SPATIAL TRANSFORMATION OF SEMICIRCULAR CANAL SIGNALS INTO ABDUCENS MOTOR SIGNALS - A COMPARISON BETWEEN GRASS FROGS AND WATER FROGS

The spatial transformation of semicircular canal signals to extraocula r motor signals was studied by recording abducens nerve responses in g rass and water frogs. Both species have similar vestibular canal coord inates but dissimilar orientations of their optic axes. Before sinusoi dal oscillation in darkness the static head position was systematicall y altered to determine the planes of head oscillation in pitch and rol l associated with minimal abducens nerve responses. Measured data and known canal plane vectors were used to calculate the abducens response vector in canal coordinates. The abducens vector deviated from the ho rizontal canal plane vector in grass frogs by 15 degrees and in water frogs by 34 degrees but was aligned with the pulling direction of the lateral rectus muscle in each of the two species. Lesion experiments d emonstrated the importance of convergent inputs from the contralateral horizontal and anterior semicircular canals for the orientation of th e abducens response vector. Thus, the orientation of the optic axis an d the pulling directions of extraocular muscles are taken into account by the central organization of vestibule-ocular reflexes. Horizontal and vertical canal signals are combined species-specifically to transf orm the spatial coordinates of sensory signals into appropriate extrao cular motor signals.