Sight and Visual Perception

The eye is the sense organ responsible for vision. The structure of the eye is critical to understanding how light is converted into a neural signal. The innermost layer of the wall of the eye, the retina, contains several layers, including pigmented cells, photoreceptors, and neurons. Light reflects off the surface of an object and enters the eye through the cornea, a transparent outer cover on the eye that allows light to enter. Next, light passes through the pupil, an opening in the eye formed by the iris that changes diameter to control the amount of light entering the eye. The iris is the ring of colored muscle tissue surrounding the pupil that controls the size of the pupil. Finally, light passes through the lens, a curved transparent structure found behind the iris that bends light to focus on the retina. The point of clearest focus in the retina is called the fovea.

The retina is made of many layers, including blood vessels and cells to aid in waste removal. Sensing light involves three neural cell layers. The innermost layer, which is actually the first layer to process incoming light, is made up of photoreceptors. Photoreceptors are sensory receptors that respond to light and are one of two types: cones and rods. A cone is a photoreceptor that responds best in high-light conditions to detect color and detail. They are concentrated within the fovea of the retina. A rod is a photoreceptor that responds well in low-light conditions, used to detect shape and motion. They are concentrated in the retina outside of the fovea. The middle layer includes bipolar cells, which relay signals from photoreceptors to the outermost retinal layer. The outermost layer is made up of retinal ganglion cells, a type of neuron that receives inputs from photoreceptors.

Light sensation begins when light reflected by an object passes through the open pupil. The cornea and lens help focus that light on the retina. Next, photoreceptors (rods and cones) convert that light energy into a neural signal (transduction). The neural signal is relayed from photoreceptors to the bipolar cells and then to the retinal ganglion cells. The axons of the retinal ganglion cells make up the optic nerve, a cranial nerve that transmits visual information from the retina to the brain for processing.

White light is made up of many different wavelengths of light. Each wavelength corresponds to a specific color. Objects in the environment absorb some wavelengths of light and reflect others. Information about the color of an object is determined by the wavelength of the light reflected off the object and entering the eye. A block reflecting only blue wavelengths of light will appear blue. An object that reflects all wavelengths of light will appear white. One that absorbs all wavelengths of light will appear black.

Humans are able to detect color because different photoreceptors in the eye respond to different wavelengths of light. Short wavelength cones (S-cones) detect short wavelengths corresponding to purple to blue light. Medium wavelength cones (M-cones) detect green to yellow light, and long wavelength cones (L-cones) detect orange to red light. There is some overlap in the wavelengths of light detected by different types of cones. Color perception depends on the relative activity of each type of cone. This is known as the trichromatic theory of color processing, or the Young-Helmholtz theory. Rods respond best to medium wavelengths of light but are not involved in color processing. Color perception also depends on opponent processing. Opponent processing theory proposes that cones are linked to form three color pairs that oppose one another: blue/yellow, red/green, and black/white. When one of the colors in the cone pair is stimulated, the opposing color is inhibited. Thus humans cannot perceive anything as reddish-green because only one member of the pair can be active.
The opponent processing theory is good at explaining color afterimages. For example, staring at a red circle for a long time causes red-sensing cones to undergo sensory adaptation (a decrease in response after continuous stimulation). Looking away from the red circle to a white wall will create the perception of a green circle. This occurs because red-sensing cones have reduced their activity while green-sensing cones have increased their activity. This phenomenon occurs for complex images as well. If people stare at an American flag drawn with opposing colors (turquoise, yellow, and black), when they shift their gaze to a blank space they will see the flag in the usual red, white, and blue. Both the trichromatic theory and opponent processing theory are needed to fully explain color vision.

Vision information is relayed via the optic nerve to the thalamus, or sensory relay, in the brain. From there information is relayed to a region of the occipital lobe called the primary visual cortex, or V1, the first brain region where visual perception occurs. Area V1 is specialized for detecting edges of objects. In this area there are neurons (nerve cells) that respond selectively to edges of a specific orientation. The coordinated response of these different neurons leads to the perception of the edges, and thus shapes, of different objects. From V1, information is sent to the parietal lobe, a brain region that helps process the spatial location of objects. Simultaneously, information is sent to the temporal lobe, a brain region that aids in recognition or identification of objects. Neurons in the middle temporal lobe (called MT or V5) track an object's change in position over time. These neurons also respond to speed and direction of motion.
Gestalt principles explain the human tendency to perceive whole objects as opposed to the individual parts of objects. Gestalt is German for "unified whole." These principles are a set of top-down processing principles that guide visual perception. For example, the principle of closure describes the tendency to fill in missing sections of a visual scene or object. The principle of continuity describes the tendency to group together edges or curves that have similar orientations. The principles of similarity and proximity describe the tendency to group similar objects together or to group objects together that are close to one another spatially. Lastly, the figure-ground relationship is important for determining how a person discriminates an object from its background. Figure-ground optical illusions are dark and light images that can be perceived in two different ways, depending on whether the light or dark areas are seen as the figure. The most famous example of this illusion involves a white vase shape, bounded by two dark faces in profile. Viewers can shift perspective between seeing either the white vase or the faces but cannot see both simultaneously. Depth and size of objects is perceived as a result of monocular cues, or information available about depth when a scene is viewed with only one eye. For example, the brain interprets the relative size of objects focused on the retina to determine which object is closer or farther away. An object with a larger relative size is interpreted as closer to the viewer than an object with a smaller relative size. Interposition is another monocular cue. If one object is partially blocking another, the blocked object is farther away than the object blocking it. Because of the eyes' separate physical locations on the face, each has a slightly different view of the same objects in the environment. Binocular disparity, the difference in retinal image location between the two eyes, aids depth perception.