Subcortical Memory Systems

Subcortical Memory Systems

1. Anatomy

   • The forebrain divides into 2:
     • Telencephalon -> Cerebral cortex, Hippocampus, Basal Ganglia, Amygdala
     • Diencephalon -> Thalamus, Hypothalamus
   • Grey vs White Matter
     • Grey Matter: where cell bodies predominate, the cortex has a greyish appearance
     • White Matter: Where myelinated axons predominate, the cortex has an opaque white appearance.
   • Subcortical Anatomy:
     • Basal Ganglia: Movement, Reward, Memory
     • Thalamus: Sensory inputs, Memory
     • Hippocampus: Memory
     • Hypothalamus: Regulates body function
     • Amygdala: Emotion

2. Korsakoffs Syndrome

   • Common form of amnesia, first describes by Korsakoff in 1889
   • Casued by chronic alcoholism leading to thiamine (Vitamin B1) deficiency.
   • Symptoms: first presentation as Wernicke's encephalopathy
     • Confusion
     • Ataxia (poor motor coordination)
     • Eye muscle weakness
     • Can be treated with thiamine
       • If untreated, develops into Korsakoff:'s:
         • Preserved IQ
         • Normal Digit Span
         • Preserved semantic memory
         • Preserved procedural memory
         • Personality changes (apathy, passivity)
         • Retrograde & Anterograde Amnesia
         • Confabulation: fills in gaps in memory with plausible made-up stories.

3. Diencephalic amnesia

   • Single Case Studies
     • Patient B.J.: Snooker cue inserted into left nostril - sustained mammillary body lesion. Resulted in anterograde and retrograde amnesia
     • Patient N.A.: Fencing epee inserted into right nostril. Damage to anterior thalamus (as well as mammillaryy bodies & mammillothalamic tract). Resulted in profound and enduring anterograde amnesia. Retrograde amnesia recovered.
   • Evidence from both Korsakoff's and patients with diencephalic lesions implicated mammillary bodies, mammillothalamic tract and anterior thalamus in the recall of episodic events & formation of new memories
     • BUT: damage is rarely restricted including areas outside diencephalon - e.g. KS often causes cortical damage especially in frontal cortex.
   • Damage to the diencephalon leads to an amnesic syndrome similar to Temporal Lobe Amnesia (e.g. HM). Whether this is a 'single' memory circuit or 2 distinct memory systems remains controversial.

4. Methods and Strategies of Research

   • Experimental Ablation/ Lesion Study: The removal/ destruction of a portion of the brain of a lab animal.
   • Rationale: the function of an area of the brain can be inferred from the behaviours that the animal can no longer perform after the area has been damage.
   • Excitotoxic Lesion: a brain lesion produced by intracerebral injection of an excitatory amino acid, such as kainic acid.
     • when an amino acid is injected, the chemical destroys neural cell bodies in the vicinity but spares axons belonging to different neutrons that happen to pass nearby - this selectivity permits investigator to determine whether behavioural effects are caused by the death of neutrons located there or by the destruction of axons that pass by.
   • Sham Lesions: a 'placebo' procedure that duplicate all the steps of producing a brain lesion except for the on estate actually causes the brain damage.
     • Allows us to compare lesioned animals with a group (controls are not usable because there is damage done even before turning on the lesion maker or starting the infusion)
   • Stereotaxic Surgery: Brain surgery using a stereotaxic apparatus to position an electrode/ cannula in a specified position of the brain.
     • One must first study a stereotaxic atlas: a collection of drawings of sections of the brain of a particular animal with measurements that provide coordinated for stereotaxic surgery.
     • Breg,ma: the junction of the sagittal and coronal sutures of the skull - often used as a reference point for stereotaxic brain surgery
     • Stereotaxic Apparatus: a device that permits a surgeon to position an electrode of cannula into a specific part of the brain.
   • Histological Methods: after producing a lesion & observing its effects on behaviour, we must slice and stain the brain so we can observe it under a microscope and see the location of the lesion - we have to verify the precise location of the brain damage.
     • Fixation and Sectioning: place the neural tissue in a fixative (most commonly formalin). We then slice it (using microtome) into thin sections and stain various cellular structures to see anatomical details. Slices are prepared for examination under a microscope
     • Staining: Discovery of cell-body stains bade it possible to identify nuclear masses in the brain - all cells are stained,, it is up to the investigator to detainee which is which (neutrons and glia)
     • Electron Microscopy: a beam of electrons is passed through the tissue to be examined. A shadow of the tissue is then cast onto a sheet which is exposed by the electrons. can provide info about structural details on the order of a few tens of nanometers.

5. The Basal Ganglia

   • 1) Receives cortical inputs to caudate and putamen from areas involved in processing somatosensory info and in planning/execution of motor movements.
   • 2) This projects back to the motor regions of the cortex via the globus pallid and then the thalamus
   • 3) This loop is involved in control of automatic and well practiced movements
   • 4) The internal division of globes pallid us has an inhibitory effect on motor systems. This inhibition is reduced by dopamine thereby promoting motor activity.

6. Parkinson's disease

   • Caused by Degeneration of the nigrostriatal dopamine system
   • Affects about 1% of people over age 65
   • Early symptoms are largely movement related - e.g. tremor, rigidity, slow movement
   • One treatment is L-DOPA (precursor to dopamine): allows surviving dopaminergic neutrons to produce sufficient dopamine for normal functioning.
   • Another treatment involves destroying the internal division of the globes pallid us
   • Other symptoms involve memory impairments:
     • Deficits in procedural memory - probabilistic classification task and Tower of Hanoi.
     • Procedural Memory (Habits): Using instrumental conditioning - initial acquit ion is slow and awkward, but over time behaviours become automatic and routine. Stimulus -> Response Habits.
       • Basal Ganglia: 'switch' from purposeful action to automatic habitual behaviour occurs in the caudate and putamen. Electric stimulation of the caudate improves learning od new instrumental responses. In rats, lesions to the caudate impair the learning of new instrumental responses, while lesions to the putamen disrupt established (habitual) instrumental responses

7. Double Dissociations

   • Neuropsychology looks for dissociations:
     • H.M. impaired on tests of episodic memory but not procedural
     • P.F (KS patient) impaired on tests of episodic memory but not Tower of Hanoi (procedural)
     • Parkinson's patients impaired on procedural memory tasks but have in-tact declarative memory