Memory

MEMORY

• Process of “Storage of acquired information”.

TYPES OF MEMORY:

1. Based on recalling ability –  Explicit & Implicit.

2. Based on memory duration – Short-term & Long-term Memory.

EXPLICIT MEMORY/DECLARATIVE/CONSCIOUS MEMORY:

• Requires conscious thought & recalling.
• Often associative – Ie., brain links memory together

Subtypes:

1. Episodic memory/Autobiographical memory::

• Provides crucial record of personal experiences.
• Associated areas:
• Medial temporal lobe.
• Hippocampus.
• Impairment causes “Amnestic syndrome”.

2. Semantic memory:

• Accounts for “Textbook learning”/”General knowledge”. 

IMPLICIT/NON-DECLARATIVE/UNCONSCIOUS MEMORY: 

• Does not require conscious thought.
• Deals with problem-solving ability, learning skills.
• Ie., Procedures remembered unconsciously.
• Implicit memory deficit causes “Apraxia”.

Subtypes:

1. Procedural memory:

• Enables execution of commonly learned tasks without their conscious thinking.
• Eg: Bike riding /Shoe tying/Dishwashing.
• Areas involved – Striatum of basal ganglia.

Unique feature of procedural memory:

• Involves multiple sites of brain compared to episodic memory.
• Hence, on injury one ability is lost without losing other.
• Eg: Amnesia person forgets personal life retaining procedural memory like car driving.

2. Priming/Conditioning: 

• Experiences are primed.
• Ie., On repeated exposure to one particular memory, their recalling ability is increased.
• Areas associated – Neocortex

SHORT-TERM/WORKING/INTERMEDIATE MEMORY: 

• Memory of very short time (< minute) before dismissal/transfer to long-term memory.
• Memory lasts only as long as person continues to think.

Short term memory due to –

• Continual neural activity in reverberating neuronal circuit
• Neurotransmitter causes presynaptic facilitation/inhibition.

Impairment – Causes “Antegrade amnesia”.

• Inability learning new things.
• Due to inability in converting short-term memory to long-term memory.

LONG-TERM MEMORY:

• Memory that happened more than a few minutes ago.
• Impairment – Causes “Retrograde amnesia”
• Amnesia of stored past memory.
• Ie., Inability to recall information stored before onset of illness/injury.
• Note: Only on enough activation memory traces become permanent & termed “Long-term”.

Subtypes:

1. Intermediate long-term memory:

• May last for many minutes/weeks.
• Due to temporary chemical/physical change both at synapse/presynaptic terminal/postsynaptic membrane.
• “Synaptic plasticity” –
• Synaptic conduction can be strengthened/weakened based on past experiences.

Habituation/Negative memory:

• Due to progressive Ca channel closure.

Facilitation/Positive memory:

• Due to serotonin release at facilitator synapse.

2. True long-term memory:

• Due to actual structural changes/physical restructuring/spatial synaptic organization.
• Changes produced by DNA stimulation of protein replication in presynaptic neuron.

Changes associated:

• ↑ed vesicle release sites for transmitter secretion. 
• ↑ed number of transmitter vesicles released.
• ↑ed number of presynaptic terminals.
• Structural changes of dendritic spines permitting stronger signal transmission.

Summary:

• Long-term memory – Resultant of actual structural changes.
• Short-term memory – Resultant of synaptic chemical changes.
• No obvious demarcation between intermediate long-term & true long-term memory.

MEMORY CONSOLIDATION:

• Process enables conversion of short-term memory into long-term memory.

• On repeated activation of short-term memory → Chemical/physical/anatomical synaptic changes —> Converts to long-term memory.

Areas associated:

• Medial temporal lobe – 
• Hippocampus (most important).
• Parahippocampal gyrus.
• Entorhinal cortex.

Storage site after consolidation:

• Neocortex.
• Amygdala stores emotional memory components.

Lesion:

• Damaged hippocampus leads to ”Antegrade amnesia” – 
• Causing inability to consolidate recent events into long-term memory.