The origin of order effects in the results of the bi-thermal caloric test.

An order effect, characterized by a progressive decline in response magnitude, has been observed in the caloric test, and it has been suggested that the effect may be countered by applying a correction. This study was undertaken to determine whether the effect has its origin in physiological adaptation or in calibration drift of the recording system. Thirty-two normal subjects underwent the caloric test, in which their nystagmus was monitored simultaneously by both electronystagmography (ENG) and videonystagmography (VNG). The experimental design also allowed other aspects of the caloric test sequence to be investigated. No evidence of physiological adaptation of the response was seen. However, there was a significant change in corneoretinal potential, and hence ENG calibration, during the tests. This was more marked if subjects' eyes were maintained in darkness. It is recommended that no correction be made for order effects but that the calibration of ENG-based systems is adjusted immediately prior to each caloric irrigation.