Brainstem and visual cortex control our eyes

A mouse study is illuminating how our brain quickly adapts and functions. Tracking mouse eye movements, researchers make an unexpected discovery — the part of the brain known to process sensory information, our visual cortex, is also key to spontaneous eye movements.

"This study elegantly shows how the visual cortex continues to surprise and awe," said Houmam Araj PhD, a program director at the National Eye Institute (NEI). The study, published in Nature, was led by researchers at the University of California, San Diego (UCSD) and the University of California, San Francisco (UCSF) while funded by the National Eye Institute (NEI), part of the National Institutes of Health.

Without our being aware of it, our eyes are in constant motion. As we rotate our heads, the world moves around us. Two ocular reflexes work to offset that head movement and stabilize any images being projected onto our retinas — the light-sensitive tissue at the back of our eyes.

The first is an optokinetic reflex which causes our eyes to drift horizontally from side-to-side — for example, as we watch scenery through the window of a moving vehicle. Secondly, the vestibulo-ocular reflex adjusts our eye position to offset our head movements. Both reflexes are crucial to survival. Each mechanisms allows humans to see traffic while driving down a bumpy road, or for a hawk in flight to see a mouse scurrying for cover.

The two reflexes occur automatically as a result of signals from the brainstem, an evolutionarily older part of our brain. Both are precisely coordinated to each other, when one is impaired, the other compensates. This orchestration requires adaptive plasticity, says Massimo Scanziani PhD, the study's lead investigator who was a professor of neurobiology at UCSD at the time of the study, before moving to UCSF.

Scanziani and his colleagues, first author Bao-hua Liu PhD - a post-doc at UCSD, and Andrew Huberman PhD - now associate professor at Stanford University, sought to understand the origins of adaptive plasticity by studying the eye movements in mice. In the mouse model, they disabled the vestibulo-ocular reflex which increases the optokinetic reflex. They measured reflex increase by keeping the mouse's head still while rotating visual stimuli in the form of black and white horizontal stripes around the head. Cameras recorded eye movements. More forceful eye movements indicated increased optokinetic reflex activity.

To test the role of the visual cortex in the adaptive plasticity reflexes, they used a technique called optogenetics — which uses light to turn target cells on or off. Targeted cells were the inhibitory neurons in the visual cortex. Turning them "on" silences that region of the brain.

Silencing the visual cortex led to a significant reduction in the activity of the optokinetic reflex, suggesting that the visual cortex is involved in mediating plasticity between the optokinetic and the vestibulo-ocular reflex.

It has long been observed that a collection of neural projections from the visual cortex extends into the brainstem which regulates innate motor behaviors. Lesioning these projections, a decrease occured in the optokinetic reflex. This suggests these neural projections are the anatomical structures with which the visual cortex adjusts plasticity in the optokinetic reflex.

"Most of our reflexes are encoded in our brainstem. But, from an evolutionary standpoint the ability to modify these reflexes expands our behavioral repertoire — as circumstances require. If you've heard people cough after a performance ends, you've experienced this ability to modify reflexes in action.

"It's an ability that appears to have been an attribute important to survival. After all, when you're hiding from a tiger, it would be the very worst moment to cough."

Massimo Scanziani PhD, at the time of the study, Professor, Neurobiology, University of California, San Diego, USA, and study lead investigator. Now with the University of California, San Francisco, USA.

The study was supported in part by NEI grant R01 EY025668.

Reference: Liu, Bao-hua et al. "Cortico-fugal output from visual cortex promotes plasticity of innate motor behavior." Published online in Nature, October 12, 2016. DOI:10.1038/nature19818

NEI leads the federal government's research on the visual system and eye diseases. NEI supports basic and clinical science programs that result in the development of sight-saving treatments. For more information, visit http://www.nei.nih.gov.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.
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Human eye retina.
Image Credit: Public Domain