Memory (neuroscience)

Memory is the process of encoding, storing, and retrieving experiences and knowledge. 
Different groups of neurons (nerve cells), responsible for different thoughts or perceptions, drift in and out of action. 
Memory is the reactivation of a specific group of neurons, formed from persistent changes in the strength of connections between neurons. A specific combination of neurons to be reactivated over any other combination of neurons is known as synaptic plasticity. This term describes the persistent changes in the strength of connections – called synapses – between brain cells. These connections can be made stronger or weaker depending on when and how often they have been activated in the past. Active connections tend to get stronger, whereas those that aren’t used get weaker and can eventually disappear entirely. 
A connection between two neurons becomes stronger when neuron A consistently activates neuron B, making it fire an action potential (spike), and the connection gets weaker if neuron A consistently fails to make neuron B fire a spike. Lasting increases and decreases in synaptic strength are called long-term potentiation (LTP) and long-term depression (LTD).
Changing the strength of existing synapses, or even adding new ones or removing old ones, is critical to memory formation. But there is also evidence that another type of plasticity, not directly involving synapses, could be important for memory formation. In some parts of the adult brain, such as the important memory structure known as the hippocampus, brand new neurons can be created in a process called neurogenesis. Studies in older mice have shown that by increasing neurogenesis in the hippocampus, memory can be improved. In humans, exercise has been shown to increase the volume of the hippocampus – suggesting new neurons are being created – and at the same time improve performance in memory tasks. Recalling a memory involves re-activating a particular group of neurons. The idea is that by previously altering the strengths of particular synaptic connections, synaptic plasticity makes this possible.
=Types of Memory=
Memories aren’t stored in just one part of the brain. Different types are stored across different, interconnected brain regions. For explicit memories – which are about events that happened to you (episodic), as well as general facts and information (semantic) – there are three important areas of the brain: the hippocampus, the neocortex and the amygdala. Implicit memories, such as motor memories, rely on the basal ganglia and cerebellum. Short-term working memory relies most heavily on the prefrontal cortex. 
==Explicit Memory==
There are three areas of the brain involved in explicit memory: the hippocampus, the neo-cortex and the amygdala.
===Hippocampus===
The hippocampus, located in the brain's temporal lobe, is where episodic memories are formed and indexed for later access. Episodic memories are autobiographical memories from specific events in our lives, like the coffee we had with a friend last week.
n 1953, a patient named Henry Molaison had his hippocampus surgically removed during an operation in the United States to treat his epilepsy. His epilepsy was cured, and Molaison lived a further 55 healthy years. However, after the surgery he was only able to form episodic memories that lasted a matter of minutes; he was completely unable to permanently store new information. As a result, Molaison’s memory became mostly limited to events that occurred years before his surgery, in the distant past. He was, however, still able to improve his performance on various motor tasks, even though he had no memory of ever encountering or practising them. This indicated that although the hippocampus is crucial for laying down memories, it is not the site of permanent memory storage and isn’t needed for motor memories. 
The study of Henry Molaison was revolutionary because it showed that multiple types of memory existed. We now know that rather than relying on the hippocampus, implicit motor learning occurs in other brain areas – the basal ganglia and cerebellum.
===Neocortex===
The neocortex is the largest part of the cerebral cortex, the sheet of neural tissue that forms the outside surface of the brain, distinctive in higher mammals for its wrinkly appearance. In humans, the neocortex is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning and language. Over time, information from certain memories that are temporarily stored in the hippocampus can be transferred to the neocortex as general knowledge – things like knowing that coffee provides a pick-me-up. Researchers think this transfer from hippocampus to neocortex happens as we sleep.
===Amygdala===
The amygdala, an almond-shaped structure in the brain’s temporal lobe, attaches emotional significance to memories. This is particularly important because strong emotional memories (e.g. those associated with shame, joy, love or grief) are difficult to forget. The permanence of these memories suggests that interactions between the amygdala, hippocampus and neocortex are crucial in determining the ‘stability’ of a memory – that is, how effectively it is retained over time.
==Implicit memory==
There are two areas of the brain involved in implicit memory: the basal ganglia and the cerebellum.
===Basal ganglia===
The basal ganglia are structures lying deep within the brain and are involved in a wide range of processes such as emotion, reward processing, habit formation, movement and learning. They are particularly involved in co-ordinating sequences of motor activity, as would be needed when playing a musical instrument, dancing or playing basketball. The basal ganglia are the regions most affected by Parkinson’s disease. This is evident in the impaired movements of Parkinson’s patients.
===Cerebellum===
The cerebellum, a separate structure located at the rear base of the brain, is most important in fine motor control, the type that allows us to use chopsticks or press that piano key a fraction more softly. A well-studied example of cerebellar motor learning is the vestibulo-ocular reflex, which lets us maintain our gaze on a location as we rotate our heads.
==Working memory==
===Prefrontal cortex===
The prefrontal cortex (PFC) is the part of the neocortex that sits at the very front of the brain. It is the most recent addition to the mammalian brain, and is involved in many complex cognitive functions. Human neuroimaging studies using magnetic resonance imaging (MRI) machines show that when people perform tasks requiring them to hold information in their short-term memory, such as the location of a flash of light, the PFC becomes active. There also seems to be a functional separation between left and right sides of the PFC: the left is more involved in verbal working memory while the right is more active in spatial working memory, such as remembering where the flash of light occurred.
Category:Neuroscience