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Melissa Emily Wright, Krishna D Singh, Simon K Rushton, Slawomir Kusmia, Richard G Wise, D Samuel Schwarzkopf, Tony Redmond; Exploring the Haemodynamic Response Function in the occipital lobe in glaucoma. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6110.
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© ARVO (1962-2015); The Authors (2016-present)
A full understanding of the neural correlates of visual field loss in glaucoma requires investigation of functional change in both the retina and visual cortex (e.g. with functional Magnetic Resonance Imaging; fMRI). Given that fMRI measures blood oxygenation level as a surrogate for cortical activity, any difference in haemodynamic response between individuals with and without glaucoma needs to be understood before formally comparing inferred functional activity, especially considering vascular risk factors for glaucoma. In this exploratory study, the Haemodynamic Response Function (HRF) at the occipital lobe was compared between glaucoma patients and age-similar controls with ultra-high field 7T fMRI.
Seven patients with glaucoma (median [range] age: 70.8 [59.5, 86.3] yrs; median [range] MD: -2.2 [-8.4, -0.5] dB) and 7 healthy age-similar controls (median [range] age: 63.0 [53.1, 72.3] yrs; median [range] MD: +0.2 [-1.4, +1.4] dB) underwent HRF mapping with 7T fMRI. A 1.2mm3 functional scan (repetition time, TR: 2s) was located over the occipital lobe, using the cerebellum as an anatomical landmark. To estimate the HRF response, subjects viewed an 11° radial checkerboard monocularly for 2s, then mean grey luminance for 20s. This was repeated 10 times. A structural scan (1mm3, TR: 2.2s) was also acquired. Repetitions were averaged and a HRF was fitted to the data. Peak amplitude (PA), time-to-peak (TTP), and full-width at half-maximum (FWHM) were extracted and compared between groups.
The figure shows raw blood oxygenation level dependent (BOLD) responses and fitted HRFs (individual and group average), with estimates of PA, TTP and FWHM for glaucoma patients and controls. Despite inter-individual differences in HRF in each group, there is no evidence of a difference in any attributes of the function between patients and controls (Bayes Factor [BF01, 2-sided t-test]: 1.82 [PA], 2.15 [TTP], and 1.60 [FWHM]).
The HRF is unlikely to be a confounder when comparing cortical activity between glaucoma patients with fMRI. This ultra-high field resolution enables greater weighting towards smaller blood vessels found in grey matter than nearby major vasculature, and so can be considered accurate for measuring the HRF in the target tissue.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
A: Individual BOLD responses; B: Fitted HRFs for each subject (thick lines: group average); C: peak amplitude; D: time-to-peak; E: full-width at half-maximum
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