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Zusammenfassung:
The remarkable ability of the brain to encode task salient information into working memory (WM) involves dopamine (DA) modulation of neural activity in the basal ganglia (BG) and the prefrontal cortex (PFC). We construct a two-component network model to investigate the influence of BG on PFC and the effect of DA modulation in these two areas on WM. The PFC component is a line attractor network encoding information about the angular position of a stimulus by a bump of persistent activity. The BG component is an array of medium spiny neurons (MSN) which become bistable in conditions of high DA. Projections of the MSN onto cortical neurons represent the direct pathway through the BG and thalamus. Under low DA, the BG input to the cortex stabilizes the activity bump against distractors and noise, or enables a switch in its location in response to a new visual stimulus. Under high DA, complementary neuromodulation of PFC and MS units enhances the stabilizing influence of the BG input. Further, DA modulation in the BG allows for the selective encoding of stimuli in WM by locking the pattern of activity of the MSN. This arises from the hysteresis associated with bistability, and, while DA modulation persists, prevents subsequent stimuli from corrupting the WM through the activation of different MSN. These effects illustrate the dual role of the BG in WM, both in gating information and increasing robustness against noise. DA modulation of MSN implements salience selectivity through a nonlinear increase of the threshold for reaching the active `up' state. Finally, since DA influences the modification of striatal synapses, it controls the input to MSN and thus the conditions under which this threshold is exceeded. This form of learning, in conjunction with DA-induced bistability of MSN, enhances the BG's competence at saliency-based gating.
