Psychology for Dummies (13 page)
Read Psychology for Dummies Online
Authors: Adam Cash
Tags: #Psychology, #General, #Body; Mind & Spirit, #Spirituality
When light travels from a light bulb or sound waves travel from a radio speaker, our sensing devices, or
accessory structures,
intercept them. Our eyes, ears, skin, noses, and mouths are called
accessory structures
because they provide us “access” to our environment. After the energy reaches our sensory structures, it has to get inside our brains somehow. We don’t have light, sound waves, and heat waves bouncing around inside of our heads. Well, at least I don’t. So, how do they get inside?
First, keep in mind that our brains use their own form of energy. In Chapter 3, I discuss the specific type of energy in our brains called
electrochemical energy.
This energy is how our neurons communicate with each other and operate. So, in order for our brains to understand the various forms of energy that our sense organs receive, each form of energy has to undergo a transformation process called
transduction
that turns it into electrochemical, or
neural,
energy. This process is called
transduction.
The presence of specific types of cells,
receptors,
in each of the sensory systems makes transduction possible. Each sensory system has its own type of receptor cell. After the receptor cells
transduce,
or convert, the environmental energy, a neural signal travels along a
sensory nerve,
taking the information to the part of the brain that is involved in processing and analyzing the information.
Does the music we listen to or human voices have only one tone? Does the light we see only come in one color? Of course not! Each of these sensory experiences, or
stimuli,
is made up of a complex array of wavelengths of light, frequencies of sound, intensities of smells and tastes, and so on. Never fear however, our sensory systems are on the job. Through the process of
coding and representation
our brains are able to capture the complexity of the environmental stimuli that we encounter.
The complexity of a stimulus is captured by the translation of its different features into a specific pattern of neural activity. The theory of
specific nerve energies
states that each sensory system provides information for only one sense, no matter how nerves are stimulated. In other words, there are specific parts of the brain that will always label the stimulation they receive as light or sound.
Psychologists have worked with neurosurgeons to conduct experiments with patients who for other reasons needed a portion of their skulls removed, exposing their brains. Then, they took an electrode and zapped specific parts of these exposed brains with a little jolt of electricity. When they applied this shock, a weird thing happened. The people in the experiment said, “I can hear chickens squawking.” If they zapped the part of the brain involving taste, a person might have said, “I can taste tomato soup. Mmm, that’s good!” How is this possible? When a particular part of the brain is stimulated, the brain thinks that it’s receiving sound or taste information from the sense organ, even if it’s not. So, specific sensory systems are wired into specific brain regions permitting the brain to know the difference between hearing a sound and seeing a light.
Different aspects of a stimulus are coded in the brain depending on which neurons are activated and the pattern of neuron activation. If neurons in the visual system are activated, the brain senses light. If the pattern of neural activation differs, the brain senses different wavelengths or intensities of light such as sunlight versus candlelight. The end of the sensory trail leads to a neural
representation
of the sensation in a specific region of the brain where we finally hear the music or see the colors.
Sight is one of the most important senses that we have. Although the other senses are also important, our ability to see is extremely critical for getting along in our world. In this section, I chronicle a little journey — the journey light takes through the eye and on into the brain, completing our sensation of light.
Our journey begins with
electromagnetic radiation,
more commonly known as light. Visible light occupies wavelengths between 400 and 750 nanometers. I remember from physics class that light travels in waves. The intensity of light is calculated by measuring the size of the waves, and its frequency is measured by how many peaks of a wave pass a particular point within a specific period of time. Wavelength is important because different wavelengths allow us to experience different colors.
1. Light enters our eye through the
cornea.
2. Light passes through the
pupil.
3. The
lens
focuses the light onto the
retina.
4. Light energy is converted into neural energy — light
transduction.
In order to understand the process of light transduction, a closer look at the retina is necessary. The retina is located on the back lining of the eyeball and contains some special cells called
photoreceptors
that are responsible for transduction. These cells contain chemicals called
photopigments
that are broken apart when the photons of light traveling in the lightwave make contact with them. This event starts a chemical reaction that tells the cell to fire a signal to the
optic nerve.
The signal then travels to the
visual cortex
of the brain, the part of the brain responsible for analyzing visual stimuli.
So, light is transformed into neural energy by literally breaking up chemicals in the retina, which then has the effect of causing a neural signal to occur. Next, I have to introduce an important “wrinkle” in this whole process. It’s all about two types of photoreceptors,
rods
and
cones.
Rods contain a chemical called
rhodopsin,
which is very light-sensitive. This chemical reacts to very low-intensity light and helps with our peripheral vision.
Cones contain a different chemical called
iodopsin,
which responds to different wavelengths of light and is involved in seeing color.
SynesthesiaSome people claim to hear light and see sounds. Other people report that they can feel colors.
Synesthesia
is the name of an ability that certain people have to sense one (or more) forms of energy with a sensory system other than the one typically used. This phenomenon is estimated to affect about one in every 2,000 people. Scientists have suspected that this experience is a product of some of the brain’s wiring being crossed. Baron-Cohen have hypothesized that this is made possible by the presence of extra connections in the brain that allow for otherwise separate sensory systems in the brain to interact. Whatever the cause, it sounds kind of cool! I’d love to be able to see the music when I dance because I sure can’t feel it!
Some people are colorblind to particular shades of blues, greens, and reds. This condition means that they have a hard time sensing the specific wavelengths of light associated with those colors. They lack a photo pigment that is sensitive to those wavelengths. Fortunately most of us aren’t colorblind. We get to see the world in all its rainbow-colored glory.
There are two basic theories of color vision, the
trichromatic theory
and the
opponent-process theory
.
The trichromatic theory is really basic. The idea is that the retina contains three different types of cones (photoreceptors) that each respond to different wavelengths of light, and these provide our experience of different colors.
Short-wavelength
cones respond to light around 440 nanometers, or blue light.
Medium-wavelength
cones respond to light around 530 nanometers, or green light.
Long-wavelength
cones respond to light around 560 nanometers, or red light. When each cone system is partially activated, we see variations of these three basic colors that give us colors like aquamarine and orange. But the main idea is that our experience of all colors originates from these three basic cone inputs.
The opponent-process theory of color vision states that the brain contains different types of neurons that respond differently to different colors. The idea is that these cells will fire more — when compared to their baseline, or background, level of firing — and when stimulated by one type of light and fire less when stimulated by another. If I’m looking at red, the specialized red cells will increase their firing rate. When I’m looking at green, those red cells will slow down their firing rate, while the rate of the green cells increases. There are other “cell sets” for yellow and blue as well.
This theory explains something called the
negative-afterimage effect
images in your “mind’s eye” that are different colors than the actual image. The most popular example uses a U.S. flag that is colored black where the white stars are, green where the red stripes are, and yellow where the blue background is. After looking at the image for a while, someone can close their eyes and see the flag in its real colors. That’s because the cells being stimulated by black, green, and yellow light are now recovering from that stimulation and are “seeing” white, red, and blue light instead. Try it. Stare at a 1-x-1-inch yellow square for about 30 seconds and then look at a white sheet of paper. You should see a blue square.
How can we tell how far something is away from us just by looking at it? Some people are good at eyeballing distances. Personally, I need a tape measure, ruler, land-surveyor, and a global positioning satellite to figure distances. Depth and distance are calculated by our visual systems using two inputs:
monocular cues
and
binocular cues.
Monocular cues are simple; we know that some things are bigger than other things. Dogs are bigger than mice. Cars are bigger than dogs. Houses are bigger than cars. Because we know these things from experience, whenever we see a mouse’s image on our retina that is bigger than the image of the dog in the same scene, we know that the mouse is closer to us than the dog. If we see a dog that’s bigger than a car, the dog is closer. The rule is that things that cast bigger images on our retinas are assumed to be closer. Artists use this rule all the time when they want to depict a three-dimensional scene on a two-dimensional canvas.
Binocular cues are interesting and also a little weird. Remember the Cyclopes from the Sinbad movies? He only had one eye, and according to binocular vision rules, he would have had a hard time figuring out distances. Binocular distance cues depend on having two eyes to provide information to the brain.
•
Convergence
is one such binocular cue and refers to information provided by the muscles of the eyes to the brain to help calculate distances. When your eyes are pointing inward, toward the nose, the brain knows that you’re looking at something close to you. When your eyes are pointing outward, the brain knows that you’re looking at an object that is farther away.
•
Stereoscopic
vision is the second type of binocular cue. Try this real quickly. Make a viewing frame with your hands by joining your thumbs at the tips and extending your index fingers up, while keeping the rest of your fingers folded. Then, close one eye and focus on an object around you. Frame the object in the middle of the box. Now, close that eye and open the other. What happened? The object should have moved. This happens because of stereoscopic vision. Each of our eyes gives us a slightly different angle on the same image because they are set apart. Our brains judge distance using these different angles by calculating the difference between the two images.