Interocular timing differences in the horizontal components of human saccades

The occurrence of systematic interocular differences in the time of initiation of saccades in various directions was investigated in normal human subjects (n=4). Saccades were recorded binocularly with scleral sensor coils on each of the eyes with a temporal resolution of 0.1 ms (sampling frequency 10000 Hz). Analysis was done in the velocity domain after digital differentiation. It was found that, in the initial phase of horizontal saccades, the nasalward moving eye lagged the temporalward moving eye consistently by slightly less than 1 ms. No such difference was found in vertical (upward or downward) saccades. In oblique saccades, the systematic initial lag of the nasalward moving eye was similarly present in the horizontal component of the saccade, but absent in the vertical component. It is postulated that this interocular timing difference in horizontal saccades is due to the additional synaptic delay caused by the abducens internuclear neuron in the pathway to the medial rectus muscle of the eye.