Date: Wed, 10 Dec 1997 19:27:04 -0500
Reply-To: "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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Sender: "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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From: Mait Edey <[log in to unmask]>
Subject: Binocular Rivalry
I only recently had an opportunity to catch up with November's discussion of
binocular rivalry and its implications. Some interesting issues were raised,
but then dropped as the conversation drifted to the more general topic of the
internal consistency constraint advocated by Bernard Baars and questioned by
Aaron Sloman. To recap:
Stanley Klein wrote (11/18):
> The neural mechanisms of binocular rivalry are especially interesting for
> the study of the neural correlates of consciousness (NCC) because binocular
> rivalry also occurs when the two eyes view two different static, in-focus
> scenes. In that case the eye correlated with the perception switches back
> and forth.
To which Sloman replied (11/20):
> Not always. A common counter example is seeing two fingers and the
> background at the same time when you hold up a single finger some
> distance from your eyes and look at the background (especially if you
> move the finger against a bright background??)
>
> Quite a lot of people seem to think that the ONLY possibilities are
> complete coherent fusion or binocular rivalry. E.g. Baars claims this
> unequivocally on page 89 of "In the Theater of Consciousness" (OUP
> 1996)
Baars claimed being misunderstood (11/20):
> My claim is that consciousness reflects an internal consistency
> constraint. The observations Aaron cites are not internally
> inconsistent, and do not contradict that hypothesis. The visual
> system seems to interpret those binocular effects as looking through
> a diaphanous object in one eye and seeing a solid one in the other.
Sloman (11/21):
> Not in my case: it actually looks as if I am seeing the SAME
> (inconsistent) thing in BOTH eyes! Doesn't it to you?
Baars (11/22):
> Aaron thinks we can have internally inconsistent perceptual
> experiences. Here is the test.
>
> CAN YOU SHOW THAT A PERSON CAN PERCEIVE TWO SOLID VISUAL OBJECTS IN
> THE SAME PERCEIVED SPATIAL LOCATION AT EXACTLY THE SAME PERCEIVED
> TIME?
Arnold Trehub (11/25):
> I agree with Bernie Baars who claims that two solid objects cannot be
> perceived in the same spatial location at exactly the same time. But,
> like Aaron Sloman, I can't imagine how this claim could be subjected
> to an adequate empirical test.
Sloman (11/27):
> Get two pens or pencils with different markings on their surfaces,
> and perhaps different colours. Hold them in front of you, maybe 18
> inches (45 cm) away, each sloping at about 45 degrees, one to the
> left and one to the right, so that they touch in the middle and form
> a cross, with a distant background beyond them (e.g. the far wall of
> the room).
>
> Fix your vergence by looking at a distant background, while keeping
> the pencils in focus (as if viewing a stereogram without a stereo
> viewer). Ensure that there's a good strong light on the pencils so
> that you see surface details e.g. lettering, texture, wood grain,
> very clearly.
>
> Each pencil should split into two parallel pencils with the four of
> them taking this sort of shape, where I've replaced each pencil with
> a single thin dashed line, instead of two lines, to save tedious
> drawing in a text file:
>
> A1 A2 B1 B2
> \ \ / /
> \ \/ / Pencil A is seen as A1 and A2
> \ /\ /
> \/ \/ Pencil B is seen as B1 and B2
> /\ /\
> / \/ \
> / /\ \
> / / \ \
>
> Now tilt the further pencil slightly towards you and the nearer
> pencil slightly away from you so that at the upper point of
> intersection (the place where A2 and B1 intersect) they appear to be
> at the same distance.
>
> You can wiggle them slightly or move the pencils on their long axis
> to keep relative distances unchanged, to see what's going on. I
> actually see two pencils passing through each other. I can see, quite
> clearly, the marks on both surfaces (I've learnt to focus on close up
> surfaces while my eyes converge in the distance in order to view
> various kinds of stereograms.)
>
> It's actually quite hard to describe the experience precisely,
> because it's not something for which our normal vocabulary for
> describing physical configuration is apt: we don't often meet this
> sort of thing except perhaps in ghost films. But saying that the
> pencils pass through each other comes as close as anything. Now
> because of the unclarity in Bernie's claim that he doesn't know of a
> single, solitary example
>
> > > showing that "YOU CAN PERCEIVE TWO SOLID VISUAL OBJECTS IN THE
> SAME > > PERCEIVED SPATIAL LOCATION AT EXACTLY THE SAME PERCEIVED
> TIME?"
>
> I don't know whether he does or doesn't regard this as a refutation.
>
> Certainly I do *not* get the binocular rivalry which he seems to
> think I should get, i.e. one of the pencils being suppressed at the
> point of overlap.
At this point, unfortunately, just as the conversation was getting empirical,
both major players veered off onto other topics (simultaneous inconsistent
meanings of "set"; inconsistent interpretations of the Necker cube, etc.),
leaving binocular rivalry behind. But the virtue of binocular rivalry in the
study of awareness and attention is that it requires no clever experimental
designs or controversial theoretical models. It gives immediate and
unequivocal results: something either appears or does not appear before your
very eyes.
It's possible, with a little practice, to maintain separation of the visual
fields. As with Sloman's pencils, or with the familiar finger, objects may
double; in this case both fields are partly seen. Or one field may appear and
the other vanish completely.
When the fields are separated, they overlap. An object in one field does not
fuse with the same object in the other field, as in normal binocular vision,
but overlaps some other object. For example, each finger in the foreground
overlaps a different object in the other field's background. As noted in some
posts, these doubled objects may appear transparent or diaphanous, so you may
see the transparent finger and the object it overlaps in the same "place".
Sloman argues that this observation refutes Baars. But Baars seems to want to
distinguish diaphanous objects from solid objects. (By "solid" I take him to
mean opaque.) It seems to me that each is partly right and partly wrong, and
the subtle nature of their disagreement provides a useful approach to
thinking about focus of attention.
The apparent transparency or opacity of an object in an overlapping visual
field is a function of focus of attention on that object. At every point in
the overlapping fields, two objects overlap. If you attend completely to one,
it is opaque, and the other vanishes. If you attend to both, they both seem
diaphanous or transparent. The more you focus on an object, the more opaque
it is, the more visual information you have, and the more transparent its
overlapped object in the other field is, to the point of vanishing.
Sloman is right that he can see both pencils where they intersect. But to see
them both, he must see them both as diaphanous; he doesn't see either
completely. This transparency is partly a matter of scale, depending on the
size of features which are readily discriminable. Something small is
relatively harder to maintain attention on. I venture to predict that if
Sloman holds his pencils so that the small print on each label overlaps the
other, and, more precisely, letters or even parts of letters overlap, he will
not be able to read both at once. Attention to one should clearly make the
other vanish. At this scale I think Baars is right.
I suspect also that parts of a field may be integrated in a way similar to
the way visual objects seem to fill the blind spot without leaving a hole,
even though nothing is actually seen there.
It seems to me that the ability to see through either eye at will, or to
attend at will to features in either of the separate visual fields, is a
skill worth cultivating for anyone interested in the nature of awareness and
attention. It has implications for how attention and awareness are
conceptualized. For instance, we inherit a Jamesian way of conceiving
attention and awareness on a spatio-visual model. Focus of attention is
understood as analagous to focus of vision, such that there is a center where
attention is focused, and a surrounding area where awareness is diffused or
blurry. But the defects of the spacial analogy are apparent when you can see
the effect of your attention on an object before your eyes, when focus
creates opacity, and the degree of focus or distraction becomes visually
obvious.
On another topic, Stanley Klein noted that "The neural mechanisms of
binocular rivalry are especially interesting for the study of the neural
correlates of consciousness (NCC)", and I would add that ditto for the neural
correlates of volition. Investigators have studied why attention switches
from one field to the other. The presence of salient features, brightness,
edges, interesting objects, or movement, will draw attention to one field or
the other. But it is clearly a top-down phenomenon when I can direct
attention to either field at will.
Mait Edey
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