[Photic evoked responses in the brain stem of the rat]

The appearance of photic evoked responses in various structures of the reticular formation (RF) and the thalamus was investigated in freely moving rats. Photic evoked potentials (PhEP) were recorded in all the nuclei tested. The PhEP are different in shape, amplitude and latency; they can be classified as a "primary type" with small amplitudes and without latency differences from the PhEP of the visual cortex (VC) and as a "secondary type" with large amplitudes and latency differences from the PhEP of the VC. The "primary type" was observed in thalamic, pontine and bulbar structures but the "secondary type" in posterior-thalamic and mesencephalic structures. Photic afterdischarges (PhNE) and photic recruitment (PhR) were recorded in most of the nuclei tested. These PhNE and PhR have a correlation in their frequency and peak-latency to the PhNE and PhR of the VC. It is discussed that a great part of visual information is transferred to the brain stem through the Corpus geniculatum laterale (CGL) and the VC.