Psychology : Semester I : Biological Basis of Behavior

Sections:

Introduction  |   Section 1  |  Section 2  |   Section 3 | Section 4 | Section 5 | Section 6

Psychology : Biological Basis of Behavior : Section Four Sensation Put yourself in this scene. You feel the warm sun,  you are amazed by the beautiful colors of the mountains and trees. The mint candy in your mouth tastes as refreshing as the clean air around you. All of a sudden, the sound of an approaching train can be heard in the distance.  This wondrous experience is brought to you complements of your senses by the process of sensation or  how we receive information from the environment.  The process takes in the physical stimuli from the environment and changes it into the electrical messages that can be understood and interpreted by the brain. This process is called transduction.  We do not take in all the possible stimuli that are in the environment.  First the stimulus must be strong enough to be perceived. The term absolute threshold is used to denote the least amount of stimulus that can be detected 50% of the time. The absolute threshold is the minimum about of stimulus needed for detection.  Anything below the absolute threshold is called subliminal.  The minimum change that can be detected between two stimuli 50% of the time is called the just noticeable difference. If you had ever had a hearing test, they use the just noticeable difference to assess the level of your hearing.  Sensations like change and the receptor neurons will stop firing when we experience one particular sensation  for a long period of time. This is called sensory adaptation.  Think of being in a room with a strong smell of perfume. After some time, you habituate the smell (get used to) and it does not bother you. We will look at the eye as an example of sensation.  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. The Eye The change of physical energy to neural energy: Fun Activity to find your blind spot: Try it yourself A O X   Instructions: Your face should be very close to the screen. Cover right eye and focus the left eye on the X. Now slowly move away from the screen. The O will disappear, while the A, which is further to the left, is still visible. How we perceive colors: There are two theories of how we perceive color. The first theory is called the Young-Helmholz 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 above 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. The Other Senses         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. 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. Flash Cards Next Section!

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