In the frog (Rana esculenta L.) we have tested the possible existence of visual inputs in the vestibular nuclei by recording the activity of second-order vestibular neurons receiving primarily horizontal canal inputs, while the visual system was stimulated by either light pulses or optokinetic stimulation; moreover, combined visuovestibular stimulation (horizontal rotatory stimulation in light) was performed and the response compared to the one obtained during vestibular stimulation alone (horizontal rotatory stimulation in dark). 42.2% of the neurons tested (38/90) responded to light pulse stimulation of the retinae by an increase of their discharge frequency of about 150%, while the other neurons (57.8%) did not. Optokinetic stimulation was completely ineffective in modulating the discharge frequency of all the neurons (98) recorded. Among 30 neurons tested during constant-velocity horizontal rotation of the turntable (2.5–10°/s) in the excitatory direction, 13 units (43.3 %) had a discharge rate significantly greater than the spontaneous frequency during the constant-velocity phase when the rotation was performed in light, while a few seconds after beginning of the constant-velocity phase the discharge frequency returned to its resting value when the rotation was performed in dark. Moreover, the difference between the maximum discharge frequency and the resting discharge frequency (AF) was significantly higher in light than in dark. The behavior of the 17 other neurons (56.7%) was similar whether the rotation was performed in light or in dark. When tested to sinusoidal rotation (0.05 Hz ± 40° or 0.1 Hz ± 30°), the characteristics of the responses were different in light and in dark. In light the gain was enhanced, and the phase lag relative to acceleration was greater. (The phase lead relative to velocity was smaller.) These results clearly show that vestibular nuclear neurons responded to light pulse stimulation; they also show that their responses to combined visuovestibular stimulation and to vestibular stimulation alone were different. Lacking of optokinetic response is discussed.

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