It was difficult for humans to understand the concept, and completely unknown to them before treecats and humans learned to sign. One's mind glow was immediately perceptible to treecats, just as one's look and smell was.
Humans mind glows were generally very powerful, if oddly inactive, as though the owner were sleep walking. Since humans were unaware even of its existence, odd qualities were likely inevitable. Some humans mind glows were exceptionally vivid and attractive; the Harrington Clan had produced a high proportion of these. (HHA2.2: WPD)