7.3: Vision

Learning Objectives

• Identify the key structures of the eye and the role they play in vision.
• Compare and contrast the anatomy and functions of the rods and cones.
• Describe the function of the lens and the visual deficiencies that result from its dysfunction.

Overview

Humans rely largely on vision and seeing the world around them through sight. Most non human organisms rely more on the other senses to interpret the world around them. Research indicates that a large part of the cerebral cortex in humans is dedicated to vision. As the information reaches the visual cortex in the brain, multiple neurons identify shapes, colors, and motions. When light falls on the eyes, sensory receptors begin the process of transduction. As this process occurs, individuals begin to perceive the environment around them.

Anatomy of the Eye and it's Function

Think about entering a dark theater or room if you have been outside on a sunny day; often times it takes time to adjust to the change of light intensity in the environment. Specific parts of the non-neural, accessory structure for vision play an important role in this process of adjustment and just about all other aspects of processing the incoming visual stimuli.

The visual accessory structure is the eyeball. The muscles in the iris of the eye adjust to allow more light to enter the pupil (the round opening in the center of the iris). The iris of the eye is the colored part of the eye that constricts and dilates the pupil to adjust for different light intensities that we may encounter throughout the day. The light from objects then gets magnified by the lens and "projects" onto the back part of the eye known as the retina. Photoreceptors located in the retina are the specialized sensory neurons for vision that help distinguish the different characteristics of different objects.

The types of photoreceptors in the human eye include rods and cones; rods are typically more prevalent than cones. Rods give us sensitivity under dim lighting conditions and allow us to see the contrast of objects against a background, even at night. Cones allow us to see fine details in bright light and give us the ability to distinguish color. Cones are tightly packed around the fovea (the small central region of the retina) and more sparsely elsewhere. Rods populate the periphery (the region surrounding the fovea) and are almost absent from the fovea. As organisms move about their environment, the information from light reflected off objects both near and far is encoded by the brain in order to identify colors, shapes, and motion (see Figure \(\PageIndex{1}\)). More detail on all of these elements and processes will be examined in subsequent sections.

Figure \(\PageIndex{1}\): 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. https://open.lib.umn.edu/intropsyc/c...04_s02_s01_f02

"Normal" Vision, Nearsightness, and Farsightness

Why do some many people need to correct their vision with glasses or contacts? Although much of the rest of the sections on vision in this chapter relate to neural processes, the reason for this common problem is due to issues occurring in the accessory structure for vision. Most of the time someone needs visual correction due to issues with how well certain muscles of the eye are able to adjust the structure and function of the lens . Because of this, the lens is one of the most critical features of the accessory structure in terms of producing a clear image from the very beginning of visual processing. Problems with the muscles around the lens or the lens itself can lead to what is commonly referred to as either nearsightedness or farsightedness. See Figure \(\PageIndex{2}\) below for more details.

Figure \(\PageIndex{2}\) For 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. https://open.lib.umn.edu/intropsyc/chapter/4-2-seeing/#stangor-ch04_s02_s01_f02

Attribution

• Vision sections adapted by Isaias Hernandez from "NOBA-Vision"; https://nobaproject.com/modules/vision
• Vision sections adapted by Isaias Hernandez from "Psychology-Penn State"; https://psu.pb.unizin.org/intropsych...eption-vision/
• Vision sections adapted by Isaias Hernandez from "Introduction to Psychology-University of Minnesota"; https://open.lib.umn.edu/intropsyc/c...04_s02_s01_f02
• All sections modified and adapted by Alan Keys, Ph. D., Dept. of Psychology, Sacramento City College, Sacramento, CA.

References

https://www.nei.nih.gov/learn-about-...olor-blindness