You might be proud of your 20/20 vision, even if it is corrected by glasses or contact lenses. 20/20 refers to a measurement associated with a Snellen chart, named after Hermann Snellen (1834–1908), the Dutch ophthalmologist who developed it as a means of assessing visual acuity, the ability to see fine detail; it is the smallest line of letters that a typical person can read from a distance of 20 feet (6 m). But if you dropped into the Birds of Prey Ophthalmologic Office, your visual pride would wither. Hawks, eagles, owls, and other raptors have much greater visual acuity than humans: in many cases, about 5 times greater (meaning that what the normal human can just see from 4 feet (1.2 m) away can be seen by these birds at a distance of 20 feet away), or the equivalent of 20/4 vision. Your sophisticated visual system has evolved to transduce visual energy in the world into neural signals in the brain. Humans have sensory receptors in their eyes that respond to wavelengths of light energy. When we look at people, places, and things, patterns of light and colour give us information about where one surface stops and another begins. The array of light reflected from those surfaces preserves their shapes and enables us to form a mental representation of a scene (Rodieck, 1998). Understanding vision, then, starts with understanding light.

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Visible light is simply the portion of the electromagnetic spectrum that we can see, and it is an extremely small slice (see FIGURE 4.3). You can think about light as waves of energy. Like ocean waves, light waves vary in height and in the distance between their peaks, or wavelengths. There are three properties of light waves, each of which has a physical dimension that produces a corresponding psychological dimension (see TABLE 4.3). In other words, light does not need a human to have the properties it does: Length, amplitude, and purity are properties of the light waves themselves. What humans perceive from those properties are colour, brightness, and saturation.

Eyes have evolved as specialized organs to detect light. FIGURE 4.4 shows the human eye in cross-section. Light that reaches the eyes passes first through a clear, smooth outer tissue called the cornea, which bends the light wave and sends it through the pupil, a hole in the coloured part of the eye. This coloured part is the iris, which is a translucent, doughnut-shaped muscle that controls the size of the pupil and hence the amount of light that can enter the eye.

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Immediately behind the iris, muscles inside the eye control the shape of the lens to bend the light again and focus it onto the retina, light-sensitive tissue lining the back of the eyeball. The muscles change the shape of the lens to focus objects at different distances, making the lens flatter for objects that are far away or rounder for nearby objects. This is called accommodation, the process by which the eye maintains a clear image on the retina. FIGURE 4.5a shows how accommodation works.

If your eyeballs are a little too long or a little too short, the lens will not focus images properly on the retina. If the eyeball is too long, images are focused in front of the retina, leading to nearsightedness (myopia), which is shown in FIGURE 4.5b. If the eyeball is too short, images are focused behind the retina, and the result is farsightedness (hyperopia), as shown in FIGURE 4.5c. Eyeglasses, contact lenses, and surgical procedures can correct either condition. For example, eyeglasses and contacts both provide an additional lens to help focus light more appropriately, and procedures such as LASIK physically reshape the eye’s existing lens.

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How does a wavelength of light become a meaningful image? The retina is the interface between the world of light outside the body and the world of vision inside the central nervous system. Two types of photoreceptor cells in the retina contain light-sensitive pigments that transduce light into neural impulses. Cones detect colour, operate under normal daylight conditions, and allow us to focus on fine detail. Rods become active under low-light conditions for night vision (see FIGURE 4.6).

Rods are much more sensitive photoreceptors than cones, but this sensitivity comes at a cost. Because all rods contain the same photopigment, they provide no information about colour and sense only shades of grey. Think about this the next time you wake up in the middle of the night and make your way to the bathroom for a drink of water. Using only the moonlight from the window to light your way, do you see the room in colour or in shades of grey? Rods and cones differ in several other ways as well, most notably in their numbers. About 120 million rods are distributed more or less evenly around each retina except in the very centre, the fovea, an area of the retina where vision is the clearest and there are no rods at all. The absence of rods in the fovea decreases the sharpness of vision in reduced light, but it can be overcome. For example, when amateur astronomers view dim stars through their telescopes at night, they know to look a little off to the side of the target so that the image will not fall on the rod-free fovea, but on some other part of the retina that contains many highly sensitive rods.

In contrast to rods, each retina contains only about 6 million cones, which are densely packed in the fovea and much more sparsely distributed over the rest of the retina, as you can see in Figure 4.6. This distribution of cones directly affects visual acuity and explains why objects off to the side, in your peripheral vision, are not so clear. The light reflecting from those peripheral objects is less likely to land in the fovea, making the resulting image less clear. The more fine detail encoded and represented in the visual system, the clearer the perceived image. The process is analogous to the quality of photographs taken with a 6-megapixel digital camera versus a 2-megapixel camera. (See the photographs of money below.)

The retina is thick with cells. As seen in Figure 4.6, the photoreceptor cells (rods and cones) form the innermost layer. They are beneath a layer of transparent neurons, the bipolar and retinal ganglion cells. The bipolar cells collect neural signals from the rods and cones and transmit them to the outermost layer of the retina, where neurons called retinal ganglion cells (RGCs) organize the signals and send them to the brain.

The bundled RGC axons—about 1.5 million per eye—form the optic nerve, which leaves the eye through a hole in the retina. Because it contains neither rods nor cones and therefore has no mechanism to sense light, this hole in the retina creates a blind spot, a location in the visual field that produces no sensation on the retina. Try the demonstration in FIGURE 4.7 to find the blind spot in each of your own eyes.

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Sir Isaac Newton pointed out around 1670 that colour is not something “in” light. In fact, colour is nothing but our perception of wavelengths from the spectrum of visible light (see Figure 4.3). We perceive the shortest visible wavelengths as deep purple. As wavelengths increase, the colour perceived changes gradually and continuously to blue, then green, yellow, orange, and, with the longest visible wavelengths, red. This rainbow of hues and accompanying wavelengths is called the visible spectrum, illustrated in FIGURE 4.8.

You will recall that all rods are ideal for low-light vision but bad for distinguishing colours. Cones, by contrast, come in three types; each type is especially sensitive to either red (long-wavelength), green (medium-wavelength), or blue (short-wavelength) light. Red, green, and blue are the primary colours of light; colour perception results from different combinations of the three basic elements in the retina that respond to the wavelengths corresponding to the three primary colours of light. For example, lighting designers add primary colours of light together, such as shining red and green spotlights on a surface to create a yellow light, as shown in FIGURE 4.9. Notice that in the centre of the figure, where the red, green, and blue lights overlap, the surface looks white. This demonstrates that a white surface really is reflecting all visible wavelengths of light.

The fact that three types of cones in the retina respond preferentially to different wavelengths (corresponding to blue, green, or red light) means that the pattern of responding across the three types of cones provides a unique code for each colour. In fact, researchers can “read out” the wavelength of the light entering the eye by working backward from the relative firing rates of the three types of cones (Gegenfurtner & Kiper, 2003). A genetic disorder in which one of the cone types is missing—and, in some very rare cases, two or all three—causes a colour deficiency. This trait is sex-linked, affecting men much more often than women.

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Colour deficiency is often referred to as colour blindness, but in fact, people missing only one type of cone can still distinguish many colours, just not as many as someone who has the full complement of three cone types. You can create a kind of temporary colour deficiency by exploiting the idea of sensory adaptation. Just like the rest of your body, cones need an occasional break too. Staring too long at one colour fatigues the cones that respond to that colour, producing a form of sensory adaptation that results in a colour afterimage. To demonstrate this effect for yourself, follow these instructions for FIGURE 4.10:

Were you puzzled that the yellowish patch produces a bluish afterimage? This result reveals something important about colour perception. The explanation stems from the colour-opponent system, where pairs of visual neurons work in opposition: blue-sensitive cells against yellow-sensitive (as in Figure 4.10) and red-sensitive cells against green-sensitive (Hurvich & Jameson, 1957). The colour-opponent system explains colour aftereffects. When you view a colour, let us say, green, the cones that respond most strongly to green become fatigued over time. Now, when you stare at a white or grey patch, which reflects all the colours equally, the green-sensitive cones respond only weakly compared with the still-fresh red-sensitive cones, which fire strongly. The result? You perceive the patch as tinted red or magenta.

Streams of action potentials containing information encoded by the retina (neural impulses) travel to the brain along the optic nerve. Half of the axons in the optic nerve that leave each eye come from retinal ganglion cells (RGCs) that code information in the right visual field, whereas the other half code information in the left visual field. These two nerve bundles link to the left and right hemispheres of the brain, respectively (see FIGURE 4.11 ). The optic nerve travels from each eye to the lateral geniculate nucleus (LGN), located in the thalamus. As you will recall from the Neuroscience and Behaviour chapter, the thalamus receives inputs from all of the senses except smell. From there, the visual signal travels to the back of the brain, to a location called area V1, the part of the occipital lobe that contains the primary visual cortex. Here the information is systematically mapped into a representation of the visual scene.

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One of the most important functions of vision involves perceiving the shapes of objects; our day-to-day lives would be a mess if we could not distinguish individual shapes from one another. Imagine not being able to reliably differentiate between a warm doughnut with glazed icing and a straight stalk of celery and you will get the idea; breakfast could become a traumatic experience if you could not distinguish shapes. Perceiving shape depends on the location and orientation of an object’s edges. It is not surprising, then, that area V1 is specialized for encoding edge orientation. As you also read in the Neuroscience and Behaviour chapter, neurons in the visual cortex selectively respond to bars and edges in specific orientations in space (Hubel & Wiesel, 1962, 1998). In effect, area V1 contains populations of neurons, each “tuned” to respond to edges oriented at each position in the visual field. This means that some neurons fire when an object in a vertical orientation is perceived, other neurons fire when an object in a horizontal orientation is perceived, still other neurons fire when objects in a diagonal orientation of 45° are perceived, and so on (see FIGURE 4.12). The outcome of the coordinated response of all these feature detectors contributes to a sophisticated visual system that can detect where a doughnut ends and celery begins.

Two functionally distinct pathways, or visual streams, project from the occipital cortex to visual areas in other parts of the brain (see FIGURE 4.13):

How do we know there are two pathways? The most dramatic evidence comes from studying the impairments that result from brain injuries to each of the areas.

For example, a woman known as D.F. suffered permanent damage to a large region of the lateral occipital cortex, an area in the ventral stream (Goodale et al., 1991). Her ability to recognize objects by sight was greatly impaired, although her ability to recognize objects by touch was normal. This suggests that her visual representation of objects, and not her memory for objects, was damaged. D.F.’s brain damage belongs to a category called visual form agnosia, the inability to recognize objects by sight (Goodale & Milner, 1992, 2004). Oddly, although D.F. could not recognize objects visually, she could accurately guide her actions by sight, as demonstrated in FIGURE 4.14. When D.F. was scanned with fMRI, researchers found that she showed normal activation of regions within the dorsal stream during guided movement (James et al., 2003).

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Conversely, other people with brain damage to the parietal lobe, a section of the dorsal stream, have difficulty using vision to guide their reaching and grasping movements (Perenin & Vighetto, 1988). However, their ventral streams are intact, meaning they recognize what objects are.

We can conclude from these two patterns of impairment that the ventral and dorsal visual streams are functionally distinct; it is possible to damage one while leaving the other intact. Still, the two streams must work together during visual perception in order to integrate “what” and “where,” and researchers are starting to examine how they interact. One intriguing possibility is suggested by recent fMRI research indicating that some regions within the dorsal stream are sensitive to properties of an object’s identity, responding differently, for example, to line drawings of the same object in different sizes or viewed from different vantage points (Konen & Kastner, 2008; Sakuraba et al., 2012). This may be what allows the dorsal and ventral streams to exchange information and thus integrate the “what” and “where” (Farivar, 2009; Konen & Kastner, 2008).

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