Mechanisms underlying modulations of thermal nystagmic responses in parabolic flight.
In six subjects nystagmography was used to compare the responses to cold calorization of one horizontal semicircular canal under ground-based and parabolic flight conditions. On the ground the expected individual differences in primary nystagmic responses were observed; only one subject manifested a brief weak secondary nystagmus. Aloft, the irrigation was carried out in straight-and-level flight prior to a pushover (half-parabola) that initiated a series of four to nine typical parabolas in a modified KC-135 aircraft. Thereafter, the free-fall phases of the parabolas furnished a zero baseline for measuring weight differences in endolymph due to changes in gravitoinertial force. In all subjects a secondary nystagmus was generated in addition to the primary nystagmus during the course of the parabolic maneuvers. The slow phase of the secondary nystagmus rarely exceeded 10 mm per sec. Evidence is presented that secondary nystagmus arises as a direction-specific adaptation effect countering the primary nystagmic response. Three stages were recognizable: first, when primary and secondary nystagmus alternated in step with the high and low force phases of the parabolas; second, after disappearance of primary nystagmus when secondary nystagmus appeared alone and was modulated by the changes in force; and third, when the secondary nystagmus present became independent of the highest gravitoinertial forces generated. Great individual differences were observed, suggesting that with large departures of the cupula from its functional rest position there are large individual variations in rate of restoration.