Date: Mon, 24 Nov 1997 01:19:47 +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 don't know how you find all the time, and I am not sure I can keep up,
but here goes. (I still owe Stan Klein a reply, explaining the
difference between being meaningless and having no clear meaning, to
I'll separate out several different claims about the requirement for
coherence in perception, and argue against them, while supporting a
loosely analogous argument about the strong likelihood of resource
limits in higher level parts of the architecture for an intelligent
agent. The problem of resource limits could be confused with a
requirement for a consistency constraint.
> Date: Sat, 22 Nov 1997 00:25:53 -0500
> From: "<Bernard Baars>" <[log in to unmask]>
> 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?
OK, we now have three distinct theses of diminishing strength:
1. When retinal inputs differ one or other will be suppressed unless
they fuse into a single percept as in normal stereo vision.
That strong thesis seems to believed by many people and that's what I
read into page 89 of B.Baars "In the Theater of Consciousness" (OUP
1996). It now seems that we are agreed it is false, so you didn't mean
to say what your words seemed (to me) to be saying.
Then came this less strong thesis:
2. Consciousness reflects an internal consistency constraint.
I assumed this weaker thesis was an attempt to rule out combinations in
which different, inconsistent, things are perceived in the same portion
of the visual field.
There are various counter-examples discussed in my previous messages,
and many more: e.g. a motion after-effect in which perception of upward
motion occurring at a particular location coincides with perception of
only static objects there.
But you didn't want to accept the things I listed as counter-examples,
so I suggested that perhaps nothing could be a counter example and the
claim is therefore non-empirical. You've now responded with a third,
still weaker thesis. (NB: "weak" does not mean "bad". It's just a
neutral description of the number and diversity of consequences of the
3. A person cannot "PERCEIVE TWO SOLID VISUAL OBJECTS IN THE SAME
PERCEIVED SPATIAL LOCATION AT EXACTLY THE SAME PERCEIVED TIME"
This weakest thesis is still interesting, but I am not sure whether it's
true or false or perhaps just a non-empirical piece of mathematics or
My initial reaction is that it is false, like the simplistic
interpretations of the first two theses, since, for instance, Picasso in
one of his sculptures showed us how to see a sports car and a baboon's
head in the same place at the same time. Michaelangelo encouraged a
percept of a human body struggling to emerge from a block of marble.
I don't think we are invited to see the marble as *enclosing* a separate
body. That would be a far less powerful work of art.
Or consider the well known fruit-face: assemble a pumpkin, a banana, a
couple of cherries and a carrot in the appropriate configuration, with
the help of some adhesive, and then you can see in the same place at the
same time a smiling face with two eyes a nose and a mouth, and a
collection of different items of fruit: two solid visual objects in the
same perceived spatial location at exactly the same perceived time.
Perhaps you'll say those are not counter examples to thesis 3, since in
each case we see the same 3-D bounding surface but can apply different
inconsistent classifications to it simultaneously.
Can you actually describe in any detail a type of perception which you
would accept as a counter example if it were to be produced in some
carefully contrived experimental situation?
If not, perhaps that's because what you are ruling out is actually
something incoherent. I.e. it's not a fact about the brain that it can't
do it, just as it's not a fact about the brain that it can't make the
number 27 a prime number.
For example, suppose that by "solid" you really meant "opaque" (i.e
impervious to vision). and by "two objects" you really meant
"differently shaped objects". E.g. you are asking (for example) can a
person see an opaque cube and an opaque sphere in the same perceived
spatial location at exactly the same perceived time".
Unfortunately, the problem now is not an empirical question about what
the brain can or cannot do but a conceptual question about what it could
mean for two differently shaped objects to occupy exactly the same
location. If their shapes are different they certainly cannot: if a
solid object occupies all and only the same spatial locations as a cube
then it must be a cube, not a sphere. But that's a trivial (well, nearly
trivial) matter of definition, not a fact about what brains can and
cannot do. Or, to be more precise: it's a fact of mathematics (geometry)
not a fact of biology (brain science).
But perhaps you meant something like this: can a person see two opaque
solid objects in roughly the same place, i.e. with overlapping volumes,
e.g. Michaelangelo's human bodies struggling to emerge from blocks of
marble, with which they somehow co-occur.
You seem to say No, whereas I expect the answer is yes: provided the
person, the context, and the stimuli are right. But in any case, it is
very hard to reach a negative conclusion from any number of experiments.
Imagine trying to prove by experiment that no human brain can find a
proof of Fermat's last theorem. Millions of negative experimental
results would still not give an answer to the question, which we now
know is positive. Or imagine experiments prior to Einstein&Minkowski
seeking to find out whether a person can imagine curved 3-D space, or
instantaneous events which are not unambiguously ordered in time, or...
Human brains are the most creative machines known to science, and any
generalisation about their limitations needs to be treated with
*extreme* caution. (Even that one...)
But I now have to ask, does this consistency/coherence constraint
really add anything significant to theories of perception or
Why is it felt to be important? Is it just a relic of a rationalist view
of mind, taken by granted by many philosophers (even Dennett, in weak
Maybe one of the most important things about (human) brains is that they
can transcend what's consistent, coherent, rational, etc.
Because at some stage evolution produced a powerful mechanism (I don't
necessarily mean a computational mechanism) for assembling new
information structures out of old ones, and then found all sorts of deep
ways to make use of it, or variants of it. E.g. it's crucial to planning
and much problem solving, and to communication in natural language. It
makes mathematical learning and problem-solving possible.
But, for deep mathematical reasons, it is hard or impossible to combine
such combinatorial creativity with a guarantee of consistency, and sure
enough humans are frequently seduced by incoherent thoughts, objectives,
ethical systems, religious propaganda, etc. So why not incoherent
percepts too, including visual percepts?
Some people used to think the reason was that visual percepts employed
spatial forms of representation, and spatial representations cannot
represent what is spatially impossible. We now know that is false:
examples include the Penrose triangle, Richard Gregory's 3-D
implementation of it, Escher figures, and one of my own favourites:
i.e. a round square seen edge on.
NB: the Penrose/Escher figures (e.g. impossible triangle) are not
illusions or simply ambiguous figures. Rather they are locally
consistent projections of globally inconsistent 3-D structures. When the
picture is sufficiently complex, e.g. not a triangle but a decagon, many
people will see only the local consistency, and not detect the global
inconsistency: i.e. they will see a geometrically impossible/incoherent
object. (Does that refute the consistency/coherence theory?)
So why struggle to defend such an embattled theory, as the theory that
the contents of consciousness must be coherent? I think it's a confusion
of a processing issue with a content issue, to be explained later.
Of course, the creativity of each brain is limited, but the limits
change over time. History is littered with the corpses of ideas about
what is incoherent, inconceivable, impossible: the earth moving through
space, space-filling one-dimensional curves, everywhere continuous but
nowhere differentiable functions, sets which are in one to one
correspondence with proper subsets of themselves, the square root of a
negative number, adding light to light in order to produce darkness,
humans evolving from micro-organisms, distant events being both before
and after each other, force-fields in totally empty space, curvature of
3-D space, transmission of music or pictures across empty space,
something behaving as both wave and particle, ... to mention a few of
the better known cases.
Instead of stressing the requirement for contents of consciousness to be
coherent, consistent, etc. we should perhaps be trying to understand
what sorts of mechanisms can fruitfully push their own boundaries beyond
what till then are the limits of consistency. That is what many great
scientists, mathematicians and artists have done.
Maybe every child does it as part of its normal process of development,
until we use bad schools and bad scientific text books and religious
dogma to stifle the process.
Anyhow, I am perfectly aware that this is hand-waving. The real work
remains to be done. Let's just not shut out theories because so far no
established evidence seems to support them, if there's a strong
theoretical reason for accepting them (in this case the theory that
perception, like thought, involves combinatorial creativity, at least
in some animals. Maybe not house flies?).
> My reading of
> all the research going back to Helmholtz is that the answer is no.
Until recently all the evidence was that nobody could prove Fermat's
last theorem. There were probably even more failed attempts at that task
than psychological experiments on perception of inconsistent scenes.
> All the
> claimed exceptions to the rule involve a clever visual-brain solution to the
We seem do be agreed on the essential creativity (cleverness) of
> as in the case where we perceive a hand over one eye as a
> diaphanous, filmy object, THROUGH WHICH we see another object. The visual
> brain is just brilliant in solving such conflicting inputs.
Given all that, what are the mechanisms underlying such brilliance? My
conjecture is that they depend on combinatorial creativity. And given
that, why should such creativity be constrained to fit any particular
kind of coherence criterion?
And how could it be? (It's not always easy to build semantic constraints
into the syntax of a powerful form of representation.)
> Most of the time,
> in normal vision, the dominant eye's interpretation "wins," though the
> nondominant eye provides a sense of depth.
(I must be a mutant: my eyes are symmetric on this test.)
I think that if you abandon this (arbtrary?) consistency constraint it
will enrich your theory of consciousness, make it closer to the
facts, and I suspect make it easier to produce explanatory mechanisms.
> I think Aaron is entirely right that local or peripheral hypotheses about
> such conflicting input situations don't get us very far. It's a
> multi-layered, if-one-thing-doesn't-work- try-another system, not at all
I apologise for the rhetorical flourish. Boringness is less important
than truth. Or, as J.L.Austin said, truth is more important than
> So the theoretical claim is that the consistency constraint applies not just
> to vision, not just to audition and the other senses, but to "conscious" or
> "explicit" ideas, meanings, and probably dominant intentions as well. I give
> a whole bunch of additional evidence for this general claim in my 1988 book.
I think it is important, as hinted above, to distinguish two distinct
(A) the contents of vision, consciousness, audition (motivation, ?) etc.
are constrained to be consistent. (I think this is false.)
(B) The mechanisms involved in self-awareness and in deliberation and
reasoning (or other things using a "global workspace"?) are inherently
resource limited. I.e. they can't simultaneously perform arbitrarily
many distinct, unrelated, tasks.
As far as (B) is concerned, I think it is true, for deep engineering
design reasons to do with the impossibility of implementing physical
mechanisms which violate it in a well integrated system.
Reasons for the resource limits restricting parallelism (and thereby
incidentally increasing coherence) include the following: (1) processes
involving combinatorial exploration require repeated re-use of the same
temporary storage space. (2) the high level processes use a long term
content addressible memory, which needs to use all available parallelism
for optimising retrieval speed, at the cost of ruling out answering
different questions concurrently (since cross-talk ccould result).
(3) An argument I first heard from Dana Ballard: various learning tasks
explode exponentially if too many things are done in parallel (e.g.
finding which subset of N concurrent actions produced some effect,
potentially requires considering and testing 2 to the power N possible
subsets, and it gets MUCH worse if delayed consequences are allowed).
I.e. (B) is not just an empirical fact about human consciousness, but
something deper. It could apply to robots, martians, etc.
However the status of (A) is different: checks and constraints
preserving consistency have great heuristic power, and therefore we can
expect to find them in many places. However, since they are hard to
implement in general, and since an apparent contradiction may be an
important first clue towards an expanded conceptual framework giving
access to a deeper more general ontology, as often happened in the
history of science and mathematics, we should not expect the consistency
requirement to have any sort of ABSOLUTE status.
I.e. exceptions can occur, and may be an important part of learning or
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.
> On theory, Aaron claims all ideas we have today are computationally
I didn't say "computationally inadequate". Computation is but one form
of information processing though its boundaries are somewhat ill
defined. (Does it include continuously varying systems, which are
non-computational in the sense in which computation requires a
succession of discrete states.)
When I talk about information processing mechanisms I leave open the
possibility of including mechanisms that are as different from today's
computational mechanisms as the latter were different from the cogs,
levers and strings of yesteryear.
> That may be true. People like Stan Franklin are working to push
> that envelope.
Stan's work is certainly an interesting example of a raft of
explorations of architectures for intelligent agents. (I have my own pet
models too.) But I suspect all this work will turn out to be missing
something very powerful, which is still waiting to be discovered.
(NB It has nothing to do with Godel's theorem, which I think is a
complete red herring.)
> As a person concerned primarily with evidence, I'm initially
> interested in EMPIRICAL adequacy. Then we have to try to fix problems of
> ALGORITHMIC adequacy. There are lots of examples in the other sciences that
> have worked just like this, where the mathematical algorithm became available
> after the evidence was reasonably well understood conceptually.
I personally prefer the approach of great physicists, like Newton and
Einstein: First (by inspired guesswork) produce a great new theory of
the architecture of the world, accounting for a wide range of already
known phenomena, and absorbing the best previous theories as special
cases, possibly inaccurate special cases. Then work out its new
consequences with as much mathematical precision as possible. Then do
experiments, etc. to see how the consequences fit reality, and where
they don't fit, be prepared to modify the theory, or consider
alternatives. If all the consequences are too vague or ambiguous to
allow any refutation then don't necessarily throw the theory away as
unscientific: try to sharpen it.
Algorithms provide just one sort of detail within the framework of an
architecture: don't be too impressed by them.
Nowadays any theory of adequate richness is likely to be too complex for
consequences to be derived without simulation. If simulation is
impossible, start worrying about whether that's because the theory is
too ill defined.
Enough. I've gone on too long again.
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