Effects of cortical microstimulation on confidence in a perceptual decision

Abstract

Decisions are often associated with a degree of certainty, or confidence--an estimate of the probability that the chosen option will be correct. Recent neurophysiological results suggest that the central processing of evidence leading to a perceptual decision also establishes a level of confidence. Here we provide a causal test of this hypothesis by electrically stimulating areas of the visual cortex involved in motion perception. Monkeys discriminated the direction of motion in a noisy display and were sometimes allowed to opt out of the direction choice if their confidence was low. Microstimulation did not reduce overall confidence in the decision but instead altered confidence in a manner that mimicked a change in visual motion, plus a small increase in sensory noise. The results suggest that the same sensory neural signals support choice, reaction time, and confidence in a decision and that artificial manipulation of these signals preserves the quantitative relationship between accumulated evidence and confidence.

Figure 1. Post-decision wagering (PDW) reflects confidence in the motion decision

(A) PDW task sequence (see Experimental Procedures). Red spots indicate direction targets, blue spot is the ‘sure bet’ target (T_s). (B) Probability of choosing T_s as a function of viewing duration and motion strength (color coded). Combined data from no-μS trials for two monkeys (N = 26,924 trials). Solid traces are running means (proportions) of the data sorted by viewing duration. Dashed traces in all panels are fits to the bounded accumulation model (see text and Figure 4). (C) Improvement in decision accuracy on no-μS trials when the sure bet was offered but waived. Solid traces are running means using all nonzero coherences and directions. (D) Same format as C, but broken down by motion strength (absolute value of coherence) and pooled across viewing durations. Symbols indicate the mean ± s.e.

Figure 2. Effects of μS on PDW and perceptual decisions

(A) The proportion of trials in which the monkey opted out of the direction task and chose the sure bet, comparing μS and no-μS trials (N = 63 sites). (B) Top: proportion of sure-bet (T_s) choices as a function of motion strength (percent coherence; positive = preferred direction of neurons at the stimulation site) for all sessions in both monkeys (N = 53,134 trials). Red and blue data points indicate μS and no-μS trials, respectively, combining across all viewing durations. Error bars (s.e.) are smaller than the data points. Top, inset: proportion T_s choices in separate control sessions, for trials with (red) and without (blue) an offset added to the motion coherence in lieu of μS (“Δcoh”, see text and Figure S3). Bottom: proportion of preferred-direction choices as a function of motion strength, plotted separately for the four conditions of the 2×2 design: μS present (red) or absent (blue), and T_s offered but waived (solid curves and filled symbols) or T_s not offered (dashed curves and open symbols). In both panels, smooth curves represent fits to the bounded evidence-accumulation model (see text), with the exception of the red solid and dashed curves in the bottom panel. These are the predicted μS choice functions based on a fit to the remaining observations. (C) Comparison of the effect of μS on choices (represented as an equivalent change in motion strength) on trials with and without the T_s option.

Figure 3. Stimulation with high current disrupts both accuracy and confidence

(A and B) Combined data from eight experiments using 75 μA stimulation (N = 4,483 trials). Same conventions as Figure 2B. Smooth curves are best fits of the extended model described in the Supplemental Experimental Procedures (see also Figure S2).

Figure 4. Bounded evidence accumulation model explains PDW and effects of μS

(A) Colored traces represent the accumulation of noisy motion evidence (i.e., from MT/MST) on three individual (simulated) trials. Evidence is drawn from a Gaussian distribution with mean (R) proportional to motion strength and standard deviation (σ) equal to 1. Decision formation terminates when the stimulus is turned off (green and blue trials) or when the accumulated evidence (the decision variable, x) reaches a bound at ±B (red trial). (B) The model prescribes a sure-bet choice when the logarithm of the odds of being correct is below a fixed threshold, θ, indicated by the black contours which divide the x,t-plane into low and high confidence regions. For example, the blue trial in panel A, but not the green trial, would have ended in a sure-bet choice. (C and D) The probability density of the decision variable across time is shown for a particular motion coherence (3.2%, i.e., weak preferred-direction motion), either with (C) or without (D) μS. Bias in this example was set to zero for simplicity. Microstimulation shifts the density upward, thereby decreasing the probability of a sure bet and increasing the probability of a preferred choice (see Figure S4). A key assumption of the model is that the brain applies the same mapping between accumulated evidence and the expected log odds of being correct (i.e., confidence), and the same criteria for opting out (black contours in B–D), irrespective of the presence of μS.