readings> visual cortex variation

Here is something odd. It's well known that the layout of the cortex varies from person to person. But still I was shocked to find that there is a three-fold range in the size of V1, the primary visual cortex. Your striate cortex could be three times larger - or smaller - than mine. Which is surprising given that our brain volumes would be unlikely to vary by more than a third at most.

What does it mean? Is there also a three-fold range in basic visual acuity? And is this wide variance deliberate or accidental? You might have thought that such a foundational part of the visual processing hierarchy as V1 must be under tight genetic control. But it's hard to see that evolution would have selected for this degree of variability. So perhaps it says neurodevelopment is a much more haphazard process than we ever realised?

The three-fold variance in striate cortex is not itself exactly news. Brodmann and other neuroanatomists remarked on it in the 1900s. Yet it has come back into the spotlight with recent efforts to show how genes construct brains.

There are many intriguing new findings. First, both post-mortem and functional imaging studies have provided evidence that it's not just the primary visual cortex but the whole visual pathway that gets scaled up or down. If you have a large V1, you are also likely to have a much greater density of cones and ganglion cells in the retina, a thicker optic tract, a larger lateral geniculate nucleus and even a larger V2. But not V3. It seems from the most recent functional imaging study (Dougherty et al, Journal of Vision 2003; 3: 586-598) that the size of the higher visual centres may not correlate with V1. So whatever causes the variation in the lower visual hierarchy, at least it seems like a purposeful bias. And significantly, a similar level of variance has been found in the retinas and striate cortex of monkeys. Thus it is not simply an isolated peculiarity of us humans.

A second fact is that more visual cortex probably does equate to sharper vision. I don't know why but, glasses and colour-blindness apart, we tend to assume that one person's subjective visual experience is much the same as another's. Yet recent psychophysics studies have proven that people are actually quite varied in their ability to do basic things like judge angles, spot changes in direction, or detect colour contrasts. The same researchers are now doing functional imaging studies that appear to show performance may indeed correlate with V1 size.

A more difficult question is whether the variation in V1 is really out-of-line with variance in the cortex generally. The striate cortex is unusually easy to measure because of its striking cytoarchitectural boundaries. However some post-mortem results exist for the primary motor and primary somatosensory cortex (areas 4 and 3) and they show markedly less variance - under 100 percent.

Another confounding finding comes from recent MRI scans that compared the brains of twins. You might perhaps have expected that - large or small - their striate cortices would be more matched in size than the higher cortical regions. Surely genes would have tighter control over the evolutionarily more ancient and foundational parts of the cerebrum? But quite the opposite was the case. The frontal lobes and language centres were extremely similar in terms of gray matter volumes while the lower visual areas may just about as well have been from the brains of two strangers.

No-one seems to be able to draw a sensible conclusion from these strange results. Some argue that vision is so important to primates that a wide individual variance is built-in to allow for a rapid evolution of visual systems to suit different environments. Yet it makes just as much sense to say that a strong selective pressure on such a critical faculty ought to narrow any inter-individual differences. After all, both the retina and striate cortex are particularly dense and costly structures in metabolic terms. Such wide variance seems a wasteful luxury.

Another theory is that the striate cortex actually has the same underlying variance as any other part of the cortex. It is simply that because it is by far the largest single cortex "module", being twice the size of most other areas, the resulting range of variance is also at least double. However this does not really say why the retinal elements and thalamic relay vary in line. So none of these answers really wash and, for the moment, V1 variability just has to be chalked up as yet another one of the mysteries of the brain.

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