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.