Introduction to Human Vision
The human visual system gives the human body the ability to see our physical environment. The system requires communication between its major sensory organ - the eye - and the core of the central nervous system - the brain - to interpret external stimuli into sight images. Humans have evolved into highly visual creatures, so we have developed an incredibly complex sensory organ system.
Sensory Organs and the Process of Sight
Vision depends mainly on one sensory organ—the eye. Eye constructions vary in complexity depending on the needs of the organism. The human eye is one of the most complicated structures, and it requires many components to allow our advanced visual capabilities. The eye has three major layers:
- the sclera, which maintains, protects, and supports the shape of the eye and includes the cornea
- the choroid, which provides oxygen and nourishment to the eye and includes the pupil, iris, and lens
- the retina, which allows us to piece images together and includes cones and rods (Figure 1).
The easiest way to understand the component pieces of the eye and how they contribute to human sight is to follow the normal processing of an image. All vision is based on the perception of electromagnetic rays. These rays, in the form of light, must pass through the cornea, which focuses the rays. They then enter the eye through the pupil, the black aperture at the front of the eye. The pupil acts as a gatekeeper, allowing as much or as little light to enter as is necessary to see an image properly. The pigmented area around the pupil is the iris. Along with supplying a person’s eye color, the iris is responsible for acting as the pupil’s stop, or sphincter. Two layers of muscles contract or dilate the pupil to change the amount of light that enters the eye. Behind the pupil is the lens, similar in shape and characteristics to a camera lens. Together with the cornea, the lens adjusts the focal length of the image being seen onto the back of the eye, the retina. Visual reception occurs at the retina where photoreceptor cells called cones and rods give an image color and shadow. The image is transduced into neural impulses and then transferred through the optic nerve to the brain for processing. The visual cortex in the brain interprets the image to extract form, meaning, memory and context.
Color Vision and Depth Perception
Human beings are capable of highly complex vision that allows us to perceive colors and depth in intricate detail. Visual stimulus transduction happens in the retina. Photoreceptor cells found in this region have the specialized capability of phototransduction. There are two types of these cells: rods, which are responsible for scotopic or night vision, and cones, which are responsible for photopic or daytime vision (Figure 2).
Color vision is a critical component of human vision, and plays an important role in both perception and communication. Color sensors are found within cones which respond to relatively broad color bands in the three basic regions of red, green, and blue (RGB). Any colors in between these three are perceived as different linear combinations of RGB. The eye is much more sensitive to overall light and color intensity rather than changes in the color itself. Colors have three attributes: brightness based on luminance and reflectivity, saturation based on the amount of white present, and hue based on color combinations. Sophisticated combinations of these receptors signals are transduced into chemical and electrical signals that are sent to the brain for the dynamic process of color perception.
Depth perception refers to our ability to see the world in three dimensions. With this skill, we are able to interact with the physical world by accurately gauging the distance to a given object. While depth perception is often attributed to binocular vision (vision from two eyes), it also relies heavily on monocular cues (cues from only one eye) to function properly. These cues range from the convergence of our eyes and accommodation of the lens to optical flow and motion.