Intra- and extracranially recorded auditory evoked potentials in the cat. I. Source location and binaural interaction.

Auditory field potentials (AFPs) were recorded stereotactically from the superior olivary complex and from the exposed inferior colliculus in the Nembutal anaesthetized cat. The brain-stem evoked response (BSER) was recorded simultaneously between an electrode on the dura mater at the vertex and an electrode on one bulla or in the neck musculature. A closed condenser microphone sound system was used to deliver monaural and binaural clicks. The binaural difference potential (BDP) was calculated by subtracting the sum of both monaurally evoked potentials from the binaurally evoked potential. The first binaural interaction was a reduction of BSER wave P4, arising in the superior olivary complex. The large extracellular AFPs generated in both inferior colliculus and superior olivary complex do not correspond to BSER waves, whereas the small volleys preceding the main waves do. These small waves show a much smaller change with recording distance than do the main AFP waves, i.e., the sources and sinks generating these waves appear to be more widely separated. We suggest that, in contrast to extracellular field potentials, the BSER is generated by action potentials in fibre tracts rather than postsynaptic potentials in nuclei. The implications for stimulation and recording laterality of late BSER waves are discussed.