Organisation of lateral line and auditory areas in the midbrain of Xenopus laevis.

Lateral line areas in the midbrain of Xenopus laevis were identified by recording evoked potentials and neural activity elicited by stimulating anterior and posterior lateral line nerves. Spike activity was found in the lateral half of the optic tectum, ventrolateral tectum, and torus semicircularis. Contra- and ipsilateral lateral line pathways to these regions were identified. Spike discharge was associated with an evoked potential (EP) consisting of a large negative-positive wave sometimes preceded by a small positive-negative deflection. EP depth profiles varied according to electrode position within the lateral line midbrain projection field. In the middle of the field a dramatic increase in EP growth occurred as the electrode passed through the torus semicircularis, with peak amplitudes being achieved 900-1,100 micron from the surface within nucleus principalis and magnocellularis. Tracks at the lateral edge of the field showed a steady growth of EP, with peak amplitudes around 600 micron as the electrode passed through ventrolateral tectum. Auditory responses to tone pips were found in the nucleus laminaris and principalis in caudomedial regions of the torus semicircularis, in areas lying medial to the main centers of lateral line evoked activity; this is a similar organisation to that found in teleost fish. The results indicate the torus semicircularis and deep layers of the lateral tectum to be involved in lateral line processing Some topographic separation of the representation of anterior and posterior lateral line systems is indicated. The possible involvement of these areas in lateral line stimulus localisation is discussed.