In November, Stan Dehaene is
coming to Maryland to give the annual Baggett Lectures on language and
cognition. To “prepare” myself, I have just finished reading his last book, Reading
in the Brain, which I can highly recommend. It appears that our friends
in cog-neuro have begun to understand the underlying mechanisms behind our
ability to read, tracing it to a confluence of capacities lodged, not
surprisingly, in the visual system and FL. The reading trick, again not a
surprise, is to figure out how to link morphemes to graphemes (I’m talking about
alphabetic reading systems here) and this problem turns out to piggy back on
some rather deep facts about the mechanisms that the visual system uses to
interpret the physical environment and how different alphabets express the
relevant morphemes in a language. It
seems that letters like ‘T’ and ‘F,’ ‘K,’ ‘Y,’ and ‘L’ are “proto-letters” and they
exploit capacities central in parsing a visual scene:
The shape T, for example, is
extremely frequent in natural scenes. Whenever one object masks another, their contours
always form a T-junction. Thus neurons that act as “T-detectors” could help
determine which object is on front of which.
Other characteristic
configurations, like the shapes of a Y and an F are found at places where
several objects of an object meet…All of these fragments of shapes belong to
what is known as “non-accidental properties” of visual scenes because they are
unlikely to occur accidentally in the absence of any object…(loc 2138 e-book
version).
These “natural” shapes find their way into many alphabetic
systems thereby allowing the capacities of the visual system to be recycled to
undergird the capacity to read.[1]
The second leg of the reading capacity lies in tying
graphemes to morphemes. This turns out to be rather difficult. I was surprised to find out (remember, I come
from a philosophy department so I know virtually no phonology and, come to
think of it, very little else) that the emerging consensus opinion concerning
dyslexia is that stems from “an anomaly in the phonological processing of
speech sounds” (loc 3779). It seems that the majority of dyslexic kids have
trouble processing phonemes in general (i.e. independent of reading) and that’s
why they have trouble matching graphemes (letters) to morphemes in reading. In other words, it seems that dyslexia is
largely a speech processing problem
(loc 3801). Dehaene calls this is a
“revolutionary idea,” one that seems “barely credible,” but he argues that the
evidence points to dyslexics having a problem with “phonemic awareness” and hence
have trouble with the necessary phoneme-grapheme mapping mastery of which is
required for fluent reading (loc 3801).
Interesting to me was the information that dyslexia appears
to be far less apparent in some cultures than in others. For example, it seems
that “dyslexia is hardly ever diagnosed in Italy” (loc 3876), whereas it is a
pretty common syndrome in French and English reading cultures. Could dyslexia
be nothing more than a cultural “disease”?
Seems unlikely. And indeed, it is
not so.
Rather, the biological propensity is rather stable across
readers of different languages but the practical reading problem becomes acute
only in cases where “writing systems [are] so opaque that they put a major
stress on the brain linking vision to language” (loc 3898).
How this was demonstrated was rather neat. A research group in Milan (headed by Eraldo
Paulesu) scoured Italy for reading impaired individuals who superficially did
not seem particularly impaired. However, careful testing showed they were; in particular,
“when compared to normal Italian readers, their scores were as deviant as those
of groups of French and English dyslexics as compared to control subjects in
their respective countries” (loc 3898). In other words, the absolute impairment Italian dyslexics
suffer from is less than that afflicting English or French dyslexics though the
relative impairment is the same. Conclusion: there is no underlying difference
between these populations despite their very different behaviors. I love these
kinds of discoveries, ones that penetrate beneath the surface glare to unpack
common features of the underlying mechanisms.[2]
Let me end this post noting one more thing that caught my
syntactician’s eye. Chapter 7 is a long discussion of symmetry effects in reading.
Dehaene reports on “mirror reading” (where (young) readers/writers “spontaneously
confuse left and right”). He attributes this to a basic structural feature of
the brain, viz. It encodes a symmetry principle “deeply buried in the structure
of our cortex” wherein “[o]ur visual brain assumes that nature is not concerned
with left and right…” (loc 4228).
It should be obvious why I found this interesting. The
Minimalist Program (MP) has taken the position that grammars care exclusively
about hierarchical dependencies, treating left/right linear order as a late
addition that arises when hierarchical grammatical structures are sent to the
S&M system for articulation. It is
curious do find out that the disregard for left/right order is a design feature
of certain parts of the nervous
system. Specifically, Dehaene recounts the following accepted wisdom: the
visual system has two main networks, a ventral what system, which functions to “recognize and label objects,” and
a dorsal how system that does things
(executes actions) with the objects so identified. Distinguishing left from
right, Dehaene notes, likely arises from the dorsal how system and symmetry is a core feature of the ventral what system.
This dorsal/ventral cut has also made an appearance in the
cog-neuro of language. Hickok and Poeppel have relatively recently
distinguished a ventral and a dorsal pathway for language, the former mapping
sound onto meaning and the latter mapping sound onto articulators (see here). My impressionistic self would love to
speculate that FL’s disregard for left/right information is related to its
living in a part of the brain that is blind to this kind of information (i.e.
maybe the part of FL that maps syntax to meaning (to CI) lives in the ventral
stream!). This comports with the basic
MP conceit that FL exploits (in part) structures from extant brainware used for
other (non-linguistic) cognitive tasks. So, if the FL mapping to “meaning”
lives in the symmetrical (ventral) part of the brain (where high level “object
recognition” also resides) then the fact that this mapping ignores left/right
information (see here)
is what we might expect (is this tenous enough for you?). We might also expect linear
(left/right) info to be prominent in the dorsal stream, the part of the brain, which
maps representations onto articulatory based representations.
Now, all of this is VERY stream of consciousness and as you
all know I am far from being competent to do anything more than ramble here
(but hey, what’s a blog for!). However,
it is neat to have discovered that some parts of the brain, as a matter of
fundamental organization (one view: symmetry is “inherent in the geometry of
our interhemispheric conncections” (loc 4444)), ignore left/right info and that
some parts of the language system, the ones mapping to meaning, appear to live
in this general neighborhood.
There’s lots more in the book, and I cannot recommend it
highly enough. I will blog one more post in the near future on another topic
that Dehaene takes up in the penultimate chapter. But for now, if you have a
couple of days of pleasure reading you are looking to fill, reading about
reading is a good way to idle away the hours.
[1]
Recycling is the star idea in this book. The term is self-explanatory:
cognitive circuits that typically serve one function can be repurposed to serve
other ends, an idea congenial to modern day minimalists.
[2]
Dave Kush’s analysis of island violations in Swedish has a similar structure.
He noted that the relative unacceptability of island violations was similar in
Swedish and English (i.e. the same sentence enjoyed the same relative standing
in the two languages), despite the fact that what is deemed ok or ? by speakers
of Swedish is considered * by speakers of English. Like Paulesu, Kush has
argued that the same mechanisms are at work wrt islands in both grammars
despite these absolute differences in acceptability ratings. Of course, why this latter difference exists is
well worth exploring (and Kush does) but the important common point is that
these easily noticeable differences often mask deeper important commonalities.
