Medial prefrontal cortex predicts internally driven strategy shifts

Abstract

Many daily behaviors require us to actively focus on the current task and ignore all other distractions. Yet, ignoring everything else might hinder the ability to discover new ways to achieve the same goal. Here, we studied the neural mechanisms that support the spontaneous change to better strategies while an established strategy is executed. Multivariate neuroimaging analyses showed that before the spontaneous change to an alternative strategy, medial prefrontal cortex (MPFC) encoded information that was irrelevant for the current strategy but necessary for the later strategy. Importantly, this neural effect was related to future behavioral changes: information encoding in MPFC was changed only in participants who eventually switched their strategy and started before the actual strategy change. This allowed us to predict spontaneous strategy shifts ahead of time. These findings suggest that MPFC might internally simulate alternative strategies and shed new light on the organization of PFC.

Figure 1. Spontaneous Strategy Switch Task

Panel (A) shows an example sequence of trials in different conditions. The stimulus consisted of a white frame, which was partially filled with small, randomly distributed, colored squares (all either red or green). In standard trials, the patch of colored squares was closest to only one corner of the white frame. In ambiguous trials, the patch was at equal distance to all corners of the frame. In Late Go trials, the white frame appeared with a delay of 2000ms. Numbers below condition names show trial proportions and durations. (B) Participants were instructed to react to the corner of the patch within the white frame (left, S-R = stimulus-response). Alternatively, after the first 10 minutes participants could detect the correlation between color and response and decide to use color to generate the answer (right).

Figure 2. Behavioral results

During the experiment, participants spontaneously separated into two groups of either color (blue) or corner users (grey lines, group membership determined by questionnaire). (A) Average percent of choices consistent with color when stimulus position was ambiguous across blocks (84 trials). The shaded areas on the left and right indicate blocks before color correlated with corner and blocks with instruction to use color, respectively. Background color indicates the binomial probability of choices assuming random behavior. (B) Color users showed abrupt onsets of color use. For each participant, all data were aligned to the individual change-point (dashed line). (C – F) Several independent behavioral markers showed significant Group × Time interactions, including a transient RT slowdown before the switch (C, upper panel shows RTs, lower panel lag-2 RTs, i.e. RT differences between blocks n and n-2), reduced errors and congruency effects following the switch (D+E), as well as increased anticipatory key-presses in delayed trials (F). All Error-bars/shadings represent mean +/− SEM. See also Figure S1.

Figure 3. Stimulus color decoding

Classification accuracy was analyzed separately for either all blocks (mean), only blocks immediately before the switch or after the switch (see gray shading in B, see also Figure S1) (A) Color maps show areas in which stimulus color could be decoded (p_FWE < .05, cluster corrected). All three time windows showed distinct brain areas. Evidence for mean (time constant, see right) color representation was found in visual areas only, whereas color information emerged in MPFC immediately before the switch (left), and was at last found in the Insula and DLPFC (medial frontal gyrus, MFG, BA10). (B) Time courses of decoding accuracy from shown clusters (smoothed with run. avg. of 2). See also Figure S2. Peak locations of individual subjects can be found in SI, Table S1.

Figure 4. Stimulus corner decoding in color users

Corner classification was analyzed in separate time windows (indicated by the grey background areas on the time course plots; p_FWE < .05, cluster corrected). The presented results stem from time windows that included either all blocks before (−5 to 0, “Early”) or immediately before (“Before”, same as in Figure 3). Time windows after the switch did not show any significant results and hence are not shown (see also Figure S2). (A) Evidence for corner representation could be found initially in frontal brain areas (medial frontal gyrus, MFG, BA10), as well as in middle cingulate gyrus (MCC), and transiently before the switch in superior parietal lobule (SPL) (B) Time courses from shown clusters, gray background area indicates relevant time window. See also Figure S2.

Figure 5. Relations of information encoding to performance

Evidence for color representation in MPFC could be used to determine group membership, even before participants showed behavioral signs of using the color. Time courses of color classification accuracy were extracted from brain areas encoding color during the switch (A) and used to classify subjects into color and corner users as a function of time (B; smoothing as in Figs.3 & 4). See also Figures S3 and S4.