Robust Midbrain Connectivity May Be Key to Paying Attention

Connectivity of midbrain regions to cerebral cortex influences visual attention.

Posted Sep 13, 2019

A previously under-investigated midbrain region called the superior colliculus (SC) may play a vital role in the ability to pay attention, according to a new study by neuroscientists at the Indian Institute of Science centre (IISc). The findings were published on September 6 in the journal Proceedings of the National Academy of Sciences.

Historically, forebrain mechanisms within the frontal lobes have been widely studied as playing a role in visuospatial attention. However, until now, the role midbrain regions might play in attention has been relatively unexplored, and these mysterious midbrain mechanisms have stayed under the radar.

For this pioneering non-invasive brain research, Devarajan Sridharan, Assistant Professor at IISc's Centre for Neuroscience, and one of his Ph.D. students, Varsha Sreenivasan, mapped the structural connectivity of the human SC midbrain regions with neocortical regions in both hemispheres of the cerebral cortex.

Notably, the researchers found that the strength of these connections strongly predicted the robustness of someone’s visual attention. 

As the first step of this experiment, Sreenivasan and Sridharan quantified hemispheric asymmetries between the “left brain-right brain” based on white matter connectivity of the SC with various cortical regions in the cerebral cortex. In general, they found stronger SC connectivity with the cerebral cortex of the left hemisphere.

Cortex means “bark of a tree” in Latin. For centuries, neuroscientists have associated the cerebral cortex with higher brain functions such as language, logic, focus, decision-making, and creativity. Neuroscientists have only recently begun to explore how midbrain regions such as the superior colliculus interact with various parts of cerebral “bark” that contains the gray (or "grey" in British English) matter of the cerebrum.

"SC is an evolutionarily conserved midbrain structure that can be found in all vertebrates, including fish, lizards, birds, and mammals. It is usually studied for its role in controlling eye movements," Sreenivasan explained in a statement.

When the researchers analyzed the results of their human experiments, they noticed robust SC midbrain connectivity to a part of the cerebral cortex called the parietal cortex; previous research on monkeys linked these regions to visual attention.

Taken together, these recent findings (2019) in humans, along with previous animal studies, suggest that the human SC midbrain region may play an evolutionarily conserved role in paying selective attention to visual cues.

Surface map of connections of the Superior Colliculus (SC) with the cortex. Stronger superior SC connectivity with the left hemispheric cortex. (A, Top) Thresholded surface maps (averaged across subjects) showing the voxelwise distribution across the cortex of connection weights with the SC. (Left and Right) Left and right hemispheres, respectively. (Top and Bottom): Lateral and medial views, respectively.
Source: Sreenivasan and Sridharan, PNAS, 2019 DOI: 10.1073/pnas.1902704116

As you can see in the neuroimages from this study (above), SC connectivity with the cerebral cortex exhibited clear laterality between the left and right cerebral hemispheres.

In the discussion section of their paper, the authors conclude with more questions than answers. Sridharan and Sreenivasan write:

“More generally, our findings on structure–behavior relationships motivate other key questions on white matter plasticity and its consequences for attention. Can subjects learn to selectively modulate one component of attention and not the other? Would such learning generate structural changes in specific connections? Can structural plasticity induced by neurostimulation, in turn, alter only one of the two components of attention? Addressing these questions could provide deeper insights into key mechanisms of selective attention in the brain."

In the near future, Sridharan and Sreenivasan are planning to study SC activity using functional magnetic resonance imaging (fMRI) to see if they can identify increased blood oxygen levels in specific brain regions that are activated during visual attention tasks.

"Through fMRI, we will investigate if SC's activity correlates with behavioural measures of sensitivity and bias. Our approach can also help understand if SC-cortex connection asymmetries are predictive of certain kinds of attention disorders such as ADHD," Sreenivasan said.

Blogger’s note: As the son of a 20th-century neuroscientist, Richard Bergland (1932-2007), who wrote The Fabric of Mind in the 1980s, I grew up surrounded by discussions about "left brain-right brain," the cerebral cortex, white matter functional connectivity, and gray matter volume.

Unfortunately, subcortical brain regions and the cerebellum (Latin for "little brain") were often overlooked and underestimated by neuroscientists of my father's generation. After he retired, Dad was on a mission to put the human cerebellum center stage.

In the early-21st century, my father and I created a split-brain model (up brain-down brain) that shifted the spotlight away from myopically focusing on cortical regions of the cerebrum and "left brain-right brain" lateralization of the cerebral cortex.

Illustration and photo by Christopher Bergland circa 2009.
Source: Illustration and photo by Christopher Bergland circa 2009.

In 2009, I clumsily drew a brain map (above) using Sharpie markers and highlighter pens that attempts to illustrate the importance of functional connectivity between all four brain hemispheres and positions the midbrain as a critical "bridge."

Since drawing this simplistic, color-by-numbers map over a decade ago, I've kept my antennae up for new research that might fill in the blanks of why this map could have some "all four brain hemispheres working in concert" significance.

Suffice to say, the moment I read about the new research by Sreenivasan and Sridharan this morning—and saw their brain maps—the first thing that sprung to mind is that my "educated-guess" brain map also shows the importance of midbrain connectivity. e.g., White matter tracts are represented by the yellow and green bidirectional Asteroids-video-game-inspired arrows within the "Super Eight" passing through the midbrain.

The latest study (2019) on midbrain connectivity to the cerebral cortex and attention doesn't explore (or mention) the cerebellum. That said, as a science reporter, who keeps my finger on the pulse of neuroscience trends, it seems that research about the importance of functional connectivity between the left and right hemispheres of the cerebrum and cerebellum is increasingly being published by peer-reviewed journals.

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Varsha Sreenivasan and Devarajan Sridharan. "Subcortical Connectivity Correlates Selectively with Attention’s Effects on Spatial Choice Bias." Proceedings of the National Academy of Sciences of the United States of America, PNAS (First published: September 6, 2019) DOI: 10.1073/pnas.1902704116