Entrainment of the rat's activity rhythm by cyclic light following lateral geniculate nucleus lesions.

The rhythmic running-wheel activity of rats is entrained by sensory input via the visual system. The retinal projection transmitting visual information essential for entrainment of the activity rhythm is unknown. Lesions of the lateral geniculate nucleus (LGN) disrupt two projections, the primary optic tract (POT) and the superior accessory optic tract (AOT-SF). It was found that activity remained entrained to a light cycle following LGN lesions. This indicates that the POT and the AOT-SF are not necessary for entrainment. The two remaining projections, the retinohypothalamic pathway and the inferior accessory optic tract (AOT-IF), are sufficient to maintain entrainment. In light of other investigations, it is suggested that the retinohypothalamic pathway is the key projection for synchrony of the activity rhythm with environmental illumination. It was also found that LGN lesions severely impaired the subjects' ability to learn a brightness discrimination.