The neurological map of memory is referred to as the engram.
We understand a great deal about the engram from various case histories. The most important of these is the case of H.M. He was a Connecticut man whose Temporal lobe was removed to control epileptic seizures. The operation controlled his seizure but the unusual side effects gave important clues to the working of memory. He could not form new memories. He could recognize himself in old photographs but not in the mirror. His conception of himself was stuck in the past. It was soon realized that an area of the brain within the temporal lobe, the hippocampus was central to memory system.
The memory system is one of the most important systems in the mind. It is a system of recall and creation more than a system of facts recalled. Memory systems are of primary importance to the creation and functioning of the self.
Memory is normally separated into two systems explicit or declarative memory and implicit memory.
Explicit memory is that which can be verbally recalled and described for instance the phrase I can play the piano is a declaration stemming from memory systems.
Implicit memory governs how we act and feel more than what we know. So far instance the physical ability to play the piano is an example if implicit memory and the discussion about that ability is an example of explicit memory.
The main sensory systems translate the exterior world into synaptic pathways and maps. These form representations in translation. From the various sensory systems within the neo cortex there is a massive convergence zone in the rhinal cortical region also known as the parahippocampal region. It is act this point that the different sensory information is integrated before it is sent to the hippocampus. In the rhinal cortex representations become multi sensory. Here sight is put with smell and sound in global memory formation. This system has come to be known as the medial temporal lobe memory system.
The entry system into the hippocampus from the rhinal area is called the dentate gyrus. The main synaptic pathway through the hippocampus is called the trisynaptic circuit. The memory information patterns exit the hippocampus through the subiculum. These information circuits then re-enter the rhinal cortex.
At the convergence zones perceptions become conceptions and abstract thought becomes associated with sensory stimulation.
The synaptic map that form memories are returned to the area of the neocortex n that originally produced them. Each time the memory is recalled it passes through the hippocampus then is reinstated in the neocortex. The hippocampus is the channeled of the stored synaptic representations. Once the memory circuit had been recalled a certain number of times the explicit memory acts without the hippocampus. The circuit then moves from short-term memory to long-term memory.
It appears that this movement of memory from the neocortex to the hippocampus takes place at night and is the process by which new memories are stored.
Implicit memories are more about general states, moods assumptions of the self and habits. Implicit memories use many different circuits for their creation and storage and are not so dependent on the hippocampus.
Implicit memory circuits are very old pathways and often operate below consciousness. These systems control the non-declarative assumptions of ourselves.
We do not have as clear a picture for how the neural circuits for implicit memory work. However it is thought that the amygdala and the cerebellum play a central role.
The frontal lobe while being central to personality is also crucial to short-term memory. It is also crucial for the comparison and development of memory systems and construction of sense.
Short-term memory is more frequently referred to as working memory. The Frontal lobe is the area primarily associated with working memory. This is another massive convergence zone with connections to sensory systems.
Working memory is best thought of as the constructed experience of the present. It enables one moment to appear to move seamlessly to the next in a seamless cognitive flow. Yet the system can only focus on one item at a time. The executive functions are the process that are performed to make sense of the flow of information and the retrieval from long term memory systems. STM holds many tasks in the present. It is thought to have a capacity of about seven pieces, yet through bundling have a tremendously increased capacity.
Executive functions let the mind process and execute the steps necessary to maintain the cognitive present.
The system works by bottom up input from the visual cortex to the PFC and top down processing from the PFC to the VC. These instruction may include directions to keep focused on a particular subject or follow its movements.
Decision-making is central to executive function. We are seldom aware of the process only the outcome of the system. It is important to realize that the sensation of a memory is only assembled at the time of its retrieval yet it is possible to hold memory present in the mind as well as other information and various tasks.
Executive systems are directly involved in scheduling the sequence of events in the cognitive present.
It is interesting to look at two pathways from the VC to the PFC.
The limbic system is the mechanism of the so-called old brain and is thought to be the system partially responsible for emotions. This is in contrast to the neo cortex, which is suggested to be the area responsible for cognition. The mental constant of the mind is often broken down into emotion, motivation and cognition.
—Peter Nadin
