5.3 Vision

ANATOMY OF THE VISUAL SYSTEM

   The eye is the major sensory organ involved in vision (figure below). Light waves are transmitted across the cornea and enter the eye through the pupil. The cornea is the transparent covering over the eye. It serves as a barrier between the inner eye and the outside world, and it is involved in focusing light waves that enter the eye. The pupil is the small opening in the eye through which light passes, and the size of the pupil can change as a function of light levels as well as emotional arousal. When light levels are low, the pupil will become dilated, or expanded, to allow more light to enter the eye. When light levels are high, the pupil will constrict, or become smaller, to reduce the amount of light that enters the eye. The pupil’s size is controlled by muscles that are connected to the iris controlled by cranial nerves II and III, and is the colored portion of the eye.

The anatomy of the human eye.

   After passing through the pupil, light crosses the lens, a curved, transparent structure that serves to provide additional focus. The lens is attached to muscles that can change its shape to aid in focusing light that is reflected from near or far objects. In a normal-sighted individual, the lens will focus images perfectly on a small indentation in the back of the eye known as the fovea, which is part of the retina, the light-sensitive lining of the eye. The fovea contains densely packed specialized photoreceptor cells (figure below). These photoreceptor cells, known as cones, are light-detecting cells. Cones are much less sensitive to changes in light compared to rods and make no contribution to night vision. Cones are much faster at detecting changes in the day time environment compared to rods, are very sensitive to acute detail and provide tremendous spatial resolution. They also are directly involved in our ability to perceive color. The retinal center of gaze is specialized for daytime vision as this area of the retina used for visual acuity, the fovea, is densely packed with cone cells. At night or during times of darkness, this center of cones provides little to no help in navigating the environment, which is what ancient astronomers documented by noticing that in order to see faint distant stars they have to look to areas around the star instead of directly at it. Additionally while walking through the forest on a dark night it would be more beneficial to look to one side or the other of an unfamiliar sound in order to visually identify the potential threat as an aggressive predator, an inquisitive Sasquatch, or a friendly fellow adventurer.

While cones are concentrated in the fovea, where images tend to be focused, rods, another type of photoreceptor, are located throughout the remainder of the retina. Rods are specialized photoreceptors that work well in low light conditions, and while they lack the spatial resolution and color function of the cones, they are involved in our vision in dimly lit environments as well as in our perception of movement on the periphery of our visual field. The astronomers discussed above were using their rods by averting the focus of their gaze to an area outside the focus of a distant star or potential threat in terms of the hikers in the nighttime wilderness. Rods as their names suggest are long cylindrical cells made up of stacks of disks which are highly sensitive to light. As the light level in a room or the environment increases, the electrical  response of the rods becomes saturated and the cells cease to response to variations in light intensity. Primates and humans have only one type of rod, but three types of cones (as discussed earlier in terms of the trichromatic perception of color).

The two types of photoreceptors are shown in this image. Cones are colored green and rods are blue.

   We have all experienced the different sensitivities of rods and cones when making the transition from a brightly lit environment to a dimly lit environment. Imagine going to see a blockbuster movie on a clear summer day. As you walk from the brightly lit lobby into the dark theater, you notice that you immediately have difficulty seeing much of anything. After a few minutes, you begin to adjust to the darkness and can see the interior of the theater. In the bright environment, your vision was dominated primarily by cone activity. As you move to the dark environment, rod activity dominates, but there is a delay in transitioning between the phases. If your rods do not transform light into nerve impulses as easily and efficiently as they should, you will have difficulty seeing in dim light, a condition known as night blindness.

Rods and cones are connected (via several interneurons) to retinal ganglion cells. Axons from the retinal ganglion cells converge and exit through the back of the eye to form the optic nerve. The optic nerve carries visual information from the retina to the brain. There is a point in the visual field called the blind spot (also known as the optic disc) where we are not able to perceive or take in any information due to a lack of receptor sites due to this being the location where the bundles of retinal ganglion cells exit the retina. We are not consciously aware of our blind spots for two reasons: First, each eye gets a slightly different view of the visual field; therefore, the blind spots do not overlap. Second, our visual system fills in the blind spot so that although we cannot respond to visual information that occurs in that portion of the visual field, we are also not aware that information is missing.

The optic nerve from each eye merges just below the brain at a point called the optic chiasm. As the figure below shows, the optic chiasm is an X-shaped structure that sits just below the cerebral cortex at the front of the brain. At the point of the optic chiasm, most of the information from the right visual field (which comes from both eyes) is sent to the left side of the brain, and most of the information from the left visual field is sent to the right side of the brain. It is important to notice in the figure below that the outside visual field maintains a ipsilateral (same side) path on its way to the mid-brain where it is processed in the lateral geniculate nucleus (LGN) and the pulvinar, and finally to the primary visual processing areas in the occipital lobe, whereas the inside (closer to the nose) visual field crosses the optic chiasm to the contralateral (opposite) side where its information is processed by the contralateral LGN and pulvinar before being sent to the contralateral primary visual processing area of the occipital lobe.

This illustration shows the optic chiasm at the front of the brain and the pathways to the occipital lobe at the back of the brain, where visual sensations are processed into meaningful perceptions.

   Once inside the brain, visual information is sent via a number of structures to the occipital lobe at the back of the brain for processing. Visual information is processed in through two pathways which can generally be described as the “what” or ventral pathway, and the “where/how” or dorsal pathway. The ventral pathway is involved in object recognition and identification. Activity spreading through ventral pathway travels from the primary visual area in the back of the occipital cortex through the inferior temporal lobe and along the bottom of the temporal cortex toward the anterior inferotemporal cortex. As the activation travels from the primary visual cortex of the occipital lobes to later areas of the inferior temporal cortex, receptive fields of neurons increase in size, latency of activation, and complexity of information they are tuned to response to. The dorsal pathway plays an important role in the classification and conceptualization of elements in the visual world, and also is recruited while judging the significance or relevance of the information in focus (Mishkin, Ungerleider & Macko, 1983). Interruption of this pathhway has been shown to disrupt object discrimination, without  affecting perception of spatial relations between objects (Jeannerod & Jeannerod, 1997). The dorsal pathway by contrast progresses along the central superior (top) portions of the occipital lobe moving through the posterior parietal cortex and is involved with location in space and how one might interact with a particular visual stimulus (Milner & Goodale, 2008; Ungerleider & Haxby, 1994). For example, when you see a ball rolling down the street, the “what pathway” identifies what the object is, and the “where/how pathway” identifies its location or movement in space. The dorsal stream functions to provide a detailed map of the visual environment and also detect and analyze movements. Areas of the parietal cortex where the dorsal stream projects to is essential for the perception and interpretation of spatial relationships between yourself and the environment, maintaining self body image and sense of self space, and the learning of tasks involving coordination of the body in the environment (Paradiso, Bear & Connors, 2007). Interruption of the dorsal pathway creates visual spatial disorientation characterized not only by misperception of the relative positions of spatial landmarks, but also by locating deficits during object-oriented action (Ungerleider & Mishkin, 1982).

Functional areas related to the processing of spatial and semantic information. Based on the Goodale and Milner, 1992.

COLOR AND DEPTH PERCEPTION

   We do not see the world in black and white; neither do we see it as two-dimensional (2-D) or flat (just height and width, no depth). Let’s look at how color vision works and how we perceive three dimensions (height, width, and depth).

Stare at the white dot for 30–60 seconds and then move your eyes to a blank piece of white paper. What do you see? This is known as a negative afterimage, and it provides empirical support for the opponent-process theory of color vision.

 

   But these two theories—the trichromatic theory of color vision and the opponent-process theory—are not mutually exclusive. Opponent process works to create hues based on the amount of information receptors are translating. Opponent processing of color vision has been demonstrated in various species other than human such as monkeys (De Valois, 1960), and birds (Maturana & Varela, 1982) allowing for a better understanding of the appearance of the visual world for other species. Cones are maximally responsive to three different wavelengths that represent red, blue, and green (Trichromatic signaling), and once the signal moves from the cones through the bipolar cells and into the brain by way of the ganglion bundles in the optic nerve, the cells respond in a way consistent with opponent-process theory (Land, 1959; Kaiser, 1997).

Watch this video to learn about color vision in more detail.

Depth Perception

   Our ability to perceive spatial relationships in three-dimensional (3-D) space is known as depth perception. With depth perception, we can describe things as being in front, behind, above, below, or to the side of other things. Gibson and Walk (1960) demonstrated children as young as 6 months are hesitant in being coaxed to crawl over a visual cliff. This suggests humans develop depth perception while the vision system matures and therefore depth perception is not an acquired or learned capability.

Our world is three-dimensional, so it makes sense that our mental representation of the world has three-dimensional properties. We use a variety of cues in a visual scene to establish our sense of depth. Some of these are binocular cues, which means that they rely on the use of both eyes. One example of a binocular depth cue is binocular disparity, the slightly different view of the world that each of our eyes receives. To experience this slightly different view, do this simple exercise: extend your arm fully and extend one of your fingers and focus on that finger. Now, close your left eye without moving your head, then open your left eye and close your right eye without moving your head. You will notice that your finger seems to shift as you alternate between the two eyes because of the slightly different view each eye has of your finger. A 3-D movie works on the same principle: the special glasses you wear allow the two slightly different images projected onto the screen to be seen separately by your left and your right eye. As your brain processes these images, you have the illusion that the leaping animal or running person is coming right toward you.

 

 

Celebrities don 3D glasses during the Grammys in 2010.

 

Another binocular cue, convergence is the brains interpretation of eye muscle contraction, leading to the perception of closer objects when both eyes are focusing on stimuli closer to the nose compared to stimuli farther away. The perception of distance in contrast from the perception of depth as discussed above is based on experiences with objects and stimuli we have encountered in our lives to where we learn associations between estimated differences and differences in eye muscle tension.

Although we rely on binocular cues to experience depth in our 3-D world, we can also perceive depth in 2-D arrays. Think about all the paintings and photographs you have seen. Generally, you pick up on depth in these images even though the visual stimulus is 2-D. When we do this, we are relying on a number of monocular cues, or cues that require only one eye. If you think you can’t see depth with one eye, note that you don’t bump into things when using only one eye while walking—and, in fact, we have more monocular cues than binocular cues.

An example of a monocular cue would be what is known as linear perspective. Linear perspective refers to the fact that we perceive depth when we see two parallel lines that seem to converge in an image (figure below). Some other monocular depth cues are interposition, the partial overlap of objects, and the relative size and closeness of images to the horizon.

 

We perceive depth in a two-dimensional figure like this one through the use of monocular cues like linear perspective, like the parallel lines converging as the road narrows in the distance. (credit: Marc Dalmulder)