Cellular and system-level memory consolidation - week 7 (Nicholas)

Question 1

Mueller & Pilzecker (1900)

Answer 1

New memories initially fragile, but soon resistant to interference.
Ss learnt a list of paired-associate syllables (AB) and were tested in cued recall (using the first syllable, A).
Interpolating another list (CD) impaired memory of the first list (AB).
Found temporal gradient, whereby the closer in time the interfering list to the target list, the stronger its amnestic effect.
Memories require time to consolidate.
Retroactive interference compromises the integrity of recently formed—but not yet consolidated— memories.
Interference is ‘nonspecific’, i.e. the interfering material does not have to be similar to the target material (i.e., AB vs. CD).
Mental exertion is the interfering force.

Question 2

The “clay sculpture” metaphor (Wixted & Cai, 2014)

Answer 2

Memory initially at its finest, but fragile; over time, it becomes resistant to interference (i.e., it shows less and less potential for damage).

Question 3

Shape of the forgetting function (Wixted, 2004b)

Answer 3

The clay metaphor, i.e., the idea that memories become more and more resistant to interference fits well with forgetting curves.
The rate with which we forget is not constant; this would be exactly the property of memoryless systems.
Ebbinghaus (1885): the forgetting function is such that we forget less and less as time goes by.
Jost (1897): If two memories have equal strength but different ages, the older trace will decay at a slower rate.
This continuous reduction in the forgetting rate is a sign of consolidation!

Question 4

Cellular consolidation

Answer 4

Occurs at the neuron level (not at the whole-brain “systems” level)
Takes place during the first hours (and perhaps days) after initial memory formation in the hippocampus
Fits well with the idea of a trace-hardening physiological process put forward as far as Mueller and Pilzecker
Corresponds to the discovery of long-term potentiation (Bliss & Lømo, 1973)

Question 5

Long-term potentiation (Bliss & Lømo, 1973)

Answer 5

Long-lasting enhancement of synaptic efficacy induced by a tetanus (short burst of high-frequency stimulation) to the presynaptic neuron.

Question 6

Retrograde facilitation

Answer 6

If subsequent encoding (mental exertion) interferes with memory consolidation, then factors blocking new encoding should promote memory stabilization.
- Alcohol (Bruce & Pihl, 1997; Carlyle, Dumay, et al., 2017)
- Benzodiazepines (anxiolytic drug; Weingartner et al. 1995);
- Slow-wave (non-REM) sleep; Yaroush et al. 1971).
In all, the resulting anterograde amnesia is accompanied by a retrograde facilitation: memories formed prior to drug intake/sleep are forgotten to a lesser degree than memories formed prior to placebo/wake.

Question 7

Systems consolidation

Answer 7

HM’s bilateral medial temporal lobe (MTL) resection:
- (1) anterograde amnesia: inability to form new declarative memories (the ‘what’)
(2) temporally graded retrograde amnesia: impairment of memories formed prior to surgery, stronger for young than old memories (Scoville & Milner, 1957) (Ribot’s law, 1881).
Declarative memories becoming independent from the hippocampus and more dependent on the neocortex is referred to as ‘systems consolidation’.

Question 8

Temporal gradient of semantic memory ─ behaviour

Answer 8

Bayley et al. (2006; Manns et al. 2003): 6 amnesic patients with damage limited to the hippocampal region.
Smith & Squire (2009): 160 questions on news events over 30 years
Systems consolidation can occur very quickly!
Richness of the memorized materials may determine how fast systems consolidation occurs.

Question 9

What is the significance of this shift of activation from the medial-temporal lobe to the neocortex?

Answer 9

Fragments of declarative memories stored in neocortex from the outset (e.g., sensory and semantic areas) as they travel through it.
Hippocampus acts as a relay station and binds these fragments together.
Over time, cortico-cortical associations develop, such that these memories become independent of the hippocampus.

Question 10

Complementary learning systems
(Marr, 1971; McClelland et al. 1995)

Answer 10

a) A fast learning system that holds information only temporarily (medial temporal lobe, i.e., the hippocampus)
b) A slow learning system that serves as long-term store (i.e., the neocortex)
Main idea:
- Information initially stored in the hippocampus is progressively fed back into the neocortex (via ‘neural replay’), so that pre-existing knowledge can accommodate newly learnt information. The hippocampus is seen as the internal sparring partner of the neocortex.
- As both systems are used to encode new learning, this reactivation/redistribution has to occur offline, for instance during sleep.