BBB - Vision Lesson

Learning Targets:

• Identify the properties of visible light and describe the parts of the eye responsible for focusing this light.
• Describe how rods and cones process visual data and outline the pathway this information takes from the eye to the brain.
• Describe the mechanisms by which we perceive color in our environment.
• Locate and explain the role of feature detectors in the visual system.
• Describe how the brain employs parallel processing to create visual perceptions.

Courtesy of the AP psychology course and exam description, effective fall 2024. (n.d.). Links to an external site.

 

Vision

Vision is our most dominant sense and one that many know little about. The organ for the sense of vision is our eyes, which we use to process the physical energy of light. Look at the image of the color spectrum. This illustrates all the various kinds of electromagnetic energy waves. However, as you will notice, humans are only able to see a tiny portion of the entire spectrum.

The waves that we do see are called visible light. The electromagnetic energy from these waves is transduced into neural impulses. Before discussing transduction, let's discuss what makes up a light wave.

A light wave has three properties: hue, saturation, and brightness. The hue varies with the wavelength of light from violet to red. Saturation describes the purity of the wavelength. The wavelength is the distance from the peak of one light wave to the peak of the next. The longer the distance, the closer the color is to the red end of the spectrum. Brightness is the intensity or amplitude of the light wave. The higher the wave of light, the brighter the perception of color.

Color vision is an additive process, which is determined by the wavelength of light an object reflects. When we view something white, it reflects all visible wavelengths with the absorption of none. Black, on the other hand, absorbs all wavelengths of visible light and reflects none. If you are wearing a green T-shirt, the cloth reflects only the wavelengths of light that correspond to that portion of the spectrum while absorbing all others.

Anatomy of the Eye and Transduction

Before discussing transduction, let's look at the eye structure and the role each part plays.

Learn more about the anatomy of the eye in the activity below. 

Once light reaches the retina, transduction begins to take place. The retina is the back of the eye and is the most important structure. The retina comprises several layers of cells, all of which must be passed for transduction to occur.

The first layer activated by light is called rods and cones. Rods are long and thin with blunt ends and outnumber about 20 to 1 cones. They are more sensitive to light and provide us with night vision. They adapt slowly to changes in light, taking as long as thirty minutes. Cones are short and fat in shape with tapered ends. They are sensitive to color wavelengths and adapt quickly to changes. Cones are in the fovea (or focal point of the retina).

Once the rods and cones fire, the information is sent to the second layer of cells, called bipolar cells. Bipolar cells pass the information on to the ganglion cells, which, with their axons, make up our optic nerve. This is how information is sent to the brain. The optic nerve carries the messages to the thalamus, which will then send them to the occipital lobes.

Once the information reaches the occipital lobe in the visual cortex, we then process the image. Our visual cortex contains many specialized neurons that distinguish lines, angles, edges, and movement. The concept of specific nerve cells in the brain responding to unique features of stimuli is called feature detectors. Here, the brain also processes several aspects of a problem simultaneously through parallel processing.

Test your knowledge of the anatomy of the eye below.

Please take a moment to view this video on Feature Detection and Parallel Processing.

 

Vision Problems

If your lens is abnormally shaped it may not focus light onto your retina properly. When this happens, the retina is unable to read images as a whole. Instead, the images are converted into neural impulses which are constructed in the brain. This affects our acuity or ability to distinguish the sharpness of an image.

Myopia (nearsightedness) is a condition in which the light from rays from distant objects is focused in front of the retina. This causes objects that appear up close to be clear, but those far away to be fuzzy. Hyperopia (farsightedness) is when light rays reach the retina before they have produced a focused image. This condition causes us to not be able to focus well up close and often overstrain our eyes. Wearing glasses, Lasik surgery, and contacts can all reshape the cornea to change how light enters the eye, and where it hits, and thus correct many vision problems.

Video Credit: Video from https://www.khanacademy.org/science/health-and-medicine/nervous-system-and-sensory-infor/sight-2014-03-27T18:45:34.237Z/v/feature-detection-and-parallel-processing 

Color Vision

There are two major theories that explain how we see color. Each can explain some aspects of the process, but neither can explain all aspects of color vision.

Young-Helmholtz Trichromatic Theory

The Young-Helmholtz Trichromatic Theory states that we have three varieties of cones in our retinas. Cones that detect red, long wavelengths, cones that detect green, medium wavelengths, and cones that detect blue, short wavelengths. Any given cone will be overly sensitive to one of the wavelengths and slightly to others. Other colors on the spectrum are achieved by stimulating a combination of the cones, creating millions of color combinations.

According to this theory, people who are colorblind lack cone receptor cells for one or more of these primary colors. The most common type of color blindness is red/green. This is when someone cannot discern between the two colors. Their blue-sensitive cones are normal, but their other cones are either red-sensitive or green-sensitive, not both. This creates a situation in which red and green both look the same.

The image below illustrates a common test for colorblindness called Ishihara. A number is projected amongst many different colored dots. What number do you see?

Opponent Process Theory

While the Young-Helmholtz Trichromatic Theory explains many aspects of color vision, it is unable to explain what is called the afterimage effect (a visual experience that occurs after the original source of stimulation is no longer present). Afterimages can, however, be explained by the Opponent Process Theory of Color.

In this theory, there are four basic color combinations divided into two pairs of color-sensitive neurons. Red/green, yellow/blue, and then black/white which absorbs and reflects all light. If one of the colors in the pairing is stimulated, the other becomes dormant. So, if red is stimulated, green will remain dormant. However, due to sensory adaptation, if the color that is being stimulated is removed, we will see its opposite.

Understanding Vision Problems

Vision problems can vary widely among individuals, impacting their ability to see clearly. One common type of vision issue is color blindness, where individuals have difficulty distinguishing between certain colors. Dichromatism is a specific type of color blindness where individuals have trouble distinguishing between two specific colors. On the other hand, monochromatism is a rarer condition where individuals see the world mostly in shades of gray. These conditions can affect everyday tasks and activities, highlighting the importance of regular eye check-ups and understanding different types of vision problems.

Understanding Feature Detectors in the Visual System

Research conducted by David Hubell and Torsten Rundtlund Wiesel led to the discovery of feature detectors, which are nerve cells located in the occipital lobe's visual cortex. These specialized neurons play a crucial role in responding to specific visual features such as edges, lines, angles, and movements within a scene. For example, when you look at a tree, certain feature detectors in your brain become activated, allowing you to perceive the tree's shape, texture, and movement. These feature detectors receive information from individual ganglion cells in the retina and then pass this specific information to other areas of the brain where more complex patterns are processed. This intricate process of deconstructing and reassembling visual images helps us make sense of the world around us.

Understanding Visual Perception Through Parallel Processing

Our brains are exceptional multitaskers, engaging in parallel processing to tackle various sub-dimensions simultaneously. For instance, when you admire a beautiful sunset, your brain is processing the colors, forms, depths, and motions of the scene all at once. This remarkable ability allows us to swiftly create a coherent and detailed visual experience. However, when parts of the brain, like the occipital lobes, which are vital for vision, are damaged, it can lead to intriguing disorders such as prosopagnosia (face blindness) or blindsight, shedding light on the intricate role of parallel processing in creating our visual perceptions.

 

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