What have neural networks achieved?

[Jay McClelland]

Max Coltheart <max at currawong.bhs.mq.edu.au> writes:

   To account for surface dyslexia (reading YACHT as "yatched", they
   stopped the training of the network before it had successfully learned
   low-frequency exception words such as this one, and postulated that in
   the normal reader such words can only be read aloud with input to
   phonology from a second system (semantics).  Two problems with this:
   (a) it involves giving up the very thing that Jay says was an
   achievement: a single mechanism that can read aloud all exception
   words plus nonwords and (b) it predicts that anyone with severe
   semantic impairment will also show surface dyslexic reading, which is
   not the case; several recent papers have documented patients with very
   poor semantics but very good reading of exception words
   (e.g. Cipolotti & Warrington, J Int Neuropsych Soc 1995 1 104-110).

The problems here are more apparent than real.  First regarding (b),
because of the fact that the spelling-sound mechanism in our model IS
capable of learning both the regular and exception words correctly,
our model is able to handle cases in which there is severe
semantic impairment and no surface dyslexia (See Plaut 97 citation
below).  We view the extent of reliance on semantics in reading words
aloud as an premorbid individual difference variable.  Regarding (a),
we do not relax the claim that a single (spelling-sound) mechanism CAN
accout for reading of both regular and exception items, we only
suggest that readers CAN ALSO read via meaning, and this allows the
spelling-sound system to be lazy in acquiring the hardest items,
namely the low frequency exceptions; the extent of the laziness
becomes parameter dependent, and thus a natural place for individual
differences to arise within the context of the model.

All agree that our models should account for disorders as well as
unimpaired performance.  Our model does account for one thing that the
standard dual route model does not account for, which is the fact that
all fluent (see note) surface dyslexia patients show spared reading of high
frequency exceptions.  According to the dual-route approach, it ought
to be possible to eliminate exception word reading entirely, but there
are no fluent surface dyslexia patients who exhibit this pattern.


         -- Jay McClelland and Dave Plaut



note: we hope we all agree that non-fluent SD patients are not relevant to
this debate... sorry if this begins to get technical!

@article	( Plaut97,
key	=	"Plaut" ,
author	=	"David C. Plaut" ,
year	=	"1997" ,
title	=	"Structure and Function in the Lexical System: {Insights} from
		 Distributed Models of Naming and Lexical Decision" ,
journal	=	LCP ,
volume	=	12 ,
pages	=	"767-808" ,
keywords=	"semantics, reading" 
)