Purchase this article with an account.
M. D. Crossland, M. P. Feely, E. von dem Hagen, A. B. Morland, G. S. Rubin; Fixation Stability and Functional Magnetic Resonance Imaging of Human Visual Cortex. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3846.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Fixation stability has traditionally been measured for short trials (<30sec) using point or cross targets. Assessing visual cortical signals with functional magnetic resonance imaging (fMRI) frequently requires participants to fixate for far longer periods. The purpose of this study was to quantify fixation stability while participants viewed stimuli used in retinotopic mapping procedures, and to determine the effect of poor fixation stability on cortical activation in V1 elicited by viewing the same stimuli.
Ten subjects with normal vision and no neurological or ophthalmological disease were assessed. Subjects were asked to fixate the centre of an expanding ring of flickering checks for 8 minutes. Fixation position was measured using an infrared gazetracker. Median fixation stability (MFS) was quantified by calculating the area of a bivariate contour ellipse encompassing 68% of fixation points using a 30 second moving window starting every second. The area of cortical activity in V1 in response to expanding ring stimuli was measured.
Median MFS value for control subjects was 23 500 minarc2 (IQR: 5 520 - 81 700 minarc2). For comparison, published values of MFS for short trials are around 1 200 minarc2. Fixation stability did not deteriorate over the 8 minutes of the trial (Repeated measures MANOVA, p=0.71). No difference in MFS was found between older and younger subjects (Welch ANOVA, p=0.27). Poorer fixation stability was associated with a reduction in the area of activated V1.
Fixation stability is far poorer for dynamic stimuli used in retinotopic mapping than for short trials fixating point targets. Considerable variability exists among subjects. Poorer fixation stability is associated with a reduction in the extent of stimulus driven activation in primary visual cortex. Therefore it is important to quantify fixation stability in patients and controls if their cortical responses, measured with retinotopic mapping procedures, are to be compared.
This PDF is available to Subscribers Only