Sensation
Sensation is the process of receiving energies from the external environment and transforming the energies into neural energy. The brain gives meaning to sensation through perception.
Perception
Perception is the process of organizing and interpreting sensory information so that it makes sense.
In other words, sensations are the first stages in functioning of the senses to receive stimuli from the environment and perception is a higher brain function about interpreting events and objects in the world.
Sensation
We will look at the eye as an example of sensation because that is the sense that we know the most about.
We will go through the parts and see how the physical energies from our environment are changed to the neural energies needed for interpretation by the brain.
Light waves enter the eye and first pass through the cornea, which is a transparent, protective covering over the eye.
Next it passes through an opening called the pupil. The pupil can change size because it is controlled by a pigmented muscle called the iris.
The pupil gets smaller when the light is bright and larger in the dark to allow the best amount of light as possible to reach the retina.
Next the image passes through the lens that focuses the image on the retina.
The retina is a layer on the back of the eye that contains the rod and cones cells. The image that gets to the retina is upside down.
The retina is the place of transduction, or where physical energies are changed to neural energies.
Rods are responsible for the detection of black and white colors. The majority of the rods are located around the periphery of the retina and they allow us to see at night.
The cones are concentrated in the middle of the retina, the fovea, and detect color.
Rods and cones synapse with neural tissues called bipolar cells and they synapse with ganglion cells that come together and form the optic nerve.
At the point where the optic nerve leaves the eye there are no rods and cones and it is called the blind spot.
Color Theory
There are two theories of how we perceive color.
The first theory is called the Young-Helmholtz trichromatic color theory.
This theory states we have three types of cones: red, blue and green. All colors are then a combination of the three.
The second theory is Edward Herring's opponent-process theory. This theory states that colors are analyzed in opponent colors (red/green and blue/yellow and in rods white/black). The thalamus turns on and off the colors.
You can see red at one point in the retina but not green at the same point at the same time. Color vision is thought today to be a combination of both theories.
Stare hard at the flag below and then look at a white background. You should see an after-image in red, white and blue. This is an example of the opponent - process theory.
Watch More than an Image to learn more about how your eyes receive visual information and your brain makes sense of it.
Sensation
Three of the other senses, hearing, taste and touch undergo transduction as does the eye.
The ear has the cochlea, the tongue has the taste buds and the skin has sensory nerves. Whereas other senses rely mostly on a single stimulus, pain results from many different stimuli.
For example, pain can be caused by intense pressure, heat, loud noises, and so on.
There are also two kinds of pain sensations; a sharp pain that you feel immediately after an injury or the dull pain that comes later.
Sometimes the sensation of pain can be lessened by shifting our attention away from the pain impulses or by sending a competing signal which creates a sort of competition between non-pain and pain impulses.
This may be a reason that athletes are able to continue playing until the game is over even though they are injured.
The only sense that does not undergo transduction is smell. It goes directly to the brain, by an organ called the olfactory bulb, to get interpreted.
There are pictures a person may look at for a minute, look away, and then describe. Yet, the same person may look at the same picture again and see different things in both viewings.
The image has many brilliant colors and patterns. One may see an image of one person at first glance and then see images of several faces during a second look.
Perception
Every time our senses get engaged, millions of pieces of information are transferred to the brain. The brain has to reassemble and make sense of the millions of pieces of information that it receives. It is similar to the brain putting together a puzzle that has millions of tiny pieces.
This is the process of perception and occurs from all of our senses every second that we experience our world.
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