**5. Conclusions**

Our conclusions are based on data from a single, very rare, subject, and are speculative. This paucity of cases should not detract from the theoretical importance of E.C.'s synesthesia, however, in which—as is otherwise almost never the case—an all-or-nothing sensory manipulation could be applied to the concurrent. We hope that more cases of this sort can be found, so that the necessity or otherwise of an active bottom-up 'tonic' signal can be more firmly established.

**Author Contributions:** Conceptualization, C.C.-L.; Data curation, C.C.-L.; Formal analysis, A.R.; Writing—original draft, C.C.-L.; review & editing, A.R.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank E.C. heartily for many hours of voluntary participation in the research.

**Conflicts of Interest:** The authors declare no conflict of interest.
