The retino-tecto-isthmo-retinal feedback circuit has been studied. A description of the tractus isthmo-opticus (TIO) has been given, using the Nauta technique, and the extra retinal projections of the nucleus isthmo-opticus (NIO) are considered. The physiological reality of the circuit has been proven, the response of the NIO to light recorded, and the conduction rates in the TIO evaluated. The NIO receives information primarily from the ipsilateral tectum but also from the contralateral one. The transfer is performed through the intertectal commissures; it is blocked by application of KCl on either tectum, and amplified by application of strychnine on the contralateral tectum. It has been shown that the nucleus mesencephalicus lateralis, pars dorsalis, responds to clicks but is without connections to the NIO. The function of the isthmo optic system seems to be to provide an enhancement of the retinal response. Activation of the NIO by pulses delivered either directly to it or on to the tectum, yields an optic tract response to light of higher amplitude. Section of the TIO suppresses the enhancing effect. A possible functional role of the system is proposed. © 1971.
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