A model for sound lateralization.

Recent studies of multiple sclerosis (MS) and stroke patients suggested a correlation between two patterns of abnormal performance in lateralization tasks and two sites of pontine lesions. Most patients who had lesions below or at the superior olivary complex (SOC) perceived all interaural differences in binaural stimuli as small, while most patients who had lesions above the SOC perceived all interaural differences as large. The two abnormal performance patterns occurred for interaural time differences (ITD) and/or for interaural level differences (ILD). The present model proposes a multi-level hierarchical brainstem structure that estimates ITD and ILD. The first level seeks dissimilarity between the left and right inputs and a second level looks for similarity between the two sides' inputs. Each level is modeled as an ensemble of neural arrays in which each unit performs a logic or arithmetic function. The inputs are simulations of auditory nerve responses to broadband stimuli. Simulations yield good correspondence to the effect of both locations of pontine lesions on binaural performance.

Sound lateralization and interaural discrimination. Effects of brainstem infarcts and multiple sclerosis lesions.

Subjects with brainstem lesions due to either an infarct or multiple sclerosis (MS) underwent two types of binaural testing (lateralization testing and interaural discrimination) for three types of sounds (clicks and high and low frequency narrow-band noise) with two kinds of interaural differences (level and time). Two major types of abnormalities were revealed in the lateralization performances: perception of all stimuli, regardless of interaural differences (time and/or level) in the center of the head (center-oriented), or lateralization of all stimuli to one side or the other of the head (side-oriented). Similar patterns of abnormal lateralization (center-oriented and side-oriented) occurred for MS and stroke patients. A subject's pattern of abnormal lateralization testing was the same regardless of the type of stimulus or type of interaural disparity. Lateralization testing was a more sensitive test than interaural discrimination testing for both types of subjects. Magnetic resonance image (MRI) scanning in three orthogonal planes of the brainstem was used to detect lesions. A semi-automated algorithm superimposed the auditory pathway onto each MRI section. Whenever a lesion overlapped the auditory pathway, some binaural performance was abnormal and vice versa. Given a lateralization test abnormality, whether the pattern was center-oriented or side-oriented was mainly determined by lesion site. Center-oriented performance was principally associated with caudal pontine lesions and side-oriented performance with lesions rostral to the superior olivary complex. For lesions restricted to the lateral lemniscus and/or inferior colliculus, whether unilateral or bilateral, just noticeable differences (JNDs) were nearly always abnormal, but for caudal pontine lesions JNDs could be normal or abnormal. MS subjects were more sensitive to interaural time delays than interaural level differences particularly for caudal pontine lesions, while stroke patients showed no differential sensitivity to the two kinds of interaural differences. These results suggest that neural processing of binaural stimuli is multilevel and begins with independent interaural time and level analyzers in the caudal pons.

Effects of localized pontine lesions on auditory brain-stem evoked potentials and binaural processing in humans.

OBJECTIVES AND METHODS: Four sets of measurements were obtained from 11 patients (44-80 years old) with small, localized pontine lesions due to vascular disease: (1) Monaural auditory brain-stem evoked potentials (ABEPs; peaks I to VI); (2) Binaural ABEPs processed for their binaural interaction components (BICs) in the latency range of peaks IV to VI; (3) magnetic resonance imaging (MRI) of the brain-stem; and (4) psychoacoustics of interaural time disparity measures of binaural localization. ABEPs and BICs were analyzed for peak latencies and interpeak latency differences. Three-channel Lissajous' trajectories (3-CLTs) were derived for ABEPs and BICs and the latencies and orientations of the equivalent dipoles of ABEP and BICs were inferred from them. RESULTS: Intercomponent latency measures of monaurally evoked ABEPs were abnormal in only 3 of the 11 patients. Consistent correlations between sites of lesion and neurophysiological abnormality were obtained in 9 of the 11 patients using 3-CLT measures of BICs. Six of the 11 patients had absence of one or more BIC components. Seven of the 11 had BICs orientation abnormality and 3 had latency abnormalities. Trapezoid body (TB) lesions (6 patients) were associated with an absent (two patients with ventral-caudal lesions) or abnormal (one patient with ventral-rostral lesions) dipole orientation of the first component (at the time of ABEPs IV), and sparing of this component with midline ventral TB lesions (two patients). A deviant orientation of the second BICs component (at the time of ABEPs V) was observed with ventral TB lesions. Psychoacoustic lateralization in these patients was biased toward the center. Rostral lateral lemniscus (LL) lesions (3 patients) were associated with absent (one patient) or abnormal (two patients) orientation of the third BICs component (at the time of ABEPs VI); and a side-biased lateralization with behavioral testing. CONCLUSIONS: These results indicate that: (1) the BICs component occurring at the time of ABEPs peak IV is dependent on ventral-caudal TB integrity; (2) the ventral TB contributes to the BICs component at the time of ABEPs peak V; and (3) the rostral LL is a contributing generator of the BICs component occurring at the time of ABEP peak VI.

Lateralization and binaural discrimination of patients with pontine lesions.

Lateralization and just-noticeable difference (jnd) measurements relative to the center were tested in a large group of patients with pontine lesions caused either by stroke or multiple sclerosis. Stimuli included binaural clicks, and low- and high-frequency narrow-band noise bursts. Two major types of abnormalities were revealed in the lateralization performances: perception of all stimuli, regardless of interaural differences (time and/or level) in the center of head (center-oriented), or lateralization of all stimuli to one side or the other of the head (side-oriented). The only significant correlation between jnd and lateralization performances was that an elevated jnd was always manifested in abnormal lateralization, while abnormality in lateralization did not necessarily indicate an abnormal jnd. Center-oriented lateralization was observed either for both interaural differences or only for one of them, and was found in both MS and stroke patients. All side-oriented stroke patients were similarly unable to center binaural stimuli for both time and level cues, whereas only one MS patient had this abnormality for interaural time differences, while his level performance was normal. More abnormalities were detected in the narrow band stimuli tests, although in some cases performance was more degraded for click stimuli. Lateralization tasks with high-frequency stimuli were more sensitive detectors of abnormality than jnd for any kind of stimulus, or lateralization tasks with low-frequency stimuli or clicks.