A spatial hearing deficit in early-blind humans

An important issue in neuroscience is the effect of visual loss on the remaining senses. Two opposing views have been advanced. On the one hand, visual loss may lead to compensatory plasticity and sharpening of the remaining senses. On the other hand, early blindness may also prevent remaining sensory modalities from a full development. In the case of sound localization, it has been reported recently that, under certain conditions, early-blind humans can localize sounds better than sighted controls. However, these studies were confined to a single sound source in the horizontal plane. This study compares sound localization of early-blind and sighted subjects in both the horizontal and vertical domain, whereas background noise was added to test more complex hearing conditions. The data show that for high signal-to-noise (S/N) ratios, localization by blind and sighted subjects is similar for both azimuth and elevation. At decreasing S/N ratios, the accuracy of the elevation response components deteriorated earlier than the accuracy of the azimuth component in both subject groups. However, although azimuth performance was identical for the two groups, elevation accuracy deteriorated much earlier in the blind subject group. These results indicate that auditory hyper-compensation in early-blind humans does not extend to the frontal target domain, where the potential benefit of vision is maximal. Moreover, the results demonstrate for the first time that in this domain the human auditory system may require vision to optimally calibrate the elevation-related spectral pinna cues. Sensitivity to azimuth-encoding binaural difference cues, however, may be adequately calibrated in the absence of vision.