Microsaccades decrease large and increase small blink-induced fixation errors.

<p>(<b>A</b>) Vertical eye-position traces after 11 randomly-selected blinks that led to large vertical errors ([0.64–0.66 deg], monkey Y). (<b>B</b>) Vertical eye-position traces after 11 randomly-selected blinks that led to small vertical errors ([0.14–0.16 deg], monkey Y). (<b>A, B</b>) Grey band: range of final eye positions resulting in a positive CR. Brown traces: microsaccades decreased the blink-induced error. Orange traces: microsaccades increased the error. [We note that, although we considered all blinks in our analyses, blinks that took the eye below the fixation point were relatively infrequent (∼18%). Thus, this figure illustrates the more typical situation where blinks induced errors above the fixation point].</p

Blink-induced fixation errors.

<p>(<b>A</b>) Eye position trace showing an example of a blink (green) followed by drift (red) and then a microsaccade (blue). The black cross represents the fixation target. The eye position at the end of the blink (2) does not match the fixation target (black cross). A microsaccade corrects the error by bringing the eye from position (3) to (4), closer to the fixation target. (<b>B</b>) Vertical eye position for the same trace. The dashed line represents the fixation target. (<b>C</b>) Cartoons of corrective and non-corrective microsaccades. BE (dashed line) indicates the blink-induced fixation error and D (dotted line) the distance between the eye position at the end of the microsaccade and the fixation target. Left: the microsaccade reduces the blink-induced eye position error (D is shorter than BE). Right: the microsaccade increases the eye position error (D is longer than BE).</p

Blink-induced error and microsaccade properties.

<p>(<b>A</b>) Normalized magnitude distributions of blink-induced fixation errors, post-blink microsaccades, and post-blink drifts across non-human primates. The distribution of post-blink microsaccade magnitudes matches closely that of blink-induced fixation errors. (<b>B</b>) Polar histogram of the directions of blink-induced fixation errors, post-blink microsaccades, and post-blink drifts. Blink-induced fixation errors are more likely directed upward. Post-blink microsaccades tend to move the eye downward, thus counteracting the error introduced by the blink. (<b>C</b>) Latency distribution for post-blink microsaccades (all monkeys combined): 74.12% of post-blink microsaccade onsets occurred in the initial 400 msec after the end of the blink. (<b>D</b>) Blink-induced error as a function of time, from the end of the blink onward. We calculated the blink-induced error at every point in time, whether there were concurrent microsaccades or drifts. The blink-induced error declines gradually, showing the largest decrease in the initial 400 msec interval, simultaneous to the highest production of post-blink microsaccades. Shaded area indicates the SEM across monkeys (<i>n</i> = 5 monkeys).</p

Average magnitude of fixation errors induced by different types of ocular events.

<p>Fixation errors associated with blinks tended to be larger than those associated with (all) microsaccades or drifts, but not significantly so.</p><p>Average magnitude of fixation errors induced by different types of ocular events.</p

Blink-induced error correction by fixational eye movements.

<p>(<b>A</b>) Correction ratio for microsaccades and drifts (solid lines) versus random permutations (dotted lines) as a function of error magnitude. Asterisks indicate statistical significance (two-tailed paired <i>t</i>-test between microsaccade or drifts and permutations, Bonferroni corrected <i>p</i><0.01). Microsaccades correct blink-induced fixation errors better than chance (i.e. random permutations; see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110889#s2" target="_blank"><i>Methods</i></a> for details). Drifts are not significantly different than permutation. Microsaccades corrected large blink-induced errors (>0.2 degrees) better than small blink-induced errors. Error bars and shaded areas indicate the SEM across monkeys (<i>n</i> = 5). (<b>B</b>) Average correction ratio for microsaccades and drifts (filled bars) compared to chance (striped bars). Asterisks indicate statistical significance (two-tailed paired <i>t</i>-test, <i>p<</i>0.01) (<i>n</i> = 5 monkeys).</p

Fixational Eye Movement Correction of Blink-Induced Gaze Position Errors

<div><p>Our eyes move continuously. Even when we attempt to fix our gaze, we produce “fixational” eye movements including microsaccades, drift and tremor. The potential role of microsaccades versus drifts in the control of eye position has been debated for decades and remains in question today. Here we set out to determine the corrective functions of microsaccades and drifts on gaze-position errors due to blinks in non-human primates (Macaca mulatta) and humans. Our results show that blinks contribute to the instability of gaze during fixation, and that microsaccades, but not drifts, correct fixation errors introduced by blinks. These findings provide new insights about eye position control during fixation, and indicate a more general role of microsaccades in fixation correction than thought previously.</p></div

Characteristics of non-human primate blinks.

<p>Characteristics of non-human primate blinks.</p

Latency of microsaccades after blinks.

<p>(<b>A</b>) Relationship between post-blink microsaccadic latency and correction ratio. Microsaccades occurring shortly after blinks were more corrective than microsaccades occurring later in time. (<b>B</b>) Relationship between blink-induced error magnitude and post-blink microsaccadic latency. Microsaccade latencies were shorter after large than small errors. Shaded areas indicate the SEM across monkeys (<i>n</i> = 5).</p

Examples of primate SWJs.

<p>Each trace represents 3 seconds of horizontal (top) and vertical (bottom) eye position recordings containing SWJs. Primarily horizontal (left), vertical (middle), and oblique (right) SWJs are displayed. All time scales are as in the bottom trace.</p

Peak velocity-magnitude relationship for SWJ saccades and non-SWJ saccades.

<p>Main panel: Each dot represents a saccade with the peak velocity indicated on the y-axis and the magnitude on the x-axis. Color determines whether the saccade was part of a SWJ (red) or not (blue). Bottom panel: Average saccade magnitude distribution across monkeys (<i>n</i> = 5). Left panel: Average peak velocity distribution across monkeys (<i>n</i> = 5). Saccade magnitude and peak velocity were greater for SWJ saccades than for non-SWJ saccades (Z-values = 2.02; <i>p</i>-values = 0.04). The slopes of the peak velocity-magnitude relationships for SWJ saccades and non-SWJ did not differ statistically (Z-value = 0.14; <i>p</i> = 0.89).</p