Eye movements during fixation as velocity noise in minimum motion detection

The functional roles and perceptual consequences of fixational eye movements are argued. The retinal image motions due to these eye movements are viewed as normally unnoticed velocity noise that limits performance of minimum motion detection without reference. When the motion detection threshold and the variability of eye velocity during fixation were measured for a group of normal adult observers, an interobserver correlation was established between psychophysical and oculomotor data. In particular, when both eyes were open, the threshold of unreferenced motion was positively correlated with the fixation instability of the eye, making larger drifts. Preliminary data also suggested the possibility that the fixation instability of this eye still dominates the detection threshold if this eye was occluded during the task. Possible schemes of living with such velocity noise as originating from fixation instability are discussed.