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Take a series of still, sequential images.

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Let's look at them one by one.

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Faster.

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Now, let's remove the gaps,

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go faster still.

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Wait for it...

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...bam!

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Motion!

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Why is that?

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Intellectually, we know we're just looking

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at a series of still images,

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but when we see them change fast enough,

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they produce the optical illusion

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of appearing as a single, persistent image

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that's gradually changing form and position.

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This effect is the basis for all motion picture technology,

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from our LED screens of today

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to their 20th century cathode ray forebearers,

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from cinematic film projection

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to the novelty toy,

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even, it's been suggested,

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all the way back to the Stone Age

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when humans began painting on cave walls.

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This phenomenon of perceiving apparent motion

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in successive images

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is due to a characteristic of human perception

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historically referred to as "persistence of vision."

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The term is attributed

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to the English-Swiss physicist Peter Mark Roget,

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who, in the early 19th century,

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used it to describe a particular defect of the eye

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that resulted in a moving object

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appearing to be still when it reached a certain speed.

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Not long after,

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the term was applied to describe the opposite,

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the apparent motion of still images,

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by Belgian physicist Joseph Plateau,

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inventor of the phenakistoscope.

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He defined persistence of vision

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as the result of successive afterimages,

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which were retained and then combined in the retina,

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making us believe that what we were seeing

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is a single object in motion.

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This explanation was widely accepted

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in the decades to follow

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and up through the turn of the 20th century,

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when some began to question

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what was physiologically going on.

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In 1912, German psychologist Max Wertheimer

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outlined the basic primary stages of apparent motion

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using simple optical illusions.

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These experiments led him to conclude

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the phenomenon was due to processes

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which lie behind the retina.

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In 1915, Hugo Munsterberg,

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a German-American pioneer in applied psychology,

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also suggested that the apparent motion

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of successive images

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is not due to their being retained in the eye,

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but is superadded by the action of the mind.

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In the century to follow,

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experiments by physiologists

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have pretty much confirmed their conclusions.

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As it relates to the illusion of motion pictures,

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persistence of vision has less to do with vision itself

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than how it's interpreted in the brain.

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Research has shown that different aspects

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of what the eye sees,

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like form,

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color,

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depth,

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and motion,

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are transmitted to different areas of the visual cortex

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via different pathways from the retina.

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It's the continuous interaction

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of various computations in the visual cortex

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that stitch those different aspects together

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and culminate in the perception.

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Our brains are constantly working,

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synchronizing what we see,

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hear,

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smell,

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and touch

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into meaningful experience

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in the moment-to-moment flow of the present.

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So, in order to create the illusion

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of motion in successive images,

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we need to get the timing of our intervals

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close to the speed at which our brains process the present.

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So, how fast is the present happening according to our brains?

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Well, we can get an idea

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by measuring how fast the images need to be changing

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for the illusion to work.

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Let's see if we can figure it out

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by repeating our experiment.

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Here's the sequence presented

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at a rate of one frame per two seconds

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with one second of black in-between.

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At this rate of change

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with the blank space separating the images,

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there's no real motion perceptible.

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As we lessen the duration of blank space,

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a slight change in position becomes more apparent,

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and you start to get an inkling of a sense of motion

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between the disparate frames.

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One frame per second,

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two frames per second,

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four frames per second.

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Now we're starting to get a feeling of motion,

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but it's really not very smooth.

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We're still aware of the fact

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that we're looking at separate images.

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Let's speed up,

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eight frames per second,

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twelve frames per second.

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It looks like we're about there.

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At twenty-four frames per second,

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the motion looks even smoother.

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This is standard full speed.

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So, the point at which we lose awareness of the intervals

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and begin to see apparent motion

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seems to kick in at around eight to twelve frames per second.

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This is in the neighborhood

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of what science has determined

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to be the general threshold of our awareness

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of seeing separate images.

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Generally speaking, we being to lose that awareness

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at intervals of around 100 milliseconds per image,

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which is equal to a frame rate of

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around ten frames per second.

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As the frame rate increases,

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we lose awareness of the intervals completely

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and are all the more convinced

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of the reality of the illusion.