Effect of filtering in the computer analysis of saccades

The effect of sampling rate and lowpass filters on saccades - A modeling approach

The study of eye movements has become popular in many fields of science. However, using the preprocessed output of an eye tracker without scrutiny can lead to low-quality or even erroneous data. For example, the sampling rate of the eye tracker influences saccadic peak velocity, while inadequate filters fail to suppress noise or introduce artifacts. Despite previously published guiding values, most filter choices still seem motivated by a trial-and-error approach, and a thorough analysis of filter effects is missing. Therefore, we developed a simple and easy-to-use saccade model that incorporates measured amplitude-velocity main sequences and produces saccades with a similar frequency content to real saccades. We also derived a velocity divergence measure to rate deviations between velocity profiles. In total, we simulated 155 saccades ranging from 0.5° to 60° and subjected them to different sampling rates, noise compositions, and various filter settings. The final goal was to compile a list with the best filter settings for each of these conditions. Replicating previous findings, we observed reduced peak velocities at lower sampling rates. However, this effect was highly non-linear over amplitudes and increasingly stronger for smaller saccades. Interpolating the data to a higher sampling rate significantly reduced this effect. We hope that our model and the velocity divergence measure will be used to provide a quickly accessible ground truth without the need for recording and manually labeling saccades. The comprehensive list of filters allows one to choose the correct filter for analyzing saccade data without resorting to trial-and-error methods.

Algorithm for automatic analysis of electro-oculographic data

BACKGROUND

Large amounts of electro-oculographic (EOG) data, recorded during electroencephalographic (EEG) measurements, go underutilized. We present an automatic, auto-calibrating algorithm that allows efficient analysis of such data sets.

METHODS

The auto-calibration is based on automatic threshold value estimation. Amplitude threshold values for saccades and blinks are determined based on features in the recorded signal. The performance of the developed algorithm was tested by analyzing 4854 saccades and 213 blinks recorded in two different conditions: a task where the eye movements were controlled (saccade task) and a task with free viewing (multitask). The results were compared with results from a video-oculography (VOG) device and manually scored blinks.

RESULTS

The algorithm achieved 93% detection sensitivity for blinks with 4% false positive rate. The detection sensitivity for horizontal saccades was between 98% and 100%, and for oblique saccades between 95% and 100%. The classification sensitivity for horizontal and large oblique saccades (10 deg) was larger than 89%, and for vertical saccades larger than 82%. The duration and peak velocities of the detected horizontal saccades were similar to those in the literature. In the multitask measurement the detection sensitivity for saccades was 97% with a 6% false positive rate.

CONCLUSION

The developed algorithm enables reliable analysis of EOG data recorded both during EEG and as a separate metrics.

Characteristics of braking saccades in congenital nystagmus

Several of the characteristic waveforms of congenital nystagmus (CN) contain braking saccades. We test the hypothesis that braking (including foveating) saccades, while not always satisfying the standard relationships for saccades, are normal; any differences are due to the presence of high-velocity, slow-phase eye movements. Better measurements of saccadic properties, including position- and velocity-based measures and skewness, can eliminate some of this apparent distortion. We also evoked an analogous effect in normal subjects by use of a ramp-step-ramp stimulus. Finally, we used a model to further demonstrate this distortion in the saccades of normals, deviating from their intended magnitude as a function of the magnitude of the opposing velocity. The saccadic analysis methods developed herein are applicable to all saccades made during ongoing eye movements, whether normal or pathological. The above findings support the hypothesis that the braking saccades integral to many CN waveforms have normal characteristics and are the result of a normal saccadic system's responses to a slow-eye-movement oscillation.