A new interpretation of the generators of BAEP waves I-V: results of a spatio-temporal dipole model.

Brain-stem auditory evoked potential (BAEP) scalp distribution, recorded in 10 normal subjects, was analysed using a spatio-temporal dipole model. This model can simulate surface wave forms due to overlapping activity from multiple dipolar sources within a 3-shell head model. The conventional 5-peak (I-V) hypothesis could not fully account for the experimental BAEP distribution, particularly around waves I- and III-. When the temporal course of dipole strength was modelled according to a triphasic compound action potential, 6 dipolar sources were sufficient to fit all BAEP wave forms. Location, orientation and latency of the dipoles indicated generation of dipole 1 at the distal end of the auditory nerve (AN), of dipole III in, or near to, the cochlear nucleus (CN), of dipole III- in the trapezoid body and of dipoles IV and V in the superior olivary complexes and in the lateral lemnisci. Dipole I-, peaking only 0.65 msec after wave I, is suggested to result from the electric inhomogeneity at the porus acusticus internus. Wave II required no extra model dipole, but was attributable, in part, to the second peak of dipole I. Estimation of latencies based on AN conduction velocity, synaptic delay and differences in AN length amongst species confirmed that second order neuronal activity cannot arise before wave III. Second order axons, spreading widely through the brain-stem, apparently cause major contributions also to waves III-, IV and V. The new source hypothesis must leave open questions concerning the amount of contribution from third order neurones or from ipsi- versus contralateral structures to the IV/V wave complex.