Wiktionary
subthreshold
a. 1 Describing a stimulus that is not strong enough to elicit a response 2 Below a threshold. 3 (context medicine English) subsyndromal
Usage examples of "subthreshold".
Supposedly, a subthreshold shock can affect the core conduits without causing a direct impulse.
But if subthreshold shocks are repeatedly applied, the cumulative affect is shock artifact--indistinguishable from a stimulating impulse.