Experiments on the Perception of Motion in the World and Movement of the Self (Gibson, 1979, Chapter 10)

Chapter 9 was about perceiving persistence in the environment. Chapter 10 is about perceiving change, and the evidence that this also might be direct. 

In contrast to retinal image based theories, Gibson proposes that change is perceived because of 'disturbances of structure in the ambient array' (pg 162). A key point here is that 'Disturbances of structure can specify events without being similar to them' (pg 162); this is a key fact about information. It does not get its meaning because it is somehow a copy of the world; it gets its meaning because of the laws that govern it's creation. 

Gibson reviews some standard methods. The stroboscope presents a series of still images in rapid succession to produce 'apparent' motion, which Gibson reframes as showing motion perception is about change, not about the contents of images. Moving belts of texture and spinning disks viewed through slots reveal, for Gibson, the importance of occluding edges for disturbing an array's continuity. Shadow projections (and the trend towards moving computer displays) are also quite revealing, although they hadn't been fully scientifically laid out, and they are displays (pictures) of motion which introduce weirdnesses Gibson will get to at the end. 

He then mentions the kinetic depth effect, where motion of a flat image can produce the perception of motion. These were typically talked about in terms of sensations on retinas, but Gibson points to the work by Johansson on structure from motion (for example from a point light display). One of the key results is that changes in the array that are coherent are perceived as such; so if dots are moving in such a way that they could be coherent rigid motion in depth or non-rigid/non-coherent in the frontal plane, we perceive the former. Gibson later points to related work (pg. 170-171) on perceiving rigid vs elastic motions; he was very influenced by what Johansson had been doing at Uppsala. 

Experiments with progressive growing or shrinking show we perceive things moving towards or away from us; things looming, not growing, for example (another example of the idea that events and their information are lawfully related but not necessarily similar). This connects to a now-common research paradigm, looking at interception and time to contact perception (e.g. Lee's work on tau), and he later (pg. 173) refers to Runeson's work on perceiving collision events. Gibson also notes here that work on looming has revealed similar perceptions (as measured by behaviour) in a variety of animals, including fiddler crabs who have compound eyes that never form an image. Chambered and compound eye are just two ways to sample optic arrays, not two fundamentally different approaches to vision. 

Progressive transformations (e.g. projecting a shape onto a screen and rotating it) produce perceptions of rigid surfaces rotating and people are very accurate at judging the slant (even though the 'image' is narrowing and widening. This work illustrates the idea that vision is not based on forms, but on invariants over transformation that are themselves formless (again, information is not a copy of the event in the world). Another example is when texture elements lose their 'adjacent order'; two elements that were flowing together might stop doing so if there is an occluding edge between them, or if their surfaces become separated in depth. People again perceive what the information specifies, not the formless transformations in the array. 

Gibson then turns his attention to the visual perception of self-motion (visual kinesthesis). He identifies one of the key features of the optic array, namely that it flows globally during self-motion. Even a small translation of the point of observation (in, say, postural sway) creates global flow specific to that motion, with a focus of expansion where the motion is headed and a focus of contraction at the opposite pole, behind the person. Vision is about ourselves, and not just the external world. Gibson reviews experiments using, for example, moving rooms that create the optical information for self-motion independent of actual motion of the self, and create the perception of self-motion (Lee again did the pioneering work here). 

There are therefore lawful invariant information variables that specify self and other motions, and support the perception of those motions without ambiguity. The perception of change can also be direct.