The secondary discharge (SD) of negative polarity recorded from the posterior part of the convexity of the cortex was studied in cats and dogs under deep barbiturate anesthesia. The peripheral origin of sensory impulses was exclusively the sciatic nerve.
1. The first sign of the negative SD appeared 30 to 60 msee after the sciatic stimulation. The peak of the negativity was reached 100 to 130 msec after the stimulation, with a delay of 30 to 50 msec when compared with the peak of the positive SD of the sensorimotor area.
2. The negative SD was composed of the intrinsic and extrinsic components. The intrinsic component which in most of the cases was found superimposed upon the declining limb of the entire negative SD complex was affected by topically applied KCl, γ-aminobutyric acid and strychnine in about the same manner as was the primary evoked potential. The part of the negative wave which escaped from the actions of the chemicals was called the extrinsic component and supposed to come from an evoked potential in deep structures such as the anterior brain stem through physical spread.
3. When a microelectrode was thrust beneath the cortical surface, the negative SD's were recorded with augmented amplitude from the layers below a certain depth. The deep negative SD was also found to consist of the intrinsic and extrinsic components.
4. In association with the negative SD there was found facilitation of the primary evoked potential produced by stimulation of the specific thalamic projection nuclei. The maximal facilitation was obtained at the time intervals of 130 to 170 msec between sciatic and thalamic stimulations. This facilitation, at least a part of it, was supposed to depend upon enhanced excitability of the thalamus due to the sciatic stimulation.
We are indebted to Dr. Tomoaki Asano and Mr. Kozo Ohsaki for their kindly reading the manuscript. This study was supported by a grant from the Ministry of Education.