  Even when we look at stationary objects, involuntarily our eyes perform
miniature movements and do not stand perfectly still. Such fixational eye
movements (FEM) can be decomposed into at least two components: rapid
microsaccades and slow (physiological) drift. Despite the general agreement
that microsaccades have a central generating mechanism, the origin of drift is
less clear. A direct approach to investigate whether drift is also centrally
controlled or merely represents peripheral uncorrelated oculomotor noise is to
quantify the statistical dependence between the velocity components of the FEM.
Here we investigate the dependence between horizontal and vertical velocity
components across the eyes during a visual fixation task with human observers.
The results are compared with computer-generated surrogate time series
containing only drift or only microsaccades. Our analyses show a binocular
dependence between FEM velocity components predominantly due to drift. This
result supports the existence of a central generating mechanism that modulates
not only microsaccades but also drift and helps to explain the neuronal
mechanism generating FEM.
