Prefrontal Theta Oscillations Promote Selective Encoding of Behaviorally Relevant Events

Increasing prefrontal neuron activity promoted the formation of specific temporal stimulus associations. A, Schematic diagram showing stimulus contingency used during differential, reversal learning, and set-shift stages. Daily sessions consisted of two epochs during which rats received pairings of either tone or light (CS1, CS2) with eyelid shock (US) in two distinct conditioning environments (Contexts A and B). Red color shows stimuli that were predictive of the US. B, In the differential learning stage (left), CNO-treated hM3Dq-expressing rats (red; n = 10; mean ± SEM) acquired CRs to the CS that was paired with the US (CS1+) faster than saline-treated rats (blue; n = 9) in both epochs. In contrast, CR expression to the other CS that was presented alone (CS2−) did not differ between saline- (cyan) and CNO-treated (magenta) rats. In the reversal stage (middle), both groups increased CR% for the newly reinforced CS2+ while concurrently decreasing CR expression for the currently non-reinforced CS1−. In the set-shift stage (right), both groups showed comparable CR% to two CS in each of two epochs while differentiating CR% to the same CS between the two epochs. ∗∗∗p < 0.001 (mixed ANOVA). C, CNO-treated rats developed differential responding faster than saline-treated rats during differential learning, but not the other two stages. Individual data shown with the median (line). *p < 0.05 (Wilcoxon rank sum test). D, Averaged normalized EMG amplitude revealed that the temporal pattern of EMG activity was comparable between the saline-treated (blue and cyan) and CNO-treated (red and magenta) rats in both epochs across the three learning stages. Gray bars depict CS presentation and black bars mask the artifact generated by the US. DL- Differential Learning; RL- Reversal Learning; SS- Set-shift.

CS-evoked theta amplitude was selective for relevant temporal stimulus correlations. A, Representative image of electrode tip location in the prelimbic region of mPFC (top) and histologic reconstruction of electrode locations (bottom) in the CNO-treated (red) and saline-treated (black) hM3Dq-expressing rats included in neurophysiological analyses. The locations of shorter and longer tips of bipolar electrodes are depicted as circles and triangles, respectively. Scale bar, 500 μm. B, Spectrograms during the differential learning stage showing the power of oscillations averaged across sessions and rats in each group. Two white lines show CS onset and offset. Black bars mask the artifact generated by the US. C, The degree of differentiation of the CS-evoked oscillatory amplitude between the reinforced and unreinforced CS during the differential learning stages (mean ± SEM; saline: n = 8 rats; CNO: n = 9 rats). The amplitude of theta band activity (4–12 Hz) differentiated two CS types, and the differentiation was stronger in CNO-treated (red) than saline-treated (black) rats. *p < 0.05 (mixed ANOVA).

Increasing the mPFC activity promoted the development of ramping theta responses selective for behaviorally relevant stimuli. A, Normalized theta amplitude to the originally reinforced CS (CS+; left) and unreinforced CS (CS−; right) in saline-treated rats is plotted over time within trials (x-axis) across days (y-axis, descending from top to bottom) in Epochs 1 (top) and 2 (bottom). Horizontal white lines separate the three stages of learning. Two vertical black and white lines show the timing of the CS and US, respectively. Black bars mask the artifact generated by the US. During the differential learning stages, theta amplitude was transiently increased during the CS+ but not the CS−. Although this phasic response did not track the subsequent change in stimulus contingency, theta amplitude came to ramp up toward the expected onset of the US during the reversal learning and set-shift stages (white arrows). B, The same as A for CNO-treated rats. Within a few sessions in the differential learning stages, theta amplitude came to ramp up toward the expected US onset, and the ramping responses tracked changing stimulus contingency during the reversal learning and set-shift stages (white arrows). C, The Differential Index of the phasic responses during the first three (early phase) and latter three (late phase) sessions of each learning stage (mean ± SEM; saline: n = 8 rats; CNO: n = 9 rats). ***p < 0.001 (main effect of phase in mixed ANOVA). D, the same as C for the ramping responses. During differential learning, both groups improved the differentiation of the ramping responses across two phases, and the differentiation was stronger in CNO-treated rats than saline-treated rats. *p < 0.05, ***p < 0.001 (main effect of stage); #p < 0.05 (main effect of group). DL- Differential Learning; RL- Reversal Learning; SS- Set-shift.