The limbic system’s ability to determine “this is it –this is truth,” is vital to creation of our mental realities. As “guardian at the inner threshold” it opens the heart to new understanding and facilitates the process of recollection and learning.
In the human mind, perceptions presented by the FIVE SENSES are compared to memory perceptions. Through its instrument, the limbic system, the faculty of imagination harmonizes inner and outer perceptions. The images created by imagination then become material for the intellect. Thus, imagination is the intermediary between perception, memory, and thought. Indeed, thought and learning are made possible by the image making part of the soul.
The sixth sense; in primitive species, the only function of the limbic system is the regulation of the sense of smell. As the brain becomes more complex, the limbic system diversifies to regulate aspects of behavior, such as emotional expression, while retaining its tie to the olfactory system. It is interesting to note that ANUBIS- the jackalheaded god of Egypt, the guardian of the threshold, and symbol for the limbic system –had a particular acute sense of smell.
Scientists have long sought the physical instrument wherein resides the capacity for imagination, memory, and learning. Many believe that these faculties are located in the outer brain, or the two cerebral lobes. In one famous experiment the American psychologist, Karl Lashley, searched for the elusive site of memory storage. He found that rats did not suffer significant deterioration of their ability to thread their way through a learned maze even though they were missing up to 90 percent of their cerebral lobes.
From this and other experiments one may theorize that each specific memory is distributed over the brain as a whole. Perhaps the images of imagination and memory are developed in the brain in a manner analogous to a hologram. What is apparent from the study of much neural structure is that the brain relies on patterns of increasing refinement, simplicity, elegance, and wholeness.
If the images of memory are experienced over the entire surface of the outer brain and perhaps even throughout the brainstem as well, how are we able to evoke those memories which are important to us? What physical structures participate in our ability to recall images by processes of order and association? To investigate this question we must search more deeply into the inner mysteries of the brain. Deep within the temporal lobes of the outer brain we must seek out those structures comprising the limbic system.
The portion of the limbic system which appears to be especially concerned with facilitating memory and learning is called the HIPPOCAMPUS, or sea horse. The hippocampus is a rather large structure reaching a peak size in man. The internal architecture of the hippocampus is curious, resembling a series of leaves like the pages of a book.
Viewed a great number of circuit-boards arranged in stacks. The input lines from the sense organs run through the stack of leaves and make contact with the neurons [brain cells] in each leaf. The output lines connect with forebrain, other portions of the limbic system, mammillary bodies, thalamic and hypothalamic nuclei –all structures participating in the facilitation of memory and learning.
IMPORTANCE TO MEMORY
Damage to both sides of the hippocampal portions of the limbic system result in a severe form of AMNESIA, the inability to evoke particular memory images. The memory-image selection process is affected in a random way.
The patient may experience the memory of the loss of favorite pet, but not that of a favorite uncle. These patients can experience new information in the present, but the ability to recall is lost when the attention shifts. A few minutes after dinner, patients cannot recall what they had eaten or even whether they had dined.
Damaged to just one side of the hippocampal portion of the limbic system does not produce such drastic effects. The degree of impairment appears to depend on the extent of hippocampus are respectively concerned with verbal and non-verbal memories, in line with the roles of the two sides of the outer brain cortex.
Order or special memory is important to many animals. Birds can remember which flowers they have already visited. Rats placed on an eight-arm maze can remember which arm contained food. Rats can also do this on a seventeen-arm maze, through with more mistakes.
Various experiments show that they remember by making a mental map. Electronic records from the hippocampus of a free-running rat show that there are specific hippocampal cells which fire only when the animal is at specific point within a maze.
In addition, these hippocampal cells fire consistently when three or four cues are present, but they fire erratically if only one cue is present. Other studies show that learning is associated with hippocampal theta rhythms [4-7 cycles per second]. If, on the other hand, the hippocampal connections are damaged, performance is profoundly impaired. So it appears that one of the functions of the hippocampus is to compare memories of special arrangements. This conclusion conforms with the evolutionary evidence.
Fish have no hippocampus. In the ocean there are few landmarks and fish are generally guided by broad environmental factors like temperature and salinity. Amphibians and reptiles have primitive hippocampus. Once creatures climbed on land there was an advantage to remembering where food, nests, and lairs could be found. The fully organized hippocampus appears in early mammals.
Russian scientists tested a range of animals for their ability to recognize the order in which three signals were given. These signals consisted of sound and light. Goldfish never learned. Turtles recognized the patterns some of the time. Birds were terrific. Rabbits which have a good hippocampus but a poor cortex needed much tutoring and were easily foxed by a change of order. Dogs and baboons were perfect.
While imagination uses the limbic system to harmonize inner and outer perceptions, the quality of the experience depends on the development of the instruments employed. Thus, in addition to the limbic system, a well-developed cortex seems necessary for greater clarity and realization of the images presented by imagination.
Some investigators speculate that the hippocampus is needed for the appreciation of music and speech. The hippocampus is the tool whereby mental maps are surveyed in serial order. If we try to find our way we have to remember what came after what. Experiencing music and speech also necessitates remembering which of the many possibilities or portions proved rewarding. This constitutes learning. In this regard, the hippocampus has many connections with the brain’s pleasure and reward centers. Some of the leaves of the hippocampus may hold a sort of “gold star” indicating “this is fun.”