the clue in the mute inaccessible cortex
He always gets the feeling there's some clue to it on the tip of his tongue, some mute and inaccessible part of the cortex, and then he always feels vaguely sick, scanning for it.
Context
Hal believes his own loneliness sermon, which unsettles him, and when he tries to reach the thing underneath it, the reason the secrecy matters more than the high, he runs into a wall inside his own head: a clue on the tip of the tongue, lodged in some mute and inaccessible part of the cortex, that scanning only makes him sick to hunt for. The self cannot get a message from its own depths; "I am in here" turns out to be true of Hal even to Hal.