Neural activity, alertness and visual orientation in intact and unilaterally labyrinthectomized rabbits

Electrical brain activity and eye movements were recorded in 10 intact and 6 unilaterally labyrinthectomized rabbits, each with implanted electrodes. The frequency of optokinetic nystagmus (OKN) and a 10-second aggregate of theta waves in the dorsal hippocampus were determined after exposure of the animals to sound, vibration and optokinetic stimulation. Each stimulus used caused significant increases in OKN and theta activity. Vibratory stimuli caused stronger increases in OKN and theta activity than did sound stimuli. Not only was OKN significantly reduced after unilateral labyrinthectomy (UL), but alerting the rabbit with sound or vibration failed to restore the responses to normal, and hippocampal EEG responses to sound, vibration or optokinetic stimulus were significantly reduced. The findings indicate that a visual orientation reflex is normally improved by simultaneous sensory stimulation. Reduced response to stimuli after UL affects not only orientational reflexes but other sensory responses as well, and is accompanied by a reduced state of alertness.