Date: Thu, 27 Nov 1997 13:58:43 +0000
Reply-To: "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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Sender: "PSYCHE Discussion Forum (Biological/Psychological emphasis)"
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From: Aaron Sloman <[log in to unmask]>
Subject: Re: Baars on Binocular Rivalry (was Re: Illusions)
I am (belatedly) responding to these three
> Date: Mon, 24 Nov 1997 02:18:00 -0500
> From: "<Bernard Baars>" <[log in to unmask]>
> Date: Tue, 25 Nov 1997 00:28:48 -0600
> From: George McKee <[log in to unmask]>
> Date: Tue, 25 Nov 1997 16:49:21 -0500
> From: Arnold Trehub <[log in to unmask]>
All are reactions to my message posted Mon Nov 24 01:19:46 GMT 1997
I guess I was both unclear in what I wrote and also intemperate in how I
wrote it. Apologies. Comes in part from always being in too much of a
[Bernie Baars wrote]
> I'm afraid Aaron is not reading the plain meaning of my words. Let me quote
> from In the Theater of Consciousness, p. 89 ......
> .... But it's meant to be read with precision.
> Internal consistency is a basic property of conscious experience, but the
> raw input to the brain may not start off as consistent.
I have a number of different problems which I had not thought through
clearly, and which caused me to focus not on the words in this bit
of text, but the ones I earlier quoted (see below).
But first let's look at the general problem I have about the notion that
conscious experience has to be internally consistent.
If someone told me he had designed some kind of general information
processing system which could contain information about the environment
and was constrained to be internally consistent I would not believe him,
unless the mechanism was very limited in the information it could hold.
There are two related reasons.
(a) in general telling whether something is consistent or not is a
problem which cannot be decided by any sort of mechanism. (Is it or is
it not inconsistent to say that there's a largest pair of twin primes,
i.e. prime numbers which differ by exactly 2, like 11 and 13? As far as
I know, that's an unsolved problem, though very simply stated. Maybe
some people believe there is a largest pair and others believe there
isn't. One belief is inconsistent, though nobody knows which.) Any
precisely defined, sufficiently rich, system cannot have a decision
procedure (Goedel, circa 1931. Compare the halting problem in
(b) Even the cases that are decidable, e.g. expressions in propositional
logic, are not tractable in general. E.g. a logical expression involving
N variables requires a truth table of size 2 to the N to be explored to
check consistency, and exponential functions grow very fast. (E.g. an
expression involving a mere 10 propositons can require over a thousand
combinations to be checked.)
So I'd be amazed if the brain had any *general* method for detecting
(and eliminating or avoiding) inconsistency, and as I remarked there are
cases where it fails, e.g. perception of a picture of an impossible
polygon, with many sides. The devil's pitchfork (shown in Baars' book)
and the Penrose triangle are easily seen to be impossible, but a more
complex picture of the same general sort looks globally consistent when
it isn't. The inconsistency is not detected by a human visual system.
So I could not believe Bernie was making a *general* claim about
consistency in the global workspace.
I focused on his more specific comments, e.g. this:
...But binocular fusion breaks down if there are significant
differences between the input to the two eyes. .... if one image is
slightly offset from the other, binocular rivalry results, and to
obtain coherence the visual system will suppress one image in favour
of another. [Baars, p89, quoted previously]
This looks unambiguous and precise to me. And false, since rivalry
requires additional conditions, as I indicated a couple of messages ago,
and Stan Klein seemed to agree. I.e. the claim is too strong. But I
think Bernie also agrees with that. He has offered a different claim
which I'll come back to in a minute.
[George McKee writes]
> I have to agree with Aaron Sloman in his debate with Bernard Baars about
> consistency constraints in awareness. To put it facetiously, Bernie needs
> to spend more time in looking-glass land with the Red Queen, believing
> six impossible things before breakfast.
> Baars appears to have a very precise notion of where "consciousness" resides
> in the system of stages between sensory transduction and muscular action --
> a notion that Sloman and I don't share.
I think there's more to it than that. It has to do not just with a
factual disagreement (as there was over rivalry), but with the problem
how to make sense of some of the claims which at first look as if they
are saying something clear, but (to me) lose their clarity when examined
As I've indicated in the past, I don't have a global disagreement with
Bernie: we are, I think, putting forward closely related ideas. I think
human intelligence involves a "central" architecture with (at least)
three importantly different kinds of mechanisms (reactive, deliberative,
meta-management) which evolved at different times, which often interact
strongly, which are shared (to varying extents) with different animals
(insects, rats, ....), which develop within an individual over time
between conception and adulthood, which are not necessarily directly
manifested in physical brain structure (because they may involve virtual
machines), which provided evolutionary pressure for a similar layering
of perceptual and motor mechanisms, which manifest themselves in three
quite different classes of emotions, which play different roles in
relation to skilled and unskilled performances, which use information in
different forms, which involve very different learning mechanisms, etc.
etc. (This is a crude and partly inaccurate summary.)
I think the global workspace idea has a lot in common with some features
of these ideas about architecture (especially the mechanisms involved in
meta-management), and goes much further in attempting to map details
onto brain structure.
But the consistency constraint looks like the wrong kind of thing to
include (for reasons indicated above), and it results in some very
For example Bernie wrote
> > So I just don't see the problem. I don'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?"
At first I thought this was an empirical claim, but as indicated in my
previous message it began to evaporate as I looked more closely. E.g.
it's simply logically impossible for a solid cube and a solid sphere to
be in the very same spatial location at exactly the same time. If they
occupied the same 3-D volume they would both be cubes or both spheres,
or both something else.
So if being in the same perceived location means occupying the very same
spatial volume then I have no idea what sort of visual experience is
being excluded. If someone claimed to have such a visual experience,
rather than treating that as an empirical refutation of Bernie's theory
I would treat it as evidence of his misusing the words describing the
I can easily imagine visual experiences in which a cube and a sphere
will be seen to have the same *centre*, but each will protrude from the
other. I.e they are partly in the same place at the same time.
Probably such objects already exist. The curved and flat surfaces could
be given different colours. You might then see a red cube and a blue
sphere in the same place at the same time, each partly obscuring the
But I expect Bernie would say that's not a refutation because it's a new
coherent experience of a geometrically possible object which is wrongly
described as two objects.
So what about binocular presentation of cube in one eye and sphere in
the other? I haven't tried, but I have tried something similar which is
a variant of the experience of seeing your finger and the distant
background at the same time, when the finger splits into two.
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
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
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
Of course I may have entirely misunderstood what he (like many others
who talk about rivalry) is saying. I am not a brain scientist.
Or maybe I am a mutant after all.
(Bernie is that a logical possibility according to your theory? I.e.
could there be a sub-species whose global workspace mechanism does not
use the consistency constraint? Or have you ruled it out on
I wonder if deep down he is ruling it out aprior:
> As a mathematician, Aaron may be acutely aware of contradictions that we can
> be conscious ABOUT --- but that is vastly different from consciously
> experiencing two sources of contradictory information at the same time. Just
> try thinking of electrons as waves and particles AT THE SAME INSTANT IN TIME.
> If you think you can do it we've got to get you into a cognitive laboratory.
Er, I think many physicists now do that all the time. It's not
particularly hard for people used to thinking about wave packets, etc.
In any case, "consciously experiencing two sources of contradictory
information at the same time" (my pencil example) is very different from
thinking of electrons as having wave-properties and particle properties
at the same time. The latter need not involve any sensory modality.
Space-filling curves are harder (was that Sierpinski's work?). That's
because they exist only as the *limit* of a process of growing them, and
the notion of the limit of an infinite sequence of steps as something
actually achieved has its own problems.
Thinking of an infinite set as the same size as a part of itself was a
bit difficult at first, but soon became easy, even though there's a sort
of contradiction (It's the same size as something smaller!)
Arnold Trehub writes about one of my examples:
> > For me, perhaps the most compelling evidence that human vision does not
> > rigidly exclude inconsistent or incoherent percepts are the motion
> > after-effects in which one sees simultaneously motion, and nothing
> > moving, in the same location.
> My experience of motion after-effects (and that of others who have
> reported their experience to me) is quite different. I see all
> (objectively) stationary stimuli in the field of induced motion as streaming
> in a direction opposite to the motion of the inducing stimuli. I *believe*
> the test stimuli are not moving but, at the same time, I *see* them in a
> peculiar kind of motion.
Let's make sure we are talking about the same thing. I view a text
window with the text smoothly scrolling up. Then when it stops I can see
downward motion even though I see no text or other visual object
actually changing its location either relative to the window frame or
relative to the direction in which I am looking or relative to anything
else. I.e. there's both visible motion and nothing moving visibly, i.e.
no change of absolute or relative location.
If you and your informants see the text moving relative to the frame, or
relative to the direction of gaze, i.e. letters actually changing their
location, then it's a different sort of motion after effect from the
kind I am referring to. (However, I suspect it may be the same one, not
inspected with sufficient care, because the relevant questions were not
asked -- a common problem with psychological experiments.)
(I sometimes get a similar effect while flying if a plane banks while I
am sitting still gazing at the bulkhead. I presume proprioceptive
sensors and pressure sensitive sensors, etc. send signals to my brain
recording a tilt and I *see* a tilt happening, yet nothing actually
moves in my visual field. It's quite strange. Some other people confirm
that they have had a similar experience.)
> Sloman offered a long list of visual situations that might be violations
> of Baars' strong consistency-of-perception claim. I could add more, such
> as low-frequency periodicity pitch tones where you can perceive (well,
> *I* can...) the pulses as individuals and/or as a continuous tone. I was
> particularly struck by the example of paintings such as the vegetable face
> or trompe-l'oeil paintings in general. I don't feel any perceptual strain
> when seeing these as multiple classes of objects simultaneously, or even
> as seeing them in two and three dimensions simultaneously.
This is very important. Although I don't yet know much about the types
of information structures and information mechanisms involved in what I
call the meta-management system (which can attend to internal states),
or in the global workspace, one thing seems to be clear: the structure
is not anything like a spatial model or replica of some external reality
since if it were there would have to be a homunculus perceiving it and
we'd get the familiar infinite regress. Rather it's got to be something
more like a richly and immediately accessible collection of "pointers"
to sensory contents (perhaps analysed at different levels of
abstraction) combined with perhaps something like "pointers" to
predicates/predicates/concepts/schemata providing interpretations and
classifications (including identified possibilities for motion,
functional and causal relationships etc., i.e. Gibson's
This perspective (admittedly still far too vague to be a theory or
specification for a mechanism) allows the possibility of combining
inconsistent predicates/concepts/schemata, though it's not surprising
that in SOME cases the inconsistency is detected and rejected, and one
E.g. if there are already mutually inhibitory linkages between two of
them they can't both occur in the same structure at the same time. If
there are not (e.g. because the inconsistency has not yet been noticed,
or because frequent co-occurrence shows that the inconsistency needs to
be endured) then the consistency constraint will not apply.
I suspect the mechanism allows enough plasticity to permit cultural and
individual variation, and even variation within one individual over
time. e.g. do very young children see the penrose triangle as
impossible, or does that come later? Some people may have learnt to see
it as a Gregory model (Richard Gregory built a 3-D structure which, when
viewed from a particular direction, looks like a Penrose triangle. I.e.
it looks impossible because two things which are actually at different
distances look as if they form a junction, from that viewpoint. It takes
time to learn to think about curved 3-D space, or wave-like particles.
> The background
> image I've installed on my PC (a rocky seashore scene) was chosen expressly
> because it generates a percept of great depth -- but that doesn't mean that
> I lose the semi-flat percept of the front of the CRT when I pay attention
> to the represented far horizon.
I.e. two classes of inconsistent schemata/concepts or whatever are
simultaneously appled to the same sensory contents. No problem for us.
How many other animals can do it?
> Does this mean that my nervous system is abnormal? Should I consult a
> neurologist about this?
I'd say it's a manifestation of a type of combinatorial mechanism
outlined in my previous message and above, which contributes to the
power of human brains, and which makes consistency detection possible in
special cases and intractable in general. (I suspect mathematicians,
composers, novelists, programmers, .... have it to a higher degree than
most ordinary people.)
Maybe there are different kinds of brains, and George and I (and others)
share some gene not everyone does?
Is that possible? I've certainly met people who appear to be totally
incapable of viewing stereograms which I (and others) see easily. Is it
possible that in some people the Baars consistency constraint operates
in a more powerful fashion, so that rivalry occurs? I wonder what
happens to them in normal binocular vision?
There certainly are differences in visual systems. Many people can see
colours which I can't.
> The standard way science has of breaking out of conceptual boxes like
> the one Baars has described is to look at the system in more detail.
> But a naive conception of detail as microscopic space or time doesn't
> work with a holistic phenomenon like consciousness.
I agree and have been trying to make similar points in a different
thread, regarding different temporal frameworks for different classes of
He goes on with some interesting comments on how consistency constraints
might break down in certain neural mechanisms.
> ....What happens in my mind (and probably in Aaron's)
> is that a higher-order process ....
> .... has learned the shape of both attractors
> and can hold the percept exactly on the separatrix between them.
> This usually takes more effort than it's worth, but it can be done.
I don't know if this is right. But if it is right, it may be that this
kind of "effort" is exactly what is required for the kinds of human
creativity referred to in my previous message.
> I'm not sure where higher-order processes fit into GW theory, but
> it seems to me that they are essential to a fully-developed theory
> of consciousness. I also think that they are incompatible with any
> strong claim about the unity of consciousness at our current level
> of sophistication. .....
Agreed. The unity of consciousness is not something "directly" observed
as often supposed, but, I suspect, something imposed in culturally
influenced introspective states, just as perception of external
phenomena (e.g. words on a sheet of paper, forms of dance, etc.) can be
strongly influenced by cultural learning.
PS: in case anyone wants to look back at my earlier comments referred to
above I've put my messages on this topic in a plain text file at
including this message.
Aaron Sloman, ( http://www.cs.bham.ac.uk/~axs/ )
School of Computer Science, The University of Birmingham, B15 2TT, UK
EMAIL [log in to unmask]
Phone: +44-121-414-4775 (Sec 3711) Fax: +44-121-414-4281
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