Decision making as a window on cognition

Abstract

A decision is a commitment to a proposition or plan of action based on information and values associated with the possible outcomes. The process operates in a flexible timeframe that is free from the immediacy of evidence acquisition and the real time demands of action itself. Thus, it involves deliberation, planning, and strategizing. This Perspective focuses on perceptual decision making in nonhuman primates and the discovery of neural mechanisms that support accuracy, speed, and confidence in a decision. We suggest that these mechanisms expose principles of cognitive function in general, and we speculate about the challenges and directions before the field.

Figure 1. Psychophysics and signal detection theory

A. Psychophysics deploys simple behavioral measures of detection, discrimination and identification to deduce the conversion of physical properties of stimuli to signals in low level processes. The approach identifies a decision stage that connects low level processors to a behavioral report. L(x,y,t) represents the pattern of luminance as a function of space and time. Adapted from (Graham, 1989) B. Application of signal detection theory to a direction discrimination task. A weak rightward motion stimulus gives rise to samples of evidence from the rightward and leftward preferring neurons, respectively, conceived as random draws of average firing rate from the two pools. The decision rule is to choose the direction corresponding to the larger sample of the pair. For Most sample pairs will exhibit the correct sign of the inequality (right sample > left sample), but the overlap of the distributions occasionally gives rise to the opposite, hence an error. C. The depiction in B can be simplified by subtracting the left sample from the right to construct a single difference (Δ) in firing rate. The distribution has a mean equal to the difference of the two means in B and a variance equal to the sum of the two variances. The decision rule is to choose rightward if Δ>0. The error rate is the total probability of Δ<0, that is the area to the left of the criterion line at Δ=0.

Figure 2. Bounded evidence accumulation explains decision accuracy, speed and confidence

A. Drift-diffusion with symmetric bounds. Noisy momentary evidence for/against hypotheses h₁ and h₂ is accumulated (irregular trace) until it reaches an upper or lower termination bound, leading to a choice in favor of h₁ or h₂. In the motion task, h₁ and h₂ are opposite directions (e.g., right and left). The momentary evidence is the difference in firing rates between pools of direction-selective neurons that prefer the two directions. At each moment, this difference is a noisy draw from a Gaussian distribution (inset) with mean proportional to motion strength. The mean difference is the expected drift rate of the diffusion process. The process reconciles the speed and accuracy of choices with two parameters, bound height (±A) and mean of e. If the stimulus is extinguished before the accumulated evidence reaches a bound, then the decision is based on the sign of the accumulation. B. Competing accumulators. The same mechanism is realized by two accumulators that race. If the evidence for h₁ and h₂ are opposite, then the race is mathematically identical to symmetric drift-diffusion. The race is a better approximation to the physiology, since there are neurons in LIP that represent accumulated evidence for each of the choices. This mechanism extends to account for choices and RT when there are more than two alternatives. If the stimulus is extinguished before one of the accumulations reaches a bound, then the decision is based on the accumulator with the larger value (as in Fig 1B). C. Certainty. The heat map displays the correspondence between the state of the accumulated evidence in panel A and the log of the odds that the decision it precipitates will be the correct one. The mapping depends on the possible difficulties that might be encountered. This corresponds to the possible motion strengths in the direction discrimination experiments. The mapping does not depend on presence or shape of the bound. Notice that the same amount of accumulated evidence supports less certainty as time passes. Cooler colors indicate low certainty (e.g., log odds equal to 0 implies that a correct choice and an error are equally likely). In the post decision wagering experiment, the monkey opts out of the discrimination and chooses the sure-but-small reward when the accumulated evidence is in the low certainty (cooler) region of the map. Adapted from Gold & Shadlen, 2007 and Kiani & Shadlen, 2009.

Figure 3. Neural and behavioral support for bounded evidence accumulation

A. Choice-reaction time (RT) version of the direction discrimination task. The subject views a patch of dynamic random dots and decides the net direction of motion. The decision is indicated by an eye movement to a peripheral target. The subject controls the viewing duration by terminating each trial with an eye movement whenever ready. The gray patch shows the location of the response field of an LIP neuron. B. Effect of stimulus difficulty on choice accuracy and decision time. The solid curve in the lower panel is a fit of the bounded accumulation model to the reaction time data. The model can be used to predict the monkey’s accuracy (upper panel). The solid curve is the predicted accuracy based on bound and sensitivity parameters derived from the fit to the RT data. C. Response of LIP neurons during decision formation. Average firing rate from 54 LIP neurons is shown for six levels of difficulty. Responses are grouped by motion strength (color) and direction (solid/dashed toward/away from the RF). Firing rates are aligned to onset of random-dot motion and truncated at the median RT. Inset shows the rate of rise of neural responses as a function of motion strength. These buildup rates are calculated based on spiking activity of individual trials 200–400 ms after motion onset. Data points are the averaged normalized buildup rates across cells. Positive/negative values indicate increasing/decreasing firing rate functions. D. Responses grouped by reaction time and aligned to eye movement. Only T_in choices are shown. Arrow shows the stereotyped firing rate ~70 ms before saccade initiation. Adapted from Shadlen et al. (2006) and Roitman & Shadlen (2002).