Physiology of Memory

Certain areas of the brain are believed to be involved in memory processing, like the amygdala (emotional memory), hippocampus(declarative learning) and mammillary bodies.To determine which areas of the brain are involved in memory processing, researchers study animal models and patients with damage to those brain areas. Memory, however, is not dependent on a particular region; adjacent areas and pathways and synaptic changes are also implicated in learning and memory. Memory building starts with encoding, which is the perception of data through smell, sound, taste, touch and sight. Information is then encoded and stored in the form of electrical charges and chemicals.

Communication between neurons (brain cells) occurs via the synapse, the spaces between neurons and center of brain activity. The firing of pulses across the synapse triggers the release of chemical messengers called neurotransmitters, which attaches to neighboring cells. These power linkages form a network between cells and groups of cells, but are temporary. Stronger connections between cells are possible through repetition. In fact, practice is the basis of how the brain organizers itself. For example, drawing or playing a piece of the music everyday enforces brain connections that ensures you get better at the task with time. Skipping a day or a week of practice, however, slows down the process to perfection. The brain “forgets.”

Parts of the Brain Involved in Memory Processing

Hippocampus. The hippocampus (Greek term for ‘seahorse’, which the structure resembles) is part of the brain’s limbic system. It contains mental maps and plays a part in the formation of explicit memory, memory consolidation and the conversion of short term memory into long term memory. Pre-processed information from different parts of the cortex feeds into the hippocampus, which also sends out information to different areas of the brain. Without the hippocampus, it is possible to form semantic but not episodic memories.Damage to the hippocampus may cause loss of memory and impaired memory storage.

Cerebellum. The cerebellum is a structure at the base of the brain that looks like a smaller replica of the cerebral cortex. Learning to drive a car and play the guitar are motor skills needing procedural memory. The cerebellum also coordinates balance and movement.

Amygdala. The amygdala is located below the hippocampus and is associated with emotions and memory. Neurons in the amygdala help in encoding emotional memories. Studies show that intense emotional activity associated with events increase the chances

of the events being remembered. For instance, people with damage to the amygdala do not show this memory enhancement effect.Research showsthat injection of hormones cortisol or epinephrine (causing stimulation of the amygdala) enhances memory storage. However, excessive cortisol may impair memory storage.

Basal ganglia. The basal ganglia are located in the medial temporal lobe and are associated with learning, memory and unconscious memory processes. Damage to the basal ganglia results in some type of motor function impairment.

Frontal Lobe. The frontal lobe of the cerebral cortex is important in working memory; it helps select memories relevant to an event. If you need to go to part of town you’ve never seen before, you combine information that you already have (traffic routes, information from a map) to get to your destination. The frontal lobe helps us remember what needs to be done in the future, like attending a concert or showing up for a doctor’s appointment.

Parietal Lobe. Thisregion performs many functions, including sensation, perception and construction of a coordinate system or a map to represent the world. Along with frontal lobe regions like the dorsolateral prefrontal cortex, the parietal lobe is also involved in short term memory processing (neuronal communication). Damage to the left parietal lobe results in writing and mathematical difficulties, while damage to the right parietal lobe causes sufferers to treat body parts and some objects in the visual field as though they do not exist.

Temporal Lobe. This area is associated with autobiographical and recognition memory, the ability to identify an item that was recently encountered, such as a person. Temporal lobe damage can affect long-term memory.

Occipital Lobe. The occipital lobe plays a key role in vision. It is found in the back of the brain underlying the occipital bone. Damage can result in hallucinations, impaired movement and color discrimination. Left side damage can cause language problems while right side damage can cause non-verbal problems.

The Engram

An engram is a hypothetical memory trace in the form of biochemical or biophysical changes in the brain. They can also be neural networks or memory fragments, reinforcing the notion that memories are not localized in a particular area of the brain. Much of engram research focuses on its exact mechanism and location. Many neuroscientists believe thatwhile certain types of information are localized in specific brain regions, memories involved in complex tasks (such as rats running in mazes) are likely spread out among neural systems. Richard Thompson studied the engram in the cerebellum using classical conditioning in rabbits. He introduced air on the rabbit cornea along with a tone that produced a blinking response. Eventually, the rabbits blinked when only the tone was introduced, allowing Thompson to study possible engram locations in the rabbits’ brain.

Thompson’s team found that deactivation and reactivation of the lateral interpositus nucleus (LIP) affected the conditioning response, showing that the LIP is involved in the engram for the eyelid response. Since the cerebellum is a more primitive region, the study only examined automatic responses. Recent studies performed on mice have also that is possible to localize certain memories into specific neurons, explaining how and why the brain organizes memories and allocates a specific memory to a group of neurons.According to an MIT study, a specific memory can be expressed in a mammal by physically activating a specific group of brain cells by light. This shows that some memories are localized in certain regions of the brain. Other researchers have shown that the limbic system and cortical regions are involved in declarative memory processes, but there is still much to be done in this area. Studies on humans are limited or do not exist, making it unclear what neuronal changesoccur with higher cognition processes.

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