Review: Brain Folding and the Size of the Human Cerebral Cortex

The paper reviewed here is ‘Brain Folding and the Size of the Human Cerebral Cortex’ by Paus and colleagues. In the abstract the authors state that

‘The expansion of the cerebral cortex, and in particular that of its prefrontal region, is a major evolutionary landmark in the emergence of human cognition. Our results suggest that this may be, at least in part, a natural outcome of increasing brain size‘

In the introduction, the authors write that when the logarithmic function is applied to the ratio of the surface area of the cortex to the cortical volume there is a linear result. However humans buck the trend with their expansion of both cortical volume and cortical folding and the resulting ratio. The researchers have developed a novel method for estimating the degree of cortical folding which they use here.

Methods

Firstly the researchers have used a 1 Tesla MRI scanner for structural Magnetic Resonance Imaging. Using T1 weighted images they have acquired ‘140-160 saggital slices’. The subjects for the study were from the Saguenay Youth Study. In the initial phase they included the results from 408 people aged 12-20. After exclusion of scans were cortical thickness could not be reliably estimated the images from 314 subjects remained (164 female 140 male). I couldn’t identify the method by which they excluded images in this manner which thus acts as an inclusion criterion. It would be interesting to see if those excluded could be characterised. The demographics otherwise are not included in this paper although they cite another paper in which the original study was reported. The age range within these subjects is fairly narrow and I was surprised to see no significant age related changes in either the hemispheric surface area or hemispheric volume given that this is a relatively important stage in development (although it was obviously not longitudinal).

A critical part of the study was the use of the novel method for estimating cortical folding. The authors identify the method they use for these purposes. They firstly identify a point on the cortical surface. I wasn’t clear on how this was identified. This might seem a somewhat trivial point but there are in theory an infinite number of points on the cortical surface. For instance if we choose two points on the cortical surface we can take another point midway between them and then repeat this ad infinitum. I presume there are some limitations here determined by the pixel resolution of the scanned image. Moving onto the next stage, the researchers then use the point to construct both a sphere and a circle. At this stage I was slightly confused. The researchers are using the ratio of the pial surface area to that of the disc to produce the ratio of interest. However since it is a sphere that is being considered I was not clear if they were rotating the disc within the sphere and repeating the calculations for each part of the rotation. If so, how many rotations are being performed (presumably the volume is being sampled from a finite number of rotations). Furthermore I was unclear on how the surface area was calculated. It seemed as though the most sensible way was for the software to divide up the circle into small squares and count how many of them were filled and estimate the fill from these. The smaller the squares used for the image the more accurate would be the results.

Results

I wasn’t clear on the meaning of the described formula for estimating cortical folding and so felt a bit uncomfortable reading the results as I could have misinterpreted their meaning. Essentially the researchers go on to state their findings that in larger brains there was a greater degree of folding, that the cortical folding was most pronounced in the prefrontal cortex and then infer that in humans, the pronounced cortical folding in the prefrontal cortex may be a result of the large increase in cortical volume relative to other species.

Conclusions

The researchers’ stated results are undoubtedly interesting although as stated above I haven’t fully understood the meaning of the technique that was used to determine this. This part of the method is described in a number of lines and has an accompanying diagram although I would have preferred a more detailed description spelling out the individual steps. However it may be that other readers were able to understand this without too much difficulty or that there are space restrictions in a printed version of the journal. Here I had a thought. Perhaps it would be useful if in the medical sciences (and biological sciences) researchers were able to record a video of their methodology. This could be encouraged by the journals. Recording a video of the methodology could be accomplished using a digital camcorder. If there is not one available in the department it could be obtained from another department. If even this is not available then a mobile video camera could be used. The video could then be uploaded onto YouTube provided it is 10 minutes or less, or if more then it could be broken down into a series. Once on YouTube, the audience would be able to see a more detailed explanation and demonstration of the techniques that are being used. The ability to use comments on YouTube would make this quite interactive and rapidly so. If sufficient videos of this nature are uploaded then certain videos explaining a particular methodology might prove more successful and researchers could simply refer to that video rather than duplicate the material. In this manner, the audience of the scientific paper would have a better understanding of the approach and the scientists and journals would have a powerful tool for communicating the science overcoming the restrictions of a paper based medium alone.

References

Toro R, Perron M, Pike B, Richer L, Veillette S, Pausova Z and Paus T. Brain size and folding of the human cerebral cortex. Cerebral Cortex. 2008. 18. 2352-2357.

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