Despite its complexity, the brain maintains some key actions. The brain senses, processes incoming signals, stores elements of this information and input, and acts on the incoming information.
How information enters, and is processed by, your brain. Image courtesy of Bruce D. Perry, M.D., Ph.D.
In order to keep us alive, the brain uses a set of sensory organs (eyes, ears, nose, tongue, skin) to tell us some of what is going on in the outside world. Remember, we can't hear like a dog, smell like a bear, or see like a hawk. In fact, when compared to the rest of the animal kingdom, human senses seem quite limited.
- Our ears hear sound only within a certain range
- Our eyes see light in the visual range but not infrared or ultraviolet light
- Our perception of touch requires a certain level of pressure
- Our sense of smell only helps us if a scent is powerful or nearby
Despite these limitations, our powerful human brains can still integrate the information from all of our different senses and use it to create an internal representation of the external world.
Everything we experience is filtered by our senses. All sensory signals (sound, sight, taste, touch) initiate a cascade of processes in the brain that alter brain structure and function. This process of creating some internal representation of the external world (i.e., information) depends upon the pattern, intensity, and frequency of neuronal activity produced by sensing, processing, and storing signals.
Experience creates a processing template through which all new input is filtered. The more frequently a certain pattern of neural activation occurs, the more indelible the memory becomes. All living organisms have mechanisms to sense and respond to changes in their environments. These mechanisms respond continually and are designed to keep our body's systems in a state of equilibrium or homeostasis.
We have sensory mechanisms to tell the brain what is going on in the internal world of the body. For example, we have special sensory apparatuses that tell the brain the concentration of oxygen in the blood. Other systems sense the concentration of salts (e.g., too much salt causes a sensation of thirst) or gases such as carbon dioxide. These internal sensory mechanisms, like the five senses for the external world, help the brain continuously monitor and act to maintain life.
Once our sensory apparatuses have translated physical or chemical information from the outside (or inside) world into neuronal activity, this set of signals travels up into the brain to be processed. Sensory information from the external environment and the internal environment enters the central nervous system at the level of the brainstem and midbrain.
As this primary sensory input arrives, it is matched against previously stored patterns of activation. If the pattern is unknown, or is associated with previous threat, the brain will activate a set of responses that are designed to help promote survival. (This alarm response is at the heart of the post-traumatic symptoms seen in so many maltreated children.)
The human brain, divided into its four interconnected areas. Image courtesy of Bruce D. Perry, M.D., Ph.D.
Your Brain is Conservative
Throughout life, the brain is making memories that correspond to various sights, sounds, smells, tastes, and movements. It creates templates of experience against which all future experience is matched. In this regard, the brain is a conservative organ. It does not like to be surprised. All unknown or unfamiliar environmental cues are judged to be threatening until proven otherwise.
Therefore, what you recognize as safe and comfortable has only become so through your experience. Something in your safe and comfortable present moment matches the associated, stored memories of previous safe, pleasing, or rewarding experiences. The same theory applies to feelings of terror or threat.
Sensory integration (putting the sight, sound, smell, and feel of an event together) is a crucial step in healthy development. There can be disruption of this capacity by even minor "timing" errors. If the signals coming from the neural systems responsible for hearing do not get into the thalamus and cortex in a synchronous way, there can be confusion, disorganization, and abnormal functioning. (See sidebar.)
At each level of brain organization, the incoming signal (called afferent) is categorized. When the brain compares incoming information to previously stored patterns, mistakes sometimes occur. For the Vietnam vet, a loud firecracker can induce a startled response and anxiety even though he knows it is only a firecracker. The incoming loud sound is categorized in the brainstem as being previously associated with threat and danger even before the signal can get to the cortex.
At each level of processing, a categorization process takes place. This immediate, localized processing can be crucial for survival. Your brainstem and spinal cord will tell you to withdraw your hand from a fire even before your cortex knows that you have been burned.
Another key step in processing experience is organizing information. Because the brain cannot possibly create a unique neural imprint or pattern of change to store every element of every experience, the brain stores "template" patterns based upon the first set of organizing experiences. All future incoming input is matched against these stored templates and, if sufficiently different from the original pattern, the brain will create a memory reflecting that difference.
Inherent to the processing of information coming into the brain is the capacity to store elements of these incoming signals. At the heart of our survival neurobiology is the capacity to make and store internal representations of the external world. Internal representations are your memory. The ability of the brain to create memories is due to the capacity of neurons and neural systems to change from one homeostatic state to another. Neuron and neural system changes are "use-dependent," only occurring if a new or extreme situation forces them.
This has important implications for understanding how we "create" memories of traumatic experiences. For adults, most experiences have only a small component that is new or unique. Typically, the majority of places, faces, words, sounds, smells, tastes, etc. in any given moment are familiar. In the classroom, for example, a lecture may result in cortical activation but will cause little new emotional, motor, or arousal activity.
You will recall that the neuronal pathways sending signals into a brain area are called afferent. The pathways sending neuronal signals out are called efferent. These efferent pathways regulate actions resulting from the process of sensing, processing, and storing incoming signals. Your brain mediates and controls the actions of your body. By regulating and directing the activities of the neuromuscular, autonomic, endocrine, and immune systems, your brain controls your every move.
Stay tuned for Lesson 2, where we'll learn about brain organization and parts like the cortex, and who MacLean was in the world of the brain.
Again, don't hesitate to use the Message Board for any questions you come up with as we move through the course. It's not every day you'll read about, say, the medulla , and not at all surprising that you might have questions about it.
See you in Lesson 2.