Sensory modality interdependence in the octaval system of an elasmobranch

Vibration-sensitive units with labyrinth input are studied in the medullary magnocellular region, and in the mesencephalic torus semicircularis and oculomotor nucleus of the thornback ray. Neuronal activity is recorded during delivery of vibratory and acoustic stimuli, and during presentation of static tilt and sinusoidal pitch movement. These stimulation procedures are subsequently combined in order to gain some insight into the potential biological function of these units. The units described fall into three categories. The first group (called VIB I) are highly sensitive to vibration. They project to the torus semicircularis and are probably activated by the macula neglecta. Their responses to vibratory shocks or acoustic click sounds are not affected by simultaneous presentation of static tilt or movement of pitch. However, if exposed to a background of vibration, these units yield responses phase-locked to sinusoidal pitch; they respond poorly to pitch alone or even fail to do so at all without simultaneous delivery of vibration. Since it is uncertain exactly which natural stimuli excite these units, their biological function is still unclear. The units of the second group (VIB II) are less sensitive to vibration, project to the oculomotor nucleus, and possibly receive input from the anterior two thirds of the macula sacculi and/or the lacinia utriculi. The units respond to both tilt and pitch, provided that continuous vibration is delivered at the same time. Unlike the preceding class, their responses to vibratory bursts and acoustic clicks are suppressed during presentation of pitch and static tilt. It is suggested that this type of unit is more likely to be used for vestibular input of tilt and pitch rather than for vibratory and acoustic information. The third group of units (VIB III) is least sensitive to vibration. These units appear to receive input from the macula utriculi and/or the posterior third of the macula sacculi. They discharge poorly during vibratory stimulation, and are able to respond to pitch even without simultaneous delivery of vibration. Their responses to vibratory stimulation are enhanced by static tilt. Apparently, these units are involved in the processing of vestibular input.