Chapter 6: Events and the information for perceiving events
Thus far, Gibson has been talking about how we perceive objects. But, things in the world often move around, so we'd better be able to perceive things that are extended in time as well. First off, Gibson describes the properties of surfaces that are relevant to events. He then identifies a number of things that can happen to surfaces during events. The nature of these changes will determine what types of optical information might specify particular events (this will be discussed in Chapter 6 Part 2).
Surfaces differ in their resistance to deformation and substances differ in their persistence in a given form. Surfaces can be large (e.g., table) or small (e.g., salt granule) and substances can be solid (e.g., rock), liquid (e.g., water), viscous, viscoelastic, and particulate. Gases aren’t substances, they are components of the medium. Some objects are attached to the ground and some are not. These properties are all relevant to events on an ecological (rather than microscopic or astronomical scale).
A classification of terrestrial events
For the moment we’re assuming that these are terrestrial events and that neither the motion of the sun across the sky nor changes in shadows and light is relevant. What remains are three main types of events that can happen to surfaces: changes in layouts, changes in colour or texture, and change in existence.
Change of layout due to complex forces
When the layout of a surface changes, it is due to some force acting on it to change its shape. Some familiar changes include moving detached objects from one place to the next. This is a translation of an object’s spatial position (this includes falling and spinning objects, as well). Another important type of change involves deformations of surfaces, such as pouring water, molding clay, or a change in the surface of a moving animal (see p. 95 for a list of changes and examples). It doesn’t make sense to reduce these events to interactions between elementary particles of matter. Dropping a rock is an event, itself and it is not understood any better by invoking atoms or molecules.
These events can combine in various ways. Throwing a ball against a wall involves a collision between two objects of different elasticity. Some events cause others (think of Rube Goldberg machines). And, sometimes, lots of things happen at once (think of the workings of a clock).
When an event causes a break in the continuity of a surface it is usually a big deal. The ball hits a window instead of the wall and the glass shatters. The surface of an animal is torn by a predator. At the extreme, a surface completely disintegrates. This is a change in state, rather than layout.
Only some events are reversible. We can walk backward and forward, but we can’t re-make the window pane.
Change of colour and texture due to change in composition
The idea that colour, texture, and shape are independent is kind of misleading. Colour and texture are both consequences of a substances composition, and texture is really a kind of small-scale shape. Chemical reactions change composition, so they also influence colour (chromatic and achromatic) and texture. These changes can indicate changes in affordances. For example, an apple that changes from green to red goes from sour to sweet – it changes in edibility. Chemical changes with animals can lead to physical transformations – adult birds have different feathers than juveniles.
Waxing and waning of a surface due to change in the state of matter
When a substance becomes gaseous it becomes part of the medium – it is desubstantialised. Critically, it isn’t specified in the optic array, so for all intents and purposes, it no longer exists for us. Surfaces going out of or coming into existence are important because they indicate a radical change in affordance. If a pond evaporates (changes from liquid to gas) or freezes (change from liquid to solid) the terrain might now afford crossing by an animal.
Summary: What shall we take as an event?
Thinking about what constitutes an event can be difficult if we get too hung up on the role of time. Regardless of what time is, it is events that we perceive, not the passing of time. Similarly, objects do not populate an otherwise empty space. There is no such thing as empty space and never was. Gibson argues for a distinction between sequential and adjacent events and objects. Events are ordered sequentially – things happen one after another in a particular order that is often non-reversible. Objects are adjacent – they occur next to one another and quite often they can be rearranged.
Events may recur (stepping, sunrises), but they do not recur identically. One sunrise is always a bit different from all others, and not in a silly poetic way. Only some events are reversible. Generally, you can reverse a change in position, but not a change in state. This contrasts with the formal physical laws that (except for thermodynamics) are directionless. But, on an ecological scale this isn’t true – a broken egg isn’t going to be re-made.
Events are nested. An episode of interest can consist of many sub-events and can, itself, be nested within a super-event. Gibson says,
“Some of the best examples of a nested hierarchy of sequential events are found in the human production of events such as speech, music, and the theatre. If we can understand these nested sequences, it may be possible to understand how it could be that in some cases the outcome of an event sequence is implicit at the outset - how the end is present at the beginning – so that it is possible to forsee the end when an observer sees the beginning” (p. 102).
Events have affordances. For example, a fire affords warmth, a looming ball affords collision, and an approaching deer might afford being eaten. Of course, we don't perceive the world, we perceive information about the world. So, in order for events to have meaning we have to be able to perceive specifying information. In the next part of this chapter, Gibson talks about what information might be available in the optic array to specify various types of events.