![]() The optic nerve is a collection of millions of ganglion neurons that sends vast amounts of visual information, via the thalamus, to the brain. 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. As light falls on the retina, it first activates receptor cells known as rods and cones. The retina contains layers of neurons specialized to respond to light (see Figure 5.9, “The Retina with Its Specialized Cells”). Eyeglasses solve the problem by adding a secondary, corrective lens. ![]() For people who are nearsighted (centre), 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. For people with normal vision (left), the lens properly focuses incoming light on the retina. Figure 5.8 Normal, Nearsighted, and Farsighted Eyes. Eyeglasses and contact lenses correct this problem by adding another lens in front of the eye, and laser eye surgery corrects the problem by reshaping the eye’s own lens. Receptor cells on the retina send information via the optic nerve to the visual cortex.Īccommodation is not always perfect (Figure 5.8) if 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. The lens adjusts to focus the light on the retina, where it appears upside down and backward. Light enters the eye through the transparent cornea, passing through the pupil at the centre of the iris. Furthermore, the image projected on the retina is flat, and yet our final perception of the image will be three dimensional. 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. Visual accommodation is the process of changing the curvature of the lens to keep the light entering the eye focused on the retina. As our eyes move from near objects to distant objects, a process known as visual accommodation occurs. Complete adaptation to the dark may take up to 20 minutes.īehind 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. When we enter a dark movie theatre on a sunny day, for instance, muscles in the iris open the pupil and allow more light to enter. The pupil is surrounded by the iris, the coloured part of the eye that controls the size of the pupil by constricting or dilating in response to light intensity. The light then passes through the pupil, a small opening in the centre of the eye. The Sensing Eye and the Perceiving Visual CortexĪs you can see in Figure 5.7, “Anatomy of the Human Eye,” light enters the eye through the cornea, a clear covering that protects the eye and begins to focus the incoming light. 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. As you can see in Figure 5.6, “The Electromagnetic Spectrum,” electromagnetic waves vary in their wavelength - the 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. The air around us is filled with a sea of electromagnetic energy: pulses of energy waves that can carry information from place to place. Once this visual information reaches the visual cortex, it is processed by a variety of neurons that detect colours, shapes, and motion, and that create meaningful perceptions out of the incoming stimuli. Seeing begins when light falls on the eyes, initiating the process of transduction. A large part of our cerebral cortex is devoted to seeing, and we have substantial visual skills. Whereas other animals rely primarily on hearing, smell, or touch to understand the world around them, human beings rely in large part on vision. Summarize how the eye and the visual cortex work together to sense and perceive the visual stimuli in the environment, including processing colours, shape, depth, and motion.Identify the key structures of the eye and the role they play in vision.
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