BRAIN ASYMMETRY IMPROVES PROCESSING OF SENSORY INFORMATION
Dreosti E., Llopis, N., Carl M., Yaksi E. & Wilson S.W.
Original paper reference:
Left-right asymmetry is required for the habenulae to respond to both visual and olfactory stimuli (2013)
Many of us are aware that the left and right sides of the brain are believed to have slightly different roles in cognition and in regulating behavior. However, we don't know whether these asymmetries actually matter for the efficient functioning of the brain. For instance, if your brain was symmetric, would it work any less well than it normally does. This is potentially an important issue as brain-imaging studies in various neurological conditions have shown alterations in normally asymmetric patterns of neuronal activity.
However, to date, it has not been possible to determine if these abnormalities are a cause or a consequence of disease. In a study published in Current Biology, Elena Dreosti from Steve Wilson's lab in collaboration with Emre Yaksi's group in Belgium address how alterations in lateralization affect the brain's ability to respond to visual and olfactory sensory stimuli. To do this, they have studied activity of neurons in a part of the brain called the habenulae in larval zebrafish (Figure 1). This region of the brain shows asymmetries across all vertebrates and is involved in mediating many different behaviours. The development of brain asymmetry is better understood in zebrafish than in other vertebrates and so Elena and her colleagues were able to use this knowledge to manipulate brain asymmetries. They generated fish in which habenular asymmetry was reversed and fish with double-right and double-left sided habenulae and then asked how the habenular neurons responded to visual or olfactory stimuli in these different conditions. They found that in the normally lateralized brain (Figure 2), most habenular neurons responding to light are present on the left, whereas neurons responding to odour are more frequent on the right. If the direction of brain asymmetry was reversed, the functional properties of the habenular neurons were also reversed, whereas double-left and double-right sided brains lacked habenular responsiveness to odour or light respectively (Figure 2).
These results show that loss of brain asymmetry can have significant consequences upon sensory processing and circuit function. Their study raises the possibility that defects in the establishment of brain lateralization could indeed be causative of cognitive or other symptoms of brain dysfunction.
Figure 1: Image of the left and right-sided habenular nuclei of larval zebrafish showing left/right structural asymmetries in the processes of neurons (pink) and their connections (blue). Image by Ana Faro and Tom Hawkins in Steve Wilson's group at UCL.
Figure 2. Examples of neuronal activity in the left and right habenular nuclei (L Hb and R Hb) of four different four-day old fish with normal left-right habenular laterality (LR), reversed laterality (RL), symmetric double-right (RR) and symmetric double-left (LL) habenulae showing lateralization of neuronal responses to light and odour. Neuronal cell bodies are shown as dots colour-coded in red, blue, violet, or white depending on whether they responded to light, odour, both light and odour, or were non-responding.