Two-dimensional sound-localization behavior of early-blind humans

To investigate whether the visual system is crucial for adequate calibratio n of acoustic localization cues, sound-localization performance of early bl ind humans was compared with that of sighted controls. Because a potential benefit of vision is mainly expected for targets within the two-dimensional (2D) frontal hemifield, localization was tested within this target range, while using sounds of various durations and spectral content. Subjects were instructed to point, in separate experimental sessions. either with their left arm, or with their nose, in the direction of the perceived target posi tion as accurately as possible. The experiments required the use of all ava ilable sound-localization cues such as interaural differences in phase and intensity, as well as the complex spectral shape cues provided by the pinna e. In addition, for long-duration stimuli, subjects could have had access t o head motion-induced acoustic feedback. Moreover, the two pointing methods allowed us to assess different response strategies for the two groups. In an additional series, subjects were instructed to respond as quickly as pos sible. The results show that, in general, 2D sound-localization performance of blind subjects was indistinguishable from that of sighted subjects, bot h for broad-band noise and for pure tones. In the fast head-pointing task, the latency distributions of both groups were equal. These findings suggest that visual feedback is not required to calibrate the available localizati on cues - including the idiosyncratic and complex spectral shape cues for e levation. Instead, the localization abilities of blind people show that the putative supervising role of vision may be supported, or taken over, by ot her non-visual feedback systems. The results do not provide support for the hypothesis that blind people can hypercompensate for the loss of vision in the frontal hemifield by developing superior sound-localization abilities. Despite the general correspondence in localization behavior, some specific differences related to pointing strategies as well as to those between bli nd and sighted subjects were apparent. Most importantly, the reconstructed origin (bias) of arm pointing was located near the shoulder for the blind s ubjects, whereas it was shifted and located near the cyclopean eye for the sighted subjects. The results indicate that both early blind and sighted hu mans adequately transform the head-centered acoustic target coordinates int o the required reference frame of either motor system, but that the adopted response strategy may be specific to the subject group and pointer method.