MEMRI-based functional mapping of the visual system in response to color stimuli

Manganese-enhanced MRI (MEMRI) visualizes neuronal activity via manganese uptake. While intravitreal MnCl₂ injection is effective, it is invasive and carries toxicity risks. This study aimed to evaluate a noninvasive topical MnCl₂ approach for detecting functional activation in the rat visual pathway in response to color-specific light stimuli.

Sprague-Dawley rats (N = 24) were randomly assigned to red, green, blue, or dark-exposed control groups (n = 6 per group). Each animal received 5 μL of 0.1 M MnCl₂ topically to the right eye, followed by 10 h of exposure to monochromatic LED light or darkness. T1-weighted MRI was performed on a 1.5-T scanner. Image processing included region-of-interest (ROI) analysis and voxel-wise statistical parametric mapping (SPM) using AlphaSim-based cluster correction (voxel-wise p < 0.005, minimum cluster size = 16 voxels).

MEMRI signal enhancement was observed in the retina, optic nerve, and superior colliculus in the green light group only. ROI analysis showed a significant increase in signal-to-noise ratio (SNR) for green light, and to a lesser extent for blue light, compared to control (p < 0.05), while red light elicited no significant changes. SPM analysis confirmed localized signal enhancement in the contralateral postchiasmatic optic tract and superior colliculus in the green light group.

Topical MnCl₂ combined with MEMRI successfully detected color-dependent neuronal activation in the rat visual system. These findings support the utility of noninvasive MEMRI combined with color stimulation and voxel-wise analysis as a promising tool for functional mapping of the sensory pathways.