Correlation of vision loss with tactile-evoked V1 responses in retinitis pigmentosa.

Neuroimaging studies have shown that the visual cortex of visually impaired humans is active during tactile tasks. We sought to determine if this cross-modal activation in the primary visual cortex is correlated with vision loss in individuals with retinitis pigmentosa (RP), an inherited degenerative photoreceptor disease that progressively diminishes vision later in life. RP and sighted subjects completed three tactile tasks: a symmetry discrimination task, a Braille-dot counting task, and a sandpaper roughness discrimination task. We measured tactile-evoked blood oxygenation level dependent (BOLD) responses using functional magnetic resonance imaging (fMRI). For each subject, we quantified the cortical extent of the tactile-evoked response by the proportion of modulated voxels within the primary visual cortex (V1) and its strength by the mean absolute modulation amplitude of the modulated voxels. We characterized vision loss in terms of visual acuity and the areal proportion of V1 that corresponds to the preserved visual field. Visual acuity and proportion of the preserved visual field both had a highly significant effect on the cortical extent of the V1 BOLD response to tactile stimulation, while visual acuity also had a significant effect on the strength of the V1 response. These effects of vision loss on cross-modal responses were reliable despite high inter-subject variability. Controlling for task-evoked responses in the primary somatosensory cortex (S1) across subjects further strengthened the effects of vision loss on cross-model responses in V1. We propose that such cross-modal responses in V1 and other visual areas may be used as a cortically localized biomarker to account for individual differences in visual performance following sight recovery treatments.

Quantitative analysis of retinal structures using spectral domain optical coherence tomography in normal rabbits.

PURPOSE: The purpose of this study is to quantify the normal retinal thickness (RT) and retinal nerve fiber layer thickness (RNFLT), and to obtain cross-sectional area measurements of the main retinal artery and vein in the rabbit animal model using spectral domain optical coherence tomography (SD-OCT). MATERIALS AND METHODS: Fifteen eyes of 15 adult rabbits were used to obtain SD-OCT scans. RT, RNFLT and cross-sectional areas of the main retinal artery and vein were measured with a custom-made software grading tool (OCTOR) on selected B-scans at the edge of the optic nerve head and at 1, 2, 3 and 4 mm from the nerve. Statistical comparisons were made using the analysis of variance test with post hoc comparison. RESULTS: In the nasal and temporal retina, the average RT and RNFLT decreased significantly with eccentricity from the optic disc. There was a statistically significant greater RT and RNFLT at the edge of the optic disc as compared with values at 1, 2, 3 and 4 mm from the disc edge (p < 0.01). Mean cross-sectional area of the primary retinal vein was significantly larger than the retinal artery (p < 0.01). CONCLUSIONS: We have quantified the cross-sectional dimensions of RT, RNFLT and the major retinal vessels in the rabbit retina with SD-OCT. These findings are novel anatomic features of the rabbit retina, which should be considered in any studies using this animal model.