Move It, Lose Sight of It

In The Incredibles, viewers were introduced to Dash, the speedster child of Mr. and Mrs. Incredible. His super speed allowed him to move from one place to another and back again in split seconds. With motion being carried out that fast, the human eye and its related neurons for vision did not have enough time to process the difference in location caused by movement. This was why Dash was able to carry out school pranks effectively, much to the annoyance of his teacher Bernie Kropp. In one instance, Mr. Kropp’s gut instinct told him that it was Dash who placed a thumbtack on his chair. But even if Mr. Kropp focused on Dash the entire class time, his brain and eyes were not able to process that Dash ever even left his seat. The idea that motion may influence visual perception is involved in silencing.

Bernie Kropp

Dash the Speedster

Silencing is a concept that refers to how movement of an object results to failure to detect change. It has been shown to persist even if the observer consciously attends to the object and already knows that it is supposed to be changing.

The science of silencing was experimentally studied by Suchow and Alvarez (2011). The optical illusion demonstrating it has been considered compelling in nature and was recognized as 2011 Best Illusion of the Year by the Vision Sciences Society.

Three experiments were conducted for the study. Experiment 1 looked into the effect of speed on silencing. Suchow and Alvarez (2011) created a series of movies which featured 100 dots arranged in a ring around a central fixation mark. Two phases, stationary and moving, alternated in each movie. In the former, dots changed rapidly in hue, luminance, size or shape. In the latter, the ring of dots was rotating around the fixation mark while the dots themselves were still changing at the same rate as the stationary phase. Six observers were tasked to adjust the rate of change during the stationary phase to match the apparent rate of change in the moving phase. Results showed that as the rate of the ring’s rotation increased, the slower the dots seemed to change. When the fastest rotation was reached (0.33 Hz), observers barely detected any changes at all; thus, indicating nearly complete silencing. The clips that follow are samples of the movies that Suchow and Alvarez (2011) used in their research. These were taken from Harvard’s Vision Lab.

Motion silences awareness of color changes

Motion silences awareness of brightness changes

Motion silences awareness of size changes

Motion silences awareness of shape changes

Experiment 2 focused on determining which of the two factors caused silencing: motion in space or motion on the retina. A movie with 100 uniformly displayed dots of changing hue was made. It also featured two fixation points at the center. There were four variants in which the movie was displayed: (1) the dots move while the observer’s gaze remains fixed on a stationary fixation point (motion in space and on the retina); (2) the dots move and the eyes follow one of the fixation points moving in the same direction as the dots (motion in space only); (3) the dots don’t move, but the eyes follow one of the fixation points moving to the right (motion in the retina only); and (4) neither the eyes nor the dots move (no motion in space nor on the retina). The six participants involved here were asked to perform the same task in Experiment 1. Comparison from results across the four conditions revealed that motion in the retina (the third variant) produced as much silencing alone as when combined with motion in space (the first variant).  In the third variant, participants had difficulty determining any changes in hue of the dots. This is because as the retinotopic detectors on the eyes move with the traveling fixation point, they are only exposed briefly to the changing hues of each dot. The time may not be enough to detect any changes in the dots’ appearance.

Experiment 3 delved into determining which of the two accounts of silencing underlies the research study’s findings: freezing or implicit updating. Eight participants watched movies that were identical to the ones with changing hue used in Experiment 1, except that the ring rotated 180O in one direction once and at this point, the fixation point changed from white to black. At the time when the fixation point changes color (reversion), the participant must respond whether there has been a “change” or “no change” in the hue of the dots in the ring. In one condition, the ring continued to rotate after reversion until a response was made. In another condition, the ring remained still after the fixation point has changed color. In the final condition, the first 100 milliseconds after reversion contained a mask (a colorful noise pattern). If freezing were responsible for silencing, it means that the observer always sees the original color, never updating his percept to incorporate the new hue. He has erroneously kept hold of an outdated status. On the other hand, if implicit updating were to account for silencing, the observer always sees the current hue of the ring of dots (at the time when the fixation point has changed color) but is unaware of dots having changed color at all. Results show that regardless of condition, silencing is due to implicit updating.

Given all these findings, Suchow and Alvarez (2011) have shown that silencing demonstrates the tight coupling between motion and object appearance—two concepts often researched independently of each other in vision studies. The experimenters have shown that by simply changing retinotopic coordinates—moving the object or the eyes—awareness of visual changes may be silenced. This means that objects that had once been obviously dynamic (changing in size, hue, luminance, or shape) suddenly appear static (featuring the same characteristic while moving in a ring).

While the study may be considered groundbreaking, I feel that it could have been further improved by using a larger sample size for each experiment—they barely reached the double digits. The study could have been subjected to less scrutiny of finicky people, at least those who are heavily focused on statistical validity, if Suchow and Alvarez (2011) increased their sample size to at least 30 participants.

Nevertheless, I find that the research work of Suchow and Alvarez (2011) truly deserves the attention it has garnered from the scientific community. They have managed to introduce a new way of studying visual perception—looking at the interaction of motion and object appearance, as opposed to considering them singly. This may influence further research work and may serve as basis for developing a better understanding of motion-produced cues. Their findings may also be applied for better understanding of visual perception in many other fields—from transportation (the implications of drivers’ traffic motion perception on driving behavior) to abnormal psychology (how combined motion and object appearance is mentally processed by those with brain damage).

On the whole, I personally feel that the research of Suchow and Alvarez (2011) shows what scientific experiments should be like—interesting, exciting, and triggers the potential for new research directions.

Reference:

Suchow, J. W., & Alvarez, G. A. (2011). Motion silences awareness of visual change. Current Biology, 21, 1-4.

Sources of Media:

http://images.wikia.com/pixar/images/2/29/BernieKropp.jpg

http://visionlab.harvard.edu/silencing/

http://youaredoingthatwrong.files.wordpress.com/2010/12/film-the_incredibles-dash.jpg

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~ by myfivesenseworth on August 27, 2011.

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