Psychology 371
The Neurobiology of Learning and Memory

• Amnesia — What is it?
• Taxonomy and operational definitions
• Case Studies of Global Amnesiacs
• H.M., R.B., N.A., B.Y.
• Korsakoff’s Syndrome
• The Common Link
• Knowing How vs. Knowing What
• The Enigma of Consciousness

• With modern technology, we are often able to image the brains of healthy individuals in order to localize functions within the brain.
• Another method is to infer function from disfunction.
• In other words, when a person suffers some form of brain trauma, we can correlate their behavioural deficits with the brain region affected.

• Amnesia is a condition that affects the memory processes of individuals.
• This could be the inability to transfer new information into long term stores, or the inability to recall past information.
• Amnesia is a non-unitary syndrome which is classified according to several criterion.

• Amnesia is classified into two types:

• Physical trauma to the brain
• Brain tumors
• Surgical ablations
• Strokes
• Degenerative processes

• Following brain damage, memory impairments may be associated with other perceptual and cognitive impairments—these non-dissociated amnesias are difficult to study.
• We want to identify those individuals that show dissociated amnesia.
• That is, we are interested in finding individuals who perform normally on intelligence tests (WAIS) but below normal on memory tests (WMS).

• In studying amnesia, memory is often classified into three categories:
• immediate memory:
• measured by immediate repetition tasks
• recent memory:
• typically memorize a list and then recall it 5 minutes later
• remote memory:
• based on biographical or general factual knowledge

• Lesions in appropriate neocortical association areas lead to modality-specific performance deficits.
• Such a deficit is termed agnosia
• an acquired impairment in recognizing a previously known sensory stimulus, unaccompanied by perceptual deficits.
• For example, bilateral lesions of the occipitotemporal cortex leads to prosopagnosia—impaired recognition of faces.

• Damage to the limbic structures and the frontal lobes causes impairments in "global" memory.
• The location and magnitude of the lesion determines the severity, endurance, and degree of multimodality of amnesia.
• There is a large literature on both types of amnesia, but we will concentrate on the global form.

• Amnesia is classified into two categories depending on the time window of affected memory based on amnesia onset.
• Retrograde (premorbid):
• loss of memory for events that occurred prior to the onset of amnesia
• Anterograde (postmorbid):
• loss of memory for events that have occurred after the onset of amnesia

• Amnesia can be transient or permanent.
• Transient amnesia is often the result of Electroconvulsive Shock Therapy (ECT).
• Permanent amnesia is caused by accidental, surgical, or degenerative ablations within the brain.
• Transient amnesia often has some permanent amnesia associated with it and permanent amnesia can partially recover.

• H. M. is probably the most famous of all amnesiacs (although he does not know it!)
• He underwent surgery to alleviate intractable epilepsy.
• Surgery was bilateral removal of medial-temporal lobe, including the hippocampus, and the amygdala.
• This resulted in anterograde amnesia with partial premorbid loss.

• H. M. scores above average on the WAIS but 3 S.D. below normal on the WEM.
• Both verbal and non-verbal memory are impaired.
• Performance in immediate memory was not affected.
• LTM was most severely affected.
• He can complete and learn routine perceptual and motor responses.

• R. B. suffered amnesia as a complication of heart surgery.
• He showed normal WAIS but a marked anterograde amnesia in both verbal and non-verbal domains.
• This was accompanied by slight retrograde amnesia.
• R. B.’s lesion was isolated to the entire bilateral CA1 field of the hippocampus .

• N. A. suffered amnesia following a fencing accident.
• Damage was restricted to a small lesion of the left dorsomedial thalamic nucleus.
• N. A. showed severe anterograde amnesia and retrograde amnesia for about 2 years prior to the accident.
• N. A. showed more severe problems with verbal than non-verbal tasks.

• B. Y. amnesia followed an infarction.
• Damage was a bilateral medial thalamic lesion involving parts of the dorsomedial, centromedian, and ventro-posteromedial nuclei..
• Although B. Y. showed both anterograde and retrograde amnesia, retrograde amnesia improved within weeks.

• Korsakoff’s syndrome is the most common organic brain disease causing global amnesia while sparing (relatively) other cognitive functions.
• It is associated with chronic alcoholism.
• Patients display severe retrograde and anterograde amnesia.
• Retrograde amnesia shows a temporal gradient with remote memories less affected.
• Damage is dorsothalamic medial nucleus and/or mammillary bodies.

• All of the previous examples of global amnesiacs have one thing in common: damage to the limbic circuits.
• Behavioural defects are in recent and remote memory, but sparing of immediate memory.
• This implies that the limbic system is not necessary for the initial stages of the formation of experience-dependent representations, but is necessary for the consolidation of these representations into a long-term form.

• The extent and location of the limbic lesion are critical determinants in the behavioural profile of the resulting amnesia.
• Bilateral limbic lesions cause multimodal, relatively permanent amensias.
• Unilateral limbic lesions lead to transient amnesias that are hemispheric dependent
• left affects verbal memories
• right affects visual memories

• Recently, it has been proposed that amnesias can be classified into "medial-temporal" (H. M. & R. B.) and "diencephalic" (N. A., B. Y., Korsakoff patients) amnesias.
• Medial-temporal amnesics forget rapidly.
• Diencephalic amnesics learn slowly, but once memory is established, it is not markedly feeble.

• Even the most severe anterograde amnesics retain some postmorbid memory.
• This is normally associated with the acquisition of some procedures.
• Performance on a mirror-reading task illustrates this:
• reading speeds increased
• there was no recognition of previous words.

• There are two major memory systems within humans:

• procedural, skill, or habit memory
• served by cortico-striatal pathways

• declarative memory (episodic and semantic)
• encompasses the relevant sensory association cortex and the medial-temporal and diencephalic structures

• Consciousness has a variety of meanings, and is often subjected to severe and prolonged philosophical debates.
• We will define consciousness as phenomenological awareness.
• In other words, consciousness is being aware of something.

• One explanation of consciousness is derived from a neuropsychological model based on modules and central systems.
• The hippocampus and related memory structures bind or integrate into a memory trace the neural elements that mediate a conscious experience.
• Consciousness is becomes an intrinsic property of the memory trance.

• Memory that is mediated by non-hippocampal structures (posterior neocortex) cannot have consciousness bound within the trace by virtue of the organization of the mediating structures.
• Those structures can be modified by experience and thereby influence behaviour but without giving rise to conscious recollection

• Amnesia — What is it?
• Taxonomy and operational definitions
• Case Studies of Global Amnesiacs
• H.M., R.B., N.A., B.Y.
• Korsakoff’s Syndrome
• The Common Link
• Knowing How vs. Knowing What
• The Enigma of Consciousness