Models of the decision process for two different two-alternative decision problems.

<p>(a) Decision model for a single source two-alternative forced choice decision. A frequently used stimulus in such tasks is the random dot kinematogram (RDK), which consists of a number of dots, only some of which move in a particular “signal” direction. Subjects are typically asked to decide in which of two directions the RDK is moving. With a single source of visual information the sensory data provide evidence for both alternatives. In motion discrimination tasks this evidence comes from neurons in area MT whose activity (firing rate) is tuned to respond to motion in a particular direction. At each moment in time, evidence from two populations of neurons is used to update a single decision variable. (b) Decision model proposed by Krajbich et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078993#pone.0078993-Laming1" target="_blank">[18]</a> for a comparative two-alternative decision problem with two sources of visual stimuli. In their study, participants were asked to choose between two food items presented simultaneously in different locations on the screen. With each stimulus providing evidence about one particular alternative, the decision variable is no longer updated simultaneously by evidence for all available targets. Instead, as a target is fixated (target X in (b)) evidence supporting that target is generated and incorporated into the decision variable. During this time, the mechanisms that represent the evidence for the non-fixated target remain silent (shaded out branch for target Y in (b)).