The Concept of Affordance and GIS: A Note on Llobera (1996)
Webster, David S., Antiquity
Marcos Llobera (1996: 612) attempts to use Geographical Information Systems (GIS) in a more `humanistic way', to explore `places and spaces' and to `close the gap between theory and method'. The theory in question is that of human practices and relates to Gidens' concept of structuration and Bourdieu's concept of habitus. As a methodological tool, GIS offers, as Kvamme (1993: 91) notes, `excellent display capabilities together with embedded systems of quantitative analysis [that] can provide an ideal environment for spatial investigation'.
Gidens and Bourdieu are now regular sources for archaeological theorizing, but the American psychologist, James J. Gibson is not. Gibson studied perception and developed an `ecological psychology' based on a theory of direct perception. The concept of affordance is central to the theory of direct perception.
For Llobera, the concepts of structuration, habitus and affordance make up a trinity of key concepts that, it is hoped, will make human practices within the landscape explicable (but note; the utility of the concept of practices for social science has been severely questioned by Stephen Turner (1994)).
Finally, Llobera's approach to understanding `places and spaces' in the Wessex landscape is also influenced by the anthropology of Tim Ingold, who in turn, draws on Gibson's theory of direct perception. Llobera is therefore doubly influence by Gibson; directly by his attempt to use the concept of affordance and indirectly through the anthropology of Ingold. What, then, does Gibson's theory of direct perception amount to?
Gibson's theory of `direct perception'
Although Gibson's theory of direct perception is generally applicable to the senses, most of the experimental work he undertook explored the processes of visual perception, so I will confine my discussion to vision.
Orthodoxy (Descartes onwards) holds that light falls onto the retina producing a `retinal image' from which the mind/brain constructs representations of things in the world. Visual cognition thus amounts to manipulating these representations.
Gibson, however, contested the relevance of the `retinal image' for vision. Gibson pointed out that light entering the environment -- as rays of radiation -- is reflected back and forth off surfaces and is scattered by particulate matter in the atmosphere. In this way radiant light becomes illumination and an ambient array at every point. Thus, light comes to every point and thereby environs every point. In this way, light at the point of observation is different in intensity in different directions.
As we move through this ambient array, invariant ratios of illumination intensity presented in temporal sequence constitute a structure or texture in the form of deeply nested sets of invariant relations of illumination; these nested sets of invariant ratios of illumination are what are registered or picked up by the perceptual system. By perceptual system, Gibson means the reflexive organization of the animal -- the mobile eye on a mobile head on a mobile body. Perception for Gibson is active and exploratory. Gibson rejected the idea that the world is reassembled from a projected image on the retina, rather, the world is specified in a lawful and reflexive way by our direct interaction with it.(1)
Affordances of the environment
The lawful specification of the environment's layout given by invariant relations of illumination in ambient light carries a qualitative value for a given animal. It is this qualitative value that Gibson termed an affordance.
Gibson (1986: 127) states that The affordances of the environment are what it offers the animal, what it provides or furnishes either for good or ill. The invariants in the optical flow consist of `lower-order' and `higher-order' invariants. These two classes of invariants are interrelated, analogously, with the way ratio or interval scales (e. …