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March 1996, Week 4


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Pip Chodorov <[log in to unmask]>
Reply To:
Film and TV Studies Discussion List <[log in to unmask]>
Thu, 21 Mar 1996 19:24:23 -0500
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These two processes are commonly confused and much misunderstood, but lead to
a fascinating, almost metaphysical, theory of what film really is.
In effect, Persistance of Vision and the Phi Phenomenon can be distinguished
by the fact that the former takes place in the retina whereas the latter
happens in the visual cortex, much later in the process of visual
Persistance of Vision:
Persistance of Vision is one of two types of temporal retinal response; a
slow response, having to do with summing and integration effects, by which
the rods are replenished after having been saturated by light. The most
well-known effect is that by which the photoreceptors' activity is prolonged
some time after the end of the stimulus. It works best when the eye is
adapted to darkness, when a bright flash can sometimes last several seconds.
On oft-cited example is the 'circle of fire' we see when we turn a flashlight
in circles in the dark: though this is not entirely due to retinal
persistance, it is obvious that this effect is in no way related to film
perception, which is not the perception of a continuous movement but of a
discontinuous projection. Persistance of Vision does account for the
thaumatropes (is that the right name?), those early disks with for example a
bird cage on one side and a bird on the other. When the disk is flipped
rapidly, the two images are superimposed. Again, this is not an illusion of
movement, and as we will see, it is the phi phenomenon which is responsable
for the perception of movement, whereas persistance of vision gives an effect
of immobility.
For completeness' sake I will mention the other type of temporal retinal
response, a fast response, which is the flicker effect. a function of
brightness and speed which define a critical frequency at which we do not
perceive flicker. Visual masking occurs when the two effects, persistance of
vision and flicker effect, are combined. In effect, if two images follow each
other quickly, persistance of vision causes interactions changing the
perception of both images. Inserting a black flicker between the images
reduces the persistance of vision. In cinema this elimination of retinal
persistance helps the perception of movement.
Phi Phenomenon:
The phi phenomenon was discovered by Wertheimer in 1912. The original
experiment was as follows: a subject is presented with two different points
of light slightly separated in space which flash once each at differing
intervals. If the two flashes are close in time, the two lights are perceived
as simultaneous. If the two flashes are far apart in time, the two lights are
perceived as distinct. However, between 30 and 200 milliseconds of difference
between the two flashes, the subject perceives one single light that travels
from the first point to the second. This perception of apparent movement
explains the flashing lightbulbs on Broadway marquis that seem to define a
continuous movement, or the world headlines snaking their way around a
building in Times Square.
The phi phenomenon, which occurs not in the retina but in the occiptal lobe
of the brain, is a complex visual process by which movement is detected. It
seems that this mechanism is the same for both apparent and real movement,
one reason that filmic movement is psychologically indistinguishable from
real movement (as Christian Metz once wrote, "movement can not be recreated
but only created"). Film and reality are thus indistinguishable, film
residing on a completely innate biological process of motion detection.
Thus the film viewer is presented with a discontinuous, luminous stimulus,
which gives an impression of continuity and apparent movement within the
image, due to the phi phenomenon.
The phi phenomenon is not completely understood and a series of recent
experiments have only served to raise more questions. For example, in the
case of two points of light that are of different colors, say red and green,
the subject perceives a single point of light that travels across the field
of vision and which CHANGES COLOR IN THE MIDDLE. This presupposes that the
second light has been perceived, but that the subject is not privy to the
existence of a second, distinct light, nor to its location until after the
phi phenomenon, several milliseconds later. Some have posited the theory of a
Marxist brain that holds back information, or else of a Stalinist brain that
lies to us.
For film perception, it is clear that the apparent movement that we perceive
in the cinema occurs only between the frames, during the darkness (don't
forget that for two hours spent at the movies, we spend forty minutes of it
in complete darkness). So just as we create our dreams at night between the
days, in the darkness behind our eyelids, at the movies we create the
movement, during the darkness between the static images. The individual
frames must be similar, but not too similar, as well as different, but not
too different, for us to perceive a movement in the image. Between the
frames, the brain bridges the gap, compensating for the differences, creating
the movement.
          "Film happens between the frames" - Jonas Mekas
The wheel-turning-backwards effect, which can just as easily be perceived
under a neon light, due to its flickering, is simply a result of the phi
phenomenon causing the perception of apparent movement between discontinuous
flashes, as always. The spokes of the wheel are filmed at a certain
frequency. The camera records the spokes as they turn, capturing distinct
instants of the rotation. If in the second of two film frames a given spoke
is closer to the spoke behind it rather than the spoke ahead of it at the
moment the shutter clicked, than the apparent movement will create a
backwards turning wheel.
These perceptual effects are discussed in most textbooks on biopsychology and
sensation and perception. A good discussion of these effects as they bear on
film perception can be found in "L'Image" by Jacques Aumont, Nathan
University Press, Paris.
-Pip Chodorov <[log in to unmask]>
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