視力検査（前編）：優先視の歴史と初期検査について (Visual Acuity Testing (Part 1): History of Preferential Looking and Early Testing)

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NARRATOR: The name "Perkins" carved in stone.
Below a gothic tower a boy navigates with a cane.
A title: Visual Acuity Testing:
History of Preferential Looking and Early Testing
with D. Luisa Mayer, Ph.D.
- Preferential looking is something that a baby does
when they see something that's very interesting
next to something that isn't interesting.
So they prefer to look at something that's interesting.
I'll give you an example.
A three-month-old baby, if we show a three-month-old baby
a drawing of a face where all the features are regular--
the eyes are where they should be,
the nose is in the middle, the mouth is below--
and then you put next to that a drawing of the same features
of the face, but where everything's scrambled--
the eyes are over here, the nose is there,
the mouth is up here.
NARRATOR: We see the image of two faces side-by-side.
The image on the left is of a normal face.
And on the right the shapes depicting the eyes,
nose, hairline and eyebrows are randomly placed
within the same space.
- What's the baby going to look at?
The baby's going to look at that face that's regular
and not at the scrambled face.
We call that the baby's preference to look at a face.
So preferential looking has a really long history
as both, just natural observation of what babies do.
They naturally look at things.
Things they're interested in capture their attention
and they spend a lot of time looking at them.
And that observation started a whole field
of vision science,
studying the development of visual perception
and the development of sensory visual functions
in babies,
using modifications of this method now
that's called preferential looking.
The first person to do that formally was someone
named Robert Fantz,
who was an American developmental psychologist.
And he presented two stimuli like stripes and a bull's-eye
to babies and he looked at...
he measured which one they preferred
by having somebody just measure their looking time
to the stripes, to the bull's-eye.
He would change the position, measure their looking time,
say in a two-minute interval.
And cumulating their looking time
over that two-minute interval would tell you that the baby
preferred one or the other
if the time was sufficiently different.
NARRATOR: We see the two shapes as described,
on the left a bull's-eye of three concentric black rings
and on the right side,
six identical wide, black vertical stripes.
- And he did that starting with newborn babies,
babies shortly after birth.
To do that he had to create this funny-looking box
that he called a looking chamber,
but actually I call it the baby box.
And it's open on the bottom.
A baby is slid in lying on their back
underneath this box.
Someone is peeking over the top of the box,
looking through a little peephole.
And inside the box are either two objects or two images
that the baby's looking at.
So baby's down below looking up at these images
or these objects.
And the observer looking through that peephole
is measuring how much time the baby looks
at one or the other object.
NARRATOR: A black and white photograph
shows Fantz's apparatus.
An infant on its back gazes up at a balloon-like object
at the top of the box.
An observer stands to the left of the apparatus
on a small step stool and leans over the top of the box
to observe the baby through a small opening.
- When he did that with newborns, he found
that newborn babies actually have visual preferences.
They look at things, some things more than others.
And it turns out it's pretty complicated
and pretty interesting.
He also studied babies up to about six months of age,
sitting them upright in a little baby chair
and showing them also patterns and objects.
The bottom line of his findings with newborns
was that babies actually make visual discriminations.
They're not blind, they see differences between things.
It's fairly rudimentary, but nevertheless
they're making discriminations.
He considered that evidence for innate visual perception
in babies,
that babies are able to see forms right after birth.
They don't need experience.
Now, some would argue, well, they need some experience.
Well, yes, to refine their vision
certainly they need experience.
He also showed that babies had preferences
for certain things that changed over time.
They weren't always interested in looking at the bull's-eye
versus the stripes.
Later on they seemed to be more interested in stripes
than bull's-eyes.
But at all ages he found that babies preferred
to look at regular faces over scrambled faces.
So Fantz started doing these studies in the '50s.
And this type of work is really called
the study of visual perception in babies.
And it's been ongoing, there are all kinds
of modifications of this procedure used to study
complex visual perception and cognition in babies.
NARRATOR: Fade to black.
A graphic of the Perkins logo swoops across the screen,
revealing a chapter heading:
Visual Perception vs. Visual Sensory Function.
- I want to differentiate between what I mean
by sensory versus perceptual.
So sensory qualities of an object are things
that make up that object, that form,
such as the angles or the edges of a form,
the lines, the width of the lines that create that form.
Light/dark differences that define where that form is
in space.
Color differences and so forth.
So those are the components that go up to make a form,
and perception is seeing the form as a form,
seeing all those components put together,
and that creates an object that we can discriminate.
Now, the sensory visual functions that I'm talking about
that have been studied over many years
since shortly after Fantz's work are visual acuity,
detail vision, contrast sensitivity--
the ability to see different contrasts between objects--
color vision, temporal vision, motion sensitivity,
stereo, vernier acuity and a number of other
really basic visual functions.
The research started on these things really with babies
in the '60s, so shortly after mid-century.
Actually, there are a number of different modifications
of preferential looking that were used to test
visual sensory function.
I'm going to focus on one in particular
because it's what I know.
It's from the person I worked with,
it's where I did my graduate studies
and so you'll hear all about that.
But there were a number of other researchers
who used modifications of preferential looking
to study behavioral visual function in babies.
And I'm just going to name a couple of them
because they're prominent people,
they're people in our community
and they're good friends as well.
At MIT, Massachusetts Institute of Technology,
some of the first studies were done by Richard Held
and Jane Gwiazda on babies' vision.
And they were also very interested in clinical aspects--
so astigmatism, an abnormality of the eye
that causes visual acuity to be reduced.
And also Eileen Birch who was working in their lab
as a post-doc, and then now is at the Retina Foundation
in the Southwest and does a lot of really good research
on clinical aspects of visual function in babies.
And then there's a pair of people-- a couple--
from England who have been working
as long as everyone else:
Jen Atkinson and Oliver Braddock
from the United Kingdom.
These are just some people.
I can't name all of them.
There's a list of maybe 40 or 50 people
doing research in this area.
But the person I'm going to talk the most about, who for me
has the greatest influence-- but of course I'm biased--
is Davida Teller,
who was a scientist of vision in adults,
who was at the University of Washington in Seattle
and had an insight about how you might approach
testing visual sensory function in babies
using a modification of Fantz's preferential looking technique.
NARRATOR: Fade to black.
Early Testing: Forced Choice Preferential Looking.
- Now if you remember, Fantz showed two stimuli
and measured whether... how much looking time
the baby spent looking at each of those stimuli.
Teller realized that you really can't test sensory function
as a perceptual task, at least not in this kind of way,
and that what you needed to do was to create a stimulus setting
where there's just one stimulus.
That is, that's the stimulus that's going to measure
your sensory function.
And I'm going to give as an example a technique,
stimulus that I've been intimately involved in
and was the first that was used,
and that's test of visual acuity using stripes,
black and white stripes in a patch.
It refers to a circular patch.
So that stimulus was going to be the test of visual acuity,
varying the size of the width of the stripes
from very coarse, very wide to very fine,
and then at some point finding out what's the threshold,
or what is the acuity of the child for that stripe stimulus.
Well, okay, you've got a stripe
and now what do you do with it?
Well, what she designed was to put the stripe
either on the right or the left of center,
and in the other part there would be a stimulus
that was a circle just like the stripes were in a circle,
but it was gray and matched in average brightness
to the stripes when you couldn't see them.
So you take a measuring instrument and you measure
what's the average brightness of that patch
and you match it to this gray.
Now, that's because if this were brighter,
the baby might be just looking at it
because it's brighter, not because they saw the stripes.
NARRATOR: We see an example of how Teller's stripe stimuli
would be presented to a test subject.
Two circles have been cut out of a large gray field.
They are on opposite sides of and equidistant from
the midline of the large field.
In the center there is a small peephole,
which allows the observer to see the test subject.
In this example, the circle on the left displays a pattern
of vertical stripes,
and the circle on the right displays a solid gray color
that matches the large field.
- And she also put the stripes in the gray in a big surround
that was the same gray approximately as the gray patch,
as the blank, we call it.
And that was to reduce features that might be distracting
to the baby.
So instead of looking at stripes, they might look
at something off on the side.
Now they have only one thing to look at: stripes versus gray.
So that was probably the, you know, the first...
I want to say innovation she created.
Other people did the same thing, but her second innovation
is even more striking and a little bit more difficult
to describe.
And that is, there's a person observing,
just as in Fantz's technique, a person observing a baby
looking at these things.
And that person's job she changed.
She changed the observer's job
so there's no longer measuring looking time
but rather where are the stripes based upon what the baby does.
So are the stripes on the right or the left?
And the observer, looking at the baby,
doesn't know where the stripes are and they have to make
this judgment just on the basis of what the baby's doing.
NARRATOR: A photograph depicts an observer
behind the testing apparatus.
The woman is looking through the peephole
to observe the test subject.
The next photo we see shows a view of a baby's face
as seen through the peephole.
In this example, the face is turned to the baby's right,
suggesting that the baby is responding
to a stripe stimulus on that side.
- And she called it a forced choice judgment,
which is really quite important.
The observer had no other choice.
Where are the stripes: right or left?
On every single trial, discrete trials,
showing a whole range of stripes from very tiny to very large.
And with that procedure, she reconceived
preferential looking and she called it
forced choice preferential looking.
Pretty obvious, but pretty significant change.
And I'm going to abbreviate that as FPL.
So I'm going to say FPL and just remember
I'm talking about that technique.
NARRATOR: Fade to black.
Using the Forced Choice Method.
- Created out of this series of presentations of stripes
of varying widths is this,
what we call a psychometric function.
And "psycho" is psychology, response,
and "metric" is a metric--
that is, some measure of sensory function.
And in fact, psychometric functions are,
I would say, the building blocks or the grounding
for a field of visual psychophysics.
So there's another fancy word.
What is psychophysics?
It is the psychological responses to the physics
of the stimulus, to qualities of the physical stimulus.
So we have stripe width, that's the physical parameter.
It varies over a certain range.
We take the psychological responses of the observer to...
in the paradigm that we're talking about.
Where is the stimulus?
You can also say, what interval is the stimulus in?
Or you can say, yes or no, does the baby see them or not?
That's another way of formulating an experiment.
Any sensory function can be measured with psychophysics.
So it doesn't have to be just vision,
but it can be hearing, smell, touch, taste.
And lots of interesting experiments are done
using psychophysics for testing all kinds of sensory functions.
NARRATOR: A young girl is shown wearing a large pair
of headphones while taking a hearing test.
She gazes up at a woman in a white lab coat
who is administering the test.
- How do you measure acuity doing that?
I mean, you know, I've said stripes
and a range of widths and so forth.
The way the technique developed was
the observer's correct and incorrect judgments
on each stripe was cumulated,
and a percent correct for each stripe width was calculated.
So what you now have is you have a function
that shows the percent correct performance of the observer
as a function of stripe width.
NARRATOR: An example of a psychometric function plot
is displayed.
Along the X or horizontal axis are six examples
of the stripe stimuli presented.
The stripe width varies progressively
from wide to very fine.
The Y or vertical axis
represents the observer's correct judgments
as a percentage.
The five points that are plotted show a diminishing percentage
of correct judgments from 100% to less than 60%,
which correlate to the diminishing widths
of the stripe stimuli presented.
- When the stripes are very large, the baby looks,
makes a really strong look at them.
The observer can tell easily where the stripes are.
When they get smaller and smaller,
it may be a little more subtle, and the baby may look like this,
may just glance a little bit at the stripes.
But the observer can still tell most of the time
where the stripes are.
But when they get so small the baby can't see them,
they're just two blanks for the baby,
the observer is just guessing
and most likely is just saying, "Oh, I think it's there,
I think it's there," and is wrong half the time,
correct half the time.
NARRATOR: We see an example of a stimuli
of very small stripes being presented,
along with a solid gray circle.
In the photo, the sensory qualities
of the stripes are too subtle to be distinguished.
A slight difference in the color of the circle to the left
of the midline is the only cue.
The next photo taken through the peephole
shows a baby looking down slightly,
exhibiting no preference for either side of the display.
Fade to black.
Early Testing: Operant Preferential Looking.
- Who are the babies that can be tested
with forced choice preferential looking, FPL?
In Davida's lab the babies that were being tested
were generally about two to three months of age.
Well, my colleague Jane Allen came along and the idea was
well, let's see if we can test really young babies
just like Fantz did with his preferential
and see how old... you know, when does acuity develop
and how does it develop normally?
And the person who did that was Jane Allen,
who in '78 finished her study of FPL acuity
tested in babies between two weeks and six months of age.
And she showed that acuity matured over that time,
it got better.
But it didn't get near adult levels.
So it wasn't over.
Well, what happened after six months?
Babies get bored, basically,
sitting for all the numbers of trials
that you have to present to do full psychometric functions.
At about six months they're no longer willing to do that.
And they need some kind of reinforcement
to keep looking, at least we realized that in those days.
And I was in Teller's lab at that time, and in fact,
working on Jane Allen's study.
There was another person in the lab
who was a research associate.
Her name was Velma Dobson,
a very important person in the field of infant vision.
And she and I conceived of a method to test babies,
or we wanted to try this method using a reinforcement procedure
to see if we could test older ages
and how far we could push this technique.
And what we did was we stole from audiologists
who use this technique to test hearing sensitivity in babies.
And basically it's a little bear, an animated bear
who is playing a drum in a box that is otherwise black,
but then when it gets turned on,
the box lights up and the baby plays the drum.
We placed a box on either side of the FPL screen
and when the observer said "Baby sees the stripes,"
pushed a button, stripes were on the left or the right,
the bear in the box on the left lit up
and the bear played his drum.
And so the baby was reinforced for looking at stripes.
NARRATOR: In a photograph, an infant sits
on his mother's lap in front of an OPL display.
There are dark black boxes on either side
of the flat gray display field.
The stripe stimulus has just been presented
to the baby's left.
The next photo depicts what happened.
Because the observer judged the stripes to be on the left
based on the baby's reaction, the observer then
pressed a button to record that choice.
As a result of making a correct judgment,
the box to the left of the display lights up
and the mechanical bear plays its drum.
- When the observer was wrong, nothing happened.
So it wasn't that it happened all the time,
it only happened
contingent on the baby's looking at the stripes.
Well, that worked pretty well and it worked for babies
up to the age of five years.
We could test now children from six months--
or actually earlier than that--
all the way up to five years of age, with one exception.
Toddlers were really tough.
And we gave up on 18-month-olds.
18-month-olds are a terror.
When we measured full psychometric functions
in children over this whole age range and compared it
with Jane Allen's data, we found at overlapping ages
the results were the same.
So that said that operant preferential looking,
or OPL, wasn't biasing the results in any way.
It didn't make babies see better, it just made it
easier to test them.
So now we had techniques that would cover the whole age range,
and we could say we know now that acuity
is near adult levels at five years of age.
It's very poor at two weeks and it gradually increases
in a steady way between two weeks and five years.
NARRATOR: This graph depicts the development
of grating acuity from early infancy to preschool age.
Acuities obtained by forced choice preferential looking
between two weeks and six months are shown by open circles.
Acuities obtained by operant preferential looking
between five months and five years
are shown by filled circles.
Grating acuity improves from 20/600 at two weeks
to about 20/120 at five to 12 months,
and to 20/20 at five years.
Fade to black.
FPL and OPL Apparatuses.
- So we have this technique and we have apparatuses.
I call them apparatuses,
this FPL apparatus and OPL apparatus.
And those devices or pieces of equipment were fairly simple
and actually sort of primitive.
They were made out of gray cardboard,
mounted on wooden frames and held together with pushpins
and fishing wire and glue.
NARRATOR: In a photo, a graduate student
in Davida Teller's lab is holding a baby
in front of the FPL screen
with the black and white grating pattern on his right.
A close examination of the photo shows the many pushpins
holding the various pieces of gray cardboard together.
- Well, over the years...
I mean this work was done in the '80s, so it was...
Engineers say, "You know, you do a breadboard model
"and you get the proof of concept and then you go on
to do more sophisticated technological things,"
and so that's what happened.
NARRATOR: Now we see an OPL device
with some metal frames and glass panels
in front of the right and left display fields.
A young girl reaches out and touches the panel on the left.
Based on the toddler's reaction, the observer correctly judged
the striped stimuli to be on the left.
As a result, a reinforcement of Cheerios cereal
has been dropped from a plastic tube into a glass dish
that is within the child's reach.
- People started using equipment that was more advanced--
for example, they used cathode ray tubes,
and then eventually video displays and computers now.
And so some very interesting, important work that was done
and continues to be done using these more complicated
stimulus displays.
Still using preferential looking as the technique.
NARRATOR: We see in a photo a baby sitting up
in front of a box containing two video monitors side by side.
We can see that a display of stripes
is in one of the screens.
- Something we did do in Teller's lab
and in Children's Hospital, where I went,
was we replaced the observer and the holder as separate people,
but actually put that person into the same role.
So the observer and the holder were now the same person--
the person holding the baby, showing them the stripes
and the observer.
And how that worked was there was a video camera
behind a peephole that was showing the image of the baby
on a video monitor, that was then projected to a mirror
and the observer was able to watch the babies looking
and make judgments on the babies looking.
Now the observer, of course...
I mean the holder/observer couldn't see
where the stripes are, they shouldn't see them either.
NARRATOR: In this photo, Velma Dobson is seen
holding a baby, her three-month-old son.
He is looking at a grating on the FPL screen,
while Velma observes his face in the mirrored video image
projected by a camera behind the central peephole
in the screen.
We can see a black and white image of the baby's face
in a monitor that has been placed on a shelf.
- Over the many decades-- as I say, we're now
in the seventh decade of studies using preferential looking--
a number of different visual functions
have been tested using behavioral techniques,
using preferential looking types of techniques.
And that would include visual acuity is the most-studied,
and over the widest age range.
NARRATOR: Fade to black.