School of Computer Science THE UNIVERSITY OF BIRMINGHAM CoSy project CogX project

Two faces -- a new illusion?
(DRAFT: Liable to change)

Aaron Sloman
School of Computer Science, University of Birmingham.
(Philosopher in a Computer Science department)

Installed: 6 Mar 2013
Last updated: 6 Mar 2013
This paper is
A PDF version may be added later.

A partial index of discussion notes is in

Do the eyes in the left face and the right face look the same?

Stare at each face in turn for a few seconds at a time.

                  [Based on Figure 5 in Sloman 1986]

Most people, though not all, find that the eyes in the two faces look different -- not
geometrically different, but different in a way that appears to be part of the expression.

That's an illusion: the only things that are different in the two faces are the mouths.

I think this provides indirect evidence for part of the architectural theory developed in
the CogAff project, in particular the claims about "multi-window perception" contrasted
with "peephole perception".

I've used "peephole perception" to label the view that there's a one-way "ontologically
thin" stream of information from the sensors that triggers central cognitive processes of
grouping, segmentation, classification, and interpretation. "Ontologically thin" in this
context implies that the information in sensory streams is restricted to sensor values or
perhaps some features simply and directly derived from sensor values, e.g. intensity,
contrast, optical flow, and in the case of haptic perception pressure, temperature,
motion, and perhaps things like roughness, smoothness and other texture features. This
would contrast with seeing petals, leaves, trees, conspecifics, predators, or, in the case
of humans, seeing letters, words, or phrases.

The "multi-window" view of perception is a suggestion that one of the results of evolution
was to produce sensory systems that, in more complex organisms, perform richer forms of
processing that use more abstract and in some cases externally referring information
contents, but organised in a way that keeps the information in registration either with
sensor arrays or, in the case of vision, in registration with the "optic array" that
exists in the environment at a viewpoint, and which animals sample by rapidly moving their
eyes. (Note that this means the information is not in registration with the retina or the
primary visual cortex: for a possible mechanism see Arnold Trehub, The Cognitive Brain,
MIT Press, 1991.

An implication is that there is no sharp division between perception and cognition: the
two sets of mechanisms overlap, or extend into each other.

The combination of more abstract derived information (e.g. the location of a corner where
two edges meet) with more "primitive" sensory information can be useful in driving/guiding
the further processing of incoming information, and in some cases also useful in
controlling actions (using visual servo-control). Some examples were provided in
connection with the description of the Popeye program in Chapter 9 of Sloman 1978,
available online here:
Some simple examples were the "place tokens" that David Marr suggested were added to the
'Primal Sketch' recording inferred entities, such as a line constituted by the collinear
edges of a group of line segments, and the "illusory contours" studied by Gaetano Kanizsa

A Kanizsa-inspired example from Chapter 9 of Sloman 1978 is

   [Examples of 'illusory contours']

What I am claiming is that even more abstract interpretations of sensory contents,
normally assumed to be the function of processes remote from sensors, can contribute
"downward" or "backward" information to the low level structures recording sensory
details. An example is the perception of eyes as "happy" or "sad", in this case as a
result of clues from a different part of the face.

Similar "back-projection" of high level interpretations can be seen in the
experiences produced by moving light-points attached to humans performing
various actions in the dark, studied by Gunnar Johansson.

Maintained by Aaron Sloman
School of Computer Science
The University of Birmingham