Single-unit study of lateral line cells in the optic tectum of Xenopus laevis: evidence for bimodal lateral line/optic units.

Responses of single units in the Xenopus tectum to stimulation of the contralateral anterior lateral line nerve (cALLN) and optic nerve were studied. Cells responded to cALLN stimulation with a phasic burst of spikes that was repeatable between trials; latencies ranged from 4 to 23 msec. The most excitable cells were located in layer 6 of the ventrolateral tectum. Cells responding to stimulation of the ipsilateral ALLN were far less numerous and robust, and showed latencies 3-10 msec greater than those of contralateral responses. Tectal cell responses to cALLN nerve stimulation grew progressively to saturation with stimulus voltage and paralleled the growth of the ALLN compound action potential; cells responded to stimulation of either supra- or infraorbital branches of cALLN. These observations indicate convergence of primary lateral line afferents in the medulla and/or tectum. Lateral line tectal cells showed strong habituation at interstimulus times less than 8-20 seconds. Experiments on bimodal cells revealed facilitatory and inhibitory interactions between optic and lateral line inputs. Some cells responded to stimulation of either lateral line or optic nerves, with combined stimulation producing responses exceeding the sum of responses to separate nerve stimulation. In other cells the response to optic nerve stimulation was markedly increased by lateral line nerve stimulation, despite the absence of response to lateral line nerve stimulation alone. Facilitation was also evident in cells that responded only to combined stimulation of lateral line and optic nerves. Some cells exhibited an early (5-10 msec) and late (20-40 msec) response to optic nerve stimulation; lateral line nerve stimulation, despite eliciting no response itself, produced strong facilitation of the early but almost complete suppression of the late optic nerve response.