Sound localization in acallosal human listeners

In order to evaluate the callosal and hemispheric involvement in sound localization, the present study examined response accuracy to auditory targets in acallosal subjects. The primary interest was to determine whether the congenital absence of the corpus callosum affects auditory localization, especially for sounds situated near the midline of auditory space or moving across it. A corollary objective was to examine the possible existence of an hemispheric asymmetry on audio-spatial localization tasks. Four subjects with callosal agenesis paired to four age and IQ-matched controls and 16 normal control subjects were asked to locate broad band noise bursts at fixed intensity (52 dB sound pressure level) in the horizontal plane in an anechoic chamber. Broad band noise bursts were delivered randomly through 16 loudspeakers, which were mounted at ∼10° intervals on a perimeter frame. Two conditions were tested: (i) localization of a fixed-sound source; (ii) localization of the beginning and the end of a simulated moving stimulus. Two response modes were used. Listeners reported the apparent stimulus location either (i) by pointing with the ipsilateral index finger or (ii) by calling out the estimated angles indicated on the calibrated sound perimeter. Aiming accuracy was assessed by calculating the mean deviation of the response from the objective target position. The results indicated that the responses of the acallosal subjects were less accurate than those of the controls. The deficit was observed not only at the midline but throughout the auditory field. This points to possible compensatory mechanisms following the early absence of the corpus callosum which are, however, limited. The results obtained with manual pointing were generally more precise than those obtained through oral responses. This difference suggests that the remapping of spatial positions onto a verbally based coordinate system involves a supplementary cognitive step which affects the precision of the response. Comparing the performance to stimulus presentation in the left and right fields indicated that no hemispheric asymmetry was apparent under any of the conditions for either the acallosal subjects or the IQ-matched and normal control subjects.