Comparison of olfactory bulb projections in pigeons and turtles

The projection targets of the olfactory bulb in pigeons and turtles were investigated using autoradiographic techniques. Despite the relatively smaller size of the olfactory bulbs in pigeons, the projection targets of the olfactory bulb are very similar to those in turtles. In both pigeons and turtles, the olfactory bulb projects to the entire rostrocaudal extent of a portion of the dorsolateral telencephalon (which is here recognized as the pyriform cortex in both birds and reptiles) and to portions of the medial telencephalic wall including the medial septal region. In addition, a projection to the olfactory tubercle of the ventral telencephalon is clearly present in turtles and also appears to be present in pigeons. Pigeons and turtles do differ significantly, however, in the extent of the projection to the amygdaloid region. In turtles, olfactory bulb input encompasses the entire mediolateral and rostrocaudal extent of the amygdaloid region, while in pigeons the input is restricted to a small dorsomedial portion of the amygdala termed nucleus taeniae of the archistriatum. The present results suggest that the olfactory bulb projections in birds are generally similar to those in reptiles, with the exception that secondary olfactory bulb projections to the amygdala may be much reduced in birds compared to those in reptiles. The functional significance of the reduction in olfactory input to the amygdala is presently uncertain.