The effects of fixation target size and luminance on microsaccades and square-wave jerks

A large amount of classic and contemporary vision studies require subjects to fixate a target. Target fixation serves as a normalizing factor across studies, promoting the field’s ability to compare and contrast experiments. Yet, fixation target parameters, including luminance, contrast, size, shape and color, vary across studies, potentially affecting the interpretation of results. Previous research on the effects of fixation target size and luminance on the control of fixation position rendered conflicting results, and no study has examined the effects of fixation target characteristics on square-wave jerks, the most common type of saccadic intrusion. Here we set out to determine the effects of fixation target size and luminance on the characteristics of microsaccades and square-wave jerks, over a large range of stimulus parameters. Human subjects fixated a circular target with varying luminance and size while we recorded their eye movements with an infrared video tracker (EyeLink 1000, SR Research). We detected microsaccades and SWJs automatically with objective algorithms developed previously. Microsaccade rates decreased linearly and microsaccade magnitudes increased linearly with target size. The percent of microsaccades forming part of SWJs decreased, and the time from the end of the initial SWJ saccade to the beginning of the second SWJ saccade (SWJ inter-saccadic interval; ISI) increased with target size. The microsaccadic preference for horizontal direction also decreased moderately with target size . Target luminance did not affect significantly microsaccades or SWJs, however. In the absence of a fixation target, microsaccades became scarcer and larger, while SWJ prevalence decreased and SWJ ISIs increased. Thus, the choice of fixation target can affect experimental outcomes, especially in human factors and in visual and oculomotor studies. These results have implications for previous and future research conducted under fixation conditions, and should encourage forthcoming studies to report the size of fixation targets to aid the interpretation and replication of their results

V1 neurons respond differently to object motion versus motion from eye movements

International audienceHow does the visual system differentiate self-generated motion from motion in the external world? Humans can discern object motion from identical retinal image displacements induced by eye movements, but the brain mechanisms underlying this ability are unknown. Here we exploit the frequent production of microsaccades during ocular fixation in the primate to compare primary visual cortical responses to self-generated motion (real microsaccades) versus motion in the external world (object motion mimicking microsaccades). Real and simulated microsaccades were randomly interleaved in the same viewing condition, thereby producing equivalent oculomotor and behavioural engagement. Our results show that real microsaccades generate biphasic neural responses, consisting of a rapid increase in the firing rate followed by a slow and smaller-amplitude suppression that drops below baseline. Simulated microsaccades generate solely excitatory responses. These findings indicate that V1 neurons can respond differently to internally and externally generated motion, and expand V1's potential role in information processing and visual stability during eye movements

Building a US company to manufacture solar PV mounting systems

This paper describes the process of developing a product for the solar industry. It is the story of starting a business in the solar market by designing a product, manufacturing the product and growing sales to over $1 million USD in 2011 and 2012. The author is describing the actual details of a manufacturing company that produces solar racking systems in the USA. The author founded the company in 2009 and left the company at the end of 2012. The document describes the changing landscape of the racking sector of the US PV market, and makes the case for industry standards in solar module dimensions. The range of current sizes of solar modules is described. The inconsistency in sizes creates additional overhead for manufacturers to accommodate different sized parts to hold the different solar panels. A uniform standard size would result in cost reductions for the end customers

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

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

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 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