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Summary: Behavioral/Systems/Cognitive
A Recurrent Network Mechanism of Time Integration in
Perceptual Decisions
Kong-Fatt Wong and Xiao-Jing Wang
Volen Center for Complex Systems, Brandeis University, Waltham, Massachusetts 02454
Recentphysiologicalstudiesusingbehavingmonkeysrevealedthat,inatwo-alternativeforced-choicevisualmotiondiscriminationtask,
reaction time was correlated with ramping of spike activity of lateral intraparietal cortical neurons. The ramping activity appears to
reflect temporal accumulation, on a timescale of hundreds of milliseconds, of sensory evidence before a decision is reached. To elucidate
thecellularandcircuitbasisofsuchintegrationtimes,wedevelopedandinvestigatedasimplifiedtwo-variableversionofabiophysically
realistic cortical network model of decision making. In this model, slow time integration can be achieved robustly if excitatory reverber-
ationisprimarilymediatedbyNMDAreceptors;ourmodelwithonlyfastAMPAreceptorsatrecurrentsynapsesproducesdecisiontimes
that are not comparable with experimental observations. Moreover, we found two distinct modes of network behavior, in which decision
computation by winner-take-all competition is instantiated with or without attractor states for working memory. Decision process is
closely linked to the local dynamics, in the "decision space" of the system, in the vicinity of an unstable saddle steady state that separates
the basins of attraction for the two alternative choices. This picture provides a rigorous and quantitative explanation for the dependence
of performance and response time on the degree of task difficulty, and the reason for which reaction times are longer in error trials than
incorrecttrialsasobservedinthemonkeyexperiment.Ourreducedtwo-variableneuralmodeloffersasimpleyetbiophysicallyplausible
framework for studying perceptual decision making in general.
Key words: sensory discrimination; intraparietal cortex; reaction time; computational modeling; dynamical systems; NMDA
Introduction
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