Eye movements between saccades: Measuring ocular drift and tremor

•Guidelines to assess the precision and resolution of eyetracking systems are defined.•This assessment is crucial when attempting to study small eye movements.•High-precision DPI and search coil systems can reliably measure tremor and drift.•Ocular drift characteristics are remarkably similar in humans and non-humans primates.•Tremor is a deviation from the 1/F2 fixational motion spectrum in the range 50–100 Hz.

Intersaccadic periods of fixation are characterized by incessant retinal motion due to small eye movements. While these movements are often disregarded as noise, the temporal modulations they introduce to retinal receptors are significant. However, analysis of these input modulations is challenging because the intersaccadic eye motion is close to the resolution limits of most eyetrackers, including widespread pupil-based video systems. Here, we analyzed in depth the limits of two high-precision eyetrackers, the Dual-Purkinje Image and the scleral search coil, and compared the intersaccadic eye movements of humans to those of a non-human primate. By means of a model eye we determined that the resolution of both techniques is sufficient to reliably measure intersaccadic ocular activity up to approximately 80 Hz. Our results show that the characteristics of ocular drift are remarkably similar in the two species; a clear deviation from a scale-invariant spectrum occurs in the range between 50 and 100 Hz, generally attributed to ocular tremor, leading to intersaccadic retinal speeds as high as 1.5 deg/s. The amplitude of this deviation differs on the two axes of motion. In addition to our experimental observations, we suggest basic guidelines to evaluate the performance of eyetrackers and to optimize experimental conditions for the measurement of ocular drift and tremor.