Neuromagnetic Evidence for the Timing of Lexical Activation: An MEG Component Sensitive to Phonotactic Probability but Not to Neighborhood Density Sarah Laszlo The Cognition and Brain Laboratory For Psych 593SG 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 Wherever there is electrical current, there is a magnetic field: There is bioelectric current in post-synaptic neurons, so there is a magnetic field associated with their firing.

10 Minute MEG Wherever there is electrical current, there is a magnetic field: There is bioelectric current in 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 associated magnetic 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 The M350 MEG component Before Trial Averaging Pylkkänen et al, 2002, 2003

The Matter at Hand The M350 MEG component After Trial Averaging Pylkkänen et al, 2002, 2003

The Matter at Hand The M350 MEG component - Left Temporal generator msec peak latency Pylkkänen et al, 2002

The Matter at Hand The M350 MEG component - Left Temporal generator 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 There is no past in MEG research *wink, wink* There are however several ERP studies which have asked this same question about the N400

The tumultuous past There is no past in MEG research *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 past The Masked Priming Paradigm - Behavioral evidence suggests that semantic priming does not require awareness of the prime: &&& DOG &&& + CAT 150 msec 10 msec 150 msec 500 msec LEXICAL DECISION

The tumultuous past The Masked Priming Paradigm - Behavioral evidence suggests that semantic priming does not require awareness of the prime: Conclusion: Behavioral semantic priming must be an automatic process

The tumultuous past The Masked Priming Paradigm - Behavioral evidence suggests that semantic priming does not require awareness of the prime: 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? + 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 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 N400 DOES show a semantic priming effect when primes are masked below perceptual threshold (Deacon et al, 2000)

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 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 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 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 + CIT

Lexical decision with a twist All stimuli vary simultaneously in phonotactic probability and phonological neighborhood density

Lexical decision with a twist All stimuli vary simultaneously in phonotactic probability and phonological neighborhood density High probability / density Low probability / density Word [bell line][page dish] Nonword [mide pake][jize yush]

Lexical decision with a twist 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

Lexical decision with a twist All stimuli vary simultaneously in phonotactic probability and phonological neighborhood density High probability / density Low probability / density Word [bell line][page dish] Nonword [mide pake][jize yush] *There were no high probability / low density or low probability /high density items

Lexical decision with a twist All stimuli vary simultaneously in phonotactic probability and phonological neighborhood density High probability / density Low probability / density Word [bell line][page dish] Nonword [mide pake][jize yush] *There were no high probability / low density or low probability /high density items

Lexical decision with a twist WHY make up this complicated task?

Lexical decision with a twist 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)

Lexical decision with a twist WHY make up this complicated task? 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.”

Lexical decision with a twist 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:

Lexical decision with a twist 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. WHY make up this complicated task? 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?

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??

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

Thanks for listening!