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Ecological psychology was founded by J.J. Gibson,
who advanced the theory of direct perception.
It maintains, e.g., that perception and action are one an the same thing, and that an
observer and the environment are an inseparable pair. In principle, it uses
real-live stimuli under natural environmental conditions. Important concepts are
those of invariances, affordances, and pi-numbers (see below). Ecological psychology, which
arose after the second world war, is a reaction against the more traditional
psychological approach that predominantly used simple stimuli under controlled
experimental conditions. This more traditional psychological approach is
referred to as empiricism.
Some preliminary observations in ecological psychology:
- Prolonged distortion leads to recovery. When one puts on
distorting spectacles, e.g., so that straight lines get curved, the visual
system adapts to this. After some time, a few hours or days, the lines
are seen as straight again.
- Perception is not the sum of simple sensations. How an observer
recognizes visual objects such as human faces or animals, can not be derived
from research on the perception of simple points and lines.
- In laboratory conditions, stimuli are empoverished and poor in
information, and the percepts represent marginal phenomena. Perception must be studied in real-life conditions, not in laboratory
conditions. In real-life conditions perceptual information is rich in
structure and, hence, in information.
Critics on "empiricism"
In contrast with real-life situations, in "empiricist" experiments the
stimuli used are extremely impoverished and, hence, are represented imperfectly.
In these conditions, identical images can have different projections on the
retina, e.g., when an object is moved in the directoin of or away from the eye.
Or different images can have the same projection on the retina, e.g., the
circles of a cone with the eye in its top. As a consequence, in the
perceptual process information must be added based on memory, habit, experience, etc. Sensory inputs are represented as images, schemata, models.
Empiricism presumes that
• Sensations are stored in memory;
• Memory is iconic, short term, long term, etc., and involves various stages
between which information flows;
• The mental and the physical are separated. The physical world is meaningless
and neutral;
• Behaviour can be divided into perception, cognition, and action.
The ecological approach
According to the ecological approach, e.g., Gibson (1979), light enters the visual system as an
optic array with is highly complex,
but structured, and rich in
information. One may compare this with a hologram in which the
information is distributed over a large array; if the hologram breaks in
two, each piece still contains all information about the scene recorded by the
hologram. Moreover, by moving around in the environment, the flow of
information over the senses is considered to be the essential source of
information for the organism. The organism scans the environment in the
course of perceiving. Hence, the observer and the environment are an
inseparable pair; and perception and action cannot be separated from each other.
In an animal, the environment expresses itself in its wings, gills, feet, hands,
etc. The environment is characterized by the organisms that live there.
The concept of perceptual constancy
In infinitely many different environmental conditions and, hence, for
infinitely many different
sensory patterns that enter the sensory organs, we perceive an object with more
or less constant perceptual properties.
Some examples:
• Brightness constancy
• Colour constancy
• Size constancy
• Etc.
So, we perceive the colour, the brightness, and the size of an object as
constant, although the lighting conditions change and the observer moves around.
When an object moves away from the observer:
• The projection on the retina gets smaller
• Its texture gets finer
• Other objects can obscure it
• It will move closer to the horizon
• Etc.
In spite of these, sometimes rapid, changes, the perceptual attributes of the
objects such as their brightness, colour, and size, remain the same. Note
that, on the other hand, these changes contribute to visual distance and motion perception.
In summary, the changes mentioned above are processed as changes in the distance
of the object, and the percepts of size, brightness and colour do not change.
The question is now, what properties of the optic array remain constant with
size, brightness, and colour, as it enters the perceptual system?
Multiple sources of information
The idea is now that the perceptual system, as with distance perception, uses
multiple sources of information from the very structured and rich optic array as
it enters the perceptual system. This makes it a very robust system. If one
source of information is missing, the perceiver can use another. Moreover,
information from a large sensory array can be used. If part of the array is obscured, the perceiver can use other
parts. Indeed, the fact that information from one object is obscured by
another object, is information in itself about the relative positions of the
objects. In addition, surfaces at the back of an object are obscured by
surfaces at the front. By moving around, obscured parts of surfaces get
visible, and visible parts get obscured. This is pre-eminently the
information used by an observer to explore the environment. Hence,
perception is an active process.
But if the perceptual attributes remain constant, what remains invariant
under all these different conditions of stimulation? Or, what are the invariants?
Invariants
An example of a possible invariant will be described for what is called
time to contact, or
t, the time is takes for
an observer to make contact with an object when moving with a constant speed
v in the direction of an object. It can be derived that, if A
is the area of a projection on the retina of a surface on the object,
t = 2A/A'. Here, we will
derive a similar formula for the time to contact
t as a function of the sound intensity
of a sound source, when a listener approaches this sound source with a constant
speed v. We will use the fact that the intensity of the direct
sound that arrives at the listener is inversely proportional to the square of
the distance to the object.
If I(t) is the sound intensity at time t, d(t) is
the distance from the object, v is the speed in the direction
of object, and c is a constant:
I(t) = c(d(t)-d(0))-2 =
c(vt-d(0))-2 (1)
I'(t) =
-2c(vt-d(0))-3v (2)
Combining (1) and (2) gives: -2I(t)/I'(t) =
-2c(vt-d(0))-2/ -2c(vt-d(0))-3v =
(vt-d(0))/v = t - d(0)/v.
Time to contact
t = d(0)/v,
hence -2I(t)/I'(t)
= t - t, or
t = 2I(0)/I'(0).
Or, in general, time to contact
t
= 2I/I'.
Now we do not perceive sound intensity, but we perceive loudness.
Applying Stevens' law R = Ia
where R is the perceived magnitude, i.c. loudness , gives
t = a
R/R'. Hence, when an oberver and a sound source approach each other,
a R/R' is an invariant of time to contact
t.
Transformational invariants
Transformational invariants reside in patterns of change that give information about what happens to
an object. For vision t = 2A/A' is an
example of a transformational invariant. For sound
t
= a
R/R' is an example.
Structural invariants
Structural invariants reside in patterns that remain constant under changing conditions.
An invariant underlying size constancy is the proportion of the distance between
the heighest point of the object and the horizon, and the distance between the
lowest point of the object and the horizon. For instance, if the heighest
point of the object is below the horizon the object is smaller than the
observer; if it is above the horizon, it is taller.
Affordances
Affordances of an object are the meanings the object has for an observer.
The environment cannot be considered separately from the observer, as the
observer cannot be considered separately from the environment.
Examples of affordances:
• A path is something one can walk on, is "walk-on-able".
• A chair is an object one can sit on, is "sit-on-able".
• A spherical object with the size of a small apple is something one can throw,
is "throwable".
Affordances are perceived directly without prior synthesis or analysis from the patterns of stimulation originating
from an object. They are derived directly from the invariant properties of that
stimulation associated with these affordances. For instance, when the presence of a
tennis ball can be derived from the optic array of an observer, this
observer first sees something which can be picked up, is "pick-up-able", can be
thrown, is "throwable", is relatively soft, and can be hit by a racket.
The conclusion "it is a tennis ball" comes later if it comes at all, and is only
relevant when indeed the observer picks up the ball and starts playing tennis
with it.
Intrinsic measures and pi numbers
In many cases, the affordances of an object can be derived from its physical
dimensions relative to the physical dimensions of the observer. In
ecological psychology various measures are distinguished:
• Extrinsic measures
– Physical measures expressed in physical units
• Intrinsic measures
– Dimensionless measures, scaled to parts of the body
• Pi numbers
– Intrinsic measures describing a fit between body and environment
These pi numbers define the fit of the measures for the affordances of the
object. The pi number that defines an optimum fit, gives an
optimum point, associated with maximally
stable and efficient behaviour. As the pi number is further removed from
its optimum point, there will come a critical point,
where the affordance can no longer be effected. Here the behaviour shows a
critical boundary. For instance, for
stair climbing, the height of a single stair is presented relative to the length
of the leg of the climber. The height perceived as optimum defines the
optimum point. A critical point is the height above which the climber is
no longer able to climb te stairs without using other body parts such as the
hands. So there is a cricitcal boundary between stair climbing without
using the hands and stair climbing with using the hands.
Attunement and information pickup
"A perceptual system does not respond to stimuli (although a receptor does) but
extracts invariants" (Gibson, 1976).
Perception is a process of resonance. The observer is tuned to the invariant
properties associated with the affordances of an object. This attunement has
developed during the evolution of the species and during learning processes in
the course of growing up. The perceptual system picks up the information
it is tuned to (Compare a radio).
The computations necessary to perform the required analyses are built into the
system. (Compare this with a connexionist network.)
Note: These ideas were developed before we could make connectionist networks!
Criticism
There have been various sources of criticism on the ecological approach to
perception. For instance,
• The meaning of "direct" in direct perception
• The detection of invariants
– Invariants appear very difficult to extract computationally.
• The nature of affordances
- What invariants are associated with being edible, for instance.
• Resonance
• Direct perception and its relation with traditional, laboratory-based research
Some examples of studies in ecological psychology
• Plummeting behaviour of gannets
• Ball catching by baseball players
The psychology of design
Gibson's theoretical contribution has played a very important role in the
design of user interfaces, e.g. Don Norman's "Psychology of everyday things"
(1988), and Bill Gaver's ecological psychology of hearing (1993a; b), to be
discussed.
Literature
Bruce, V., Green, P.R. & Georgeson, M.A. (1996)
Introduction to the Ecological Approach to Visual Perception.
In: Visual perception: Physiology, Psychology, and Ecology, 3rd ed.
Hove, East Sussex, UK: Psychology Press, pp. 255-266.
Gaver, W.W. (1993a)
What in the world do we hear? An ecological approach to auditory source perception.
Ecological Psychology 5, 1-29.
Gaver, W.W. (1993b)
How do we hear in the world? Explorations in ecological acoustics.
Ecological Psychology 5, 285-313.
Gibson, J.J. (1979)
The ecological approach to visual perception.
Boston, MA: Houghton Mifflin.
Gordon, I.E. (1996)
Direct perception and ecological optics: the work of J.J. Gibson.
In: Theories of visual perception, 2nd ed.
Chichester, UK: John Wiley & Sons, pp. 180-220.
Li, X., Logan, R.J. & Pastore, R.E. (1988)
Perception of acoustic source characteristics: Walking sounds.
Journal of the Acoustical Society of America 90, 3036-3049.
Norman, D. (1988)
The psychology of everyday things.
New York, NY: Basic Books.
Warren, H. & Verbrugge, R.R. (1984)
Auditory perception of breaking and bouncing events: A case study in ecological acoustics.
Journal of Experimental Psychology: Human Perception and Performance 10, 704-712.
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