According to David N. Lee (Lee and Aronson, 1974; Lee
and Lishman, 1975), the optic array (such as what we are in a movie theater)
contains two sorts of information: information about the layout of objects in
the environment, which is called exteroceptive2 information,
and information about the location of our body and its parts in the environment,
which is called exproprioceptive3 information. Although
the exproprioceptive information we receive is usually consistent, in a movie
theater we receive contradictory exproprioceptive information from two sources:
Our eyes tell us that we are moving with the camera while the pressure receptors
in our skin tell us that we are sitting quietly in our seats. Whether we undergo
both experiences at once or whether they alternate we do not know. It seems
that in a movie theater our experiences are mostly due to the visual input;
the visual source that tells us we are moving overrides the source that tells
us we are sitting.
A similar type of visual dominance can be observed in the preservation of equilibrium,
which is served by the semicircular canals in our inner ears, the sensors of
pressure in our feet, and the visual sense. Without disturbing the semicircular
canals or moving the feet, it is possible to cause a standing person to sway
and on occasion to fall in an effort to compensate for the movement of the walls
of a "swinging room" such as depicted in Figure 10.2.
Have someone trace a b or a d on your forehead
and try to identify which of the two letters was traced. You will most likely
feel somewhat uncomfortable carrying out this task, because you weren't told
whose point of view to adopt: the writer's or your own. Now have the person
trace one of these letters on the back of your head. You will probably not hesitate
and report the letter as seen from behind your head. Now why is there some question
regarding the correct point of view to adopt when a letter is traced on your
forehead and no question regarding which point of view to adopt when the letter
is traced on the back of your head? After all, if you are "reading"
the letter from within your head, the same ambiguity should arise when the letter
is written on the back of your head: to read from within looking backward or
to read from outside looking forward. There is one way of making sense of this
dilemma: Suppose we do not mind moving our vantage point in or out of our heads,
but we try to avoid two things: turning our vantage point backward and "reading"
through the skull. In the case of writing on the forehead, we cannot avoid doing
one of the things we wish to avoid; in the case of writing on the back of the
head, we can simply move our vantage point behind the head and "read"
the letter from there.
To find out why we resist turning our vantage point backward compared to our
avoidance of reading through the skull, we must refine our technique somewhat.
Instead of simply asking people to report which letter was traced on the head,
we can assign a vantage point to them and tell them to adopt one of four vantage
points.
Rear vantage point. Wherever a letter is traced on the head, it should
be interpreted from a vantage point behind the head looking forward. If the
letter is traced on the forehead, it should be read as if the head were transparent
and the letter was written in opaque ink.
Front vantage point. Wherever a letter is traced on the head, it should
be interpreted from a vantage point in front of the head looking backward. Letters
traced on the back of the head should be read as if the head were transparent.
Internal vantage point. Wherever a letter is traced on the head, it should
be interpreted from a vantage point inside the head looking radially outward
through the transparent skull. Thus the vantage point faces forward to read
a letter traced on the forehead and backward to read a letter traced on the
back of the head.
External vantage point. Wherever a letter is traced on the head, it
should be interpreted from a vantage point outside the head looking radially
inward at the skull. Thus the vantage point faces forward to read a letter traced
on the back of the head and backward to read a letter traced on the forehead.
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Fig.10.3 Predictions for speed of "reading"
letters traced on the head for four assigned vantage points. Top panel:
under assumption that reading is mostly hindered by turning vantage
point backward. Bottom panel: under assumption that reading is mostly
hindered by having to read through skull.
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Now suppose we measured the amount of time it took to
read letters traced on the forehead and on the back of the head for each of
the four vantage points. We could then find out whether the correct interpretation
of the traced letter is slowed down more by having to read through the skull
or by having to turn one's vantage point (see Figure 10.3). To clarify this
figure, let us discuss the predictions for the rear and front vantage points
under the two hypotheses. If of the two obstacles to reading, turning the vantage
point is a greater hindrance than reading through the skull, then the letters
should be read rapidly regardless of whether they are traced on the forehead
or the back of the head; if, on the other hand, reading through the skull is
the greater obstacle, then letters traced on the back of the head should be
read more quickly than letters traced on the forehead. Considering now the front
vantage point, if facing backward is a greater hindrance than reading through
the skull, then the letters should be read slowly wherever they are traced;
if, on the other hand, reading through the skull is the difficulty, then letters
traced on the forehead should be read more quickly than those traced on the
back of the head. Similarly, this reasoning applies to the remaining two vantage
points.
In an experiment begun with Janice Marcus (a graduate student at Yale) and concluded
with David Turock and Thomas Best (graduate students at Rutgers) (Kubovy et
al., unpub.), instead of having letters traced on just one position on the forehead
and one position on the back of the head, there were eight positions, three
of which we consider to be on the back of the head. The results were unequivocal
in their support of our first hypothesis, namely, that it is harder for us to
turn our vantage point than to read through the skull.
