Alternative Reactionary Input
Often referred to as subliminal cues, these are the use of other tools available to the game developer to replicate the role of audio in indicating something important is happening. This could mean things like the screen turning red as the character is increasingly wounded, flashing when the character is low on health, or using the vibration offered by a standard console controller to have a meaning that Deaf gamers can interpret. Alternative reactionary inputs allow for the user to be more in tune with what is currently happening in the game without needing to hear the environment, or read that something is happening.
In World of Warcraft, the edges of the visual area flashes red at an increasing rate once the character reaches less than 20% health.
In Call of Duty blood appears around the screen and the clearness of the screen blurs as the character takes additional damage. As regeneration occurs, the bloodied screen clears up. Both of these are accompanied with an audio cue as well.
These are non-audio cues that allow the gamer to interpret important game information, in this case character health, without having to hear anything. To the non-disabled gaming crowd, these types of alternate warnings are becoming more commonplace as an additional means of providing immersion and understanding of the environment. To a hearing impaired gamer, these types of warning systems are an essential feature to level the playing field.