Classic graphics #4: Cortical wiring

In the last installment of this series, I talked about cortex. And here we are again at the cortex.

I got the idea for looking at this graphic from the recent Neuroscience meeting, where I saw this diagram in at least two of the featured late afternoon talks. So I reckoned that qualified it as a classic. Of course, it probably didn't hurt that one of the authors of the paper that featured this diagram was the current president of the Society, David Van Essen. I was able to track it down, and it's freely available online in the original paper (it's figure 4, on page 30).

It also didn't hurt the fame of this diagram that it was printed in the very first article of a brand new journal, I imagine.

And I'm pretty sure I saw this picture prominently featured in a commentary by Nobel laureate and DNA structure describer Francis Crick in Nature. He used it as an example of something we know in monkey, but we should know in humans. We discussed the Crick paper when I was a grad student at our weekly "neurolunch" seminar. (Checking this now, it was Crick and Jones, actually.)

This figure shows the wiring diagram of the part of the brain responsible for visual processing in macaques -- which, because primates are visual animals, and because it's easy to control visual stimuli, is one of the best understood regions of the brain. 32 areas, 10 hierarchical, levels, and 187 linkages, most two-way between connected areas.

I should say, though, that the original was published in colour, based on comments in the text. That the PDF online now is in black and white is probably an oversight.

This diagram is clearly not famous because of its elegance. It's very hard to interpret and looks like the electrical wiring from the Chilton's manual of the car you hope you never own. Heck, even the authors write, "The sheer complexity of Figure 4 makes it difficult in many places to trace the lines representing specific pathways." (They go on to describe a computer representation that allows you to highlight specific connections. Sadly, that diagram does not appear to have made its way online, though I haven't looked hard).

But then, that's the point. It's considered a classic, not despite its complexity and difficulty in interpretation, but because of it. It emphasizes the tremendous complexity of the cortex and how different areas are connected to others.

And make no mistake: this diagram certainly represents a nearly heroic compilation of experimental results. And perhaps that admirable feature has helped people view it favourably over time.

Looking at the text, though, I'm struck by the several qualifiers, provisions, caveats, and tentative interpretations about the information that went into making this figure. The diagram, in a way, is often shown as factual, but is in fact somewhat hypothetical. Something which is not often mentioned when this picture is shown. It's possible, I suppose, that all the hypothesis have been shown correct in the following 16 years of research -- though I doubt that.

Next in this series, I will probably be looking at some graphics by Jerison on brain size.

References

Felleman DJ, Van Essen DC. 1991. Distributed hierarchical processing in the primate cerebral cortex. Cerebral Cortex 1: 1-47.
http://cercor.oxfordjournals.org/cgi/content/abstract/1/1/1-a

Crick F, Jones E. 1993. Backwardness of human neuroanatomy. Nature 361: 109-110. http://dx.doi.org/10.1038/361109a0