Non-visually evoked activity of isthmo-optic neurons in awake, head-unrestrained quail.

Changes in the internal state of the brain may modulate retinal function. In birds, most neurons in the isthmo-optic (IO) nucleus project their axons topographically into the contralateral retina, and activity in IO neurons enhances visual responses of retinal ganglion cells in the target retinal region. To elucidate the significance of this pathway, we recorded spikes of IO neurons in four awake Japanese quail using an implanted electrode assembly while recording unrestrained head movements. The IO neurons fired passively in response to visual stimuli in receptive fields and non-visually without visual stimuli or eye-head movements. Non-visually evoked activity was observed in the middle of eye-head fixation, as well as at about 200 ms before the onset of head saccades. Intensity of activity before onset of head saccades depended on the direction of motion of subsequent head saccades. Local retinal output may be enhanced by centrifugal signals before gaze shifts.