Fooled By Our Eyes

Saccades are rapid, ballistic movements of the eyes that abruptly change the point of fixation.  They range in amplitude from the small movements made while reading, for example, to the much larger movements made while gazing around a room.  Saccades can be elicited voluntarily, but occur reflexively whenever the eyes are open, even when fixated on a target.  Saccade control is the ability of the eye(s) to move quickly from one point of interest to the next after an appropriate time of fixation (100 to 300 msec).  To obtain a complete picture of the visual field a normal adult has to perform between 3-5 saccades or ‘snap-shots’ per second, including the corresponding breaks (periods of no eye movements), in order to bring all the visual field into focus.  Details are captured in these serial images.  The brain organizes this temporal sequence so that it appears to us as an unbroken image.  The visual system not only creates these sequences by means of controlled saccades, it also must be able to cope with the speed of sequences of images.

Saccades refer to the eye’s ability to quickly and accurately shift from one target to another.  This is a critical skill in reading, involving very specific eye movements.  The eyes must move left to right along a straight line without deviating up or down to the lines above or below.  In addition, when we reach the end of a line, our eyes must make a difficult reverse sweep back to the beginning of the next line.  At the moment when the decision to move the eyes is taken, the area of focused attention will be centered on a point whose distance from the target depends on the level of visual processing that has been reached.

Human eyes have constant breaks in perception whenever they flit about, in blind jumps called saccades. To experience this temporary blindness, look at your own eyes in a mirror and shift your sight focus from one eye to the next.  Although a person standing next to you and watching you will effortlessly see your eyes flit and shift from one direction (line of sight) to another, you will never be able to see your own eyes move, even though they do move and are indeed seen as such by the other person.  This blindness is due to the saccades.

The Spinning Dancer, also known as the silhouette illusion, is a kinetic, bi-stable optical illusion resembling a pirouetting female dancer.  The illusion involves the apparent direction of motion of the figure.  Some observers initially see the figure as spinning clockwise (viewed from above) and some counterclockwise.  Additionally, some may see the figure suddenly spin in the opposite direction.

Click to see dancer spinning

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