Electrophysiology of the pigeon's habenular nuclei: evidence for pineal connections and input from the visual system

Single unit electrical activity was recorded from neurons in the habenular nuclei of pigeons. The neurons were initially classified according to their responses to single shock stimulation of the pineal organ and subsequently according to their response following stimulation of the optic chiasma. The cells were also tested for changes in the level of spontaneous activity in response to illumination of the lateral eyes and the exposed dorsal surface of the brain. In response to electrical stimulation at the two sites, cells were either orthodromically activated or inhibited or unaffected by the stimulus. No antidromic responses to stimulation of either site were observed. The results show that habenular neurons receive excitatory and inhibitory afferent fibres from both regions. Evidence is provided for a substantial convergence of either excitatory or inhibitory inputs onto many of these cells. Both excitatory and inhibitory responses to photic stimulation were observed, although almost half of the neurons were unresponsive. No significant relationships were found between response combinations to electrical stimulation at either site and the effects of illumination. This suggests that neither of the afferent pathways demonstrated are primarily concerned with the transfer of photic information.