Posterior parietal cortex plays a causal role in perceptual and categorical decisions

Abstract

Posterior parietal cortex (PPC) activity correlates with monkeys' decisions during visual discrimination and categorization tasks. However, recent work has questioned whether decision-correlated PPC activity plays a causal role in such decisions. That study focused on PPC's contribution to motor aspects of decisions (deciding where to move), but not sensory evaluation aspects (deciding what you are looking at). We employed reversible inactivation to compare PPC's contributions to motor and sensory aspects of decisions. Inactivation affected both aspects of behavior, but preferentially impaired decisions when visual stimuli, rather than motor response targets, were in the inactivated visual field. This demonstrates a causal role for PPC in decision-making, with preferential involvement in evaluating attended task-relevant sensory stimuli compared with motor planning.

Fig. 1.

Behavioral task. (A) Monkeys reported their categorical (MDC task) or directional (MDD task) decisions about visual motion stimuli by choosing either the green or red saccade target. The positions of red and green targets were randomly chosen on each trial. Using an RT design, monkeys could initiate their saccade as soon as they had made their decision. (B) The motion direction categorization task (MDC) required grouping ten motion directions (indicated by the direction of the arrows) into two categories (indicated by the color of the arrows) defined by a learned category boundary (black dashed line). In the motion direction discrimination task (MDD), two motion directions (the two category center directions from the MDC) were shown at three coherence levels. (C) Three spatial stimulus configurations tested LIP’s role in sensory evaluation (S_IN), saccade planning (T_IN), and a control condition assessing non-spatial aspects of the tasks (B_OUT). The dark shaded area and the dashed circle represent the inactivated visual field and the position of motion stimulus, respectively. The red and green spots indicate the positions of the saccade targets, and the yellow spot indicates the position of fixation.

Fig. 2.

Causal evidence for decision-related sensory evaluation in LIP. (A) Psychometric curves for the S_IN condition of the MDC task. Task accuracy pooled across both monkeys is plotted as the proportion of choosing the primary category, defined as the category for which each monkey showed a greater decrease in accuracy (on average across all sessions) following LIP inactivation (see Fig. S6 and S7 for data shown for each monkey separately). (B) Chronometric curves are shown for the S_IN condition of the MDC task. (C-D) The psychometric and chronometric curves in the B_OUT condition of the MDC task, (same format as A and B). (E-F) Comparisons of the averaged behavioral deficits following LIP inactivation in S_IN and B_OUT conditions of the MDC task. (G-L) Monkeys’ behavioral performance in MDD task is plotted for inactivation and control sessions. Psychometric and chronometric curves for S_IN (G-H) and B_OUT (I-J) conditions are shown in the same format as the MDC task (A-D). Monkeys showed a significantly greater deficit in S_IN than B_OUT conditions in the MDD task (k-L). (M-P) Paired comparisons between inactivation and control sessions for choice bias and threshold in the S_IN conditions of the MDC (M-N) and MDD tasks (O-P). The open and filled symbols denote the inactivation sessions in which the majority of the recorded neurons at the targeted cortical locations preferred the primary (open) and non-primary (filled) category/direction (see Supplementary Materials). The black stars indicate the statistical significance (*: p<0.05, **: P<0.01, ***: p<0.001/, unpaired t-test, multiple tests in A-D and G-J are Bonferroni corrected). The error bars denote ±SEM. P: primary, NP: non-primary, cat: category, dir: direction, H: high, M: middle, L: low, S_IN: stimulus-in, B_OUT: both-out.

Fig. 3.

Causal evidence for decision-related saccade planning in LIP. (A) Psychometric curves for T_IN condition of MDC task. The choice accuracy is plotted as the proportion of contralateral saccades relative to the inactivated hemisphere. Data from both monkeys were pooled based on target location. (B) Chronometric curves for T_IN conditions of MDC task. (C-D) Comparisons of behavioral impairments following LIP inactivation between ipsilateral and contralateral target trials in T_IN condition. Monkeys’ saccade choices were biased toward the ipsilateral target following LIP inactivation, shown by both accuracy(C) and RT(D). (E-F) Comparisons of overall behavior deficits following LIP inactivation between S_IN and T_IN conditions of MDC task. Monkeys showed significantly greater behavioral impairment in the S_IN than T_IN condition, in their accuracy(E), but not RT(F). (G-L) Monkeys’ behavioral performance in T_IN condition of MDD task. Psychometric and chronometric curves (G-H) are in the same format as the MDC task (A-B). Monkeys showed consistent saccadic choice biases following LIP inactivation in both MDD and MDC tasks (I-J). In the MDD task, a greater deficit was observed following LIP inactivation in the S_IN than T_IN condition in accuracy but not RT (K-L). (M-P) Paired comparisons between inactivation and control sessions for choice bias and threshold in the T_IN conditions of the MDC (M-N) and MDD tasks (O-P).

Fig. 4.

LIP activity reflects decision-related sensory evaluation. (A-B) Activity is shown for each motion-coherence level for two example direction-selective LIP neurons in the MDD task. The motion stimulus but not the targets appeared in neurons’ RF. The zero coherence trials were grouped based on the monkeys’ choices. The two vertical dashed lines mark the time of target and motion stimulus onset, respectively. (C) Average normalized population activity across all direction-selective neurons is shown for each motion coherence level, aligned to stimulus onset (left panel) or saccade onset (right panel). Activity shown in the left panel is truncated at the monkeys’ mean RT for each coherence level. (D) The motion direction selectivity (DS) (determined by ROC) for different coherence levels is shown as in (C). (E) Average activity on low-coherence trials is shown for neurons’ preferred and non-preferred directions, separately for correct and error trials. (F) Neuronal selectivity on low-coherence trials is compared for the monkeys’ decisions about motion direction compared to the physical direction of stimulus motion. The black stars indicate time periods in which there was a significant difference (P < 0.01, paired t-test). (G-H) DS on low coherence trials (G) and choice selectivity on zero coherence trials (H) is compared between faster RT and slower RT trials. Shaded areas denote ±SEM. (I-J) Partial correlation analysis. (I) The value of r-stimulus (partial correlation between neuronal activity and stimulus direction, given the monkeys’ choices) and r-choice (the partial correlation between neuronal activity and monkeys’ choice, given the stimulus direction) are plotted across time. (J) Correlation between r-stimulus and r-choice. Note that most LIP neurons showed a congruent sign between r-stimulus and r-choice values.