Seeing Left, Smelling Right

We’ve all heard about the “left-brain/right-brain” hype, which, to be honest, is really just a bunch of malarkey. Supposedly, a bigger right hemisphere means you’ll be a great artist, and a larger left indicates a penchant for science. If the dancer spins clockwise, you’re right-brained, while if you’re left-brained she twirls counter-clockwise.

Fortunately, all of these neural conspiracy theories have been largely debunked. However, the fact does remain that we do have two hemispheres that are connected but divided – a cortical “separate but equal," if you will. And oftentimes, one of these hemispheres is larger than the other, the smaller being situated slightly behind. Now, again, this is not to say that the bigger hemisphere is better, simply that they are asymmetric, and presumably this asymmetry has evolved for a reason.

Researchers from University College London have investigated the purpose of this neural asymmetry on a much smaller scale using the zebrafish, a common animal model used for investigating basic but deceptively complex brain-related phenomena thanks to their simplified central nervous system. Published in the journal Current Biology, the researchers discovered that neuronal asymmetry lends itself towards enhanced processing of sensory information in the zebrafish, and that a symmetrical brain can result in an impairment of the processing of visual or olfactory stimuli.

The researchers focused on the habenula – an area located near the thalamaus that is a type of way station in the brain, processing sensory information. The habenula receives inputs from around the brain and helps to designate the appropriate neurochemical output for neurons further down the line. However, cells in the left and right habenula react differently to different types of stimuli, resulting in separate projections to other areas of the brain.

In the current study, cells in the right habenula were largely responsible for receiving odor information, while the left-sided neurons processed visual information. Very few neurons responded to both types of stimuli. These left and right neurons also had distinct outputs, the left heading to the dorsal, or top, interpeduncular nuclei (IPN), while the right had outputs to the ventral, or bottom, IPN. These ventral and dorsal IPN neurons subsequently had their own distinct outputs as well, meaning the entire operation of processing visual and olfactory information was distinct, divided between the two hemispheres.

The real test of any scientific phenomenon though, is what happens when you disrupt this process (scientists really just like to mess things up to see what will happen). Will the other hemisphere take over, or will that function be entirely lost?

To find out, the researchers “shocked” the fish with cold – meaning when the fish were still embryos, they exposed them to extreme cold with the hopes of disrupting their typical gene expression and thus their cell development. In fact, using cold shock was so successful, it resulted in a complete reversal of many of the fishes' neurons, meaning that what was right was now left, and left was right. Not only did this lead to a switch in the processing of sensory information, but the entire assembly line from the habenula neurons on down was reversed, a mirrored reflection of the fishes’ normal cell functions. Light information was now processed on the right side, however, the projections to the IPN remained the same. So light processed on the right side projected to the dorsal IPN, whereas previously the dorsal IPN had been activated by the left habenula light response.

The final step was to find out what happens when asymmetry is completely lost, to ascertain whether there was a functional benefit to this lateralization (again, scientists really just like to mess with a perfectly good brain process). To do this, the researchers manipulated the fishes’ neurons so that the habenula cells were either all right or all left. That isn’t to say that all the neurons were located on either the left or the right side, but rather the cells acted like they were all “right” neurons or “left” neurons, receiving inputs and creating outputs from and to their respective sources.

This complete lateralization resulted in a loss of the opposite side’s function, meaning the “double-left” fish had exceptional vision but were unable to process odors, while the “double-right” fish were blind to the light but had a super-power sense of smell.

Finally, even fish that were raised in complete darkness still showed this laterality when it came to processing visual information, meaning that the brain’s left-right organization was dependent on gene expression, not the cells’ experience or exposure to light.

From this, the researchers concluded that it doesn’t actually matter which side the cells are on, so long as each type of cell and its connections are in place. But a loss of those neurons, even if others are in their place, leads to complete functional disruption. And really, this makes sense; it is not the location of the cell but its connections that truly matter, dictating its function.

Yet another instance of science proving cool stuff that, if we really thought about it, we already kind of figured to be true.

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Dreosti et al. Left-Right Asymmetry is Required for the Habenulae to Respond to Both Visual and Olfactory Stimuli. Current Biology 24 1-6 (2014).

Image Credit: Nobuyuki Kayahara, Gray's Anatomy and Azul (via Wikimedia Commons)