Vestibular caloric responses and behavioral state in the fetal sheep

Functional activity of the vestibular system in relation to behavioral state of fetal sheep in utero was studied by cooling and heating of the fetal middle ear and skin (control) with implanted copper-tube heat exchangers. Eye movements and fetal cortical activity were assessed before, during, and after 2 min irrigations with water at 6, 46, or 39.5 degrees C (isothermic). Cold water induced slow-phase eye movements toward the irrigated ear followed by saccades toward the opposite ear after a delay of several seconds. The direction of the response reversed with warm water, and saccades were absent during irrigation with body-temperature water. Cold-water irrigations of the skin over the jaw did not result in nystagmus. Arousal-like responses were elicited with thermal stimulation of the ear or facial skin while the fetus was in either rapid eye movement (REM) or non-REM states. Circulation of 39.5 degrees C water through the ear also produced arousal-like responses, possibly due to turbulence-induced noises in the heat exchanger or slight deviations between the irrigation temperature and the actual fetal inner ear temperature. These results suggest that mechanisms responsible for saccade suppression during depressed levels of consciousness (i.e. sleep) are inactive in utero. Fetal behavioral state responsiveness to vestibular and somatosensory thermal stimulation may be of great significance, especially in the premature neonate.