6 - shortterm and working memory

• Information content —> What?

• Time course —> How long?

short-term memory (passive sensory trace)

—> temporary storage of information

working memory (active exploitation of long-term memory)

storage and manipulation of information

long-term —> > minutes

• implicit: procedural memory (skills like riding bike, learning to swim, …)

  tied to subcortical areas (basal ganglia)

• explicit: allow to store facts and episodes

  tied to hippocampus

three-component system:

• Phonological loop: verbal material 

• Visuo-spatial sketch pad: visual material 

• Central executive: control stage 

—> Core cognitive function for intelligent behavior

brain’s RAM chip:

• store information over short time periods

• manipulate information to think and act

• core cognitive function for intelligent behavior

• Persistent activity

  • single-unit responses

• Transient activity

  • gamma/ beta bursts

• Silent activity

  • altered synaptic weights

single-unit responses —> individual prefrontal cortex (PFC) neurons fire persistently for memorized items

—> Experiments:

• Fuster and Alexander (1971):

  • monkey remembers in which box a fruit was

    • Cue: show fruit

    • Delay: remember

    • Prompt: decide

• Kandel (2013)

  • monkey remembers if first shape = second shape -> Delayed Match to sample (DMS)

Similar monkey memory experiment:

—> Gamma bursts (~75Hz) are only found at informative spiking

—> Gamma burst —> Beta Burst —> Gamma …

• metabolically less expensive

• encode multiple items

• explains limits on working memory capacity

• better at resisting distractors

—> But: lack of correlation between gamma bursting and animal performance in tests

Task-relevant input is stored in the network by altered synaptic weights

—> Computational model -> no real experimental proof yet

—> cooperation of many high-level cortical parts from frontal, parietal, and temporal lobe

• PFC and parietal activity during working memory tasks

  • —> Lesion leads to absence of signal and worse performance

• PFC and parietal signals have distinct channels

  • PFC —> theta (~3Hz)

  • Parietal —> alpha (~8-12Hz)

• PFC timing of signal (phase) is important

  • in phase of parietal: target

  • out of phase: distractor

• Stimulation of cortex with frequencies

  • PFC with theta —> better performance

  • Parietal with alpha —> better performance

Working memory content can be decoded from the visual cortex

-> storage and active manipulation of information for organizational behavior