The sensory projections to the frontal association cortex in the dog

Afferents to the frontal association cortex (FAC) from structures involved in olfactory, visual and auditory functions were studied in 37 dogs using the method based on the retrograde axonal transport of horseradish peroxidase. Separate injections to FAC or to the mediodorsal thalamic nucleus (MD) showed that sensory afferents could reach FAC by two channels: by direct cortico-cortical projections and via MD. The olfactory input originating in the primary and secondary olfactory cortex run to FAC through both transcortical and subcortical pathways, while some subcortical structures, such as the anterior olfactory nucleus and olfactory tubercle, send abundant afferents to FAC only via MD. The visual projections taking their rise from a few separated areas of the visual association cortex reach FAC only transcortically. Most significant subcortical projection to FAC from the lateral geniculate nucleus, were transmitted via MD. On the contrary, auditory information was sent to FAC mainly through direct cortico-cortical pathways originating in the auditory association areas. Auditory input related by MD was very weak and originated from a limited region of the some cortex. Thus, independently from the way of transmission, auditory and visual projections terminated always in the dorsal zone of FAC, whereas the olfactory projection terminated in its ventral zone.