Properties of cortical somatosensory evoked potentials in the awake rabbit

By appropriate experimental procedures it was possible to distinguish the components of the contra- and ipsilateral cerebral somatosensory potentials evoked by electrical stimulation of an extremity in the unrestrained rabbit. The components of the contralateral response occur in the following order; an early positive primary wave (P1), a positive associative wave (P2), and ultimately a late negative N wave. The components of the ipsilateral response occur in the following order; a small positive wave P, the latency of which is intermediate between those of the contralateral P1 and P2 waves, and a large negative wave N, similar to the contralateral N wave. The different topographical distributions of these waves were elucidated by the use of insulated, chronically implanted electrodes glued onto the cortical surface. The properties of the waveform components were studied by various methods such as varying the stimulation parameters, simultaneous application of somesthetic and acoustic stimuli, and administration of narcotic drugs. The properties of P1 were similar to those of P28 in humans; the properties of P2 can be compared to those of P45; and, finally, the N wave resembles the late negative components observed in humans. Inconstant small positive waves of shorter latencies, which will be discussed in a following paper, may also be seen. Interestingly enough, no early negative wave such as that observed in humans (N20) was ever found. If, as is presently thought, this wave is, in fact, due to the folding of the cortical surface (Broughton, 1969), its absence is to be expected in the rabbit because the cortical surface of this species is lissencephalic and thus devoid of gyri.