VESTIBULAR CALORIC RESPONSES AND BEHAVIORAL STATE IN THE FETAL SHEEP

Functional activity of the vestibular system in relation to behavioral state oi fetal sheep iu utero was studied by cooling and heating of t he fetal middle ear and skin (control) with implanted copper-tube heal exchangers. Eye movements and fetal cortical activity were assessed b efore, 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 oi the response revers ed with warm water, and saccades were absent during irrigation with bo dy-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 sl ight deviations between the irrigation temperature and the actual feta l inner ear temperature. These results suggest that mechanisms respons ible for saccade suppression during depressed levels of consciousness (i.e. sleep) are inactive in utero. Fetal behavioral state responsiven ess to vestibular and somatosensory thermal stimulation may be of grea t significance, especially in the premature neonate. (C) 1998 Elsevier Science Ireland Ltd. Ail rights reserved.