Spatio-temporal distribution of auditory-evoked far field potentials in rat and cat

Short latency (less than 5 msec) auditory-evoked 'far field' potentials were mapped over the head of anesthetized rats and cats using a monaural 'click' stimulus. Significant activity was found over most of the head and, in the rat, on other parts of the body. Different components of the observed waveforms have different spatial distributions, supporting earlier evidence that they come from different generators. No area was found on the head which could be considered as an electrically neutral reference point for all the waves. There was considerable asymmetry in the spatial distribution of the early waves, with the area near the contralateral ear showing larger magnitudes than the area near the ipsilateral ear. It was found that most of the head of the rat is not in the far field, as previously defined. Significant activity was found on the tongue in the cat, which tended to increase the apparent magnitude of waves I and II at the vertex when the tongue was used as a reference. The area near the ear canal was also found to show significant activity, raising questions as to the use of the ear lobe as a reference point in human studies. We conclude that the placement of the electrodes can markedly influence the waveforms obtained, in some cases enhancing detection of early components.