Neuromagnetic Evidence for the Timing of Lexical Activation: An MEG Component Sensitive to Phonotactic Probability but Not to Neighborhood Density Sarah

Neuromagnetic Evidence for the Timing of Lexical Activation: An MEG Component Sensitive to

Phonotactic Probability but Not to Neighborhood Density

Sarah LaszloThe Cognition and Brain Laboratory

For Psych 593SG

9.22.2005

Liina Pylkkänen, Andrew Stringfellow, and Alec Marantz

10 Minute MEG

30 Second MEG

MEG is like EEG, but with magnetic fields instead of electrical current, much better spatial resolution and much more expensive equipment

+ + MEG

10 Minute MEG

Wherever there is electrical current, there is a magnetic field:

10 Minute MEG


There is bioelectric currentin post-synaptic neurons,so there is a magnetic field associated with their firing.

10 Minute MEG


There is bioelectric currentin post-synaptic neurons,so there is a magnetic field associated with their firing.

Just as we measure that bioelectric current with EEG,we measure the associatedmagnetic field with MEG

10 Minute MEG

MEG shares some of the pitfalls of EEG:- These magnetic fields are small and tend to cancel each other out, so we can only measure them when they are produced by a large population of neurons firing nearly simultaneously and in a serendipitous configuration

10 Minute MEG

And has additional pitfalls all of its own:- The magnetic field of the earth is many orders of magnitude greater than any signal made in the brain, so MEG facilities have to be magnetically shielded (adding to the $$ factor)

10 Minute MEG

The Payoff:-In addition to EEG quality temporal information, MEG can provide millimeter quality spatial information

10 Minute MEG

The Payoff:-In addition to EEG quality temporal information, MEG can provide millimeter quality spatial information

(Because the skull and skin do not distort the magnetic signal the way they do the electrical signal)

10 Minute MEG

The Apparatus:

® Elekta Corporation

The Matter at Hand

The M350 MEG component

Pylkkänen et al, 2003® Elekta Corporation

The Matter at Hand


Before Trial Averaging

Pylkkänen et al, 2002, 2003

The Matter at Hand


After Trial Averaging

Pylkkänen et al, 2002, 2003

The Matter at Hand

The M350 MEG component- Left Temporal generator

- 300-450 msec peak latency

Pylkkänen et al, 2002

The Matter at Hand

The M350 MEG component- Left Temporal generator

- 300-450 msec peak latency

- Sensitive to many of the same factors as the N400

- NOT entirely homologous to the N400

- peaks earlier, narrower waveform

Pylkkänen et al, 2002

N400

Federmeier and Laszlo, in prep

Not Homologous, but Informative

Elbow-nudging, winking, sneaky, critical assumption of this paper:- Even though the M350 is not directly homologous to the N400, it is at least an early component of the N400, so knowing more about the M350 can inform theories of the N400.

The goal of the study

“ … to determine whether the M350 MEG response component reflects automatic

lexical activation or subsequent processing.”

The tumultuous past

There is no past in MEG research

The tumultuous past


*wink, wink*

There are however several ERP studies which have asked this same question about the N400

The tumultuous past


*wink, wink*

There are however several ERP studies which have asked this same question about the N400

AND there are behavioral tasks which can identify and discriminate between automatic lexical and controlled post-lexical processes

The tumultuous past

The Masked Priming Paradigm- Behavioral evidence suggests that semantic priming does not require awareness of the prime:

The tumultuous pastThe Masked Priming Paradigm

- Behavioral evidence suggests that semantic priming does not require awareness of the prime:

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DOG

&&&

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CAT

150 msec

10 msec

150 msec

500 msec

LEXICAL DECISION



Conclusion: Behavioral semantic priming must be an automatic process



Conclusion: Behavioral semantic priming must be an automatic process

Can semantic priming effects on the N400 also be elicited without awareness of the priming stimulus?

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unprimed

primed

The tumultuous past

Can semantic priming effects on the N400 also be elicited without awareness of the priming stimulus?

If yes, the N400 must be automatic (lexical)

If no, the N400 must be controlled

(post-lexical) ?

The tumultuous past

The N400 is a controlled process

The N400 is an automatic process

The N400 DOESN’T show a semantic priming effect when primes are masked below perceptual threshold (Brown and Hagoort, 1993)

The tumultuous past



The N400 DOESN’T show a semantic priming effect when primes are masked below perceptual threshold (Brown and Hagoort, 1993)

The N400 DOES show a semantic priming effect when primes are masked below perceptual threshold (Deacon et al, 2000)

The tumultuous past



The N400 DOESN’T show a semantic priming effect when primes are masked below perceptual

threshold (Brown and Hagoort, 1993)

The N400 DOES show a semantic priming effect when primes are masked below perceptual

threshold (Deacon et al, 2000)

Deacon et al’s explanation:

-We threw out subjects who showed no priming effect for unmasked primes

The tumultuous past



The N400 DOESN’T show a semantic priming effect when primes are masked below perceptual

threshold (Brown and Hagoort, 1993)

The N400 DOES show a semantic priming effect when primes are masked below perceptual

threshold (Deacon et al, 2000)

Deacon et al’s explanation:- We threw out subjects who showed no priming effect for unmasked primes

- Brown and Hagoort used a between subjects design, preventing a similar exclusion system, and obfuscating the semantic priming effect they surely would have seen to masked primes with a similar system in place

The enlightened present (2002)

Let’s do away with this barbaric and uninterpretable masked priming procedure

The enlightened present (2002)

Let’s do away with this barbaric and uninterpretable masked priming procedure

New Paradigm:

Lexical decision with a twist


Lexical decision:Participants see words and nonwords and push one button for ‘word’ and one button for ‘nonword’

CAT

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CIT


All stimuli vary simultaneously inphonotactic probability and phonological neighborhood density



High probability / density

Low probability / density

Word [bell line] [page dish]

Nonword [mide pake] [jize yush]






High phonotactic probability: /be/ is more likely than /dZi/High neighborhood density: more words sound like ‘line’ than dish







*There were no high probability / low density or low probability /high density items







*There were no high probability / low density or low probability /high density items


WHY make up this complicated task?



Because high probability items facilitate lexical activation, but high density items inhibit lexical decision (Vitevich and Luce, 1998, 1999)



Because high probability items facilitate lexical activation, but high density items inhibit lexical decision

Remember the goal of the study:

“ … to determine whether the M350 MEG response component reflects automatic lexical activation or subsequent processing.”


If the M350 is a lexical component, it should onset earlier for high than low probability items.If the M350 is a post-lexical component, it should onset later for those same items because they are also high density.

The predictions:


If the M350 is a lexical component, it should onset earlier for high than low probability items.If the M350 is a post-lexical component, it should onset later for those same items because they are also high density.


What a sophisticated and informative design!

Informative!

The behavioral results:- HIGH items were responded to more slowly in the lexical decision task

This is expected, because high density items inhibit lexical decision

Informative!

The behavioral results:- HIGH items were responded to more slowly in the lexical decision task

This is expected, because high density items inhibit lexical decision

The big question: Does M350 latency track RT for HIGH items?

Informative!

Peak latency is reduced for high probability words

Remember: high probability items facilitate lexical processing, but inhibit post-lexical

processing

Words Data

Informative!

Wow! Even though RT to HIGH items was LONGER, M350 latency was SHORTER!

Informative!

Wow! Even though RT to HIGH items was LONGER, M350 latency was SHORTER!

Conclusion: The M350 must be an automatic lexical component

Informative AND thought-provoking

- Nope, sorry, I still think this design is overly complicated and / or entirely uninformative?



- You left out my favorite result! Why didn’t you talk about it??



- You left out my favorite result! Why didn’t you talk about it??

- I’m still unclear about aspect X of the MEG recording and optimistically hope you might be able to elaborate?

Thanks for listening!

Documents

Neuromagnetic Evidence for the Timing of Lexical Activation: An MEG Component Sensitive to Phonotactic Probability but Not to Neighborhood Density Sarah