Cognitive Neurostimulation: Learning to Volitionally Sustain Ventral Tegmental Area Activation

Abstract

Activation of the ventral tegmental area (VTA) and mesolimbic networks is essential to motivation, performance, and learning. Humans routinely attempt to motivate themselves, with unclear efficacy or impact on VTA networks. Using fMRI, we found untrained participants' motivational strategies failed to consistently activate VTA. After real-time VTA neurofeedback training, however, participants volitionally induced VTA activation without external aids, relative to baseline, Pre-test, and control groups. VTA self-activation was accompanied by increased mesolimbic network connectivity. Among two comparison groups (no neurofeedback, false neurofeedback) and an alternate neurofeedback group (nucleus accumbens), none sustained activation in target regions of interest nor increased VTA functional connectivity. The results comprise two novel demonstrations: learning and generalization after VTA neurofeedback training and the ability to sustain VTA activation without external reward or reward cues. These findings suggest theoretical alignment of ideas about motivation and midbrain physiology and the potential for generalizable interventions to improve performance and learning.

Figure 1. Task Design

Pre-test and Post-test: all groups completed identical test runs. During ACTIVATE trials, participants tried to increase motivation using only internally generated thoughts and imagery, without reward cues or rt-fMRI neurofeedback. During COUNT baseline trials, participants counted backward. Training: during ACTIVATE trials, participants in VTA and NAcc Feedback groups tried to increase motivation and received veridical neurofeedback from either VTA or NAcc. FF participants received noise neurofeedback they were told was veridical. VC participants viewed predictable patterns indicating the duration of the ACTIVATE period. During REST trials, each group’s thermometer display presented a random (VTA Feedback, NaCC Feedback, FF groups) or predictable (VC group) pattern. COUNT trials were identical across all runs. An inter-trial interval ranging from 3.5–5.5 s separated all trials.

Figure 2. Significant VTA Activation and Group Differences Emerged in Post-test following Feedback Training

(A) VTA ROI defined in an independent sample of 50 participants. Color scale denotes probabilistic weighting of the ROI. (B) Test run × group interaction plot (p < 0.05) representing percentage signal difference for mean ACTIVATE > COUNT values. Pre-test: no significant activations or group differences. Post-test: VTA Feedback group self-activated the VTA relative to baseline (p < 0.005) and to Control (p < 0.0005) and FF (p < 0.05) groups.

Figure 3. Consistent VTA Activation and Group Differences Emerged with Feedback Training

ERA time courses for ACTIVATE >COUNT during Test and Training trials. Waveforms represent percentage signal difference from baseline (shading, ± SEM). The time course for both ACTIVATE and COUNT is calculated relative to the preceding 3-s inter-trial interval. To compare the time series, we subtracted COUNT from the ACTIVATE time series. Time courses were segmented at 10 s to examine sustained activation (solid horizontal bars represent means). Pre-test: no significant positive activations or group differences. Training: VTA Feedback group showed greater VTA activation than the VC group in both early (p < 0.0001) and late phases of trials (p < 0.05; i.e., across the entire 20 s), but did not significantly differ from FF group (p > 0.1). Post-test: the VTA Feedback group sustained greater activation relative to baseline (early, late, and overall p < 0.05), relative to the VC group (early, late, and overall p < 0.005), and relative to FF group (late and overall p < 0.05). Post hoc t tests (p < 0.05) are denoted by the keys below the time courses. Center white circle, baseline; orange, VTA Feedback; blue, VC; gray, FF; black line, a significant difference.

Figure 4. No Significant NAcc Activation or Group Differences in Test Runs

(A) NAcc ROI defined by Greer et al. (2014). (B) Non-significant test run × group interaction plot (p > 0.1) representing percentage signal difference for mean ACTIVATE > COUNT values. Pre-test: No significant corrected positive activations or group differences were observed. Both control groups were significantly deactivated relative to baseline (p < 0.05). Post-test: The groups did not significantly differ from each other (p ≥ 0.09) and no group self-activated the NAcc relative to baseline (p ≥ 0.1).

Figure 5. No Significant NAcc Activation or Group Differences prior to, during, or following Feedback Training

ERAs for ACTIVATE > COUNT during Test and Training trials. Waveforms represent percentage signal difference from baseline (shading, ± SEM). The time course for both ACTIVATE and COUNT is calculated relative to the preceding 3-s inter-trial interval. To compare the time series, we subtracted COUNT from the ACTIVATE time series. Time courses were segmented at 10 s to examine sustained activation (solid horizontal bars represent means). Pre-test: no significant corrected activations or group differences. Training: no significant activations or group differences. Post-test: no significant activations or group differences. Significant mean differences from baseline (p < 0.05) are denoted by the keys below the time courses. Center white circle, baseline; green, NAcc Feedback; blue, VC; gray, FF; black line, a significant difference.

Figure 6. Functional Connectivity Significantly Increased in Mesolimbic Networks following VTA, but Not NAcc, Feedback

In the VTA Feedback group (left), both the VTA and the NAcc ROIs exhibited significantly greater Pre-test to Post-test connectivity with the bilateral HPC (p < 0.05). There were no significant connectivity changes for the NAcc Feedback group (p > 0.1; right), resulting in a significant Run × Group interaction for these ROIs (see Table S2 in the Supplemental Information). Line thickness denotes the change in correlation strength from the Pre-test to Post-test (Z scored). Line color indicates significant/non-significant changes in connectivity (dark/light gray). The line pattern indicates the direction of change in Z-scored r values (solid lines, increased connectivity from Pre-test to Post-test; dotted lines, decreased connectivity from Pre-test to Post-test).