Mark Moran
Prof. Norma Graham
Psychology W3001
March 12, 1999
The Evolution of Two Visual Systems
The most important sensory input for humans and other primates is vision. Vision is so crucial to humans’ view of themselves that many people say they would rather give up any other sense (hearing, taste, smell, touch, etc.) than lose their sight. Neuroscience research has revealed that a huge area (25-40%) of the cortex and the majority of neurological signals within the brain relate to vision, so much so that some scientists consider the eyes an extension of the brain. Humans and other primates have forward-facing, stereoscopic color vision. This precise vision is what allows us to appreciate movies, television, sporting events, and most other daily activities. The forward-facing aspect of our vision links us with other predators in our ability to focus and zone-in on a moving target, and it distinguishes us from the more defensive vision of prey animals who have eyes on the sides of their head. Two crucial aspects of human vision are color recognition and spatial orientation processing. (Class Lecture).
Like language in humans, color vision is possible in primates and birds because of anatomical and neural structures that facilitate it. Our eyes and brains have special structures for processing colors, which is an example of biological preparedness. This allows the species-specific behavior of how animals with color vision obtain food. Both primates and birds rely on their vision in order to catch or gather food, e.g. our ability to easily distinguish red berries in a green bush or for an eagle to spot a brown mouse in a yellow field. Color recognition is especially important for birds and primates because our other senses like smell and hearing are less useful to us in catching food than they are in color-blind animals. (Indeed, color-blind animals tend to have much better hearing or smell than primates and birds do.) This is an ultimate explanation rather than a proximate explanation of why color vision might have involved in birds and primates. (Gray 75-77, 247-252).
Color vision involves so many complex structures that it is surely not the result of chance or genetic drift. The retina contains three distinct types of cones for processing long, medium, and short wavelengths of light. Each unique ratio of wavelengths represents a different color. (People who are missing one or two of the types of cones are unable to distinguish certain wavelengths. This is often caused by a recessive defect on the X-chromosome, which is why color-blindness occurs much more frequently in men, who only have one X-chromosome.) These three types of cones feed into ganglion cells, which are specialized to different opponent-processes, or groups of opposite colors. Neurons that process different colors farther up in the cerebral cortex also show this opponent-process distinction, which is why opposing colors like yellow and blue or red and green cancel each other out. (Gray 249-251).
Another hallmark of our visual system is the ability to recognize the orientation and texture of objects. Indeed, the primary evolutionary purpose of our visual system is to recognize and identify objects, whether they are potential food or predators. Color vision is helpful to the extent that it separates objects from their backgrounds. Even more fundamental, however, is our ability to distinguish the objects’ borders and thus shape based on different levels of brightness. We perceive objects by their edges or contours, which are the lines formed between different levels of brightness. Along with color vision, this ability almost certainly evolved to facilitate finding food and avoiding predators. (Gray 252-253).
The system which enables this to happen is as complicated as color vision and is even more vital. Within the retina, cones and rods are arranged in concentric circles or ovals which respond to varying light intensities. These impulses feed into horizontal cells, then bipolar cells, then ganglion cells, and then into the brain. By comparing different levels of on and off neural impulses, the visual cortex is able to determine the presence of an edge. The visual system exaggerates levels of contrast to further improve our ability to distinguish objects’ borders. Additionally, within the visual cortex, there are many different receptor fields which respond to a range of different contour orientations. All of these different edge orientations is what allows us to identify the shapes of objects as well as their rotation and position. Furthermore, additional fields identify the various spatial frequencies within a given surface, which allow us to see the texture of objects as well. (Gray 252-257).
Color vision and oriented receptive fields are thus two critical aspects of our visual system that allow us to recognize and interpret the world we see around us. These systems involve incredibly complex structures that have proven nearly impossible to replicate in a computer, and which are clearly the result of millions of years of evolution. Because evolution has no foresight, every variation or step along the way toward more complex structures must have had some immediate benefit for the organism. With vision, every little bit of information is advantageous to natural selection, so one can imagine how and why our visual systems kept improving and evolving. (Gray 69).
Gray, Peter. Psychology, Third Edition. New York: Worth Publishers, 1999.