A program of investigation is outlined which will explore the effect of a new generation sonar sensory aid -- the Trisensor -- on blind people's ability to spatially "update" or keep track continually of their changing position relative to their surroundings during locomotion. In addition, the project will assess the extent to which the sonar sensory aid can substitute for vision in the control and regulation of balance and postural stability in blind people. The major study will assess the ability of congenitally totally blind adults and school-age children to learn to use the Trisensor to locate targets in large scale space (5m arc). The subjects' ability to use the Trisensor will be compared to their ability to locate the targets when they are emitting sound. The effects of both Trisensor and natural sound localization training on more generalized tests of spatial cognition will also be determined. The prototype test entails familiarizing subjects with an array of targets by walking them to each target. Subjects would then be led to one of the targets and asked to indicate the direction and distance of another target, the location of which they had not directly experienced from their new position. Blind people experience difficulty with this task presumably because they have not had past experience with dynamic visual information which is produced during locomotion and specifies an observer's changing perspective. Since the returning echoes of the Trisensor afford a moving observer acoustic flow patterns regarding the position of objects in the distal world it is hypothesized that the use of the aid will enhance spatial updating ability. It is now known that vision also plays a crucial role in controlling human balance. Because blind people have well documented difficulties with balance, it is important to determine whether the Trisensor can substitute for vision in this capacity. A battery of balance tests administered before and after Trisensor and natural sound localization training will permit the assessment of the effect of sonar sensory aid and natural sound on the control and regulation of balance and postural stability.