The brain is made up of billions of neurons, or nerve cells. Neurons
are the building blocks of the brain and all activity and thinking that
is conducted is based on these cells. The job of a neuron is to process
information sent to it and convey this information on to other neurons,
ultimately controlling all behaviors and experiences.
This page will now concentrate on the role that the network of neurons in the
brain plays in the memory process.
For further basic background on neurons and the neural network of the brain, please click here.
The foundation of the types of memory discussed and the brain structures responsible for specific aspects of memory is the neural network of the brain. To understand memory storage and retrieval, it is necessary to look at the neurophysiology of the brain. Information that enters and travels through specific parts of the brain is passed along this neural web. Nerve impulses arrive and travel through a neuron to the synapse. The impulses then travel across the synapse of one neuron via electrical activity to another neuron. Through the travelling of impulses across the synapses, information is passed from neuron to neuron. The junctions of a single nerve fiber can number up into the hundreds. The nerve impulses arriving at a neuron form a pattern similar to a microstructure of wave forms. These wave forms interact with similar, overlapping wave forms of junctioning neurons. The interaction of these wave forms together causes a new pattern of nerve impulses to result. This impulse pattern effects protein and other chemical molecules at the synaptic junctions of neurons.
Karl H. Pribram presented his Hologram theory for neuronal storage of memories in 1969. His hypothesis of memory storage in the brain is that it is similar to a hologram. In a hologram, information is recorded and distributed across a type of photographic plate as a meaningless pattern. Thus the whole plate contains parts of the original image or stimulus. The neural network works similar to this in that information is distributed across many neurons throughout a "neural hologram" of sorts. Uncovering part of an image or stored stimuli allows the rest of the image to be accessible across the hologram.
Memory and memory storage is based on the firing and travelling of impulses across patterns of nerve cells. The activity of neurons that are effected by an experience is altered in this process in order for this experience to be remembered and behavior to be modified. Thus memory consists of changes and alterations of effected neurons left from a past activity or experience. Multiple patterns of these traces are left in effected neurons and these patterns make up a memory. Thus memories are stored and reinforced by stimulation of a neuron from arriving impulses. The initial experience leaves a memory trace or pattern in effected neurons. Repetition and recall of a memory further reinforces a memory trace, making it stronger and easier to access. Further recall and response based on this memory trace becomes larger than it had previously been. This phenomenon is known as long-term potentiation, or LTP. The duration of this effect differs from hours to weeks or longer. The duration depends on the age, health, and experience of the person, as well as the properties and strength of the stimuli. The best analogy for this is a river that constantly flows and cuts a deeper, more defined impression in its channel bed. Thus changes occur at the synapses between neurons after impulses arrive and these changes effect subsequent impulses, reinforcing the memory trace left. The changes occur in the structure of the synapse and the chemical molecules released here. Thus memory storage is based on a web of neurons that are altered to leave a memory trace. This memory trace differs based on space and time. That is, the neural network in different areas of the brain is responsible for holding different types of memories for varying amounts of time.
The complexity and beauty of this system of neurons is its flexibility and plasticity. The connections that exist between neurons are constantly changing. Throughout a lifetime, the structure of neurons is constantly changing as neurons die, knowledge is accumulated, information changes, and disease, old age, and other misfortunes destroy neurons and their connections. Constant modifications are being made of the neuron webs. Furthermore, the structure and connections of neurons differs from person to person based on unique experiences and environments. Thus the architecture of every person's brain is slightly different. These differences in the brain architecture at the neuron level create a unique individual who is constantly undergoing changes; changes to better suit new demands or experiences. This plasticity of the neuronal structure is necessary for storing memories. The strengths and structure of connections among neurons determines the strength and location of a saved memory.