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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