From Dorothea Brande's 1934 classic, Becoming a Writer :
"[The writer] will only know that there are times when he must, at all costs, have solitude; time to dream, to sit idle. Often he himself believes that his mind is empty. Sometimes we hear of gifted men who are on the verge of despair because they feel they are going through a "barren" period; but suddenly the time of silence is past, and they have reached the moment when they must write.
"That strange, aloof, detached period has been called "the artisitic coma" by observers shrewd enough to see that the idleness is only a surface stillness. Something is at work, but so deeply and wordlessly that it hardly gives a sign of its activity till it is ready to externalize its vision. The necessity which the artist feels to indulge himself in solitude, in rambling leisure, in long speechless periods, is behind most of the charges of eccentricity and boorishness that are leveled at men of genius. If the period is recognized and allowed for, it need not have a disruptive effect.
"The artist will always be marked by occasional periods of detachment; the nameless faculty will always announce itself by an air of withdrawal and indifference, but it is possible to hasten the period somehwat, and to have it, to a limited extent, under one's control. To be able to induce at will the activity of that higher imagination, that intuition, that artistic level of the unconscious -- that is where the artist's magic lies, and is his only true 'secret.'"
Tomorrow: Reflections on the Artistic Coma