Brain plasticity 2: Learning, memory and amnesia

• Communication of cells through synapses
• Learning - experience changes cells
• Memory - how changes strengthen connections between cells

Patient HM

• H.M. was an epileptic who, in 1953, has his medial temporal lobes removed.
• His seizures were dramatically reduced - but so was his memory.
• Mild retrograde amnesia and severe anterograde amnesia. 
• Types of memory problems were show by which bits of brain were retained.

Amnesia

• Retrograde: Unable to remember the past (backwards acting)
• Anterograde: Unable to form new memories.
• While HM was unable to form most types of new, long-term memories, his STM was intact.

• Digit span - HM could repeat digits as long as the time between learning and recall was within the duration of STM (Add numbers and repeat).
• Block-tapping memory-span test: This test demonstrated that H.M's amnesia was global - not limited to one sensory modality. (Squarelights come and go and you have to tap them and remember order and location).
• Mirror drawing task - H.M exhibited improvement with practice. He was able to show memory for skills - demonstrating that he can learn also demonstrated by performance of the rotary-pursuit and a drawing task, although he was not aware of learning.
• HM readily 'learned' responses. 

• Medial temporal lobes are involved in memory
• STM and LTM are distinctly separate - HM was unable to move memories from STM to LTM (memory consolidation problem)
• Memory may exist but not be recalled - HM exhibited a skill he does not know he has learned. Explicit and implicit memory.
• Explicit - conscious memories, knowing-what
• Implicit - unconscious memories - knowing how (i.e. HM showed intact motor learning).

• Not everyone was this form of amnesia are unable to form new explicit long-term memories  - as with the case with HM.
• Semantic memory (general information) may function normally.
• Episodic memory ( events that one has experienced) may not function normally. Able to learn facts, but not able to remember doing so (the episode that had been learned).

The hippocampus and consolidation

• HM has retrograde amnesia.
• Perhaps the hippocampus stores memories temporarily in order to consolidate.

Object recognition memory

• Early animal models of amnesia involved implict memory and assumed the hippocampus was key. 
• 1970's - monkeys with bilateral medial temporal lobectomies show LTM deficits.
• Like HM, performance was normal when memory needed to be held for only a few seconds.

Object-recognition and Medial Temporal Lobectomy

• Bilateral removal of the rhinal cortex results in object-recognition deficits (if damaged).
• Bilateral removal of the hippocampus produces moderate effects on object recognition
• Bilateral removal of the amygdala has no effect on object-recognition.

The Hippocampus

• Rhinal cortex plays an important role in object recognition.
• Hippocampus plays a key role in memory for spatial location.

Theories of Hippocampal Function

• Cognitive map theory - constructs and stores allocentric maps of the world.
• Configural association theory - involved in retaining the behavioural significance of combinations of stimuli.
• Involved in recognizing spatial arrangements of objects.

• Each memory is stored diffusely throughout the brain structures that were involved in its formation.
• Hippocampus - spatial location.
• Rhinal cortex - object recognition.
• Medidorsal nucleus - Korsakoff's 
• Basal forebrain - Alzheimer's disease.
• Damage to a variety of structures results in memory deficits.
• Inferotemporal cortex - visual perception of objects.
• Amygdala - emotional learning - leads to lack of learned fear.
• Prefrontal cortex - temporal order of events and working memory.
• Cerebellum - sensorimotor tasks - conditioned eyeblink.