Abstract
Single neurons in cortical area LIP are known to carry information relevant to both sensory and value-based decisions that are reported by eye movements. It is not known, however, how sensory and value information are combined in LIP when individual decisions must be based on a combination of these variables. To investigate this issue, we conducted behavioral and electrophysiological experiments in rhesus monkeys during performance of a two-alternative, forced-choice discrimination of motion direction (sensory component). Monkeys reported each decision by making an eye movement to one of two visual targets associated with the two possible directions of motion. We introduced choice biases to the monkeys' decision process (value component) by randomly interleaving balanced reward conditions (equal reward value for the two choices) with unbalanced conditions (one alternative worth twice as much as the other). The monkeys' behavior, as well as that of most LIP neurons, reflected the influence of all relevant variables: the strength of the sensory information, the value of the target in the neuron's response field, and the value of the target outside the response field. Overall, detailed analysis and computer simulation reveal that our data are consistent with a two-stage drift diffusion model proposed by Diederich and Bussmeyer [1] for the effect of payoffs in the context of sensory discrimination tasks. Initial processing of payoff information strongly influences the starting point for the accumulation of sensory evidence, while exerting little if any effect on the rate of accumulation of sensory evidence.
Highlights
One of the most successful enterprises of experimental psychology and systems neuroscience has been the elucidation of mechanisms underlying simple forms of decision-making
We begin with a consideration of the behavioral performance: The probability of a target 1 (T1) choice as a function of coherence is identical in the HH and LL conditions (Fig. 11A), even though the accumulation process starts at a higher level in the HH than in the LL condition (Fig. 11C); the choice curves for HL and LH conditions are shifted to the left or right compared to either the HH and LL curves, in the simulation as in the behavioral data
We examined the dynamics of neural activity in Lateral Intraparietal Cortex (LIP) while rhesus monkeys performed a 2AFC motion discrimination task under conditions of equal and unequal payoffs signaled at the beginning of the trial
Summary
One of the most successful enterprises of experimental psychology and systems neuroscience has been the elucidation of mechanisms underlying simple forms of decision-making. While the original formulation of Green and Swets was designed only to account for the accuracy of choice data, a rich body of experimental and theoretical work subsequently extended the insights of signal detection theory into dynamical models in which evidence is accumulated gradually over time during single trials. Originating in seminal work by Laming [3], Link and Heath [4] and Ratcliff [5], these models depicted the decision mechanism as a ‘‘diffusion’’ process in which a decision variable assumes a neutral value at the beginning of a trial, ‘‘drifts’’ gradually under the influence of incoming sensory information toward a ‘‘barrier’’. As Ratcliff and McKoon [16] have recently observed: ‘‘It has probably not been realized in the wider scientific community that the class of diffusion models has as near to provided a solution to simple decision making as is possible in behavioral science.’’
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