The inflection point; the very first sign of changeover, before any nerve cell accretions have even built up. A subliminal awareness, a sensitivity not felt before. (Paraphrased from Source: Mahogany Trinrose Ch 2)
The Response Factor is the mathematical expression of the relationship between stimulus and response. At the threshold of changeover, the new Sime nervous system (even when only a few thready trails of synapses are formed) can respond to changes in the surrounding selyn fields. The magnitude of stimuls vs. response is a measure of the progress of changeover -- and a clue to the type of Sime the adult will become. A high order, Endowed, channel will have an extremely large response to a very small stimulus at an unreasonably early stage of changeover.