Introduction to Seeing...
We are so familiar with seeing, that it takes a
leap of imagination to realise that there are
problems to be solved. But consider it. We are
given tiny distorted upside-down images in
the eyes, and we see separate solid objects in
surrounding space. From the patterns of
stimulation on the retinas we perceive the
world of objects, and this is nothing short of a
miracle.
The eye is often described as like a camera,
but it is the quite uncamera-like features of
perception which are most interesting. How
is information from the eyes coded into
neural terms, into the language of the brain,
and reconstituted into experience of
surrounding objects? The task of eye and
brain is quite different from either a
photographic or a television camera
converting objects merely into images. There
is a temptation, which must be avoided, to
say that the eyes produce pictures in the
brain. A picture in the brain suggests the
need of some kind of internal eye to see it but this would need a further eye to see its
picture... and so on in an endless regress of
eyes and pictures. This is absurd. What the
eyes do is to feed the brain with information
coded into neural activity - chains of electrical
impulses - which by their code and the
patterns of brain activity, represent objects.
We may take an analogy from written
language: the letters and words on this page
have certain meanings, to those who know
the language. They affect the reader's brain
appropriately, but they are not pictures.
When we look at something, the pattern of
neural activity represents the object and to
the brain is the object. No internal picture is
involved.
Gestalt writers did tend to say that there are
pictures inside the brain. They thought of
perception in terms of modifications of
electrical fields of the brain, these fields
copying the form of perceived objects. This
doctrine, known as isomorphism, has had
unfortunate effects on thinking about
perception. Ever since, there has been a
tendency to postulate properties to these
hypothetical brain fields such that visual
distortions, and other phenomena, are
'explained'. But it is all too easy to postulate
things having just the right properties. There
is no independent evidence for such brain
fields, and no independent way of
discovering their properties. If there is no
evidence for them, and no way of discovering
their properties, then they are highly suspect.
Useful explanations relate observables.
The Gestalt psychologists did however point
to several important phenomena. They also
saw very clearly that there is a problem in
how the mosaic of retinal stimulation gives
rise to perception of objects. They particularly
stressed the tendency for the perceptual
system to group things into simple units. This
is seen in an array of dots.
Introduction to Seeing (2)
This array of equally spaced dots is seen as
continually changing patterns of rows and
squares.
We see something of the active organising power
of the visual system while looking at this figure.
Here the dots are in fact equally spaced, but
there is a tendency to see, to 'organise', the
columns and rows as though they are
separate objects. This is worth pondering, for
in this example we have the essential
problem of perception. We can see in
ourselves the groping towards organising the
sensory data into objects. If the brain were
not continually on the look-out for objects,
the cartoonist would have a hard time. But, in
fact, all he or she has to do is present a few
lines to the eye and we see a face, complete
with an expression. The few lines are all that
is required for the eye - the brain does the
rest: seeking objects and finding them
whenever possible. Sometimes we see objects
which are not there: faces in the fire, or the
Man in the Moon.
Introduction to Seeing (3)
Although we are concerned with how we see
the world of objects, it is important to
consider the sensory processes giving
perception - what they are, how they work
and when they fail to work properly. It is by
coming to understand these underlying
processes that we can understand how we
perceive objects.
Introduction to Seeing (4)
There are many familiar so-called
'ambiguous figures', which illustrate very
clearly how the same pattern of stimulation at
the eye can give rise to different perceptions,
and how the perception of objects goes
beyond sensation. The most common
ambiguous figures are of two kinds: figures
which alternate as 'object' or 'ground', and
those which spontaneously change their
position in depth.
This figure makes the point clearly. Just a set
of meaningless lines? No - it is a woman with
a bucket washing the floor! Now look again:
the lines are subtly different, almost solid they represent objects.
The seeing of objects involves many sources
of information beyond those meeting the eye
when we look at an object. It generally
involves knowledge of the object derived
from previous experience, and this
experience is not limited to vision but may
include the other senses; touch, taste, smell,
hearing and perhaps also temperature or
pain. Objects are far more than patterns of
stimulation: objects have pasts and futures;
when we know its past or can guess its
future, an object transcends experience and
becomes an embodiment of knowledge and
expectation without which life beyond the
simplest is not possible.
Here is an example which alternates in figure
and ground - sometimes the black part
appears as a face, the white being neutral
background, and at others the black is
insignificant and the white surround
dominates and seems to represent an object, a
vase or chalice.
The well-known Necker cube shows a figure
alternating in depth. Sometimes the face
marked with the 'o' lies in front, sometimes at
the back - it jumps suddenly from the one
position to the other.
Perceiving and thinking are not independent:
'I see what you mean' is not a peurile pun,
but indicates a connection which is very real.
Richard Gregory
Perception is not determined simply by the
stimulus patterns; rather it is a dynamic
searching for the best interpretation of the
available data. The data are sensory signals,
and also knowledge of the many other
characteristics of objects. Just how far
experience affects perception, how far we
have to learn to see, is a difficult question to
answer; it is one which will concern us in this
book. But it seems clear that perception
involves going beyond the immediately given
evidence of the senses: this evidence is
assessed on many grounds and generally we
make the best bet, and see things more or less
correctly. But the senses do not give us a
picture of the world directly; rather they
provide evidence for the checking of
hypotheses about what lies before us. Indeed
we may say that the perception of an object is
an hypothesis, suggested and tested by the
sensory data. The Necker cube is a pattern
which contains no clue as to which of two
alternative hypotheses is correct: the
perceptual system entertains first one then
the other hypothesis, and never comes to a
conclusion, for there is no best answer.
Sometimes the eye and brain come to wrong
conclusions, and then we suffer
hallucinations or illusions. When a perceptual
hypothesis - a perception - is wrong, we are
misled, as we are misled by science when we
see the world distorted by a false theory.