Abstract
Purpose: :
To identify the anatomical correlates of voluntary "look" optokinetic nystagmus (OKN) compared to involuntary "stare" OKN based on fMRI.
Methods: :
fMRI was undertaken utilizing a 3.0T GE system and the BOLD technique. Data analysis was carried out using FEAT (FMRI Expert Analysis Tool) Version 5.4 (FMRIB’s Software Library, www.fmrib.ox.ac.uk/fsl). Look OKN and stare OKN were generated under identical stimulus "ON" conditions (vertical sine wave grating of 0.62 c/deg and a speed of 6.4 c/sec drifting right to left with binocular viewing). The only difference between look and stare OKN conditions was the subject instructions. To minimize stimulus pattern and movement related fMRI activation, "OFF" conditions included a stationary grating (pattern only activation) or a counter phase grating (movement and pattern activation). Subjects included 7 normal adults ranging in age from 18–54 years with normal visual acuity (20/20 or better) and normal stereoacuity (40 sec of arc or better).
Results: :
Look OKN generated significantly more fMRI activation than did stare OKN in agreement with Konen et al (Exp Brain Res. 2005). As predicted, a counter phase grating in the "OFF" condition yielded less fmri activation than did the stationary grating in the "OFF" condition. Presumably, the counter phase grating minimized both pattern and movement related fMRI activation while the stationary grating minimized only pattern related fMRI activation.
Conclusions: :
These preliminary results suggest that look OKN involves more brain sites than stare OKN. Further, by choosing "OFF" conditions that minimize both stimulus pattern and stimulus movement fMRI activation, it may be possible to further isolate fMRI activation based on different types of OKN only. The anatomical correlates of look vs stare OKN will be discussed.
Keywords: imaging/image analysis: non-clinical • nystagmus • eye movements