I just read a fascinating paper and excellent comment
thereupon in Nature Neuroscience (thx
to Pierre Pica for sending them along) (here
and here).
The papers make the interesting point that, as I will argue below, illuminate
two very different views of what structure is and where it comes from. The two views have names that should now be
familiar to you: Rationalism (R) and Empiricism (E). What is interesting about
the two papers discussed below is that they indicate that R and E are
contrasting philosophical conceptions that have important empirical
consequences for very concrete research. In other words, R and E are
philosophical in the best sense, leading to different conceptions with testable
empirical (though not Empiricist) consequences. Or, to put this
another way, R and E are, broadly speaking, research programs pointing to
different conceptions of what structure is and how it arises.[1]
Before getting into this more contentious larger theme,
let’s review the basic findings. The main paper is written by the conglomerate
of Saygin, Osher, Norton, Youssoufian, Beach, Feather, Gaab, Gabrielli and
Kanwisher (Henceforth Saygin et al). The comment is written by Dehaene and
Dehaene-Lambertz (DDL). The principle finding is, as the title makes admirably
clear, that “connectivity precedes function in the development of the visual
word form area.” What’s this mean?
Saygin et al observes that the brain is divided up into
different functional regions and that these are “found in approximately the
same anatomical location in virtually every normal adult”.[2]
The question is how this organization arises: “how does a particular cortical
location become earmarked”? (Saygin et al:1250). There are two possibilities: (i)
the connectivity follows the function or (ii) the function follows the
connectivity. Let’s expand a bit.
(i) is the idea that in virtue of what a region of brain
does it wires up with another region of brain because of what it does at
roughly the same time. This is roughly the Hebbian idea that regions that fire
together wire together (FTWT). So, a region that is sensitive to certain kinds
of visual features (e.g. Visual Word From Area (VFWA)) hooks up with an area
where “language processing is often found” (DDL:1193) to deliver a system that
undergirds reading (coding a dependency between “sounds” and “letters”/”words”).
(ii) reverses the causal flow. Rather than intrinsic
functional properties of the different regions driving their connectivity (via
concurrent firing), the extrinsic connectivity patterns of the regions drives
their functional differentiation. To coin a phrase: areas that are wired
together fire together (WTFT). This is what Saygin et al finds :
This tight relationship between function and
connectivity across the cortex suggests a developmental hypothesis: patterns of
extrinsic connectivity (or connectivity fingerprints) may arise early in
development, instructing subsequent functional development.
The
may is redeemed to a does by following young kids before and
after they learn to read. As DDL summarizes it (1193):
To genuinely test the hypothesis that the VWFA owes
its specialization to a pre-existing connectivity pattern, it was necessary to
measure brain connectivity in children before they learned to read. This is
what Saygin et al. now report. They
acquired diffusion-weighted images in children around the age of 5 and used
them to reconstruct the approximate trajectory of anatomical fiber tracts in
their brain. For every voxel in the ventral visual cortex, they obtained a
signature profile of its quantitative connectivity with 81 other brain regions.
They then examined whether a machine-learning algorithm could be trained to
predict, from this connectivity profile, whether or not a voxel would become
selective to written words 3 years later, once the children had become
literate. Finally, they tested their algorithm on a child whose data had not
been used for training. And it worked: prior
connectivity predicted subsequent function (my bold, NH). Although many
children did not yet have a VWFA at the age of 5, the connections that were
already in place could be used to anticipate where the VWFA would appear once
they learned to read.
I’ve bolded the conclusion: WTFT and not FTWT. What makes
the Saygin et al results particularly interesting is their precision. Saygin et
al is able to predict the “precise location of the VWFA” in each kid based on
“the connectivity of this region even before the functional specialization for
orthography in the VWFA exists” (1254). So voxels that are not sensitive to words and letters before kids learn to read, become
so in virtue of prior (non functionally based) connections to language regions.
Some remarks before getting into the philosophical issues.
First, getting to this result requires lots of work, both
neuro imaging work and good behavioral work. This paper is a nice model for how
the two can be integrated to provide a really big and juicy result.
Second, this appeares in a really fancy journal (Nature Neurosceince) and one can hope
that it will help set a standard for good cog-neuro work, work that emphasizes both the cognition and the neuroscience.
Saygin et al does a lot of good cog work to show that in non-readers VWFA is
not differentially sensitive to letters/words even though it comes to be so
sensitive after kids have learned to read.
Third, DDL points out (1192-3) that whatever VWFA is
sensitive to it is not simply visual features (i.e. a bias for certain kinds of
letter like shapes). Why not? Because
(i) the region is sensitive to letters and not numerals despite letters and
numerals being formed using the same basic shapes and (ii) VWFA is located in
the same place in blind subjects and non-blind ones so long as the blond ones
can read braille or letters converted into “synthetic spatiotemporal sound
patterns.” As DDL cooly puts it:
This finding seems to rule out any explanation
based on visual features: the so-called ‘visual’ cortex must, in fact, possess
abstract properties that make it appropriate to recognize the ‘shapes’ of
letters, numbers or other objects regardless of input modality.
So,
it appears that what VWFA takes as a “shape” is itself influenced by what the
language area would deem shapely. It’s not just two perceptual domains with
their independently specifiable features getting in sync, for what even counts
as a shape depends on what an area is wired to. VWFA treats a “shape” as
letter-like if it tags a “shape” that is languagy.
Ok,
finally time for the sermon: at the broadest level E and R differ in their
views of where structure comes from and its relation to function.
For
Es, function is the causal driver and structure subserves it. Want to
understand the properties of language, look at its communicative function. Want
to understand animal genomes, look at the evolutionarily successful phenotypic
expressions of these genomes. Want to understand brain architecture, look at
how regions function in response to external stimuli and apply Hebbian FTWT
algorithms. For Es, structure follows
function. Indeed, structure just is a convenient summary of functionally useful
episodes. Cognitive structures reflect shaping effects of the environmental
inputs of value to the relevant mind. Laws of nature are just summaries of what
natural objects “do.” Brain architectures are reflections of how sensory
sensitive brain regions wire up when concurrently activated. Structure is a
summary of what happens. In short, form follows (useful) function.
Rs
beg to differ. Es understand structure as a precondition of function. It
doesn’t follow function but precedes
it (put Descartes before the functional horse!). Function is causally
constrained by form, which is causally prior. For Rs, the laws of nature
reflect the underlying structure of an invisible real substrate. Mental
organization causally enables various kinds of cognitive activity. Linguistic
competence (the structure of FL/UG and the structure of individual Gs) allows
for linguistic performances, including communication and language acquisition.
Genomic structure channels selection. In
other words, function follows form. The latter is causally prior. Structure “instructs” (Saygin et al’sterm) subsequent functional
development.
For
a very long time, the neurosciences have been in an Empiricist grip. Saygin et
al provides a strong argument that the E vision has things exactly backwards
and that the Hebbian Esih connectionist conception is likely the wrong way of
understanding the neural and functional structure of the brain.[3] Brains come with a lot
of extrinsic structure and this structure casually determines how it organizes
itself functionally. Moreover, at least in the case of the VWFA, Darwinian
selection pressures (another kind of functional “cause”) will not explain the
underlying connectivity. Why not? Because as DDL notes (1192) alphabets are
around 3800 years old and “those times are far too short for Darwinian
evolution to have shaped our genome for reading.” That means that Saygin et
al’s results will have no “deeper” functional explanations, at least as concerns
the VWFA. Nope, it’s functionally inexplicable structure all the very bottom. Connectivity
is the causal key. Function follows. Saygin et al speculate that what is right
for VWFA will hold for brain organization more generally. Is the speculation correct?
Dunno. But being a card carrying R you know where I’d lay my bets.
[2]
This seems like a pretty big deal to me and argues against any simple minded
view of brain plasticity, I would imagine. Maybe any part the brain can perform
any possible computation, but the fact that brains regularly organize
themselves in pretty much the same way seems to indicate that this organization
is not entirely haphazard and that there is method behind it. So, if it is true
that the brain is perfectly plastic (which I really don’t believe) then this
suggested that it is not the computational differences responsible for its
large scale functional architecture. Saygin et al suggest another causal
mechanism.
