Neuroscience

Poverty of Stimulus and Ecological Laws

A question about a term I’ve been using is a nice segue into an important moment in the history of the ecological approach.

Poverty of stimulus is a term that came from Chomsky, but the intuition has been underpinning theories of perception for as long as there have been theories of perception. The term describes a problem, in which the information required to achieve something is not present in the environment. In language, the argument runs

Certain patterns of correct language use can only be learned with exposure to negative evidence (i.e. evidence about what counts as incorrect)
Children learning languages only encounter positive evidence (i.e. evidence about what counts as correct)
Children do acquire the patterns in (1).

Chomsky accepted these premises, and the conclusion is that a) the stimulus (language exposure) is not sufficient to explain learning, so b) there must be supplementary information available from another source (innate grammar). As I mentioned when posting about Skinner this was the essence of the attack on Skinner’s behaviourist account of language acquisition; if the stimulus is not sufficient, behaviourism fails.

In perception, the argument is similar, but is rooted in the physiology of the eye (on the assumption that this is the gateway into the brain for light; any bottlenecks or limitations in the eye would be reflected in the information the brain had to work with to achieve the vision of our every day experience). One specific example is 3D information – depth perception.

The world has 3 spatial dimensions (length, breadth and depth).
The eye can only form a 2D image on the back of the retina (length and breadth; no depth).
People do see in 3D.

If you accept the premises, the conclusion is that the 2D stimulus doesn’t contain enough information to unambiguously resolve the third dimension. We do achieve this, however, so we must be supplementing (‘enriching’ is the term favoured by the Gibsons) the 2D information from another source. That source is generally some form of past experience; this gets implemented as memory, dedicated representations, etc.

The problem of depth perception has a long and noble history in philosophy of mind and psychology. The state of the art when Gibson was working was David Marr’s computational model of vision, where the primal sketch provided by the eye was supplemented by texture information into a 2.5D sketch and eventually enriched into a full 3D model of the world.

Gibson’s move was to stop thinking about the physiology of the eye. His focus on what there was to be perceived (a functionalist stance, in the fine tradition of American psychology started by William James) led him to develop ecological optics (for vision). An optic array is the description of the ever changing (flowing) visual information available to an organism, in which relations remain invariant over the transformations and thereby permit specification to work.

Skinner was ‘killed off’ by Chomsky’s review of his book. Two major names in cognitive science attempted the same thing on Gibson’s 1979 book. In 1981 Jerry Fodor and Zenon Pylyshyn published a critique of the notion of specification that is, I have to say, full of rather patronising bullshit. (A friend of mine at IU knew Fodor, who apparently still thinks he killed specification off with the paper and hasn’t thought about this in years; this is apparently pretty typical behaviour.) Unlike Chomksy’s review of Verbal Behaviour, Fodor & Pylyshyn’s paper provoked a detailed response in a paper that has effectively defined the ecological approach ever since.

The critique

The critique is this: Gibson claims that perception contains information that is specific to an object or event in the world. Perception is therefore direct. Fodor and Pylyshyn note that this simply renders the problem to be trivial (in the technical sense) because Gibson’s theory does not a priori constrain the class of things that can be detected. Without this, the ecological programme of research devolves into a tautology hunt:

All things of a certain type (e.g. shoes) have at least one property in common that is specific to them (specifically the property of being a shoe). If you wish to pick out the shoes from a line up of objects, F&P claim that Gibson could simply say ‘the perceptual system simply detects the property ‘being a shoe’’.

This is somewhat of a dick move by F&P; it ignores everything Gibson had ever said about the optic array, etc. However, if it was true, it would be fatal to the programme, and so, dick move or no, it had to be addressed.

The reply

Enter Turvey, Shaw, Reed and Mace. Their reply to F&P is simple: Gibsonian information is suitably constrained (i.e. you can’t just claim any old property can be perceived) and that these constraints are not simply ad hoc, but rooted firmly in a consideration of the physics of the world in which vision evolved. This paper is elegant and clear; in fact its elegance and clarity manage to retrospectively make F&P’s paper worthwhile because it caused Turvey et al to write this.

The constraints lie in the existence of ecological laws. A law describes a set of conditions and the necessary consequence of those conditions (if the temperature of pure water at sea level is 100°C then the water will boil). Laws have scope: the scope is laid out in the set of conditions. Within that scope, the conclusion is necessary, i.e. if the conditions obtain and the scope is correct the conclusion must be the case. Ecological laws lay out the conditions and consequences by which a pattern in (for example) an optic array can specify an object or event in the world; these conditions must relate to the physics of the situation (so ‘shoeness’ is not the kind of property we are dealing with). The programme of research from an ecological viewpoint is to elucidate these laws correctly and identify the information that specifies the relation in question. If you have yet to identify a specification relation in your research, you have yet to correctly formulate the law governing the behaviour under examination. This last point has recently become a point of contention that will be the topic of future posts.

Summary

Poverty of stimulus describes an epistemological claim; the information required for us to do something we clearly do is not contained in the task environment, and so must be provided by an alternative source. For Gibson (and especially Turvey et al) this scenario simply tells you that your research has so far failed to correctly describe the task environment. The ecological methodology therefore assumes that poverty of stimulus is not an actual state of affairs in the world but a clue that your research is incomplete. Foder & Pylyshyn attempted to show that this assumption is under-constrained and therefore trivial; Turvey et al then laid out precisely how and why the assumption is entirely scientific and justifiable. The assumption made by the ecological approach might be wrong, but this is a coherent and answerable empirical question, and it is the focus of the ecological research programme.

References

Fodor, J., Pylyshyn, Z. W (1981). How direct is visual perception?: Some reflections on Gibson's “ecological approach” Cognition, 9 (2), 139-196 DOI: 10.1016/0010-0277(81)90009-3

Turvey, M. T., Shaw, R. E., Reed, E. S., Mace W. M. (1981). Ecological laws of perceiving and acting: In reply to Fodor and Pylyshyn (1981) Cognition, 9 (3), 237-304 DOI: 10.1016/0010-0277(81)90002-0

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