Middle- and long-latency auditory evoked responses recorded from the vertex of normal and chronically lesioned cats

Abstract A prolonged sequence of auditory evoked potentials with latencies ranging from 1 to 250 msec was recorded from the vertex of the awake restrained cat. This sequence was reproduceable within and across subjects and was not altered by complete neuromuscular paralysis. The effects of click rate, pentobarbital, and chronic lesions of a number of different brain areas were evaluated for each of the potentials. Vertex waves 1–5, previously shown to originate from generators in the primary auditory pathway of the brain stem, were followed by smaller and less well defined waves 6 and 7, with peak latencies in the 6–8 msec and 10–12 msec range respectively. These potentials were not abolished by fast click rates (i.e. up to 50/sec) nor by moderate levels of pentobarbital. Correlative extra- and intracranial studies indicated that wave 6 occurred in the same latency range as the medial geniculate body, pars principalis potential, and that wave 7 occurred in the same latency range as the primary ectosylvian cortical potential. The intracranial potentials showed click recovery functions and barbiturate resistance which were similar to those of waves 6 and 7, and wave 7 disappeared following aspiration of ectosylvian cortex. These data suggest that waves 6 and 7 reflec generators in medial geniculate body and ectosylvian gyrus. In contrast to the stability of potentials 1 through 7, the longer latency waves were relatively unstable. Wave A occurred in a latency range of 17–25 msec, wave B, 35–45 msec, wave C, 50–75 msec, and wave D, 150–200 msec. All of these waves showed marked amplitude fluctuations, disappeared as click rates increased to 10/sec, and were abolished by moderate levels of pentobarbital. After bilateral aspiration of middle suprasylvian gyrus, ectosylvian gyrus, or frontal lobes, wave A continued to appear. After hemispherectomy, which removed all cortex, basal ganglia and limbic lobes, wave A was not abolished and appeared enhanced in one animal. Thus, the generator system of wave A appears to be largely independent of auditory cortex and adjacent association cortex, but may be modulated by other forebrain systems. Wave C continued to appear after aspiration of suprasylvian and ectosylvian gyri and after frontal lobectomy, but disappeared after hemispherectomy. Thus, wave C reflects a generator system which differs from that of wave A, but which also appears to be largely independent of the primary geniculo-cortical auditory pathway. These data suggest the following conclusions: waves 1 through 7, which show high fidelity, rate-resistant, barbiturate-insensitive acoustic transmission, appear to reflect activation of the primary auditory system from acoustic nerve to auditory cortex. Subsequent, longer-latency vertex potentials seem to be generated through other forebrain systems which receive auditory information in parallel from the brain stem, rather than serially from the primary geniculo-cortical pathway and association cortex relays. The relevance of data in the cat model to the human vertex potentials is discussed.