Specific spatio-temporal activities in the cerebral ganglion of Incilaria fruhstorferi in response to superior and inferior tentacle nerve stimulation.

In terrestrial gastropod mollusks (slugs and snails), olfaction is the dominant sensory modality guiding various kinds of behavior. Anatomical studies indicate that olfactory information is processed in the brain (the cerebral ganglion) in two lobes in particular: the procerebrum (PC) and the metacerebrum (MtC). This implies that olfactory functions emerge from simultaneous and cooperative processing in the PC and the MtC. However, no previous physiological study has investigated the activity in these two lobes simultaneously. In the present study, the activity evoked by electrical stimulation of the olfactory nerves, the superior and inferior tentacle nerves, was recorded optically from the whole cerebral ganglion of the terrestrial slug, Incilaria fruhstorferi. The results indicated that the evoked activity in the PC and the MtC showed two specific spatio-temporal patterns. First, when either set of nerves was stimulated, the activity of the medial neuropilar region of the MtC (the mMtC) always preceded the activity in the PC. Second, stimulation of the superior tentacle nerves activated the medial and lateral halves of the mMtC almost evenly, whereas stimulation of the inferior tentacle nerves activated the lateral half of the mMtC more strongly than the medial half. These results suggest that the activated region of the mMtC plays an important role in olfactory processing, especially with respect to the functional differences between the superior and inferior tentacles.