Localization of moving sounds by hemispherectomized subjects

The objective of the present study was to evaluate the localization abilities of the residual auditory structures in hemispherectomized subjects (Hs) during monaural and binaural listening. The monaural stimulus presentations were aimed at evaluating the relative contribution of crossed and uncrossed afferents to the perception of a moving auditory target. Three Hs and ten control subjects (Cs) were asked to localize a simulated moving sound source of fixed intensity presented on the horizontal plane. The moving stimulus was delivered randomly through 16 loudspeakers which were mounted at 10 degree intervals on a calibrated perimeter frame located inside an anechoic chamber. The apparent movement traveled in either direction over three different distances in the peri-central and lateral fields. Listeners had to report the movement trajectory by pointing with the index finger where they thought the beginning and the ending of the stimulus occurred on the perimeter. Hs were less accurate than Cs in the binaural condition, suggesting that the residual hemisphere and/or subcortical structures are not by themselves as efficient to accurately analyze motion as when the two hemispheres are present. In the monaural testing conditions, most of the subjects showed a prominent lateral displacement of the perceived localization towards the functional ear. However, all subjects, particularly the Cs, were able to perceive different lengths of trajectories although they did not discriminate the direction of the motion. For the Hs, the pattern of performance differed for each subject. One right Hs was more accurate for localizing the sources delivered on the side of the functional ear when relying on crossed-input. A left Hs was able to detect the appropriate side of the moving sound source even when it was presented on the side to the obstructed ear. The other left Hs who had developed normally until the age of 5, was more affected by the hemidecortication in both the binaural and the monaural listening conditions. These findings suggest that it is possible to detect moving sounds in the absence of binaural cues and without the integration carried out by both cerebral hemispheres. However it appears that interaural disparities and bihemispheric integration are required for finer analysis.