As a part of the study to elucidate the influence of exteroceptive stimulation on the excitability of the brain stem reticular formation (RF) in presence of flash stimuli, the effect of hippocampal stimulation on the threshold of the RF was investigated in the rabbit and th e following observed.
1) The threshold of the arousal reaction appearing in the cerebral cortex EEG and the hippocampus (HPC) EEG and the evoked muscular discharge in the fore- and hind limbs, due to stimulation of the RF with 100 Hz was decreased by stimultaneous 100 Hz (0.2-1.0V)stimulation of the HPC.
2) The threshold of the arousal reaction and the evoked muscular discharge was increased by simultaneous 3 Hz (0.5-2.0V) stimulation of the HPC.
3) The decrease or increase in the threshold of the arousal reaction and the evoked muscular discharge following stimulation (100 Hz or 3 Hz) of the HPC, was abolished by destruction of the nucleus ventralis anterior or nucleus reticularis, which belong to the diffuse thalamocortical projection system. From these results, it w as concluded that the increase in excitability of the RF by HPC stimulation with 100 Hz, and the decline by stimulation with 3 Hz, can be attributed to a decrease in indirect inhibition and rise in indirect inhibition of the RF, respectively, via the diffuse thalamocortical projection system. The findings suggest that the previously reported increase in excitability of the RF by attachment of the contact lens in the presence of the flash stimuli (I), corresponds to the effect of 100 Hz HPC stimulation while the decrease in excitability of the RF by the contact lens in the absence of the flash stimuli corresponds to the effect of 3 Hz HPC stimulation.
