Auditory and visual orienting responses in listeners with and without hearing-impairment.

Head movements are intimately involved in sound localization and may provide information that could aid an impaired auditory system. Using an infrared camera system, head position and orientation was measured for 17 normal-hearing and 14 hearing-impaired listeners seated at the center of a ring of loudspeakers. Listeners were asked to orient their heads as quickly as was comfortable toward a sequence of visual targets, or were blindfolded and asked to orient toward a sequence of loudspeakers playing a short sentence. To attempt to elicit natural orienting responses, listeners were not asked to reorient their heads to the 0 degrees loudspeaker between trials. The results demonstrate that hearing-impairment is associated with several changes in orienting responses. Hearing-impaired listeners showed a larger difference in auditory versus visual fixation position and a substantial increase in initial and fixation latency for auditory targets. Peak velocity reached roughly 140 degrees/s in both groups, corresponding to a rate of change of approximately 1 micros of interaural time difference per millisecond of time. Most notably, hearing-impairment was associated with a large change in the complexity of the movement, changing from smooth sigmoidal trajectories to ones characterized by abruptly changing velocities, directional reversals, and frequent fixation angle corrections.