The Visual System, part I (OLI)

 The Visible Spectrum

Whereas other animals rely primarily on hearing, smell, or touch to understand the world around them, human beings rely in large part on vision. A large part of our cerebral cortex is devoted to seeing, and we have substantial visual skills. Seeing begins when light falls on the eyes, initiating the process of transduction. Once this visual information reaches the visual cortex, it is processed by a variety of neurons that detect colors, shapes, and motion, and that create meaningful perceptions out of the incoming stimuli.

The air around us is filled with a sea of electromagnetic energypulses of energy waves that can carry information from place to place. As you can see in the figure below, electromagnetic waves vary in their wavelengththe distance between one wave peak and the next wave peak, with the shortest gamma waves being only a fraction of a millimeter in length and the longest radio waves being hundreds of kilometers long. Humans are blind to almost all of this energy—our eyes detect only the range from about 400 to 700 billionths of a meter, the part of the electromagnetic spectrum known as the visible spectrum.

Only a small fraction of the electromagnetic energy that surrounds us (the visible spectrum) is detectable by the human eye.

Did I get this

The property that differentiates the part of the electromagnetic spectrum that we can see from the part we cannot see is __________________.
wavelength OR intensity OR colour
Wavelength is the peak-to-peak distance of a wave. The human eye is able to detect electromagnetic energy with wavelengths from about 400 to 700 nanometers. This range is called the visible spectrum. As shown in the image above, there are other forms of electromagnetic energy with longer and shorter wavelengths we cannot see, such as radio waves (longer wavelengths) and X-rays (shorter wavelengths).

A wavelength is measured by the _____________ between one wave peak and the next wave peak.
volume OR height OR dictance
Wavelength is a measurement of the distance from a point on one wave peak to the exact same point on the next peak.

Vision: How the Eye Directs Light to the Retina

Light enters the eye through the transparent cornea, passing through the pupil at the center of the iris. The lens adjusts to focus the light on the retina, where it appears upside down and backward. Receptor cells on the retina send information via the optic nerve to the visual cortex.

As you can see in the above figure, light enters the eye through the cornea, a clear covering that protects the eye and begins to focus the incoming light. The light then passes through the pupil, a small opening in the center of the eye. The pupil is surrounded by the iris, the colored part of the eye that controls the size of the pupil by constricting or dilating in response to light intensity. When we enter a dark movie theater on a sunny day, for instance, muscles in the iris open the pupil and allow more light to enter. Complete adaptation to the dark may take up to 20 minutes.

Behind the pupil is the lens, a structure that focuses the incoming light on the retina, the layer of tissue at the back of the eye that contains photoreceptor cells. As our eyes move from near objects to distant objects, a process known as accommodation occurs. Accommodation is the process of changing the curvature of the lens to keep the light entering the eye focused on the retina. Rays from the top of the image strike the bottom of the retina, and vice versa, and rays from the left side of the image strike the right part of the retina, and vice versa, causing the image on the retina to be upside down and backward. Furthermore, the image projected on the retina is flat, and yet our final perception of the image will be three dimensional.

Accommodation is not always perfect, and in some cases the light hitting the retina is a bit out of focus. As you can see in the figure below, when the focus is in front of the retina, we say that the person is nearsighted, and when the focus is behind the retina we say that the person is farsighted. Eyeglasses and contact lenses correct this problem by adding another lens in front of the eye. Laser eye surgery corrects the problem by reshaping the eye's cornea, while another type of surgery involves replacing the eye's own lens.

Nearsightedness and FarsightednessFor people with normal vision (left), the lens properly focuses incoming light on the retina. For people who are nearsighted (center), images from far objects focus too far in front of the retina, whereas for people who are farsighted (right), images from near objects focus too far behind the retina. Eyeglasses solve the problem by adding a secondary, corrective, lens. 

Did I get this

Which structure of the eye determines whether a person has abnormal vision (e.g., is nearsighted or farsighted)?
eye muscles OR retina OR lens OR pupil
If the lens does not focus light in the right spot, the person will have blurry vision.

What area of the eye controls the size of the pupil?
iris OR retina OR lens OR cornea
The iris makes up the eye muscles that help to control the size of the pupil.

What part of the eye contains photoreceptor cells?
pupil OR retina OR lens OR cornea
The retina contains the photoreceptor cells, specifically rods and cones.

Vision: Neural Cells of the Retina

The retina contains layers of neurons specialized to respond to light (see the figure below). As light falls on the retina, it first activates receptor cells known as rods and cones. The activation of these cells then spreads to the bipolar cells and then to the ganglion cells, which gather together and converge, like the strands of a rope, forming the optic nerve. The optic nerve is a collection of millions of ganglion neurons that sends vast amounts of visual information, via the thalamus, to the brain. Because the retina and the optic nerve are active processors and analyzers of visual information, it is not inappropriate to think of these structures as an extension of the brain itself.

The Retina

When light falls on the retina, it creates a photochemical reaction in the rods and cones at the back of the retina. The reactions then continue to the bipolar cells, the ganglion cells, and eventually to the optic nerve.

Rods are visual neurons that specialize in detecting black, white, and gray colors. There are about 120 million rods in each eye. The rods do not provide a lot of detail about the images we see, but because they are highly sensitive to shorter-waved (darker) and weak light, they help us see in dim light, for instance, at night. Because the rods are located primarily around the edges of the retina, they are particularly active in peripheral vision (when you need to see something at night, try looking away from what you want to see). Cones are visual neurons that are specialized in detecting fine detail and colors. The 5 million or so cones in each eye enable us to see in color, but they operate best in bright light. The cones are located primarily in and around the fovea, which is the central point of the retina.

To demonstrate the difference between rods and cones in attention to detail, choose a word in this text and focus on it. Do you notice that the words a few inches to the side seem more blurred? This is because the word you are focusing on strikes the detail-oriented cones, while the words surrounding it strike the less-detail-oriented rods, which are located on the periphery.

Mona Lisa's Smile
Margaret Livingstone found an interesting effect that demonstrates the different processing capacities of the eye’s rods and cones—namely, that the Mona Lisa’s smile, which is widely referred to as “elusive,” is perceived differently depending on how one looks at the painting. Because Leonardo da Vinci painted the smile in low-detail brush strokes, these details are better perceived by our peripheral vision (the rods) than by the cones. Livingstone found that people rated the Mona Lisa as more cheerful when they were instructed to focus on her eyes than they did when they were asked to look directly at her mouth. As Livingstone put it, “She smiles until you look at her mouth, and then it fades, like a dim star that disappears when you look directly at it.”

Vision: From Eye to Brain

As you can see in the figure below, the sensory information received by the retina is relayed through the thalamus to corresponding areas in the visual cortex, which is located in the occipital lobe at the back of the brain. (Hint: You can remember that the occipital lobe processes vision because it starts with the letter O, which is round like an eye.) Although the principle of contralateral control might lead you to expect that the left eye would send information to the right brain hemisphere and vice versa, nature is smarter than that. In fact, the left and right eyes each send information to both the left and the right hemispheres, and the visual cortex processes each of the cues separately and in parallel. This is an adaptational advantage to an organism that loses sight in one eye, because even if only one eye is functional, both hemispheres will still receive input from it.

The Visual PathwaysThe left and right eyes each send information to both the left and the right brain hemispheres.

The visual cortex is made up of specialized neurons that turn the sensations they receive from the optic nerve into meaningful images. Because there are no photoreceptor cells at the place where the optic nerve leaves the retina, a hole or blind spot in our vision is created (see the figure below). When both of our eyes are open, we don’t experience a problem because our eyes are constantly moving, and one eye makes up for what the other eye misses. But the visual system is also designed to deal with this problem if only one eye is open—the visual cortex simply fills in the small hole in our vision with similar patterns from the surrounding areas, and we never notice the difference. The ability of the visual system to cope with the blind spot is another example of how sensation and perception work together to create meaningful experience.

You can get an idea of the extent of your blind spot (the place where the optic nerve leaves the retina) by trying this demonstration. Close your left eye and stare with your right eye at the cross in the diagram. You should be able to see the elephant image to the right (don’t look at it, just notice that it is there). If you can’t see the elephant, move closer or farther away until you can. Now slowly move so that you are closer to the image while you keep looking at the cross. At one distance (probably a foot or so), the elephant will completely disappear from view because its image has fallen on the blind spot.
- Why do we experience this blind spot?
- We have no photoreceptor cells where the optic nerve leaves our eye.

Perception is created in part through the simultaneous action of thousands of feature detector neuronsspecialized neurons, located in the visual cortex, that respond to the strength, angles, shapes, edges, and movements of a visual stimulus. [1] [2] The feature detectors work in parallel, each performing a specialized function. When faced with a red square, for instance, the parallel line feature detectors, the horizontal line feature detectors, and the red color feature detectors all become activated. This activation is then passed on to other parts of the visual cortex where other neurons compare the information supplied by the feature detectors with images stored in memory. Suddenly, in a flash of recognition, the many neurons fire together, creating the single image of the red square that we experience. [3]

The Necker Cube
The Necker cube is an example of how the visual system creates perceptions out of sensations. We do not see a series of lines, but rather a cube. Which cube we see varies depending on the momentary outcome of perceptual processes in the visual cortex.

Some feature detectors are tuned to selectively respond to particularly important objects, for instance, faces, smiles, and other parts of the body. [4] [5] When researchers disrupted face recognition areas of the cortex using the magnetic pulses of transcranial magnetic stimulation (TMS), people were temporarily unable to recognize faces, and yet they were still able to recognize houses. [6] [7]

- Why don’t we have a gap in our vision due to our blind spot?
- There are three reasons. First, our eyes are constantly moving, so they take in information from different locations. Second, one eye takes in information that the other eye misses. Third, our visual cortex fills in what is missing.

The Physical Dimensions of Color

It has been estimated that the human visual system can detect and discriminate among 7 million color variations, [1] but these variations are all created by the combinations of the three primary colors: red, green, and blue. The shade of a color, known as hue, is conveyed by the wavelength of the light that enters the eye (we see shorter wavelengths as more blue and longer wavelengths as more red), and we detect brightness from the intensity or height of the wave (bigger or more intense waves are perceived as brighter).

Light waves with shorter frequencies are perceived as more blue than red; light waves with higher intensity are seen as brighter.


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