Transcript Slide 1

Neuromputational models of
working memory
Durstewitz, D., Seamans, J. K. and Sejnowski, T.J.
2000. Nature Neuroscience, Vol. 3, 1184-1191.
Johan Engström
Working memory
• WM=The ability to hold and manipulate
information over time to guide future actions ->
requires sustained neural activation in the
absence of stimulus input
• In the real world:
– Remembering a phone numbers, mental calculation,
conversing on the cell phone
• In (monkey) experiments:
– Delayed response tasks
• Pre-frontal cortex main brain area involved
Emprical evidence from singleneuron recording (Fuster, 1973)
Sustained
activity
Stimulus
presentation
Response
(reach left or right)
Distractor (crying monkey)
What are the mechanisms behind selective sustained activity?
A neurdynamical model based on recurrent excitation
(Amit and Brunel, 1995 )
Inhibition
Sensory input
Cell assembly (strongly connected
neurons)
I
Recurrent connections


dI i
  I i   wi , j R( I j )  G  R( I j )  I aff
dt
Total synaptic input current
Bistability in recurrent networks
”Jump” to active
state (bifurcation)
Sustained without
input
Excitation
threshhold
Sensory input current
Additional mechanisms at synaptic
level
• Including modulation by dopamine in the
model stabilises sustained activity
(Durstewitz, Kelc and Güntürkün, 1999)
• How to maintain novel items in WM?
Cellular instability:
– Dopamine and acetylcholine may change
dynamical properties of the cell (induce
bistability) regardless of pre-established
synaptic weights
– Due to non-linear voltage-current dependence
of NMDA-receptors
Conclusions
• Selective sustained neural activation during
werking memory tasks can be explained in terms
of recurrent excitation neurodynamical models
• Implements a bistable dynamical system with
”on” and ”off” attractors that are robust to
perturbations
• Additional mechanisms at the synaptic level –
may explain how maintaing novel stimuli in WM
is possible