Highlights on Asymmetric Human Brain

In 1865, Marc Dax had reported the association of right hemiplegia of our brain with speech disturbance. Darwin published the Origin of Species after sixteen years and soon after that Paul Broca formulated the hypothesis that:

“Man is, of all the animals, the one whose brain is the most asymmetrical. He is also the one who possesses the most acquired faculties. Among these faculties, the faculty of articulate language holds pride of place. It is this that distinguishes us the most clearly from animals.”

Broca, P. (1877). Medicine, Mind, and the Double Brain: a Study in Nineteenth-Century Thought. Cambridge: Cambridge University Press. Quoted in pp. 65-66 Harrington A (1987).

We always had the idea that the asymmetry in the human brain is the key to various answers to evolutionary studies and that there are differences in the brain structure of humans and chimpanzees. The left cerebral hemisphere is longer and is lesser in height than the right hemisphere, but, the width is equal in both the right and left hemispheres. In contrast, the left cerebral hemisphere of the chimpanzee matches the right cerebral hemisphere closely in length, height and width.

Different regions of the human brain.

Recently, research by the University of Edinburgh highlights additional asymmetries of the human brain in contrast to chimpanzees like we never thought before. So, it can be said the asymmetries of the brain are related to the evolution of our ability to understand and use language in order to communicate.

The Study – asymmerties in the brain:

Overview of the Method:

In this section, we will understand how the group did their findings. Firstly, they imaged the brains of 223 humans and 70 chimpanzees using the Magnetic Resonance Imaging (MRI) technique. Secondly, they studied the asymmetries in the position of the brain surface by measuring the displacements between corresponding vertex pairs with respect to the mid-sagittal plane from these images. Finally, they summarized the observations by obtaining statistically meaningful conclusions. So, this study helps in understanding how humans have more asymmetries in their brain than chimpanzees.

Observations:

Now, let’s see what this group has found:

  1. The torque pattern of right-frontal and left-occipital lobes.
  2. Displacement of the anterior temporal lobe and the anterior and central segments of the superior temporal sulcus (STS) towards the left.
  3. Posteriority in the position of the left occipitotemporal surface.
  4. Clockwise rotation of the left Sylvian Fissure around a left-right axis.
  5. An area of the cortex parallel to the olfactory tract located higher on the left in females and higher on the right in males.
Average values of the obtained data from 223 humans and 70 chimpanzees.

The Road Ahead:

Many questions are associated with this recent discovery, and some of the burning questions are:

  • Do the asymmetries in the human brain have potential correlations to how our cognitive skills developed?
  • Is asymmetry an answer to various unique abilities of humans?
  • How have we evolved to be so different from our closest primate cousins?

References:

  1. Li, X., Crow, T. J., Hopkins, W. D., Gong, Q., & Roberts, N. (2018). Human torque is not present in chimpanzee brain. NeuroImage, 165, 285–293. doi:10.1016/j.neuroimage.2017.10.017

About the author

Shruti Mandal, a sophomore of IISER Kolkata is ongoing her BS-MS course, in the department of biological sciences. She likes to introduce herself as a – ‘Jack of all trades, master of none ‘. She is a regular writer at The Qrius Rhino and you can find all her blogs here.

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