Topography of responses to colour and luminance in human subcortical visual pathways as revealed by high-resolution fMRI at 7T

Abstract

Chromatic signals, are transmitted to cortex via the lateral geniculate nucleus (LGN) and potentially also via the superior colliculus. The latter has not been studied to date in humans using cone-contrast controlled stimuli. Here we investigated the BOLD-responses in these sub-cortical regions using various cone-contrast levels. We measured sixteen axial slices covering cortical, tegmental, and thalamic regions on a Siemens 7T MR-scanner (inplane-resolution 1.38×1.38mm, slice-thickness 1.5mm, TR=1s). Visual stimuli were hemifield-checkerboards (18deg) containing a central fixation marker. Two directions in cone contrast space were used with four contrast levels each (L−M: 1.75%,3.5%,7%, and 14%; L+M: 7%,14%,28% and 56%). Although hemodynamic response amplitudes were substantially lower in subcortical regions than in V1 it was possible to record reliable signals from SC and LGN. Responses to colour and luminance stimuli in V1, LGN and also the superior colliculus increased monotonically with cone contrast. Luminance contrast sensitivity was highest in the SC. At 7% cone contrast V1 and LGN responded stronger to color-opponent stimuli while the SC preferred luminance stimuli. At 14% cone-contrast, we measured approximately equal BOLD-responses to colour and luminance in all three regions, also in SC. This provides evidence for color-opponent properties in the SC. The shape of the BOLD response differed substantially across the ROIs, possibly indicating different adaptation properties of the underlying neurons. In addition, the gain of luminance responses was sufficient to separate magno- and parvo-cellular subdivisions of the LGN. Thus, ultra-high-field fMRI may serve as a valuable tool to study fine-grained topographies of subcortical visual processing non-invasively in the human brain.