Predominance of nasal over temporal saccades in fast eye movement

Quantitative analysis of saccadic eye movement caused by 20 degree amplitude gaze in horizontal plane was undertaken in 131 subjects using the Contraves' computerized oculomotor testing system. All of the subjects were free from any spontaneous nystagmus, neurological symptoms, and abnormalities of vestibular function. Both of the monocular recordings of saccade were made with a 3 sec time constant, 25 Hz high-cut filter, and digitized with a 3.9 msec sampling time. Four parameters i.e., maximum velocity, gain, latency, and duration, were analyzed. The average (S.D.) of each parameter was 337.1 (65.7) degree/sec, 1.00 (0.14), 199.1 (44.0) msec, and 128.3 (20.2) msec, respectively. We discussed the differences between nasal saccade and temporal saccade. The former was faster than the latter in maximum velocity and shorter in duration, while no differences were recognized in latency. It is suggested that the predominance of nasal saccade in fast eye movement is due to the anatomical difference of the vestibulo-ocular pathway between the medial rectus and the lateral rectus. In other words, because the synaptic inputs to the medial rectus are excitatory as opposed to the inhibitory and excitatory the inputs to the lateral rectus, the difference in muscle tone between the two muscles results in a faster maximum velocity of nasal saccades in fast eye movement.