How facilitation influences an attractor model of decision making

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How facilitation influences an attractor model of
decision making

• Larissa Albantakis

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Outline

• Facilitation and depression in cortical neurons
• How does it work and what is it good for?
• 2. Facilitation and working memory
• 3. Facilitation and decision making
• Sequential decision making
• Parallel decision making
• 4. Outlook

Lunch
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Neurophysiological mechanism
Idea - docked pool of vesicles containing
neurotransmitter - each released vesicle is
replaced with a time constant td. -
intracellular calcium responsible for release
probability of vesicles
Synaptic depression The neuron fires more
rapidly than vesicles can be replaced. ? Vesicle
depletion

• Facilitation
• APs arriving in the presynaptic terminal cause
  the accumulation of calcium ions.
• ? increase of released vesicles/AP

From Wang Y et al. 2006
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Possible benefits of facilitation and depression

• Synaptic Filter (low-, high- or band-pass)
• Decorrelation
• Burst detection
• Stabilize against noise (Gain Control)

• Persistent activity
• Depression
• Persistent activity can be maintained with
  relatively low firing rates (e.g. 15 Hz).
• Facilitation
• May increase the strength of connections above a
  critical threshold necessary for the stability of
  persistent firing.

? Brief periods of synaptic activity may
transiently shift connected cortical neurons into
a state in which recurrent excitation is
sufficiently strong to support persistent
activity.
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Mostly depression in the cortex?
Short-term synaptic depression between pyramidal
neurons has been observed in all cortical areas
that have been examined (mainly V1, sensory
areas).

• Special properties of PFC
• Heterogeneity of synaptic dynamics (40-50
  facilitation dominant).
• Facilitation is often masked by high initial
  probability of release.
• Augmentation with time constants of several
  seconds.

From Wang Y et al. 2006
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Why in PFC? Working memory!
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Why in PFC? Working memory!

• Synaptic Theory of Working Memory
• Mongillo et al. (2008) Science 319, 1543
• Memory is maintained by short-term synaptic
  facilitation.
• (mediated by increased residual calcium levels
  at the presynaptic terminals)

• Object Working memory dominated by recurrent
  inhibition
• Brunel and Wang (2001) J Comp Neurosci 11, 63-85
• Working memory based on persistent activity
  maintained through recurrent NMDA synapses with
  long time constants.
• (Synapses are permanently enhanced by previous
  long-term potentiation)

x available resources (vesicles) u fraction of
resources used by each spike (residual calcium
level) tF 1.5 s gtgt tD 0.2 s
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F-D spiking network

• integrate-and-fire neurons
• (inhibitory and excitatory)
• structured network
• (p selective populations each encoding one
  memory item)
• E-E connections weights are multiplied by
  u(t)x(t) (0 gt ux gt 1)

From Mongillo et al. 2008
? Different regimes of working memory depending
on background input
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F-D spiking network

• Memory maintained synaptically

Memory maintained by persistent firing
From Mongillo et al. 2008
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Temporary vs. permanent synaptic enhancement

• Advantages of strengthening synapses
  temporarily
• metabolically more efficient
• (conserve synaptic resources during periods of
  baseline activity)
• greater flexibility and control
• (synaptic facilitation is activity dependent and
  can vary in a graded fashion)
• regulation of persistent activity by altering
  presynaptic release probability through
  neuromodulators (e.g. dopamine)
• Quiescent memory ? decoupling from other brain
  areas

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So many advantages why not try in decision
making?
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Facilitation and sequential decision making

• Task Comparison of two vibrotactile stimuli
  applied sequentially

Problem Model of Wang 2002 (attractor network of
two competing populations) does not work in
sequential decision making
Deco et al. in preparation
Experimental findings partially differential
neurons in VPC respond to f1, show no persistent
activity, but ramp up at the end of the delay
period and are influenced by f2 during the
decision period
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Facilitation and sequential decision making
Specific input
f2
f1
delay
Global attention signal
Deco et al. in preparation
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Facilitation and sequential decision making

• During application of f2, the level of activity
  will depend on the synaptic history (uf1) and on
  f2.

Together with the neurons that encode just f2, a
decision of f1 lt f2 or f1 gt f2 can be reached
with a standard attractor model of decision
making.
f1 gt f2
f1 lt f2
Deco et al. in preparation
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Facilitation and parallel decision making

• Task Gather evidence (dot motion) and decide
  between multiple targets

Albantakis Deco 2009

• Facilitation
• Network performance and decision dynamics are
  mainly preserved.
• But greater flexibility in weights!

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Outlook
Idea Use facilitation (or combination of
facilitation and depression) for experimental
findings that are currently not completely
accounted for by the multiple choice model
without facilitation

• Log Odds (coherence-dependent difference between
  in and out choice)
• Extend the multiple choice model to more than 4
  choices

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Thanks
to my supervisor Prof.Gustavo Deco Consolider
and to everybody for listening!
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Facilitation and parallel decision making