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Language, Mind, and Brain:
Experience Alters perception
Chapter 8
The New Cognitive Neurosciences
M. Gazzaniga (ed.)
Sep 7, 2001
Relevant points from Stein et al. (Chap. 5)
• AES functions as an association area, and feeds back to the superior
colliculus (SC)
• Multisensory neurons prefer spatially and temporally coincident events
• Neonatal SC neurons do not receive cortical input.
By around 20 weeks postnatal:
– Percentage of multisensory neurons increase from ~ 0% to ~60%
– RFs for the SC neurons shrink
– Mutisensory neurons become spatially aligned and integrated
Relevant points (contd.)
• One dominant modality (e.g. vision) may first develop and influence the
development of other modalities (e.g. audition)
• Sensory integration seems to occur after development of individual
modalities
=> Initially, the modalities are developmentally related but not integrated
End of critical development for cats: ~ 135 days ~ 20 weeks
20 weeks x 7 human days/cat day = 140 days = ~ 5-6 months
(about 0.6% - 1% of lifespan)
Speculation: Basic perceptual abilities of infants may be developed by 5 months
postnatal, and integration starts occurring.
Overview of the paper
• Development versus learning
• Language as a case study
– Developmental changes in speech
– Crossmodal influences
• The perceptual magnet effect
– Acoustic structure of vowels
– Experience alters perception
• A theory of speech development (Native Language Magnet)
• Argument for NLM
– Nature of language input
– Vocal learning
– Brain correlates
• Conclusion
Development vs. Learning
• An organism’s behavior changes over time. Are the changes caused by:
– Unfolding of the genetic program (e.g. growing a leg)?
– A general learning mechanism that detects and adapts to patterns in the world
(e.g. a baby learns to use a walker)
– A combination of both (how are they related?)
• Kuhl’s four types of models:
A) Development and Learning are independent
B) Learning <=> Development
C) Development enables learning
D) Development enables learning, and learning triggers development
Issue: What is development?
• All learning involves physical changes to the brain
• Which physical changes are due to ‘development’, as opposed to
‘learning’?
• Possible distinction: developmental changes are ‘internally triggered’
– Problem: Kuhl’s eventual position is that learning sets up developmental
changes => ‘externally triggered’
• Possible distinction 2: Aspects of behavior that were not environmentally
specified are ‘developmental’.
– Problem: We are defining development wrt behavior, not physical changes in
the brain.
Language as a case study
Chomsky (Development + Selection)
Skinner (Learning +Instruction)
A child grows language, just as he grows
arms and legs.
Argument
Grammatical structure is too complex to
be learnt
Don’t need a lot of input, just some of the
right kind of input to set parameters for the
developing language
A child learns language by associating
sounds with objects and events.
Argument
Language is learnt by the same
mechanisms that are used to learn other
patterns in the world (learn associations
with feedback from a “teacher”)
Debate applies at three levels: phonology (sounds of speech)
word structure (ordering of speech sounds)
grammar (ordering of words)
Language as a case study:
Developmental changes
Cooing
1-4 months
Vowel-like sounds
Canonical
babbling
5-10 months
Consonant-vowel
strings (“bababa”)
First words
10-15 months
Consistent use of a
word to refer to
objects
Two-word
utterances
18-24 months
Usually verb +
noun pairs
Meaningful
speech
15 months
onwards
Long intonated
utterances
Deaf children follow a similar developmental pattern
Able to discriminate
between most speech
contrasts
Able to discriminate
between only native
language speech
contrasts
Language as a case study:
Crossmodal influences
The McGurk Effect
Auditory information for [b] (closure at front of the mouth)
+ visual information for [g]
(closure at back of the mouth)
? perception of [d]
(closure in the middle of the mouth)
• 18-20 week old infants prefer correlated visual+speech signals
• Robust influence of vision on speech perception
• Polymodal representation of speech
The perceptual magnet effect:
Acoustic structure of vowels
Each vowel has characteristic
resonance frequencies (spectral peaks)
Vocal tract
Spectrum
Formants = spectral peak
Quality of a vowel approximately
given by locations of the first two
Formants (F1 and F2).
F1
F2
F3
from P. Ladefoged, Elements of Acoustic Phonetics, 2nd ed
.
The perceptual magnet effect:
Language experience alters perception
Discrimination is worse near the
prototypes of phonetic categories
The better the prototype, the worse
the discrimination near it (as if
the prototype attracts the percepts
towards itself)
Effect seen in humans but
not in monkeys
1-dim MDS solutions (vertical distance
arbitrarily chosen to prevent overlap)
from Iverson & Kuhl, JASA 97(1)
The perceptual magnet effect:
Effect of native language
Adults exhibit the PME only
for native language speech contrasts
6-month old infants exhibit a similar
native language effect
The perceptual magnet effect:
Theoretical implications
“Language input scults the brain … [to highlight] contrasts used in
language, while de-emphasizing those that do not, and this happens prior
to word learning”
The PME may assist infants in chunking the sound stream into words
=> Learning (PME) promotes development (word acquisition)
Issue: Paradoxical nature of learning
Speech sounds: discriminating among frequently-encountered stimuli is
more difficult
Face recognition: discrimination among frequently-encountered stimuli is
much easier
• When does learning involve loss of ability, and
when does learning sharpen ability?
• Are both loss and sharpening caused by the same learning/
developmental process?
• What kind of neural mechanisms could provide this kind of learning?
(competitive-learning neural nets?)
The “Native Language Magnet”
model of speech development
Phase 1 (ability at birth):
Infants can hear almost all speech contrasts
Predisposition may be due to auditory abilities
Phase 2 (ability at 6 months):
Infants start representing native language information
Language-specific perceptual maps start to form
Phase 3 (stabilization of perceptual maps):
Native speech distinctions are maximized
Non-native speech distinctions are minimized
NLM
Argument for NLM:
Critical Periods
Critical periods are not be strictly timed.
During “sensitive periods”:
– Exposure to some kinds of information may be more effective,
– and thus alter the duration of the sensitive period
Why are 2 nd languages difficult to learn?
– Maturationally defined temporal window of learning
– Neural commitment resulting from early learning
Speculation
Reason for neural commitment: Interference
– Brain plasticity is governed by statistical sufficiency
– Once the speech prototypes stabilize, further data are
ignored for processing efficiency
– Thus old patterns interfere with learning new patterns.
Argument for NLM:
Nature of language input to the child
• ‘Motherese’ is hyperarticulated
• Possibly to give clearer prototypes of speech sounds to infants
and thus promote PME
Argument for NLM:
Vocal learning
• Production becomes “discrete” at around the same time PME is first seen
• By ~20 weeks, infants also more likely to imitate correctly
Argument for NLM:
Vocal learning
Possible explanation for discrete productions:
Due to perceptual magnet effects, infants speech
representations become more discrete:
=> they hear more discretely and so produce more discretely
=> Perceptual representations serve as guides for production
Argument for NLM:
Vocal learning
Argument for NLM:
Brain correlates
• Disassociation for speech and nonspeech signals
– Phonetic processing engaged the left hemisphere
– Pitch processing of same stimuli engaged the right hemisphere
• Speech becomes lateralized by ~ 4 months
Conclusion:
Learning and development are interrelated
• Development (genetic program) sets up the constraints for effective
learning:
– Auditory sensitivity and natural discontinuities
– the neural mechanisms for pattern detection
• Possible explanation:
Experience with linguistic input causes
the perceptual magnet effect, which causes
the formation of discrete perceptual representations, which allows for
word acquisition, which triggers
the left-hemisphere dominance, which allows
better representation and processing?