit tracking in amblyopic eyes

Small-amplitude, low-velocity, predictable triangular tracking was tested in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia by means of a photoelectric eye-movement recording technique. In the majority of amblyopic patients, abnormal saccadic substitution was found; that is, abnormally large saccades rather than small-amplitude smooth movements were used by the amblyopic eye to follow a spot stimulus that moved horizontally with low to high frequencies. Pursuit for the same range of stimuli was normal for binocular tracking and for monocular tracking with the dominant eye, pointing to a sensory rather than motor basis for the defect. This abnormal saccadic substitution response appeared to be related to the presence of amblyopia rather than strabismus. Several possible mechanisms responsible for causing this unusual response are discussed, including impairment of direction sense over small central regions of the amblyopic eye.