Reading Group - Gibson (1979) Chapter 4

In this chapter Gibson really steps up the pace, so hold onto your hat. First, he shows that perception can’t be based on sensation. Then he shows that we can’t see light. Finally, he shows that we don’t see images. Here we go...

Chapter 4: The relationship between stimulation and stimulus information

The first three chapters describe the environment (see here, here, and here). Now, Gibson describes the information available in the environment. For visual perception, this information comes from ecological optics. Ecological optics is not the same thing as either geometric or physiological optics.

The distinction between luminous and illuminated bodies

Luminous bodies emit light. The sun, a desk lamp, and a fire are luminous. Non-luminous bodies do not emit light, and most things in our environment are non-luminous. We can often see non-luminous bodies because they are illuminated by ambient light. We can tell the difference between luminous and illuminated bodies.

The distinction between radiation and illumination

Radiant light travels from the sun to the earth in parallel rays. Some of this light is scattered in the atmosphere. When the light hits the ground it is further broken up by scatter reflection. This scatter-reflected light bounces between the sky and the ground until the ray is completely dispersed. It is because of this property that things are illuminated even if they are not directly under the sun. For instance, a room with windows is illuminated during the day. So, radiant light travels to earth in nice parallel rays, and illumination occurs when these rays are scattered through their interaction with the ground, etc. Ambient light is a consequence of illuminated light.

The distinction between radiant light and ambient light

Light rays scatter so completely and quickly that light rays completely fill the air. “Each point in the air [is] a point of intersection of rays coming from all directions. It would follow that light is ambient at every point” (p. 51). The amount of energy from light in a space changes all the time as a consequence of changes in the source, like when the sun goes behind a cloud. However, the structure in the reverberation of light is preserved through these changes. This structure is a more reliable source of information about the environment than the absolute flow of energy.

“Radiant light from a point source is not different in different directions; ambient light at a point is different in different directions. Radiant light has no structure; ambient light has structure. Radiant light is propagated; ambient light is not, it is simply there. Radiant light comes from atoms and returns to atoms; ambient light depends on an environment of surfaces. Radiant light is energy; ambient light can be information” (p. 51).

The structuring of ambient light

The ambient optic array refers to ambient light with structure. The structure mostly comes from differences in ambient light intensity coming from different directions. Without structure there is no information. For instance, during a white-out, light reflects between tightly packed particles of snow to create more or less homogeneous illumination. Being in a situation like this is stupidly disorienting. Structure rules.

Stimulation and stimulus information

If an eye (hopefully attached to an animal or else that’s just creepy) intercepts some ambient light, then photons will stimulate photoreceptors in the retina. In a white-out, the stimulation will be uniform and the observer won’t be able to perceive anything. For perception to occur there must be structure in the light. This means that stimulation doesn’t lead to perception. In people, our two mobile eyes, set in a turn-able head, on top of a movable body comprise a perceptual system (the visual system). This system is activated in the presence of stimulus information. This construal of the visual system essentially abandons the old idea of sensation. Sensation (e.g., stimulation of photoreceptors) doesn’t tell us anything about the world. Sensation cannot be the basis of perception.

Do we ever see light as such?

The sensation-based theories of perception assert that all we can ever see is light. Light is all that gets into the eye (in the form of rays) via the retina, so this must be what we see. This would mean that we don’t see the environment directly, that we don’t see objects or surfaces. The other possibility is that we never see light, which sounds crazy. But, consider, if we see a single point of light in a dark room we don’t think “My, look at that point of light,” we think “Oh, there must be a light source somewhere in the distance.” We don’t see a beam of light in the air unless it illuminates particles in the medium. We don’t see light that fills empty space – we only see things that are illuminated. So, Gibson concludes that “all we ever see is the environment or facts about the environment, never photons or waves or radiant energy” (p. 55). The stimulation of receptors in the retina is not the data of perception – it is not sufficient for perception.

The concept of the stimulus as an application of energy

People use the word “stimuli” to mean all sorts of things. The original physiological sense of the word meant something that fires a nerve cell, or triggers a reflex arc. So, something that caused a photoreceptor to fire, that caused a sensation, would be a stimulus. Psychology generalised this meaning to include something that causes a response, as well as a sensation. However, we respond to lots of things, which means that we can call pretty much anything a stimulus. This makes it a pretty useless designation. For clarity, Gibson uses only the physiological meaning of “stimuli.” By this definition, any type of energy that causes a response, in a receptor, say, is a stimulus. Photoreceptors are very sensitive to light energy, but other types of energy will cause a response as well. This is also true for chemorecptors (in the nose) and mechanoreceptors (in the skin). So, stimulation cannot tell us anything about the source of the energy – it carries no information about the world. Stimulation is also temporary, and cells quickly adapt to sensation and decrease their firing rate. But, our experience tells us that objects in the world persist. This means that objects cannot be specified by sensations because sensations do not persist. Also, according to the physiological definition used by Gibson, objects can’t be stimuli (although this is how psychologists tend to use the term). Photons are stimuli, not the objects from which they reflect. For visual perception, the task is to find invariants in the changing structure of the optic flow.

The orthodox theory of the retinal image

The story goes like this: We form images of objects in the back of the eye, like a camera. All objects reflect light from all directions – in effect, they are covered in tiny sources of light. These rays enter the eye and are focused onto the back of the eye. This set of focus points is the retinal image, which connects object and image, point by point. So, psychologists tend to think of the stimulation of photoreceptors in the retina as an image projected on a screen. Gibson points out an interesting entailment of this conception: If the stimulation of receptors constitutes an image, then who looks at that image? Why would an image inside our eye be useful? Presumably images are meant to be seen with eyes, and putting the thing to be seen inside the eye itself seems like a dumb idea. Another, slightly more refined conception, is to think of an image as being transmitted, bit by bit, as signals along the optic nerve fibres. However, this gets back to the problem of sensation-based perception. If images are transferred as signals, bit by bit, then they would simply reflect the correspondence between brightness of things in the world and firing intensity or between colour and wavelength. This isn’t very useful and it places an enormous burden on the brain to infer what these signals could be about. And, the whole idea of the brain inferring things about signals entails something like a little man waiting to receive information from the outside world. In other words, this conception implies a separation between mind and body. Gibson’s main point is that we don’t need to think of anything being transmitted along the optic nerve during perception. The proper view is to construe vision as a perceptual system engaged in a circular process – not a one way transmission. What does this system do? It “registers invariants in the structure of ambient light”, that’s what (p. 61).

A demonstration that the retinal image is not necessary for vision

Insects have a completely different type of eye than mammals. I’m sure everyone’s heard of the compound eye, but it’s worth taking a second to appreciate how different it is from our type of eye. Unlike our eyes, which have a lens, and a sensory surface (which many people interpret as acting like a screen), insect eyes have tightly packed receptive tubes, all pointing in different directions, like in this close-up picture of a fly eye. It is simply not possible to conceive of these eyes producing anything like an image. First, there is no “screen” to project anything on to. Second, all the points would end up jumbled. Despite this, insects see quite well. What their eyes are suited for is taking lots of samples of ambient light – precisely the kind of thing you’d want to do if visual perception entailed registering invariant structure in ambient light.

The concept of optical information

It’s important to distinguish perceptual information from the common use of the word “information”. We usually think that information is something conveyed via some medium. For instance, we can convey words via speech, books, email, etc. This type of information has senders (who transmit the signal) and receivers (who, um, receive the signal). Perceptual information is not conveyed and it doesn’t necessitate a sender and a receiver.  The world isn’t trying to communicate with us – “the world is specified in the structure of the light that reaches us, but it is entirely up to us to perceive it” (p. 63).

Got it? We don’t see images or light, and sensation isn’t the basis of perception. By focusing on perceptual information, Gibson removes an enormous burden from the brain, which no longer is expected to make complex inferences on the basis of under-specified sensations. He also sidesteps the pesky mind-body dualism that emerges from sensation-based accounts of perception. The next chapter goes into detail about the ambient optic array.