Tutorial 13: The Human Eye

Intro | Cornea | Optic Disk and Blind Spot | Fovea | Iris | Lens | Muscles (to move eye) | Muscles (to adjust lens) | Optic Nerve | Pupil | Retina

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test

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The faculty of seeing or sight is of great importance to humans. Vision not only provides the early warning system for threats to our survival, but also enriches our existence by describing features of the world according to texture, color, context, and depth. At a very simple level, the visual system consists of the eye and a long chain of neural connections that extend from the retinal receptors through the visual pathway to the primary visual cortex of the cerebrum. The pattern of excitation within the retina is processed by the neuronal machinery of the visual system to form a representation of the external world in our brain. This central representation is formed in a series of interacting regions within the occipital lobe that process different aspects of the input in parallel.

The eye provides the optics to produce an image of the external world upon the retina. It is adapted to convey the electromagnetic waves of light energy to the retinal receptors. Our perception of brightness is derived from the amplitude of a light source, whereas our perception of color is derived from the wavelength of a light source. The perception of purity or richness of color is affected by the number of wavelengths present in a source of light. The human eye is capable of encoding only a portion of the light spectrum, ranging from 380 (violet) to 760 (red) nanometers.

Figure 13 illustrates the main structures of the human eye and this tutorial provides a brief description of the functions and characteristics of each. Greater detail on the structure and function of the retina will be provided with Figure 14. Whereas the visual pathways and important processing points are discussed in association with Figures 15 and 16.

The light radiating from objects in the external world enters the eye through the clear and transparent cornea. The pupil is an opening at the front of the eye that regulates the amount of light allowed to pass into the eye. The iris is the colored ring of muscle that surrounds the pupil and allows adjustment of the size of the pupil opening, working much like the diaphragm in a camera. With contraction of the iris, the pupil opening dilates (becomes larger), letting in more light but creating a less sharper image. When the iris relaxes, the pupil opening constricts (becomes smaller), letting in less light but creating a sharper image. In conditions of dim lighting, the pupils will dilate to let in more light. Although the images are less clear, more of the environment will remain visible.


The human eye is a fragile extension of the brain, encased and protected by the bones of the skull. The eye has three coats. The cornea forms the clear and transparent front portion of the outer layer, through which light passes. The remaining outer layer of the eye is formed by the tough, white sclera that protects the delicate receptors within. The second or middle layer is called the choroid and contains the blood vessels that supply the eye with nutrients. The innermost layer contains the specialized receptor cells of the retina.

The placement of the human eyes in the front of the head allows us to judge distance. Due to the separation of the eyes, each is able to see further around an object in front of it than the opposite eye. The brain receives two slightly distinct images of the same object and superimposes them to derive distance or depth. This is called stereoscopic vision.

When light reaches the eye, it is converted from stimulus to sensation to perception via three steps, reception, transduction, and coding. Reception is the process where the physical stimulus or photons are absorbed by the receptor cells. Transduction involves the conversion of the energy into a chemical reaction resulting in the initiation of action potentials or nerve impulses. Finally, the spatial and temporal characteristics of nerve impulses are coded to represent the stimulus in a meaningful way.

Suggestions for further study


Barlow, R.B.,Jr., (1990, April). What the brain tells the eye. Scientific American, 262(4), 90-95.

Koretz, J.F., et al., (1988, July). How the human eye focuses. Scientific American, 259(1), 92-99.

Leutwyler, K. (1994, March). Prosthetic vision. Scientific American, 270(3), 108.

Ramachandran, V.S. (1992) Blind spots. Scientific American, 86-91.

Rock, I., (1981, March). Anorthoscopic perception. Scientific American, 244(3), 145-153.

Stryer, L. (1987, July). The molecules of visual excitation. Scientific American, 257(1), 42-50.


(Seeing More Than Your Eye Does)
Bryn Mawr University, Serendipity Project - A demo of perceptual filling of the blind spot.

(Eye and Retina)
Washington University School of Medicine

(Low Vision)
University of Minnesota, Ligge Vision Laboratory