(A) Momentary evidence, in favour of choice ‘A’ over choice ‘B’, is accumulated over time. The time-varying accumulation of evidence is termed the decision variable (DV; black line). (B) The bounded accumulation model applied to the cylinder task with microstimulation. The momentary evidence (e) is the difference in activity between two neuronal pools, one selective for CW rotation and one for CCW rotation. Parameter k represents the sensitivity of these sensory representations to the cylinder disparity C (Palmer et al., 2005). In each trial, the DV follows a stochastic ‘drift-diffusion’ path with an average drift rate dependent on the mean of e = kC. Parameter B represents the distances to the CW and CCW decision bounds, which are assumed to be equal. Microstimulation contributes to e as additional evidence in favour of the sensory preference of the stimulated V5/MT multi-unit site (CCW in this example). The perceptual decision depends on which bound was first reached, or, if neither is reached, which bound the DV is nearest to at the end of stimulus viewing. (C) Model simulations illustrate that the slope of the psychometric function, fitted by a simple logistic model of bounded accumulation (Equation 3a), can be affected by changes in either parameter k or B; for example, an increase in either parameter will steepen the slope, indicating improved performance. (D) Model simulations illustrate how insertion of additional sensory evidence by electrical microstimulation in visual cortex biases perceptual choices toward the preferred (PREF) cylinder disparity of the stimulated multi-unit (MU), which is revealed as a shift of the psychometric function. (E) An increase in parameter B steepens the slope of the psychometric functions, but does not change the size of microstimulation shift. This is because by the evidence-accumulation stage, visually evoked and electrically evoked signals are combined and cannot be differentially affected (see panel B). (F) An increase in parameter k also steepens the slope of the psychometric function. However, improved perceptual sensitivity to the stimulus affects only the visually-evoked sensory representations (see panel B), so the relative contribution of the electrically-evoked signals is decreased, which results in a decrease in the microstimulation-induced shift of the psychometric function.