Auditory localization in subjects with central auditory processing disorders

This study investigated the auditory localization ability of children with central auditory processing disorder (CAPD) with various pychophysical auditory tests, including headphone & free-field testing. An objective electrophysiological test, the post-auricular muscle response (PAMR), was also conducted to investigate binaural interaction at the brainstem levels. For headphone tests, the left & right perceptions were provided by interaural time difference (ITD) cues & interaural intensity difference (IID) cues. The first headphone test (a "pointing" test) measured "gross" ability of subjects to localize to nine simulated sound positions around a hemifield. In the second headphone test (a just noticeable difference or JND task), subjects reported position differences for two sequential sounds ("fine" ability). A click, a low-pass noise (LPN), a high-pass noise (HPN; both at a corner frequency of 2 kHz) & a narrow-band noise (NBN, centered at 4 kHz, one third octave wide) were the stimuli. The third headphone test tested ability to identify a signal in the presence of a masker, using a masking level difference (MLD) technique (signals used were 0.5, 2 & 4 kHz pure tones). For the free-field test, subjects indicated perceived positions by pointing to loudspeakers, arranged horizontally & vertically on a semicircle. For PAMR tests, binaural, contralateral & ipsilateral responses were recorded and specific amplitude ratios were computed. Twenty normal & 15 CAPD subjects participated in the headphone tests, and the pointing test (with HPN & NBN stimuli) was found to be the most sensitive (14 CAPD children failed). The MLD test sensitivity was also better with the addition of high-frequency tones. Ten normal & 12 CAPD subjects participated in free-field & PAMR testing. The binaural ability of CAPD subjects was comparable to normal subjects. In contrast, they had significant difficulties localizing front/back & up/down positions. They also had abnormal binaural interaction processes, as revealed by the PAMR test. The sensitivity of ITD tasks have been discussed, particularly regarding multiple difficulties for CAPD children. Other issues discussed include whether CAPD & normal subjects come from two different populations, or whether the CAPD group is a deviation of the normal population. In general, ITD tasks employing a pointing method have great potential as a simple CAPD test, with most CAPD subjects having difficulties in identifying sound left/right position, but this is only apparent if the two IID & ITD cues are separated. In a free-field setting, binaural cues integrate with spectral cues, resulting in near normal left/right performance, but poorer front/back & up/down ability.