Origin of the secondary response induced by flash stimulation under barbiturate anesthesia in the rabbit

The photically evoked secondary response of the visual cortex was studied in a rabbit anesthetized with pentobarbital sodium. The following results were obtained: 1) The secondary response (P2) under anesthesia was a simple positive potential having a long latency and resembled the primary response (P1) in waveform. 2) The latency of P2 shortened gradually as the recovery from anesthesia proceeded. 3) P2 was usually smaller than P1 in amplitude when evoked with a strong flash, but larger than P1 when evoked with a weak flash. 4) Electrical stimulation of the superior colliculus (SC) induced in the visual cortex a positive potential which had a long latency comparable to that of P2. On the contrary, stimulation of the lateral geniculate body (LGB) evoked a short latency, positive potential similar to P1, but failed to evoke a positive potential comparable to P2. 5) A partial lesion of LGB led to a decrement of P1, while lesion of SC resulted in a decrement of P2. 6) Cooling of SC resulted in a rapid reduction and disappearance of P2 as well as the negative potential in SC. 7) There were a considerable number of visual cortical neurons which fired commonly during the phases of P1 and P2. These results suggest that P2 might be derived through the extrageniculo-cortical system which is relayed at SC. The mechanism by which P2 was manifested under anesthesia was discussed.