The auditory filter
Auditory Scene Analysis 1
Auditory Scene Analysis 2
Ecological Psychology - General
Ecological Psychology of Hearing
Sound Localization
Sound Examples

Dik J. Hermes

Eindhoven University of Technology
Department of Industrial Engineering & Innovation Sciences
Human-Technology Interaction
P.O. Box 513
NL 5600 MB Eindhoven
The Netherlands

Ecological Psychology - General

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 laboratory 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 direction 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?


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 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!


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.


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.