Identification of avian brain regions responsive to retinal slip using 2-deoxyglucose.

Key words: accessory optic system-pretectum-retinal slip-optokinetic nystagmus In optokinetic nystagmus (OKN) retinal slip, that is, movement of the visual world on the retina, results in stabilizing eye movements which reduce the retinal slip velocity almost to zero. The visual input for this compensatory reflex seems to be transmitted along anatomically distinct pathways to the pretectal nuclei and accessory optic system in both birds and mammals. In mammals, one of the pretectal nuclei, the nucleus of the optic tract (NOT), appears to receive visual input necessary for horizontal OKN from a class of retinal ganglion cells that respond to slow horizontal movement and presumably project directly to the NOTS,7,15. Cells of the NOT are excited by slow whole-field horizontal movement in a temporal to nasal direction and either are not activated or are inhibited by movement in the opposite directionS, 7. The most convincing evidence for the role of NOT is the finding that electrical stimulation of the NOT produced horizontal OKN while lesioning the nucleus eliminated this type of nystagmus 6. In the mammalian accessory optic system, the medial and lateral terminal nuclei (MTN and LTN respectively), appear to be involved in retinal signalling of vertical OKN. Retinal ganglion cells that are direction-selective and respond to slow stimulus movement14,15 are thought to terminate in the MTN. The cells of the MTN have large receptive fields and respond to slow vertical stimuli moving upward and posteriorlS, 20. Physiological properties of LTN are similar except that the cells prefer downward and posterior movement. The third terminal nucleus, the dorsal nucleus (DTN), is best modulated by horizontal movement in a temporal to nasal direction, a property of cells in the NOTS,7, is. In birds the nuclei considered homologous to the mammalian accessory terminal nuclei are the nuclei of the basic optic root (nBOR) 1,11 which consist of the nBOR pars dorsalis (nBORd), nBOR pars lateralis (nBOR1) and the nBOR proper. As in mammals, two of the three nuclei appear to receive vertical, or near vertical, retinal slip signals, as shown by electrophysiological studiesZ,a,12,1L Neurons of the nBOR responsive to slow whole-field upward movement are concentrated in the nBORd while neurons responsive to slow whole-field downward and anterior movement are