TMS-induced neuronal plasticity enables targeted remodeling of visual cortical maps

Experiments started with acquisition of orientation maps followed by 30 min of TMS. In most experiments, maps were again evaluated directly after TMS before visual stimulation started with prolonged exposure to a single orientation. Fig. 1A shows two examples of V1 orientation maps before (pre-) and after (post-) treatment with high-frequency 10-Hz TMS and subsequent visual stimulation. Before TMS, the recorded maps displayed the typical layout (Fig. 1A, Left), characterized by regular representation of different orientation angles around “pinwheel” centers (17). Next, we inspected the maps following 10-Hz TMS and prolonged visual exposure to a single orientation, excluding times directly after visual stimulation to avoid overlap with early adaptation effects (18). We detected that maps were dominated by the representation of the stimulated orientation, i.e., horizontal in the example in Fig. 1A, Top. Starting from a map with roughly equal representation of all orientations, the cortical area representing horizontal (orange/reddish/colors) increased by 28.9%, here measured 1–2.5 h post visual stimulation. To verify that the plastic cortical change was specific to visual stimulation, we used different orientations in different experiments. In the second example shown (Fig. 1A, Bottom), stimulus orientation was 90°, and orientation was dominance-shifted toward vertical orientations (bluish/greenish colors), again revealing an increased (18.6%) representation of the stimulated and neighboring orientations. Across all experiments, the relative augmentation of regions representing the stimulated orientation was 19.0 ± 4.0% SEM (Fig. 1B; P = 0.0257, one-sample t test, Bonferroni-corrected for eight comparisons, n = 7 experiments), with simultaneous reduction of pixels with orientation preference orthogonal (i.e., 90°) to the stimulated orientation (Fig. 1B, P = 0.0099, one-sample t test, Bonferroni-corrected for eight comparisons, n = 7). No significant changes were observed in cases where visual stimulation followed sham TMS (Fig. 1B, black bars, n = 3). Fig. 1C summarizes these findings in a polar graph (SI Appendix, SI Methods) for the individual experiments (thin lines). On average the cortical reorganization was evident in the biased occurrence of orientations neighboring the visually trained orientation after 10-Hz TMS (Fig. 1C, red lines in Left graph, average outlined in bold), notably different from pretreatment conditions (outlined stippled gray), as well as from sham pre and post visual stimulation conditions (Fig. 1C, Right graph). Further analysis revealed that the increased representation of the stimulated orientation was not randomly recruited—apparent by a systematic shift in orientation preference of the neighboring orientation domains toward the stimulated orientation (Fig. 2).

The above findings indicate that the key mechanisms responsible for the observed remodeling lie in the immediate high-frequency TMS effects. Next, we therefore directed our analysis to the time window straight after TMS treatment (“post 10-Hz TMS”). Fig. 3A, Left depicts orientation map layout, assigning reproducibility values (i.e., trial-to-trial circular variance; see SI Appendix, SI Methods) to all pixels. If a given pixel consistently responded strongest to the same orientation in each trial, it has a high reproducibility value (brightness in maps indicates degree of reproducibility) with low values (dark) for inconsistent preferred orientation across trials. Note that this method faithfully captures the location of pinwheel centers (see dark local spots within the three encircled examples) because pinwheel pixels cover (i.e., average) neurons tuned to different orientations at closest distance (17). Strikingly, following 10-Hz TMS, the reproducibility of the pixels’ orientation preferences was strongly reduced compared with the pre-TMS map (Fig. 3A, Right).

Furthermore, the rising phase of the VSD includes a small downturn (“notch”; Fig. 3B, gray trace), which was proposed to reflect inhibitory processes that sharpen orientation tuning (19). We found the notch diminished after 10-Hz TMS and a subsequently increased amplitude of cortical activity (Fig. 3B; compare gray and bluish traces in encircled region and thereafter). Consistent with the present results, we previously demonstrated that 10-Hz TMS produces an excitatory cortical state where suppression is weakened, leading to the disappearance of the notch and subsequent increase in evoked activity levels with decreased orientation-specific response components (16).

In addition, pixels with low reproducibility revealed a substantial loss in modulation depth (Fig. 3C, P = 0.0138, paired t test, n = 5 experiments, in which we imaged directly after 10-Hz TMS). In other words, the difference between preferred and orthogonal responses [and thus selectivity (20)] was reduced compared with pixels with high reproducibility. To further confirm that the effects we observed were specific to the TMS intervention, we tracked mean reproducibility values of pre- and post-TMS conditions after 10-Hz TMS and sham controls for each individual experiment (Fig. 3D). Reproducibility did not significantly change after sham TMS (Fig. 3D; P = 0.4995, paired t test, n = 4 experiments). In contrast, reproducibility was significantly reduced after 10-Hz TMS (Fig. 3D; P = 0.0155, paired t test) in all experiments, and this reduction was significantly different from sham conditions (P = 0.0086, nonpaired t test). These results validate 10-Hz TMS induction of increased variability and also suggest increased decorrelation of responses across the entire neuronal populations. To quantify the effect on the correlational structure of orientation maps, we correlated averages of resampled single-trial maps (bootstrap, 1,000 iterations) with the mean map for each orientation (Fig. 3E). The matrices displayed the expected diagonal of highest-correlation coefficients across maps of the same orientation. After 10-Hz TMS these correlations strongly declined (Fig. 3E, Top; P = 0.0078 paired Wilcoxon signed-rank test, n = 5 experiments). This decline was again significant in contrast to sham conditions (P = 0.0281, Wilcoxon rank-sum test), where orientation maps maintained initial response correlations (Fig. 3E, Bottom; P = 0.1484, paired Wilcoxon signed-rank test, n = 4 experiments). We conclude that the observed decline in reproducibility indicates a high-frequency TMS-induced state, in which the cortex is less suppressed, thus more excitable, and exhibits decreased orientation selectivity and decorrelation of neuronal responses: these phenomena together seem to set the ground for remodeling of the maps.

Do neurons after remodeling, particularly those with newly acquired orientation preference, display consistent orientation tuning? Fig. 4 (red solid line) depicts the tuning curve for pixels that coded for the stimulated orientation before the 10-Hz TMS intervention, as a Gaussian fit through values across all experiments. The half width at half height (45.1°) was comparable to tuning width obtained before TMS treatment (42.9°, gray curve, calculated across pixels with preference to stimulated orientations) and in the range as reported in a previous VSD-imaging study when averaging over hundreds of milliseconds (19). Importantly, the tuning width of pixels including neurons that underwent remodeling (stippled red curve) was similar (43.7°) to that of pixels that maintained their preference throughout the experiment, altogether suggesting consistent tuning for the newly acquired orientation after reorganization of the maps.

To finally test the hypothesis that map remodeling was specifically facilitated by 10-Hz TMS-induced increase in excitability, we performed TMS using a 1-Hz protocol, proposed to act dominantly suppressive (13, 16, 21, 22). Neither remodeling nor significant shifts in orientation preference (P = 0.2973, repeated measures ANOVA, across all orientations, Greenhouse–Geisser-corrected) were found when visual stimulation was applied after TMS with 1 Hz (Fig. 5 A–D). Computing amplitude differences of visually evoked activity between pre- and immediate post-TMS conditions we found that activity after 1-Hz TMS was significantly reduced in comparison with 10-Hz TMS (Fig. 5E). Moreover, post 10-Hz correlations across maps and map reproducibility were significantly lower compared with post 1-Hz TMS (Fig. 5 F and G, all orientations combined), further suggesting that the observed remodeling was specifically facilitated by high-frequency 10-Hz TMS along with induced decorrelation and increase of response variability.

In previous studies, intracortical microstimulation (23, 24) or retinal lesions were used (3, 25–28) to show that the intrinsic horizontal cortical network has the potential to undergo remapping of orientation preference. The latter approach revealed slowly developing and long-lasting effects after weeks and months of recovery within the deprived cortical region (3, 25, 26, 28). How stable are the reorganized maps in our case, where we noninvasively applied only a brief period of cortical perturbation by TMS? We found map stability was preserved for up to 6 h after TMS and visual stimulation (median across all experiments was 2.5 h), i.e., for as long as we could record reliable optical signals (see Methods). Using 2-photon imaging it has been shown that spine growth and retraction, as well as de novo formation of axonal boutons, can occur over tens of minutes (29), contributing to massive restructuring of neuronal circuits (30), which makes it likely that the here observed remodeling might be associated with structural plasticity. We speculate that a long-lasting establishment of functional reorganization may, however, also require repeated sessions of high-frequency TMS (12, 31) to counteract the tendency of neurons that underwent plastic changes to regain their pretreatment response properties (32). Additionally, the fact that we measured effects under anesthesia must be taken into account, as cortical responsiveness can be changed greatly, dependent on behavioral conditions (33).

Whether TMS-induced remodeling of orientation maps promotes existing intrinsic cross-connectivity through unmasking of latent inhibitory connections (26, 34, 35) or, rather, incorporates plastic changes across thalamic afferents (36) remains to be clarified. On the one hand, high-frequency TMS pulses facilitate cortical disinhibition (37, 38) and, consequently, excitability. Moreover, repetitive 10-Hz magnetic stimulation in entorhinohippocampal slice cultures was shown to reduce GABAergic synaptic strength (22). Hence, the here found 10-Hz TMS-triggered attractive shift in orientation preference toward the stimulated orientation could result from weakening of lateral suppression (39). On the other hand, increased excitability was recently proposed to evolve from TMS-induced cortico–subcortical loops (40) based on the observation of a rebound excitation phase following TMS (16, 40). Thus, postulating that increased excitability predisposes toward remodeling, the dominance of the visually stimulated orientation could also incorporate TMS-enhanced changes of thalamic input.

In contrast to the strong effect of 10-Hz TMS, 1-Hz TMS showed decreased responsiveness, along with no remodeling effects. This would support the view that low-frequency TMS facilitates suppressive mechanisms (13, 16, 20–22) and conceivably stabilizes orientation maps.

Finally, our results imply a surprising homogeneity of changes in orientation map layout after 10-Hz TMS. Note, however, that our method depicts an average population picture of upper cortical layer activity. Therefore, possibly existing subpopulations of neurons resistant to plasticity (32, 41) remain masked. Similarly, the method is blind to the possibility of involvement of distinct inhibitory cell circuits, which have been shown to change cortical state and enhance visual plasticity through disinhibition (42).

In summary, our data show that high-frequency 10-Hz TMS induces a state where visual responses are destabilized and undergo increased variability (i.e., decreased reproducibility) in orientation preference accompanied by increased excitability, most likely based on changes in excitation/inhibition balance—a potential main driving force for plastic cortical processes (43–45). In turn, the cortex may transiently be sensitized to a bias in input (23, 41, 46)—here imposed by prolonged stimulation with a single orientation—and primed to relearn connectivity patterns. Our results therefore suggest that perceptual training would be most effective immediately following high-frequency TMS application. During the immediate post-TMS phase, internal variability is produced in cortical responses (47), which may increase sensitivity to new input regularities. Plastic neuronal processes are then primed to store the newly acquired response properties. Interestingly, such a principle, that is, flexibly processing the tradeoff between internally generated variability and acquisition of novel global regularities to restructure neuronal networks, has recently been shown also for human motor-learning (48). We show here at the level of early sensory cortex that such remapping can be targeted and enhanced by specific input combined with noninvasive brain stimulation.

We thank Drs. Hubert R. Dinse and Paul R. Martin for discussion and Stefan Dobers for technical assistance. This work was supported by grants from the Deutsche Forschungsgemeinschaft, SFB 874 (TP A2), German–Israeli Project Cooperation (DIP, JA 945/3-1, SL 185/1-1), SPP 1665 (JA 945/4-1), and the Bundesministerium für Bildung und Forschung, BMBF.