Department of Systems Neurophysiology, Graduate School of Medicine, Tokyo Medical and Dental University, Yushima, Bunkyo-ku, Tokyo, Japan
Address for correspondence: Dr. Yoshikazu Shinoda, Department of Systems Neurophysiology, Graduate School of Medicine, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo, Japan, 113-8519. Voice: +81-3-5803-5152; fax: +81-3-5803-5155. yshinoda.phy1{at}tmd.ac.jp
The caudal part of the superior colliculus (SC) plays an important
role in the generation of saccades, whereas the rostral part
of the SC is considered to be involved in visual fixation. The
present study was performed to determine neural connections
from the rostral and caudal parts of the SC to inhibitory burst
neurons (IBNs) and pause neurons (PNs) in the nucleus raphe
interpositus in the anesthetized cat, and to reveal the functional
role of the rostral SC on eye movements. The intracellular potentials
from IBNs and PNs were recorded, and the effects of stimulation
of the SC on these neurons were analyzed. The results show that
IBNs receive monosynaptic excitation from the contralateral
caudal SC, and disynaptic inhibition from the ipsilateral caudal
SC via contralateral IBNs. In addition, IBNs receive disynaptic
inhibition from the rostral part of the SC on either side via
inhibitory interneurons other than IBNs. Intracellular recording
from PNs revealed that they receive convergent excitation from
the rostral parts of the bilateral superior colliculi and that
the rostral SC inhibits IBNs on both sides via PNs. The neural
connections determined in this study support the functional
independence of the rostral SC and are consistent with the notion
that the "fixation zone" is localized in the rostral SC. These
results show that the fixation zone in the rostral SC may suppress
the initiation of bilateral saccades via pause neurons.
