Thursday 7 June 2012

Specification: What It Is, and Why We Need It (Specification I)

The first thing I need to do in a discussion of specification is explain what it is and why it's important to ecological psychology. I've tried to maintain a clear logical progression in this post, building towards the need for specification. In my next post, I'll take a first swing at explaining what specification gives us, namely a reason why information means one thing and not another.

The issue of specification comes from Gibson's (1966, 1979) analysis of visual perception, so that's where I'll start too. Most descriptions of visual perception begin with the anatomy of the eye; people note that the eye resembles a camera, and that the lens seems to focus a messy, upside down image onto the retina. The retina then pixelates that image into neural activity, and this pixelated structure then shows up in primary visual cortex (this is topographic mapping). If vision does indeed begin this way, then a huge amount of work seems to be required to take this impoverished stimulus and use it as the basis for the rich, 3D visual world we experience.

Gibson's ecological theory begins with a re-evaluation of the stimulus for vision. The first three chapters of the 1979 book are about the world and what it contains, while chapter 4 is about how this world can interact with light to produce information. Only once he lays out the information available to the organism does he begin to talk about the act of perception itself; this re-evaluation of the 'job description' for a visual system is one of his most important contributions to psychology. Gibson's reanalysis leads him to conclude that action relevant properties of the world (specifically, affordances) can be specified in the optic array, and this concept underpins the directness of his theory of perception.

The issue of specification is assumed to be critical for the success of a direct theory of perception. The traditional views propose a 'many-to-one' mapping; a given pattern of stimulation on the retina is ambiguous because it could be caused by many possible states in the world. Specification is the hypothesis that there is a 'one-to-one' mapping - a given pattern in the optic array comes from one and only one state of the world. This can happen, according to Turvey, Shaw, Reed & Mace (1981) if (and only if) the creation of information about the world is a lawful process. If the projection of world into optics is underwritten by a law and thus one-to-one, then detecting the optical pattern is equivalent to detecting the property of the world: detecting the information is perceiving the world, with no additional processing work required. Perception can be direct.

A theory of direct perception will require several elements: there must be invariant structure within the endless flow across the retina that relates 1:1 to some property of the world. To be invariant, this structure must be relational, and therefore higher order. If perception is to be direct, these higher-order invariants must be detectable as a piece, and not built out of their elements in some post-perceptual process. Only if you have all this do you have the possibility of a one-to-one mapping between the world and vision, i.e. the possibility of specification.This post lays out what this all means, and  how these pieces come together in ecological psychology.

Traditional theories of psychology never thought specification was possible, because the projection on the back of the retina is constantly changing. We are never entirely still; even if you put people into a bite-board their eyes still move enough to produce sufficient change on the back of the retina that people can detect the separation in depth of surfaces using this information alone (Bingham, 1993). This ever-changing image has always been a problem for vision research. It is what creates the ambiguous relationship between what's happening in the world and what's happening on the retina. This motion is a problem that must somehow be filtered out.

Gibson was happy with the idea that there is endless flow across the retina; in fact, he demanded it, because it's only through flow that specifying variables become available. For Gibson, the endless motion of the visual perceptual system had to be a feature, not a bug - we must move to create information. While the individual elements of visual experience change enormously with any motion, relations between these elements can stay the same. These particular relations are defined as the things that remain the same while all else changes; they are, in Gibson's phrase, invariants over transformation and only they can specify properties of the world.

Higher Order Variables and Relations
Traditional theories consider the basic units for perception to be whatever the mathematically simplest elements are. If you need to perceive something like area, you must first perceive the two relevant lengths and then apply a transformation on these (multiplying them together). Area, in this example, is what is known as a higher-order variable - mathematically, the units of area are length2, which is of a higher order (2) than the units of length (length1). Systems detect simple things first, and must get to more complex variables via computation (or something like it). However, remember that the world measured by vision is not necessarily the world measured by physics and thus our analysis should never assume that it is.

Invariants-over-transformation must be higher order variables. The only thing that can remain the same while the elements change is a relation between those things, and relations are, by definition, higher order. Area is a relation between two simpler lengths, specifically the relation implemented via multiplication. You can preserve the area while the two lengths that make it change, so long as the relation between the two lengths is preserved. Another example I like is the right-angled triangle. (I've laid this example out in some detail before; go have a read, then come back. I'll wait.) Pythagoras' Theorem states that a triangle is right-angled if and only if the following relation is true: the square on the hypotenuse is equal to the sum of the squares on the other two sides. The exact lengths can vary, but as long as they vary in a way that preserves the relation, the triangle remains right-angled. This higher-order property of right-angled triangles only becomes clear as you allow the lengths to vary; the relation emerges as the thing that remains the same while everything else changes.This is precisely the kind of relational structure we're looking for in the optic array.

Simple is relative
Gibson first explains that this kind of relational, higher-order invariant structure does exist in the optic array; his theory of ecological optics is a theory of how the world projects these variables into light (see Chapter 4 and 5 of Gibson, 1979). This is all well and good, but if the system must build access to higher-order variables by transforming lower-order ones, then perception still requires internal processing. Gibson's theory of direct perception requires us to have direct access to these higher order relations, without having to go via the lower order components. Is this possible?

The proof-of-concept is the polar planimeter (Runeson, 1977 and this post). This is a device used for measuring the area of irregular shapes - you place one end somewhere outside the shape and trace the outline. If your device is correctly calibrated, the action of drawing the outline leads to a direct measurement of the area. For a polar planimeter, area is simpler than length; it has direct access to the former, and it requires a lot of quite sophisticated maths to find any lengths (maths which was only worked out well after the planimeter had been invented and put into common use, I might add. The primacy of algorithm over dynamical devices is relatively recent).

The moral is simple: what counts as the simple unit depends on the device doing the measurement, and higher order variables can be directly detected by suitable measurement devices. If the visual system is such a device, then these higher-order, specifying, invariants-over-transformation that are present in the optic array can serve as the basis of vision, and this perception can therefore be direct. Given that the information exists, and that we often act as if we are using that information, perhaps we are, indeed, the right kind of measurement device. One part of an ecological research programme is to therefore empirically establish that we are actually perceiving the higher order structure and not the components (e.g. Gray & Regan, 1999).

This chain of events (from world -> structure in light -> detection of that structure) supports direct perception because of the lawful nature of the process. Properties of the world project structure into light in a lawful manner that has to do with their physical properties (how they are moving, the way their surface reflects light, etc). This lawful process is described by Gibson's ecological optics. The structure in light these properties cause is higher-order, relational and thus able to be invariant over a transformation such as locomotion or an eye movement. Perceptual learning is then the process of differentiating this structure from the ever-changing optic flow; we are able to learn them because they are, by definition, the only things that do not change over time and thus they hang around long enough to serve as a target for a learning process.

The lawfulness of the process is key: because these variables are the result of the lawful interactions between light and properties of the world, detecting the light is equivalent to perceiving the property of the world. This equivalence is underwritten by the law, and it's important, because we have no access to the world except via perception - there is no 'peeking behind the curtain'. The law allows us to act as if we are back-tracking along the chain of events from detecting structure to perceiving the world. Without a law, that back tracking is not guaranteed to work or to take you back to the right property of the world.

These laws are ecological (Turvey et al, 1981); they don't hold universally, the way the law of gravity does. Their scope is limited to a particular ecological niche; perhaps the niche of surfaces of a medium size moving fairly slowly (the scope of Newtonian mechanics). However, this limited scope does not stop them being laws; quite the contrary, even the laws of physics require details of the range of conditions for which they are true - things break down at a singularity/black hole, for example. Ecological laws are the same, except their scope is typically much smaller than that of the laws of physics. Within their scope, however, the laws can underwrite the directness of perception as effectively as the laws of physics can underwrite the behaviour of particles, galaxies and things in-between.

The laws of ecological optics allow properties of the world to be projected into light in such a way as to create higher-order, specifying variables which are invariant over transformations caused by our motion through the world. This specification relation works to the extent that there is, in fact, a law underwriting the process. It is possible to directly measure such higher-order variables, if you have the right kind of measurement device.

Next, we will start to review some of the empirical and theoretical work that Withagen and Chemero are relying on to challenge the necessity of the law-based specification story.Their basic concern is that this lawful process is too restrictive and that it cannot produce enough information to explain everything we get up to.The question they pose is, can variables in, say, light, still be information if they don't specify
Bingham, G. (1993). Optical flow from eye movement with head immobilized: “Ocular occlusion” beyond the nose Vision Research, 33 (5-6), 777-789 DOI: 10.1016/0042-6989(93)90197-5 Download

Gibson, J. J. (1966). The senses considered as perceptual systems. Boston: Houghton Mifflin.

Gray, R., & Regan, D. (1999). Do monocular time-to-collision estimates necessarily involve perceived distance? Perception, 28 (10), 1257-1264 DOI: 10.1068/p2895

Gibson, J. J. (1979/1986). The ecological approach to visual perception. Hillsdale, NJ: Lawrence Erlbaum Associates.

Runeson, S. (1977). On the possibility of "smart" perceptual mechanisms Scandinavian Journal of Psychology, 18 (1), 172-179 DOI: 10.1111/j.1467-9450.1977.tb00274.x Download 

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 Download


  1. Sounds good to me so far.

    Many(states-of-the-word)-to-one(pattern-of-energy) is clearly a problem, as it is the foundation of most traditional arguments for indirect perception. As I read things, and you seem to agree, somehow that challenge must be overcome, or the direct perception game (at least as Gibson wanted to play it) cannot work.

    However, I never thought one-to-many was problematic. So long as the resultant 'many' has unique properties, one-to-many and one-to-one are (for these purposes) the same thing.

    This relates to it being annoying when Turvey insists on a single 'invariant' vs. a allowing formula specifying the relationship. It is trivial to name a variable, the challenge is finding the relevant relationship.

    1. I agree with the sentiment; but one-to-many is, I think, unlikely given the kind of lawful processes underpinning projection from world property into optic array. A given event produces many invariants, but each only specifies one property of that event. They may be correlated to each other (see about two posts from now when I review some empirical work by Jacobs, Michels and Runeson) but that's a different story.

  2. Revisiting this and thanks again for doing these blogs. Question:

    How might an organism distinguish between a relationship which is lawful and one which has behaved lawfully up to the present but has un-encountered contingencies?