Chapter 11 - Learning, Memory and Amnesia

Patient H.M

Underwent bilateral medial temporal lobectomy (medial portion of his temporal lobe removed) for treatment of severe epilepsy

-Involved removal of most of the hippocampus, amygdala and adjacent cortices

Experienced an inability to form new long term memories, but could still remember things before the surgery and form short-term anterograde memories

Assessment of Anterograde Amnesia in HM

1. Digit Span - 1 Test

2. Block-Tapping Test

3. Mirror-Drawing Test

4. Incomplete-Pictures Test

5. Pavlovian Conditioning

Digit-Span - 1 Test

Inability to form long-term memories was demonstrated in his performance on the digit-span + 1 test

-Tests verbal long-term memory by having the patient repeat 5 digits at 1-second intervals

-Then, the same 5 digits are presented with 1 new digit added on the end, continuously presented until another digit is added

Patient HM failed to repeat the 8-digit sequence, whereas most people can repeat 15-25 digits

Block-Tapping Test

An array of 9 blocks spread out on a board, patient asked to watch the neuropsychologist touch a sequence of them, then repeats that same sequence

-Typical person has a block-tapping span of 6, HM could not reach a sequence of 6

Thus, HM had global amnesia (amnesia for information presented in all sensory modalities)

Mirror Drawing Test

Sensorimotor task where patient draws a line within boundaries of a star-shaped target by their watching hand in a mirror

-Over the course of 3 days, patient traces the star 10 times, number of times they went outside the boundaries per trial was recorded

HM’s performance improved over 3 days, indicates retention of the task but he could not recall completing the task before

-Demonstrates that HM’s anterograde amnesia did not involve ALL long-term memories

Incomplete-Pictures Test

Non sensorimotor test of memory, using 5 sets of fragmented drawings with each set containing drawings of the same 20 objects

-Sets differ in their degree of completeness (set 1 contains more fragments, set 5 contains complete drawings)

-Patient asked to identify the 20 objects from the most fragmented set, those objects that go unrecognized are presented in their set 2 versions, continues until all 20 items have been identified

-HM could not recognize the previously performed task (the drawing set presented beforehand)

Pavlovian Conditioning

HM learned an eye-blink Pavlovian conditioning task, at a slower rate than normal

-Tone sounded just before puff of air administered, trials repeated until tone elicited an eye blink

-HM retained this conditioned response 2 years later, despite no recollection of the training

Remote Memory

Memory for experiences in the distant past

Explicit and Implicit Memory

Explicit Memory - Long-term memory that is conscious

Implicit Memories - Long-term memories demonstrated by improved test performance without conscious awareness

Medial Temporal Lobe Amnesia

Amnesia associated with bilateral damage to the medial temporal lobes

-Major features are anterograde/retrograde amnesia for explicit memories, and preserved intellectual functioning

Repetition Priming Test

Assesses implicit memory, includes the incomplete-pictures test and the mirror drawing task

Repetition priming test often involve memory for words; participant asked to examine list of words then shown series of fragmentes of words from the original list

-Asked to then complete them, participants with amnesia oftem perform equally well as regular participants

Semantic and Episodic Memories

Both are variations of Explicit Long-Term Memories;

Semantic Memories - Explicit memories for general facts or information

Episodic Memories - Explicit memories for specific moments in one’s life

-People with medial temporal lobe amnesia have difficulty with episodic memories, despite semantic memories being normal

Global Cerebral Ischemia

Interruption of blood supply to the entire brain, often results in medial temporal lobe amnesia

-Patient RB demonstrated that hippocampal damage by itself can produce medial temporal lobe amnesia

Transient Global Amnesia

Defined by sudden onset in absence of any obvious cause in normal adults

-Severe anterograde amnesia and moderate retrograde amnesia for explicit episodic memories

-Lasts only 4-6 hours, but is strong evidence that selective hippocampal damage can cause medial temporal lobe amnesia

Sudden onsets suggests they may be caused by strokes and are suggestive of ischemia-induced damage

Korsakoff’s Syndrome

Disorder of memory common in people who have consumed large amounts of alcohol

-Attributed to brain dysfunction associated with thiamine deficiency that accompanies heavy alcohol consumption

-Characterized by variety of sensory/motor problems (confusion, personality changes, risk of death from liver, gastrointestinal or heart disorders)

Lesions to media diencephalon (medial-thalamus/hypothalamus) and damage to hippocampus, neocortex and cerebellum)

Amnesia in Korsakoff’s Snydrome

Similar to Medial Temporal Lobe Amnesia;

-Anterograde amnesia for explicit episodic memories

-As the disease progresses, retrograde amnesia develops, and deficits in implicit memory (not as severe as explicit memory deficits)

Mediodorsal Nuclei

Pair of medial diencephalic nuclei in the thalamus, likely responsible for amnesia effects of Korsakoff’s Syndrome

Medial Diencephalic Amnesia

Amnesia associated with damage to medial diencephalon

-Patient NA had conventional memory deficits

Alzheimer’s Disease

Major cause of amnesia, signs begin with mild deterioration of memory with dementia eventually developing

-So severe, that eating, speaking, recognition and bladder controls cannot function

-More general memory deficits than those of medial temporal lobe damage, medial diencephalic damage, or korsakoff’s syndrome

Level of acetylcholine is greatly reduced in brains of alzheimer’s patients, reduction results from degeneration of basal forebrain (brain’s source of acetylcholine)