Mark Moran
Prof. Norma Graham
Psychology W3001
March 12, 1999
Examples of Nervous System Localization
The human nervous system, especially the brain, is highly organized and specialized for receiving, interpreting, and processing various types of input. Our bodies have special neurons and brain structures for each of our senses (sight, sound, taste, smell, touch, etc.). One such system that is found in many animals is hearing.
In order to recognize or distinguish a sound, our brains must receive signals from our ears. At a minimum, the signals must tell us what the frequency (pitch) and the volume of the sound are. All other qualities of sound, such as timbre, attack, and sustain, as well as what note or phoneme might be being made can be interpreted within our brain. But in order to encode the pitch and volume, neurons in the inner ear must be activated. This happens as follows: Sound waves enter the ear canal in the form of vibrating air, in which the vibration frequency is the sound’s frequency and the vibration intensity is the sound’s volume. This air vibrates the ear drum in the same manner, which in turn vibrates the tiny bones (ossicles) of the middle ear. The ossicles amplify the vibrations by thirty times when they vibrate the oval window on the cochlea of the inner ear. Fluid within the cochlea vibrates, causing tiny hairs (cilia) on the basilar membrane to bang against the tectorial membrane, which activates the synapses of auditory nerves that lead into the brain. Hairs at the furthest end of the basilar membrane correlate to the lowest frequency tones. (Gray 236-237).
At the other end of the auditory nerve, structures within the brain process the pattern of firings from the thousands of different cilia-neuron pairs in order to perceive a sound. Other structures within our brain then assemble the in-coming tones into a pattern of notes or phonemes, where higher structures can build the sounds into possible words, music, or what not. By comparing the different patterns from each ear, we are able to determine a sound’s location in space and thus its distal origin. Finally, recent study with guinea pigs and macaque monkeys have revealed that certain neurons in the cortex only respond to certain frequencies, or only to rising and falling pitches, or to brief bursts of sounds. This has been measured by single-unit-recording of various auditory cortical neurons, special stimuli (such as macaque calls to macaques), and permanent brain damage (e.g. guinea pig’s whose cilia auditory nerves were destroyed by continuous rock music exposure). (Gray 238-240).
Another compelling example of localization within our nervous systems is in our drive to sleep. Our desire for sleep seems to be both a function of the time of day (circadian rhythms) and how long we have been awake (stopwatch effect). These two different time keeping systems seem to be differ from each other by a small amount: our internal stopwatch seems to run on a 24 ½ hour clock. This stopwatch gets reset by our circadian clock, which responds to the time of day and the amount of daylight. (Blind people or experimenters living under ground often have sleep problems because their stopwatch does not get correctly reset by the daylight.) (Class Video, Gray 206-212).
Our circadian clock is located primarily in the pineal gland and the supra chiasmitic nucleus of the hypothalamus. It regulates our alertness, drowsiness, temperature sensitivity, and cognitive abilities by releasing different hormones such as melatonin into the blood at different times of day. Our internal stopwatch, which seems to use the same simple structure found in many other animals, is in the substantia nigra in the basal ganglia. The cortex estimates the amount of time gone by and the striatum registers it. This sense of timing is distorted by fever, adrenaline (stress), and drugs such as amphetamines, all of which speed up our internal clock and give us the sensation that time is going by slower. (Class Video, Gray 206-212).
There are many evolutionary advantages to both types of clocks as well as to our stopwatch speeding up under high stress situations. Most of what we know about sleep drives has been learned from animal experiments (e.g. preventing birds from sitting on their nests) as well as human volunteers, who undergo special stimuli such as light deprived caves and gross recording (EEGs) to monitor their mental activity both while tired and while actually sleeping. (Class Video, Gray 206-212).
A third example of neurological specialization in animals is the many structures that are involved with learning, particularly classical and operant conditioning, which have been the most studied and best understood forms of learning. Classical conditioning is the process of learning simple, relatively automatic responses to stimuli that would not normally evoke a response. When Pavlov rang a bell before serving dogs food, the dogs eventually learned or became conditioned to salivate and anticipate food whenever the bell was pressed. This kind of learned response is beneficial since it allows us to learn to fear and run when we hear the rattle of a snake we’ve heard before. Operant conditioning, on the other hand, is the process of learning to perform specific actions quickly in order to obtain a reward. This process feels more like what we intuitively think of as learning, and some people have even described all of human or animal learning in terms of operant conditioning. This powerful tool allows us to appreciate actions that have been successful in the past, such as picking berries from a tree, and to build upon them, perhaps even teaching them to our young. (Class Lecture, Gray 101-119).
Both of these types of learning involve complex areas within the cortex. Conditioning has been studied for many decades, primarily by experimenting on animals such as dogs, rats, and pigeons. Animals have simpler nervous systems and can be ethically studied in labs. The methods used for classical condition are a form of special stimuli. The brain activity is usually not studied, since behaviorists traditionally do not seek to understand the inner workings of the brain. However, some researchers have looked at gross recordings and even PET or fMRI imaging in order to study which structures in the brain seem to change through conditioning. (Class Lecture, Gray 101-119).
These are just a few examples of the many areas of the brain that are highly specialized in humans and other animals. These are also only a few of the techniques that have been used to study the brain. Research continues today on almost every imaginable area and aspect of the nervous system, but it is so complex that several more decades will likely pass before we have a thorough understanding or mapping of the system.
Gray, Peter. Psychology, Third Edition. New York: Worth Publishers, 1999.