Download An item is maintained in the working memory state by short

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Synaptic Theory of
Working Memory
Caitlin Hof & Steven Larson
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Spike Timing Measurements

Measures the firing rate of neurons

Is assumed to be a correlate to working memory

Is done with single neuron electrodes (electrophysiology)
and EEG

Has allowed for the theoretical measurement and
identification of working memory length and location
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Synaptic Theory

Memories are preserved in the short term by constant
activation of neurons

Measuring activity spikes shows what is thought to be
memory

Memories in working memory can be reactivated by a
general stimulus

Adjusting chemical balance within the synapses affects
length of working memory
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Synaptic Theory (Continued)

Short term plasticity leads to the creation of Polychronous
Neuronal Groups

PNGs form and dissipate to hold temporary memories

These are energy inefficient but hold memory in its purest
form
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Visual STM

Where it occurs are Prefrontal Cortex and Inferior Temporal
Regions

Ablations to area TE affect STM while the Perirhinal cortex
affects LTM

Activity in the PFC takes place in fewer neurons but
continues during distractors

Suggests that a circuit like series of neurons stores the
stimulus for later comparison
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Synaptic Theory of
Working Memory
Gianluigi Mongillo, Omri Barak, & Misha Tsodyks
Science Volume 319 14 March 2008
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Working Memory
 Holds
information temporarily
 Processing
 Crucial
to cognitive tasks
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Delayed response paradigm

Stimulus kept until task execution

Stimulus specific spiking activity – neuronal correlate of
working memory
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Theoretical Construct

Delay activity – intrinsic cell properties
OR

Persistent reverberations in selective neural populations
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Theoretical Construct Issues

Both theories suggest working memory might not be
entirely in spiking memory

Energetically expensive to hold information in spiking form

High metabolic cost of action potentials
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Alternative Theory

Presented alternative account

Based on properties of excitatory synaptic transmission in
prefrontal cortex
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Prefrontal Cortex

Higher order functions

Cognitive behavior

Personality

Decisions

Planning

Thoughts and actions for internal
goals

Executive function
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Alternative Theory

Prefrontal cortex implicated in working memory

Excitatory synaptic transmission is facilitatory

Sensory -> depression
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Alternative Theory
Proposed: An item is maintained in the
working memory state by short-term
synaptic facilitation mediated by
increased residual calcium levels at the
presynaptic terminal of the neurons that
code for this item
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A Little Biology
https://www.google.com/search?hl=en&q=neurons%20with%20calcium&psj=1&bav=on.2,or.r_qf.&bvm=bv.43828540,d.b2U&biw
280&bih=666&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi&ei=fAVIUazmLMXT2QWslYDYBg#imgrc=egmQIIRhiuVYM%3A%3B6dqfSBdLjAvs0M%3Bhttp%253A%252F%252Fclick4biology.info%252Fc4b%252F6%252Fimages%252F6.5%252Fs
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Alternative Theory

Removal of residual calcium from presynaptic terminals is
relatively slow

Memory can be transiently held for about 1 s without
enhanced spike activated
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Methods

Recurrent network of integrate and fire neurons

Network encodes a set of memories (items)


Composed of randomly selective populations of excitatory
neurons
Connections between the neurons coding for the same
memory stronger than connections between different
populations
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Figure 1B
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Figure 1A
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Figure 1 A
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Memory Maintenance with Synaptic
Facilitation (Figure 2A)

Stimulation – loading one item into working memory by
transient external excitation to the corresponding neuronal
population
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Memory Maintenance with
Synaptic Facilitation (Figure 2B)
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Memory Maintenance with
Synaptic Facilitation (Figure 2C)
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Memory Maintenance with
Synaptic Facilitation (Figure 2)
 The
use of residual calcium at synaptic
terminals as a memory buffer requires low
emmision rates
 Buffer
content is not substantially affected
by the neural activity of the rest of the
cortex
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Conclusions

Working memory can be maintained by short-term synaptic
facilitation

Accumulation of residual calcium in the presynaptic
terminals could carry the information about the recalled
memory in a working form, reducing the need for
metabolically costly action potentials

Memories -> spiking memory

A result of global reactivating input to the network or by virtue of
the intrinsic network dynamics
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Conclusions

Not all stimuli enter working memory

Decision to allow items into working memory is mediated by
attention – represented by the global excitatory output

Performance of human observers on memory tasks positively
correlates with the level of neural activity during the
presentation of the items

Model predicts that residual calcium at the presynaptic
terminals should be enhanced during working memory even
if there is no increase in firing rate
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Conclusions

Suppressing the spiking activity for several hundred
milliseconds should still allow for the memory to reactivate
after the suppressing input is withdrawn

Groups of neurons could exhibit brief epochs of coherent
firing

Model provides a possible target for a pharmacological
interference with working memory

Manipulations that modify the facilitation/depression
balance in the memory related cortical areas are predicted
to have a strong effect on the stability and duration of
memory