Precision of sustained fixation in trained and untrained observers

Abstract

During visual fixation, microscopic eye movements shift the image on the retina over a large number of photoreceptors. Although these movements have been investigated for almost a century, the amount of retinal image motion they create remains unclear. Currently available estimates rely on assumptions about the probability distributions of eye movements that have never been tested. Furthermore, these estimates were based on data collected with only a few, highly experienced and motivated observers and may not be representative of the instability of naive and inexperienced subjects in experiments that require steady fixation. In this study, we used a high-resolution eye-tracker to estimate the probability distributions of gaze position in a relatively large group of human observers, most of whom were untrained, while they were asked to maintain fixation at the center of a uniform field in the presence/absence of a fixation marker. In all subjects, the probability distribution of gaze position deviated from normality, the underlying assumption of most previous studies. The resulting fixational dispersion of gaze was much larger than previously reported and varied greatly across individuals. Unexpectedly, the precision by which different observers maintained fixation on the marker was best predicted by the properties of ocular drift rather than those of microsaccades. Our results show that, during fixation, the eyes move by larger amounts and at higher speeds than commonly assumed and highlight the importance of ocular drift in maintaining accurate fixation.

Figure 1

Examples of the raw eye movement data recorded in the experiments (thin lines) and filtered traces (thick lines). The two curves in each panel represent horizontal and vertical eye movements. The two panels show traces from the most accurate (subject WW; left) and the least accurate (subject DG; right) observers in our pool.

Figure 2

Precision of fixation. Probability density functions of gaze position for individual observers during sustained fixation on (a) a 4′ dot (marker condition); and on (b) a uniform field (no-marker condition). Different panels refer to different subjects. Color codes the probability that the line of sight deviated by an amount corresponding to each pixel location. Distributions were normalized by their peak value for better visualization. Here and in the following Figures and Tables, asterisks mark the experienced observers. (c, d) Average cumulative probability across all the observers in the untrained group as a function of the span of fixation in the two experimental conditions (black). The shaded gray region represents SEM. For comparison, the average cumulative probability across trained observers (green) and the equivalent estimate given by the confidence ellipse (red) are also shown.

Figure 3

Characteristics of saccades. (a, b) Average probability distributions of saccadic amplitudes in the two experimental conditions. Data represent average ± SEM of the amplitude distributions of individual subjects. Triangles indicate mean values. (c, d) Probability distributions of saccades for each individual observer during fixation on (c) a marker and (d) a uniform field.

Figure 4

Characteristics of ocular drift. (a, b) Average probability distributions across all observers of (a) drift speed and (b) index of curvature in the marker condition. Error bars represent SEM. Triangles indicate mean values. (c) 2D probability distributions of instantaneous drift velocity for individual observers in the marker condition. Maps are in polar coordinates. Color codes the probability of occurrence of instantaneous drift velocity with given direction and modulus. (d-f) same as in (a-c) for the no-marker condition.

Figure 5

Interplay between saccades and drift. Average distributions across observers of the angular differences between consecutive oculomotor events: (a, b) angles of saccades relative to the preceding drifts, (c, d) angles of drifts relative to the preceding saccades. (e) Compensation indices in the two experimental conditions for saccades after drifts (saccades) and drifts after saccades (drift). Data represent means ± SEM. (f) Compensation indices (CI) for individual observers.

Figure 6

Linear regression between the fixation span and the four oculomotor variables in the marker (a-d) and no-marker condition (e-h). Each dot represents one observer. Experienced subjects are indicated in gray. Solid and dashed lines represent linear regressions and their 95% confidence intervals, respectively. The corresponding values of the coefficient of determination (r²) and probability (p) are reported in each panel.