Purpose. To investigate the peak latency of the orbital presaccadic spike potential （SP） with horizontal eye movement in normals.Methods. Orbital SP was recorded in 28 normal subjects from 8 electrodes around the eyes with Pz as the reference while performing 5°

10°

20°

30° and 40°horizontal saccades to visual targets. SP peak latency was measured from SP onset to SP peak on averaged data aligned on SP peak.Results. Significant main effects on SP peak latency are found for saccade size （<0. 01）

saccade direction （P<0. 01） and electrode site （P<0. 05）. No significant main effect on SP peak latency is found for eye （P>0. 05）. SP peak latency increases with increasing saccade size from 5°to 40°. SP peak latency is longer with saccades back to center than away from center

and with abducting saccades than with adducting saccades. SP peak latency differs at the electrode sites with an order from shorter to longer as follows: inner canthus （IC）; inferior orbit （IO）: outer canthus （OC）; superior orbit （SO）.Conclusicos. The effects on the peak latency of orbital SP can be explained by the saccade dynamic property

volume conduction as well as physiologic and anatomic factors of the eyes and orbits. The peak latency of orbital SP can be used to reflect the temporal characteristics of ocular motor units controlling saccadic eye movement.