Human vision and the system that supports it is one of the most complex constructions of any animal capable of vision.
The main sensory organ of the visual system is the eye, which takes in the physical stimulus of light rays and transduces them into electrical and chemical signals that can be used by the brain to interpret physical images.
The eye has three main layers: the sclera which includes the cornea; the choroid which includes the pupil, iris, and lens; and the retina which includes receptor cells called rods and cones.
The human visual system is capable of complex color perception which is initiated by cones in the retina and completed by impulse integration in the brain.
Depth perception is our ability to see in three dimensions, and relies on both binocular (two-eye) and monocular (one-eye) cues.
TERMS
retina
The thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain. photoreceptor A specialized neuron able to detect, and react to light. Includes both cones (daytime and color) and rods (nighttime). phototransduction The process whereby the various bodies in the retina convert light into electrical signals
FIGURES
Image of Cones and Rods fig. 2
Cones and Rods
The retina is made up of cones and rods that perceived color and shadow in visual images.
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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
