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The Concept of a Decision Threshold in Sensory-Motor Processes Xiao-Jing Wang Department of Neurobiology Yale University School of Medicine Ramp-to-threshold (drift diffusion) Model Ratcliff (Psychol Rev 1978) Schall (Nature Rev Neurosci 2001) Mazurek et al (Cereb Cortex 2003) Sigman and Dehaene (PLoS 2005) Frontal eye field neuron Preparation of an action Hanes and Schall, Science 1996 Probabilistic decision-making in a visual motion two-alternative forced choice task Reaction Time Task LIP Roitman and Shadlen 2002 Open Questions • What is the neural mechanism that produces a long integration time for accumulation of evidence in favor of one of choice alternatives? • What is the source of neural stochasticity that gives rise to decision variability at the behavioral level? • What is the circuit dynamics underlying competition between neural pools representing different choice alternatives, or response vs no response, or a reflexive vs a voluntary response? • What is the neural substrate of a decision threshold? Membrane Time Constant=10-20 ms Lorente de Nó’s reverberatory circuit: attractor network model Levitt et al 1993 Kritzer and Goldman-Rakic 1995 Pucak et al 1996 A cortical circuit model of decision making IA IB c’=(IA-IB)/(IA+IB) A=Upward motion B=Downward motion 2-population excitatory neurons (integrate-and-fire neurons driven by Poisson input) • • • Slow reverberatory excitation mediated by the NMDA receptors at recurrent synapses Winner-take-all competition by feedback inhibition Wang, Neuron 2002 Wong and Wang J Neurosci 2006 Wang Neuron 2002 Wong & Wang J Neurosci 2006 Neural substrate of a decision threshold? A bursty neuron in superior colliculus Fixation Target D Munoz and R Wurtz A Large-Scale Network Model of Decision-Making Cortex Caudate SNr SC Lo and Wang Nature Neurosci (2006) Model Monkey Data Reaction time decreases with increasing coherence Weber's Law Data by J Roitman, J Ditterich and M Shadlen Tentative answers • • • • What is the neural mechanism that produces a long integration time for accumulation of evidence in favor of one of choice alternatives? Slow reverberation mediated by NMDA receptors in a recurrent circuit What is the source of neural stochasticity that gives rise to decision variability at the behavioral level? Fluctuating neural spiking activity What is the circuit dynamics underlying competition between neural pools representing different choice alternatives, or response vs no response, or a reflexive vs voluntary a response? Winner-take-all attractor dynamics instantiated by strong recurrent excitation and feedback inhibition What is the neural substrate of a decision threshold? A `hard-threshold’ for the input drive to downstream neurons Multiple ramping processes in a distributed neural circuit? Hikosaka, Physiol Rev 2000 SEF Basal Ganglia Sensory Thalamus (also PFC, FEF, LIP) 20 Hz 1s Schlag-Rey et al. J. Neurophys. ‘99 Tremblay et al. J. Neurophys. ‘98 Komura el al. Nature ‘01 Acknowledgments Nicolas Brunel Chung-Chuan Lo Kong-Fatt Wong The Swartz Foundation