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A Direct Fear Pathway in the Eye-Brain Neural Circuity in Small Mammals

2015-06-26

This article from Science News is behind a paywall, so forgive me if I tell the whole story to make sure you don’t have to visit the site.  By the way, in college, I was the junior member of a team that was exploring visual pathways in the brains of cats, specifically the lateral geniculate body, a lower brain first stopover for optic nerve impulses from the retina.  In the lateral geniculate are cells that respond specifically to single small spots in the cat’s visual field, so they are some of the simplest nerve cells in the brain that respond to light.

The Science News story is about the discovery of a path that fear takes to the brains of mice: the superficial superior colliculus.  In this brain nucleus, located on the top or dorsal surface of the mouse’s brain, are single cells that respond specifically to objects in the visual field that appear to be coming closer, and the more rapid the closure rate, the higher the response.  These cells project to the amygdala, the brain nucleus most closely associated with the sensation of fear, with a stopover in the parabigeminal nucleus.

The researchers were able to genetically engineer these cells to respond to blue light.  Thereafter, they were able to stimulate them at will, using an optical fiber implanted in the brain close to the superior colliculus.  They were able to cause generalized fear reactions, specifically “running around in frantic attempts to escape” followed by freezing for a minute or more, even after the stimulus was turned off.  This was followed, in some cases, by “depression” related phenomena: the stimulus “triggered fear responses, induced conditioned aversion, and caused depression-related behaviors.”

In addition, the abstract concludes:  “Approximately 20% of mice subjected to the fear-conditioning paradigm developed a generalized fear memory.”  Does that sound like post-traumatic stress disorder?

In other words, was the research involved in the induction of PTSD in mice?  Perhaps it was only 20% of them.  Otherwise, to be honest with you, I don’t see the point of the research.  It’s cool to genetically engineer specific small groups of cells in the brain, and it’s cool to make them light-sensitive, so you don’t have to use an electrical impulse to stimulate the brain experimentally (which is better because electrical stimulation over longer periods can sometimes cause a reduced response, tentatively put to “scarring”), but beyond that, I’m not sure I see anything remarkable.  Is this brain region poorly understood?  Apparently not, because they know where it receives signals from, and where it projects to.

So what is the point of this research?  On the other hand, maybe that is precisely the point: we did not yet know that there were such cell types in that part of the brain, cells that respond to “looming” objects in the visual field with fear.  Maybe they proved just that they can genetically engineer specifically that one type of cell, or at least that general type of cell, because I don’t see any claim that nothing else is stimulated by the light impulses to the brain.

Who knows?

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