Abstract
Abstract: :
Purpose: Two forms of OKN exist, look and stare OKN, which were first described by Ter Braak (Arch Neerl Physiol, 1936). Look OKN is characterised as having a high gain with large amplitude slow phases, associated with infrequent quick phases, whereas stare OKN produces a pattern consisting of low gain and amplitude with frequent fast phases. It has been postulated that look OKN is mediated through cortical pathways whereas stare OKN is thought to be subcortical in nature. We compared look and stare OKN in four different stimulus directions; vertically, up and down, and horizontally, with nasally and temporally directed targets. Methods: Ten normal healthy volunteers viewed a sinusoidal contrast grating covering a visual field of ±20o, at 33cm, with either the right or left eye. A target contrast of 50% was selected with a target velocity of 40°/s. Stare OKN was assessed by asking the subject to keep fixation in the centre of the target and ignore the moving stripes, whereas look OKN was assessed by asking the observer to actively fix and follow individual target stripes. Eye movements were recorded from each eye, using a high–resolution infrared pupil tracker (EyeLink, Sensomotoric Instruments). Slow phase OKN gains were measured. Results: The look OKN data traces revealed a combination of both look and stare OKN, with stare OKN occurring at the beginning and end of a look OKN slow phase. The data was sub–analysed to isolate the look OKN. The gain of look OKN was strongly correlated with the gain of stare OKN for downward moving stimuli (p<0.001) but not for upward moving stimuli. There was also a weak correlation between the gain of look and stare OKN for nasalward moving stimuli (p<0.05) but not temporalward moving stimuli. Conclusions: When observing downward moving targets, the gain of look OKN is significantly correlated with that of stare OKN. It is thought that look OKN is related to smooth pursuit, and is possible that the difference in the correlation between the gains of upward and downward OKN can be explained by differences in the smooth pursuit system.
Keywords: eye movements • nystagmus