Jamie Alexandre

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Grammatical Complexity The Chomsky Hierarchy

Recursion Something containing an instance of itself.

Recursion in Language The dog walked down the street. The dog the cat rode walked down the street. The dog the cat the rat grabbed rode walked down the street.

Recursion: “Stack” Memory The dog the cat the rat grabbed rode walked down the street. DOGCATRATWALKRIDEGRAB

Recursion: “Stack” Memory The dog the cat the rat grabbed rode walked down the street. DOG CAT RAT WALKRIDEGRAB “Limited performance…” “Infinite competence…”

?

SRN Simple Recurrent Network (Elman, 1990) Some ability to use longer contexts Incremental learning: no looking back No “rules”: distributed representation

PCFG Easily handles recursive structure, long-range context Hierarchical, “rule”-based representation More computationally complex, non-incremental learning Probabilistic Context-Free Grammar S  NP VP N’  AdjP N’ N’  N Adj  green … …

Serial Reaction Time (SRT) Study Buttons flash in short sequences –“press the button as quickly as possible when it lights up” Dependent measure: RT –time from light on  correct button pressed Subjects seem to be making sequential predictions RT ∝ P(button|context) also: RT ∝ -log(P(button|context)) (“surprisal”, e.g. Hale, 2001; Levy, 2008)

Training the Humans Eight subjects per experimental condition Same sequences, different mappings Broken into 16 blocks, with breaks About an hour of button-pressing total Emphasized speed, while minimizing errors

Training the Models Trained on exactly the same sequences as the humans, but not fit to human data Predictions at every point based solely on sequences seen prior to that Results in sequence of probabilities –correlated with sequence of human RTs, through surprisal (negative log probability)

Analysis

A Case Study in Recursion: Palindromes A C L Q L C A (Sequences of length 5 through 15; total of 3728 trials per subject)

PCFG SRN PCFG SRN “Did you notice any patterns?” Subjects with no awareness of pattern: “No”, “None”, “Not really” (n=5) Those with explicit awareness of pattern: “Circular pattern”, “Mirror pattern” (n=3) SRN (implicit task performance) PCFG (explicit task performance) Will this replicate?

Block Correlation (Surprisal vs RT) Implicit, didn't notice (n=8) PCFG SRN

Differences between individuals? –or actually between modes of processing? What if we explicitly train subjects on the pattern? First half implicit, second half explicit

“ This is the middle button in every sequence (and it only occurs in the middle position, halfway through the sequence): This means that as soon as you see this button, you know that the sequence will start to reverse. Here are some example sequences of various lengths: Explicit Training Worksheet

And Quiz Sheet “ Now, complete these sequences using the same pattern (crossing out any unneeded boxes at the end of a sequence):

Block Correlation (Surprisal vs RT) Fully explicit from middle (n=8) PCFG SRN (explicit instruction given here)

Before explicit instruction After

Context-free vs Context-sensitive A  A B  B C  C D  D

CFG: CSG: Explicit Instruction (after block 4)

Methods Four conditions, with 8 subjects in each –Implicit context-free grammar (CFG) –Implicit context-sensitive grammar (CSG) –Explicit context-free grammar (CFG) –Explicit context-sensitive grammar (CSG) Total of 640 sequences (4,120 trials) per subject –Sequences of length 4, 6, 8, and 10 –Around 1.5 hours of button-pressing –In blocks 9-16, 5% of the trials were “errors” A 1 B 1 C 1 C 2 B 2 A 2 D 2

Blocks 1-4 Blocks 5-8 Blocks 9-12 (errors thicker) Blocks (errors thicker)

** (6ms) ** (27ms)(2ms) ** (11ms) RT (ms)

(1ms) ** (6ms) ** (7ms) ** (3ms)

Conclusions Explicit/Implicit processing –Implicit performance correlated with the predictions made by an SRN (a connectionist model) –Explicit performance correlated with the predictions made by a PCFG (a rule-based model) Grammatical complexity –Able to process context-free, recursive structures at a very rapid timescale –More limited ability to process context-sensitive structures

Longer training More complex grammars –Determinism Other response measures –EEG: more sensitive than RTs to initial stages of learning Field studies in Switzerland or Brazil…? Future Directions

Broader Goals L2-learning pedagogy

Thankyous! Mentorship Jeff Elman Roger Levy Marta Kutas Advice Micah Bregman Ben Cipollini Vicente Malave Nathaniel Smith Angela Yu Rachel Mayberry Tom Urbach Andrea, Seana and the 3 rd Year Class! Research Assistants Frances Martin (2010) Ryan Cordova (2009) Wai Ho Chiu (2009)

Palindromes

AGL and Language Areas associated with syntax may be involved –Bahlmann, Schubotz, and Friederici (2008). Hierarchical artificial grammar processing engages Broca's area. NeuroImage, 42(2): P600-like effects can be seen in AGL –Christiansen, Conway, & Onnis (2007). Neural Responses to Structural Incongruencies in Language and Statistical Learning Point to Similar Underlying Mechanisms. –“violations in an artificial grammar can elicit late positivities qualitatively and topographically comparable to the P600 seen with syntactic violations in natural language”

Sanity Check: Effect is Local

Context-free Grammar The dog the cat the rat grabbed rode walked. S  NP VP NP  N NP  N S N  the dog N  the cat N  the rat VP  grabbed VP  rode VP  walked