Avian cerebral asymmetries: the view from the inside

Lateralized activation of Cluster N in the brains of migratory songbirds

Cluster N is a cluster of forebrain regions found in night-migratory songbirds that shows high activation of activity-dependent gene expression during night-time vision. We have suggested that Cluster N may function as a specialized night-vision area in night-migratory birds and that it may be involved in processing light-mediated magnetic compass information. Here, we investigated these ideas. We found a significant lateralized dominance of Cluster N activation in the right hemisphere of European robins (Erithacus rubecula). Activation predominantly originated from the contralateral (left) eye. Garden warblers (Sylvia borin) tested under different magnetic field conditions and under monochromatic red light did not show significant differences in Cluster N activation. In the fairly sedentary Sardinian warbler (Sylvia melanocephala), which belongs to the same phyolgenetic clade, Cluster N showed prominent activation levels, similar to that observed in garden warblers and European robins. Thus, it seems that Cluster N activation occurs at night in all species within predominantly migratory groups of birds, probably because such birds have the capability of switching between migratory and sedentary life styles. The activation studies suggest that although Cluster N is lateralized, as is the dependence on magnetic compass orientation, either Cluster N is not involved in magnetic processing or the magnetic modulations of the primary visual signal, forming the basis for the currently supported light-dependent magnetic compass mechanism, are relatively small such that activity-dependent gene expression changes are not sensitive enough to pick them up.

Timing of ascending and descending visual signals predicts the response mode of single cells in the thalamic nucleus rotundus of the pigeon (Columba livia)

Neurons of the pigeon's diencephalic n. rotundus were demonstrated to show visual responses of short and long latency representing ascending signals of the retino-tecto-rotundal system and descending signals from telencephalo-tecto-rotundal fibers. Pigeons thus provide an ideal model to investigate the convergence of ascending and descending visual processing streams at single cell level. Although it is known that rotundal responses of long latency show distinct response characteristics, dependent on the stimulus being presented monocularly or binocularly, the mechanisms underlying these response differences are still unclear. While it is possible that the simultaneity of eye stimulation produces a change of processing, it is also possible that the relative timing and order between ipsilateral and contralateral signals are the decisive variable. To test between both possibilities, we recorded from cells in the pigeon's n. rotundus while providing monocular or binocular visual stimulation and varying the delay and order of eye presentations. We revealed that the precise temporal interaction and order of ascending and descending inputs to the tectum decide about late responses with burst or tonic characteristics. When descending signals reached the tectum before the ascending signals, rotundal cells showed late responses that were characterized by burst activity patterns. When ascending input reached the tectum first, responses with tonic characteristic were observed. These effects might become mediated by intratectal mechanisms, the nucleus ventrolateralis thalami, or the bed nuclei of the tectothalamic tract and might constitute the neural basis of a bihemispheric gating function.