ABSTRACT
In this contribution, I would like to concentrate on the micromovements of the eye which occur during fixation and their role in the maintenance of vision. Figure 1, taken from Riggs and Ratliff (1951), is a sample recording that shows clearly the different components of eye movements, namely the saccades, the drift, and the tremor motions. The calibration mark is 100 sec of arc. Among the three types of movement, the saccades and the drifts are the larger, each with an average amplitude of 5 min of arc and an irregular rate of occurrence. The saccades are fast with an average duration of 25 msec, the drift is slow having a duration of 100 msec or longer. The tremor motion is small and fast with an average amplitude of 20 sec of arc, and a rate of occurrence up to 100 times per sec. These spontaneous movements of the eye result in displacements of the image in relation to the retina. It is calculated that in a typical fixation period of 4 sec the image remains in the fovea but travels back and forth across 3D-50 receptors. Therefore, many cone receptors receive stimulation with a frequency and amplitude that depend on both the characteristics of the motion and the spatial contrast properties of the image. It is reasonable to assume that such a rich variety of stimulation supplied by eye movements amplifies the response of the receptor or receptor groups, eventually aiding visual function. Indeed, for a long time after their discovery these fixational eye movements were regarded as the mechanism subserving acuity and contrast discrimination. Acuity is the sensitivity of the human visual system to the size of spatial detail in the stimulus; contrast sensitivity refers to the just discriminable level of contrast, regardless of the size of the detail.
