Download Pattern recognition and visual word forms

Pattern recognition and
visual word forms
Epigraph
“But if any see fit not to agree with the
opinions here expressed. . .still let them note
the great magnitude of experiments. . .we
have dug them up and demonstrated them
with much pains and sleepless nights and
great money expense. Enjoy them you, and if
ye can, employ them for better purposes.”
--William Gilbert, 1600
In other words: Even if you don’t like the theories,
you have to explain the data.
The great “visual word form area” debate
Is the “visual word form area” specialized for
visual word forms?
Larger debates:
Domain general vs. Domain specific
Organization-by-material vs. Organizationby-process
Roles of learning, expertise and evolution
in shaping brain function.
Fusiform Gyrus and the
“Visual Word Form Area”
Fusiform gyrus
VWFA:
Fusiform Gyrus and the
“Visual Word Form Area”
Hypothesized to “contains a
population of neurons that, as an ensemble,
is tuned to invariant stimulus properties
and structural regularities of written
words” (McCandliss et al., 2003)
Fusiform Gyrus and the
“Visual Word Form Area”
Model of functional anatomy for invariant word perception
ose a simple anatomical and functional model of the visual
word reading, which follows the general principles that govern
cognition in the visual system of primates (Fig. I). Separate
s in left and right hemispheres are integrated in the left
d Visual Word Form Area (VWFA), which mediates between
pecific input, and more abstract linguistic areas responsible for
emantic and phonological processes. Although the precise
ns from VWFA to systems involved in lexical, semantic and
VWFA:
phonological processes are currently less clearly defined, functional
areas probably include the left angular gyrus [52], left inferior frontal
cortex [53], and temporal regions anterior to the VWFA [54]. Finally,
ventral visual regions receive top-down attentional influences associated with left and right parietal regions that are likely to affect all
processing levels, and whose impairment might therefore lead to
various forms of neglect dyslexia.
Hypothesized to “contains a population of neurons that, as an
ensemble, is tuned to invariant stimulus properties and structural
regularities of written words” (McCandliss et al., 2003)
Anatomy and connectivity
Phonology
Lexico-semantic features
VWF
Visuospatial
attention
Functional properties
W-O-R-D
V4
V4
V2
V2
V1
V1
dr
oW
Visuospatial
attention
• Sequence of abstract
letter identities
• Font invariant
Wdo
• Location invariant
• Increasing receptive
field size
• Increasingly
abstract features
Word
TRENDS in Cognitive Sciences
ntative model of functional anatomical pathways involved in visual perception of words. Letter strings are first processed in ventral occipital regions (V1 to
lateral to the stimuli, building up increasingly abstract visual representations (right). For stimuli in the left visual field, information is conveyed from the right
Fusiform Gyrus and the
“Visual Word Form Area”
Response properties (McCandliss et al., 2003):
Responds reliably to letters and words.
May also respond to faces and objects.
Responds more to letters than pseudo-letters
Modality-specific (doesn’t respond to spoken words)
Invariant with regard to retinal location, letter case,
size and font (neural priming studies)
Insensitive to lexical properties (e.g. frequency)
Left Fusiform is Activated by Visual Word Forms
(Cohen et al., 2002)
Passive viewing of
words, letter strings
and checkerboards
Words & Letters >
Checkerboards in left,
but not right, fusiform
cluster.
Left Fusiform is Activated by Visual
Word Forms (Cohen et al., 2002)
Passive viewing of
words, letter strings
and checkerboards
Words > letters in left
fusiform cluster.
Left Fusiform is Activated by Visual
Word Forms (Cohen et al., 2002)
How does the VWFA become specialized?
Written language is a recent cultural development (~5400
years ago), so can’t be evolution.
Children do not show letter/word specific activation in
VWFA before learning to read.
Initial properties intrinsic to the region and its connectivity
must determine its subsequent specialization for reading.
May be specialized for foveal objects, local object
features, and invariance for position and size.
Expertise for reading in the fusiform gyrus
(McCandliss et al., 2003)
Expertise in different visual categories (e.g. birds, cars)
linked to enhanced perception of category members via
more holistic processing of the stimulus, through
functional re-organizaiton of visual areas.
Expertise for word reading may be similar.
Expertise for reading in the fusiform gyrus
(McCandliss et al., 2003)
Literate adults group letters together into a single
perceptual unit (visual word form).
Speed of word recognition is unaffected by the
number of letters for 3-6 letter words.
Suggests processed in parallel
Expertise for reading in the fusiform gyrus
(McCandliss et al., 2003)
Hypothesis: reading experience drives progressive
specialization of a pre-existing inferotemporal pathway
for visual object recognition.
Expertise for reading in the fusiform gyrus
(McCandliss et al., 2003)
Evidence:
Younger children do show word length effects for
3-6 letter words.
ERP data shows 10-year-olds have adult-like
response to high frequency, but not low frequency,
words.
Activation level of VWFA correlated with phonemegrapheme decoding ability, controlling for age.
VWFA less active in adults with developmental
dyslexia.
“The myth of the VWFA”
(Price & Devlin, 2003)
Is the VWFA really specialized for word forms?
Neuropsychological evidence:
“pure alexics” usually have much larger lesions
(including cuneus, calcarine sulcus and lingual
gyrus in addition to fusiform)
“pure alexics” often have other perceptual
problems (e.g. color naming, picture processing)
“The myth of the VWFA”
(Price & Devlin, 2003)
476
C.J. Price, J.T. Devlin / NeuroImage 19 (2003) 473– 481
Is the VWFA really specialized for word forms?
Functional imaging evidence:
Also active when subjects name familiar
objects, make manipulation responses to
pictures of unfamiliar objects, name colors and
perform auditory and tactile word processing
tasks.
“The myth of the VWFA”
(Price & Devlin, 2003)
So what does the “VWFA” do then? 3
possibilities:
1. Different populations of neurons in the
same region, one for VWFs and others for
naming, object perception etc.
Not very neurally plausible.
Would require single-cell evidence.
“The myth of the VWFA”
(Price & Devlin, 2003)
So what does the “VWFA” do then? 3
possibilities:
2. A single cognitive function, not yet
identified underlies all these multimodal
responses.
“The myth of the VWFA”
(Price & Devlin, 2003)
So what does the “VWFA” do then? 3
possibilities:
3. the same population of neurons could
support different cognitive processes,
depending on their interactions with other
cortical and subcortical areas.
Visual Perceptual Learning of Words and the VWFA Debate
(Xue & Poldrack, 2007)
Korean characters presented in pairs: same/different
judgment.
Scanned before and after training.
Difficulty controlled by amount of visual noise.
English word control task.
Figure 1. Example of the stimuli. (A) Korean characters. The top and bottom show an example character with left–right–bottom, and
top–middle–bottom structure, respectively. The letters of the character are marked with different colors. (B) English words. All English words
Visual Perceptual Learning of Words and the VWFA Debate
(Xue & Poldrack, 2007)
At pre-training scan, both words and Korean characters
strongly activated VWFA. Not sig. different.
After training, less VWFA activation for Korean
characters, both with same level of noise as pre-training
scan, and same level of performance (by increasing
noise).
Suggest that “VWFA is neither specific to words nor
sensitized by visual expertise with specific writing
systems”