X-Chromosome Insufficiency Alters the Structure and Function of the Human Visual Cortex

Post by Lincoln Tracy

What's the science?

Turner syndrome is a genetic condition in females caused by the absence of a part of, or an entire, X chromosome. Women with Turner syndrome have specific cognitive deficits relating to the ability to perceive spatial relationships between objects and the ability to select and monitor goal-related behaviors. Previous research suggests that women and girls with Turner syndrome have structural changes in the parieto-occipital cortex (the part of the brain responsible for the integration of sensory information and the processing of visual information) when compared to typically developing controls. Specifically, females with Turner syndrome are reported to have a smaller volume of parieto-occipital cortex with a thicker surface. Despite knowing that these structural changes exist, it is not known whether abnormal processing of visual information occurs in females with Turner syndrome. This week in The Journal of Neuroscience, Green and colleagues used functional magnetic resonance imaging (fMRI) and population receptive field (pRF) mapping to investigate receptive field processing in Turner syndrome.

How did they do it?

The authors recruited 24 girls (aged 7-14 years) with Turner syndrome and 28 typically developing girls of the same age to act as controls. All 52 participants had an MRI scan where they watched a screen that showed a flickering black and white checkerboard pattern in either expanding rings or rotating wedges to obtain and assess data for the pRFs. pRFs can be used to generate topographic maps of the visual field (i.e. the total area an individual can see) using representations of the polar angle (the angle from the horizontal axis) and eccentricity (the distance from fixation). Data from the MRI scans were used to determine the volume, surface area, and cortical thickness of the early visual areas V1-V3. Visual field coverage (the locations within a visual field that evoke a response from voxels within a brain map) in these areas were also drawn from the MRI data. Outside of the MRI scanner the girls completed the Picture Puzzles test, one of the tests from the NEPSY-II battery, which tests visuospatial processing. The relationship between performance on the Picture Puzzles test and pRF properties was compared.

What did they find?

First, the authors found that girls with Turner syndrome and typically developing girls showed a similar organization of polar angle and eccentricity maps. These maps were then used to define visual field maps for V1-V3. Using these maps, the authors then showed that while there were no differences in total brain volume between girls with Turner syndrome and typically developing girls, the cortical volume and surface area of the V1-V3 visual field maps were smaller in girls with Turner syndrome. There were no differences in V1-V3 cortical thickness between girls with Turner syndrome and typically developing girls. Second, they found that despite Turner syndrome and typically developing participants having similar pRF size for V1-V3, participants with Turner syndrome had less pRF eccentricity in visual areas V1-V3 (meaning that their pRF was closer to the center of their gaze). Third, visual field coverage was determined by comparing how the pRFs from V1-V3 fit together in the visual field. There were no differences between participants with Turner syndrome and typically developing girls in the visual field coverage of V1-V3, nor were there differences in the polar angle and eccentricity maps. Fourth, they found that participants with Turner syndrome performed worse in the Picture Puzzles task compared to the typically developing group. Performance in the Picture Puzzles task was found to be negatively correlated with pRF size and eccentricity in girls with Turner syndrome. There was no correlation between performance in the Picture Puzzles task and the cortical volume of V1-V3 in either the participants with Turner syndrome or typically developing girls.

What's the impact?

The authors have demonstrated that Turner syndrome may negatively affect the functional properties of brain regions responsible for visual processing, which may in turn influence behavior. The findings of this study may serve as a novel approach for investigating how other regions of the brain are affected by Turner syndrome. Furthermore, the authors provide a target for interventions to improve the visuospatial deficits observed in Turner syndrome. Taken together, these results suggest a fundamental change in our understanding of how variations in sex chromosome pairings affect visuospatial development in humans.

Green et al. X-Chromosome Insufficiency Alters Receptive Fields across the Human Early Visual Cortex. Journal of Neuroscience (2019).Access the original scientific publication here.