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 1 1 patients (44-80 years old) with small, localized pontine lesions due to vascular disease: (1) Monaural auditory brain-stem evoked potentia ls (ABEPs; peaks I to VI); (2) Binaural ABEPs processed for their bina ural interaction components (BICs) in the latency range of peaks IV to VI; (3) magnetic resonance imaging (MRI) of the brainstem; and (4) ps ychoacoustics of interaural time disparity measures of binaural locali zation. ABEPs and BICs were analyzed for peak latencies and interpeak latency differences. Three-channel Lissajous' trajectories (3-CLTs) we re derived for ABEPs and BICs and the latencies and orientations of th e equivalent dipoles of ABEP and BICs were inferred from them. Results : Intercomponent latency measures of monaurally evoked,ABEPs were abno rmal in only 3 of the 11 patients. Consistent correlations between sit es of lesion and neurophysiological abnormality were obtained in 9 of the 11 patients using 3-CLT measures of BICs. Six of the 11 patients h ad absence of one or more BIC components. Seven of the 11 had BICs ori entation 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 tone patient with ventral-ros tral lesions) dipole orientation of the first component (at the time o f ABEPs IV), and sparing of this component with midline ventral TB les ions (two patients). A deviant orientation of the second BICs componen t tar the time of ABEPs V) was observed with ventral TB lesions. Psych oacoustic lateralization in these patients was biased toward the cente r. Rostral lateral lemniscus (LL) lesions (3 patients) were associated with absent tone patient) or abnormal (two patients) orientation of t he third BICs component (at the time of ABEPs VI); and a side-biased l ateralization with behavioral testing. Conclusions: These results indi cate that: (I) the BICs component occurring at the time of ABEPs peak IV is dependent on ventral-caudal TB integrity; (2) the ventral TB con tributes to the BICs component at the time of ABEPs peak V; and (3) th e rostral LL is a contributing generator of the BICs component occurri ng at the time of ABEP peak VI. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.