Thresholds for perception of direction of linear acceleration as a possible evaluation of the otolith function

BACKGROUND Previous attempts to measure otolith function using ocular counter-rolling have shown poor sensitivity and specificity, thereby hindering a useful clinical application. We have conducted a study to investigate whether thresholds for the perception of the direction of linear acceleration might be an alternative for the clinical evaluation of otolith or statolith function. METHODS Perception of the direction of motion was evaluated in 28 healthy subjects while all external auditory and visual cues were eliminated. Whole body motion stimulus was generated by a motor driven linear sled at a stimulus frequency of 1 Hz at a linear acceleration ranging from 0 to maximum 40 cm/s2. Subjects were required to correctly indicate the direction of motion (anterior-posterior or lateral) or whether they were stationary. Both velocity and acceleration thresholds were measured. RESULTS The median acceleration thresholds for the perception of direction of linear movement for anterior-posterior movement was 8.5 cm/s2 and for lateral movement 6.5 cm/s2. According to the literature, acceleration thresholds depend on the stimulus profile whereas velocity thresholds do not. The median velocity thresholds for the perception of direction of linear movement for anterior-posterior movement was 13.5 cm/s and for lateral movement was 10.4 cm/s. The median velocity thresholds for the perception of direction of linear movement for anterior-posterior movement increased linearly with age, whereas the median velocity threshold for lateral movement was not correlated with age. CONCLUSION The thresholds found in this study are lower than reported in the literature before which may be due to the repetitive predictive sinusoidal stimulus which makes it relatively easy to lower the threshold by learning already within one test profile. The variability is large in line with the previous literature, but our experiments indicate that variability decreases after a training session. We interpret the literature and our current results that linear velocity thresholds after some training might reflect the sensitivity of the otolith system per se.