WEBVTT

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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
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
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
to the novelty toy,

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even, it's been suggested,
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 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, 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,
inventor of the phenakistoscope.

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He defined persistence of vision
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
is a single object in motion.

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This explanation was widely accepted
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
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
which lie behind the retina.

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In 1915, Hugo Münsterberg,

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a German-American pioneer
in applied psychology,

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also suggested that the apparent motion
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,
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
of what the eye sees,

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like form, color, depth, 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,
hear, smell, 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
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
by repeating our experiment.

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Here's the sequence presented
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
that we're looking at separate images.

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Let's speed up. Eight frames per second.

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12 frames per second.

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It looks like we're about there.

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At 24 frames per second,
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 12 frames per second.

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This is in the neighborhood
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 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
of the reality of the illusion.