“If all these considerations are correct, then the appearance of eyes really could have ignited the Cambrian explosion. And if that’s the case, then the evolution of the eye must certainly number among the most dramatic and important events in the whole history of life on earth.”
Nick Lane, Life Ascending: The Ten Great Inventions of Evolution
Evolution’s Big Bang
Before the Cambrian explosion, most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. Almost all present animal families (phyla) appeared during this period.
Andrew Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time, hunting and evading were both close-range affairs — smell, vibration, and touch were the only senses used. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce.
“In the land of the blind one-eyed king”
When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines, and similar defenses may also have evolved in response to vision.
Evolution of the eye
The earliest predecessors of the eye were photoreceptor proteins that sense light called “eyespots”. Eyespots can sense only ambient brightness: they can distinguish light from dark, for synchronization of circadian rhythms. They are insufficient for vision, as they cannot distinguish shapes or determine the direction light is coming from. However, eyespots can assist organisms to move towards the light for photosynthesis.
Visual pigments are located in the brains of more complex organisms and are thought to have a role in synchronizing spawning with lunar cycles. By detecting the subtle changes in night-time illumination, organisms could synchronize the release of sperm and eggs to maximize the probability of fertilization.
The basic light-processing unit of eyes is the photoreceptor cell, a specialized cell containing two types of molecules in a membrane: a light-sensitive protein (opsin), surrounding a pigment that distinguishes colors. These cells permit animals to gain only a very basic sense of the direction and intensity of light. Developing an optical system is apparently much more difficult:
Only six of the thirty-some phyla can discriminate the direction of light to within a few degrees. However, these phyla account for 96% of living species.
In humans, and a number of other mammals light enters the eye through the cornea which then the lens focuses light onto the light-sensitive membrane in the back of the eye, called the retina. The retina serves as a transducer for the conversion of light into neuronal signals. These signals are transmitted by the optic nerve, to the visual cortex and the superior colliculus.
Transduction is the process through which energy from environmental stimuli is converted to neural activity. The retina contains photoreceptors with different sensitivities call rods and cones . The cones are responsible for color perception and are of three distinct types labelled red, green and blue. Rods, are responsible for the perception of objects in low light.
In the retina, the photoreceptors synapse onto ganglion cells which will then conduct action potentials to the brain. These axons originate from different ganglion cell types:
- M cells, that are sensitive to depth, and can rapidly adapt to a stimulus.
- P cells, sensitive to color and shape.
- K cells, sensitive to color and indifferent to shape or depth.
To reconstruct the visual world, this information will be processed in parallel by the left and right halves of the brain. That is, the right side of primary visual cortex deals with the left half of the field of view, and similarly for the left brain.
Neurology of Vision.
Neurons in the visual cortex fire action potentials when visual stimuli appear within their receptive field. Any neuron responds best to a subset of given stimuli, this could be as simple as a vertical line or as complex as a certain face. The deeper the cell lies in the Visual Cortex, the more complex the stimuli it responds to.
There are two primary neural pathways processing visual information:
- The Ventral Stream or “What Pathway”, is associated with form recognition and object representation. It also deals with long-term memory.
- The Dorsal Stream or “How Pathway”, is associated with motion, representation of object locations, and control of the eyes and arms.
Both streams begin at the primary visual cortex (V1) it receives sensory inputs and responds to simple stimuli which can be best described as “edge detection”. More complex stimuli are handled by the remaining layers of the Visual Cortex:
- V2 cells are tuned to respond to other simple properties such as orientation, spatial frequency, and color. It also plays an important role on the conversion of short-term object memories into long-term memories.
- V3 cells respond to coherent motion and contain the first representation of the entire visual field.
- V4 is tuned for recognition on object features of intermediate complexity, like simple geometric shapes. Selective attention in this layer can change firing rates by about 20%.
- V5 cells are tuned to the detection of speed and direction of moving stimuli. It plays a major role in the integration of local motion signals into global percepts, and also guides some of the eye movements.
- V6 responds to visual stimuli associated with self-motion, it is likely to be important for the control of skeletomotor activity, including postural reactions and reaching movements towards objects.
Layers V3 and V4 are more related to the Ventral Stream while layers V5 and V6 are related to the Dorsal Stram.
The average number of neurons in the adult human primary visual cortex in each hemisphere has been estimated at around 140 million.