Masson disk: Meaning and Definition of

Mas'son disk"

Pronunciation: (mas'un), [key]
  1. a white disk on which a series of concentric gray circles appear to vanish intermittently when the disk is rotated, used for testing a person's fluctuation of attention and visual threshold.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.