Influence of postural changes on nystagmic elicitation induced by linear acceleration

Functional nystagmic eye movements are compensatory responses meant to stabilize the gaze during head movement. Niven et al. (1966) have reported that otolithic nystagmus can be elicited by linear acceleration in humans only along interaural (Y-axis) acceleration (Gy), but not along occipitonasal (X-axis) or head-to-foot (Z-axis) acceleration. This result was reexamined in the present study in normal subjects who elicited lateral (Gy) nystagmus frequently. The nystagmic elicitation was examined by EOG in 4 healthy subjects who were fastened tightly in the chair of a linear accelerator and instructed to look straight ahead at an imaginary target in the dark. The stimulation was five oscillations of a constant 10-m stroke at two different acceleration levels of 0.3 and 0.5 G in a step mode. The X- and Z-axis accelerations did not induce any horizontal or vertical nystagmus, while the Y-axis acceleration did, in agreement with the above report. However, it was found that nystagmic elicitation due to Gy loading tended to be lower in the supine than in the upright sitting position. The present finding that nystagmic slow-phase velocity (SPV) increased with an increase in the G-load also contradicted the above report. Upright subjects who showed G-directional preponderance (DP) in nystagmic elicitation did not always maintain DP when supine. No relation could be seen between DP and SPV, suggesting that DP might be generated by a central mechanism different from SPV control.