Chapter 4: The Psychology of Vision

What you will learn: How the interaction of colour, shape, size, proximity etc, produces an object. Why chameleons are difficult to spot during the day, why we prefer watching high-definition movies, and why a person with one eye will not be able to watch a 3-D movie. 

What aspects of an object are important in the seeing process? Building on the previous chapter, Crick dives deeper into the underlying processes that enable us to see objects as objects, including how far they are from us, as well as their colour, shape, and size.  The visual system must not only perceive an object, but also perceive it in depth. What is depth? It is seeing an object with as much detail as pragmatically and functionally possible. It is the ability of the visual system to discern an object's shape, distance, motion, colour, etc. In this chapter, Crick describes the underlying processes of the visual system that are responsible for extracting depth information from an object. From careful observation, and from previous works of the Gestaltist psychologists, he reveals specific clues during the seeing process that point to these underlying processes.

What are these clues? The German Gestaltists psychologists, which we have encountered in the previous chapters, and to which Crick builds upon some of their works, had identified interactions that are almost universally common to all visual processing. They labelled these interactions the Laws of Perception, and these include; the law of proximity, the law of similarity, the law of good continuation, and the law of good closure. These laws describe how the brain groups things together, and how it assigns identity to objects using various visual parameters. 

So, how does the brain extract depth? In a similar way that a group of people would slaughter a cow. Two people are tasked with stabilizing the hind limbs, while two more people hold the front legs, and one or two more are tasked with removing the skin and getting into the actual meat of the matter with a blade. Each person has a specific role that they are particularly designated to perform, and they perform it together (Figure 4.1). Similarly, different parts of the visual system, simultaneously work together to interpret different aspects of the visual information coming from the retina. Detecting shape requires a part of the brain that is good at extracting outlines (Figure 4.2C,D), detecting color requires photoreceptors that are sensitive to different wavelengths of light (Figure 4.2A), while detecting motion requires a part of the brain that is activated when there is movement (Figure 4.2A,B). The presence of these dedicated processes can be derived from specific clues of the seeing process (Figure 4.2). 

In a sense, the visual system can be seen to be a pragmatic application of the philosophical principle of Occam’z razor. Whereas Occam’s razor requires the simplest and fewest assumptions to be the most accurate in solving a conceptual problem, the visual system takes the simplest, most regular and symmetrical interpretation of an object as the most accurate. The Gestaltists called this phenomenom Prägnanz which roughly means “goodness”. 

Looking at Figure 4.2A, the brain finds it easy to interpret the shape on the far left side as a cone with a sphere on its apex. However, distorting the outline makes it difficult to group the different intermittent lines as belonging to the same object, making the 'true' identity of the object more elusive. This distortion and subsequent loss of object identification gives us a clue to the existence of an underlying process in the brain that picks out the outlines of an object. This particular process must have the ability to recognise fine details and less fine details. This can be appreciated more when watching any video that is low-resolution e.g. old movies are a good example since they were made by cameras that are not as good at separating the pixels. The lower the resolution, the less obvious the outlines and therefore the more unclear the object. Generally, the more defined the outline, the clearer the object, which is part of the reason why we prefer to watch movies on high definition screens than low definition ones. 

In addition to picking up outlines, a different part of the brain must be good at detecting motion. It is more difficult to spot someone at night unless they are moving. This ability is appreciated in the wild, where predators rely on it to spot the movement of their prey, and prey rely on it to detect the presence of predators. Motion makes it easier for the brain to separate figure from background, which is one of the most important operations in vision. On the other hand, differences in colour, and colour gradient, enable better identification of objects. This gives clues to an undelying process that identifies colour. The brighter the object, the easier it is to identify, because the figure easily separates from ground. In a black and white film, for example, this process is made difficult as the figure and background are easily conflated, making if difficult to identify objects.

Interestingly, the biggest source of depth information comes from having two eyes. The processes, working in parallel to give depth to the information of the images falling on the retina, are performed by each eye. However, with two eyes, we can extract depth of a 2-dimensional image into 3-dimensions, allowing us to perceive the world as such. Our ability to discern distance, size, motion, and shape become even more enhanced. With two eyes, we gain perspective clues which enable us to discern more accurately the size of objects in front of us as well as their distance. Binocular vision enables us to gain clues into occlusion–when a nearer object partly hides one farther away. We also can use gradient of texture to enhance depth information and get information on distance. This is seen in paintings e.g., google the painting “Paris Street, Rainy Day" (1877) by Gustave Caillebotte. In the painting, the painter uses different lengths of the cobblestones on the ground to give the impression of them stretching away from the viewer. 

After Crick describes the various clues that the visual system takes advantage of to extract depth information and identify images, he hopes the reader will have gained a deeper appreciation and understanding of the complexity of visual psychology. From the above information, seeing–an ability we rely on daily, but take for granted–is not a simple process. In the next chapter, he touches on two other aspects of seeing; attention, and very short-term memory. How is all this related to the emergence of consciousness? Let us find out more in the next chapter as Crick continues taking us on a psychological and neuroscientific tour to the soul.

Blogger's thoughts: This was the longest and most technically dense chapter so far. However, learning about the Laws of Perception was interesting. It made me think whether or not the brain would be able to see an object if all previous memory of seeing were lost.  Then this thought-experiment led me to something I had encountered in David Eagleman's book, Incognito: The Secret Lives of the Brain. In one of the chapters of that book, Eagleman recounts the story of a blind man who, after being cured of his blindness, was able to see for the first time in his life. But because his brain had never encountered any visual images prior to this amazing event, what the man saw for the first time might have not appeared to him as distinct objects but rather a confusing array of bright light with different colours bombarding the retina from all directions. As the story goes, the man's expression was not that of a person excited to finally see and identify objects in the world, but of a confused person not knowing what his visual system was relaying to him. With time, the brain learns to group things together and to separate figure from ground, and requires memory to do this. In this chapter Crick has emphasised the existence and workings of the processes working behind the scenes of vision. The take-away here is that we can derive clues from observing behaviour, and gain insight into the existence of the underlying processes required for the expression of some aspects of said behaviour. What this has to do with consciousness is still not self-evident. Nonetheless, let us follow the breadcrumb trail as it leads us into the Chapter 5. 

←Chapter 1: Introduction - Review

←Chapter 2: The General Nature of Consciousness - Review

←Chapter 3: Seeing - Review