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Florian Jaeger & Roger Levy LSA 2011 Summer Institute Boulder, CO 12 July 2011

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## Florian Jaeger & Roger Levy LSA 2011 Summer Institute Boulder, CO 12 July 2011

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**Computational PsycholinguisticsLecture 2: surprisal,**incremental syntactic processing, and approximate surprisal Florian Jaeger & Roger Levy LSA 2011 Summer Institute Boulder, CO 12 July 2011**Comprehension: Theoretical Desiderata**• Realistic models of human sentence comprehension must account for: • Robustness to arbitrary input • Accurate disambiguation • Inference on basis of incomplete input (Tanenhaus et al 1995, Altmann and Kamide 1999, Kaiser and Trueswell 2004) • Processing difficulty is differential and localized how to get from here… …to here? the boy will eat…**Review**• Garden-pathing under Jurafsky 1996 • Scoring relative probability of incremental trees • An incremental tree is a fully connected sequence of nodes from the root category (typically, S) to all the terminals (words) that have been seen so far • Nodes on the right frontier of an incremental tree are still “open” (could accrue further daughters) • What kind of uncertainty does the Jurafsky 1996 model of garden-pathing deal with? • Uncertainty about what has already been said**Generalizing incremental disambiguation**• Another type of uncertainty • This is uncertainty about what has not yet been said • Reading-time (Ehrlich & Rayner, 1981) and EEG (Kutas & Hillyard, 1980, 1984) evidence shows this affects processing rapidly • A good model should account for expectations about how this uncertainty will be resolved The old man stopped and stared at the statue? dog? view? woman? The squirrel stored some nuts in the tree**Non-probabilistic complexity**• On the traditional view, resource limitations, especially memory, drive processing complexity • Gibson 1998, 2000 (DLT): multiple and/or more distant dependencies are harder to process Processing Easy the reporter who attacked the senator the reporter who the senator attacked Hard**Probabilistic complexity: surprisal**• Hale (2001) proposed that a word’s complexity in sentence comprehension is determined by its surprisal • This idea can actually be traced back (at least) to Mandelbrot (1953) • (Cognitive science in the 1950s was extremely interesting -- many ideas to be mined!]**Garden-pathing under surprisal**• Another type of local syntactic ambiguity • Compare with: When the dog scratched the vet and his new assistant removed the muzzle. When the dog scratched, the vet and his new assistant removed the muzzle. When the dog scratched its owner the vet and his new assistant removed the muzzle.**Expectations versus memory**• Suppose you know that some event class X has to happen in the future, but you don’t know: • When X is going to occur • Which member of X it’s going to be • The things W you see before X can give you hints about (1) and (2) • If expectations facilitate processing, then seeing W should generally speed processing of X • But you also have to keep W in memory and retrieve it at X • This could slow processing at X**Study 1: Verb-final domains**• Konieczny 2000 looked at reading times at German final verbs in a self-paced reading expt Er hat die Gruppe geführt He has the group led “He led the group” Er hat die Gruppe auf den Berggeführt He has the groupto the mountain led “He led the group to the mountain” Er hat die Gruppe auf denSEHR SCHÖNENBerggeführt He has the group to theVERY BEAUTIFULmtn. led “He led the group to the very beautiful mountain”**Prediction**Result easy slow hard fast hard fastest Locality predictions and empirical results • Locality-based models (Gibson 1998) predict difficulty for longer clauses • But Konieczny found that final verbs were read faster in longer clauses Er hat die Gruppe geführt He led the group Er hat die Gruppe auf den Berg geführt He led the group to the mountain ...die Gruppe auf den sehr schönen Berg geführt He led the group to the very beautiful mountain**3**2 Locality-based difficulty (ordinal) 1 Locality-based models (e.g., Gibson 1998, 2000) would violate monotonicity Predictions of surprisal Er hat die Gruppe (auf den (sehr schönen) Berg)geführt Er hat die Gruppe (auf den (sehr schönen) Berg) geführt Er hat die Gruppe (auf den (sehrschönen) Berg) geführt Levy 2008**S**VP NP Vfin NP PP V Er hat die Gruppe auf den Berg geführt Deriving Konieczny’s results • Seeing more = having more information • More information = more accurate expectations NP? PP-goal? PP-loc? Verb? ADVP? • Once we’ve seen a PP goal we’re unlikely to see another • So the expectation of seeing anything else goes up • pi(w)obtained via a PCFG derived empirically from a syntactically annotated corpus of German (the NEGRA treebank)**Study 2: Final verbs, effect of dative**...daßder Freund DEMKunden das Auto verkaufte ...that the friend the client the car sold ‘...that the friend sold the client a car...’ ...daßder Freund DESKunden das Auto verkaufte ...that the friend the client the car sold ‘...that the friend of the client sold a car...’ Locality: final verb read faster in DES condition Observed: final verb read faster in DEM condition (Konieczny & Döring 2003)**Next:**NPnom NPacc NPdat PP ADVP Verb SBAR SBAR COMP COMP S VP NPnom NPdat NPacc V daß der Freund DEM Kunden verkaufte das Auto Next: NPnom NPacc NPdat PP ADVP Verb S NPnom VP NPnom NPgen NPacc V daß der Freund DES Kunden verkaufte das Auto**Model results**~30% greater expectation in dative condition once again, wrong monotonicity**Theoretical bases for surprisal**• So far, we have simply stipulated that complexity ~ surprisal • To a mathematician, surprisal is a natural cost metric • But as a cognitive scientist, it would be nice to derive surprisal from prior principles • I’ll present three derivations of surprisal in this section**(1) Surprisal as relative entropy**• Relative entropy: a fundamental information-theoretic measure of the distance between two probability distributions • Intuitively, the penalty paid by encoding one distribution with a different one • It turns out that relative entropy over interpretation distributions before and after wi= (surprisal!) • Surprisal can thus be thought of as reranking cost • Relative entropy independently proposed as a measure of surprise in visual scene perception (Itti & Baldi 2005) Levy 2008**(2) Surprisal as optimal discrimination**• Connections with diffusion model (Ratcliff 1978) and MSPRT (Baum & Veeravalli 1994) • Also connections w/ cortical decision-process models (e.g., Usher & McClelland 2001) • Many theories of reading posit lexical access as key bottleneck • E-Z Reader (Reichle et al., 1998); SWIFT (Engbert et al., 2005) • Same bottleneck should hold for auditory comprehension as well • Norris (2006)’s Bayesian Reader: lexical access involves a probabilistic judgment about the word’s identity from noisy input • Certainty takes a “random walk” in probability space, and surprisal determines starting point of the walk Decision Threshold Norris 2006**(3) Surprisal as optimal preparation**• Are all RT differences best modeled as discrimination? • Intuitively, it makes sense to prepare for events you expect to happen • Such preparation allows increased avg. response speed • Smith & Levy (2008) formalize this intuition as an optimization of response speed against (fixed) preparation costs: • Let the brain choose response times, but faster is costlier • + scale-freeness: a unit’s processing cost is sum of costs of its subunits • = surprisal, under very general conditions Smith & Levy, 2008**Is probabilistic facilitation logarithmic?**• What I’ve shown you so far: • More expected = faster • What the theoretical derivations I’ve shown promised: • More expected = fasterin a logarithmic scale • Established for frequency, not for probability • Focused look at subtleties of specific constructions may not be the best way to investigate this issue • highly refined probability distributions are challenging to estimate • we need a lot of data to get a good view of the picture • Solution: broad-coverage model, reading over free text Smith & Levy, 2008**First fixations**Frontier fixations Log-probability: methods • Dataset • the Dundee Corpus (Kennedy et al., 2003) • 50K words of British newspaper text, read by 10 speakers • Measures of interest: • “Frontier” fixations (all fixations beyond the farthest fixation thus far) • First fixations (frontier fixations falling on a new word)**Deconfounding frequency & probability**• Major confound: log-frequency, widely recognized to have linear effect on RT • Unfortunately, freq & prob are heavily correlated (=0.8) • Fortunately, there’s still a big cloud of data to help us discriminate between the two (N≈200,000)**Log-probability: results**• Facilitation is essentially linear in log-probability • True even after controlling conservatively for frequency and word-length effects nonparametric regression binned median log-probs and frontier-fixation RTs**Aggregation across words & spillover**Eye-tracking Self-paced reading**slower**faster When ambiguity facilitates comprehension • Sometimes, ambiguity seems to facilitate processing: • Argued to be problematic for parallel constraint-based competition models (Macdonald, Pearlmutter, & Seidenberg 1994) • (though see rebuttal by Green & Mitchell 2006) The daughteri of the colonelj who shot himself*i/j The daughteri of the colonelj who shot herselfi/*j The soniof the colonelj who shot himselfi/j (Traxler et al. 1998; Van Gompel et al. 2001, 2005)**himself**himself Traditional account: stochastic race model • Sometimes the reader attaches the RC low... • and everything’s OK • But sometimes the reader attaches the RC high… • and the continuation is anomalous • So we’re seeing garden-pathing ‘some’ of the time NP NP PP RC the daughter P NP who shot… of the colonel (Traxler et al. 1998; Van Gompel et al. 2001, 2005)**NP**NP NP RC NP PP who shot… NP PP the daughter P NP NP the daughter P NP RC of of the colonel who shot… the colonel Surprisal as a parallel alternative • assume a generative model where choice between herselfandhimself determined only by antecedent’s gender • Surprisal marginalizes over possible syntactic structures self herself**Ambiguity reduces the surprisal**daughter…who shot… can’t contribute probability mass to himself But son…who shot… can**Ambiguity/surprisal conclusion**• Cases where ambiguity reduces difficulty aren’t problematic for parallel constraint satisfaction • Although they may be problematic for competition • Surprisal can be thought of as a revision of constraint-based theories with competition • Same: a variety of constraints immediately brought to bear on syntactic comprehension • Different: linking hypothesis from probabilistic constraints to behavioral observables**Competition versus surpisal: speculation**• Swets et al. (submitted): question type can affect behavioral responses to ambiguous RCs: “Did the colonel get shot?” • Asking about RC slowed RC reading time across the board • And speed of response interacted with question type • RC questions answered slowest in ambiguous condition • Speculation: • Comprehension is generally parallel & surprisal-based • Competition emerges when comprehender is forced into a serial channel**Memory constraints: a theoretical puzzle**• # Logically possible analyses grows at best exponentially in sentence length • Exact probabilistic inference with context-free grammars can be done efficiently in O(n3) • But… • Requires probabilistic locality, limiting conditioning context • Human parsing is linear—that is, O(n)—anyway • So we must be restricting attention to some subset of analyses • Puzzle: how to choose and manage this subset? • Previous efforts: k-best beam search • Here, we’ll explore the particle filter as a model of limited-parallel approximate inference Levy, Reali, & Griffiths, 2009, NIPS**The particle filter: general picture**• Sequential Monte Carlo for incremental observations • Let xi be observed data, zibe unobserved states • For parsing: xi are words, zi are incremental structures • Suppose that after n-1observations we have the distribution over interpretations P(zn-1|x1…n-1) • After next observation xn, represent the next distribution P(zn|x1…n) inductively: • Approximate P(zi|x1…i) by samples • Sample zn from P(zn|zn-1), and reweight by P(xn|zn)**Particle filter with probabilistic grammars**S * S * * NP VP * * * NP VP * * * * N RRC V * * * * * * N * V N * * Part N * * * * * * * tripped women brought sandwiches tripped women sandwiches brought 0.3 0.7 0.1 0.3 0.7 0.4 0.3**Resampling in the particle filter**• With the naïve particle filter, inferences are highly dependent on initial choices • Most particles wind up with small weights • Region of dense posterior poorly explored • Especially bad for parsing • Space of possible parses grows (at best) exponentially with input length input**Resampling in the particle filter**• We handle this by resampling at each input word • With the naïve particle filter, inferences are highly dependent on initial choices • Most particles wind up with small weights • Region of dense posterior poorly explored • Especially bad for parsing • Space of possible parses grows (at best) exponentially with input length input**Simple garden-path sentences**The woman brought the sandwich from the kitchen tripped • Posterior initially misled away from ultimately correct interpretation • With finite # of particles, recovery is not always successful MAIN VERB (it was the woman who brought the sandwich) REDUCED RELATIVE (the woman was brought the sandwich)**Solving a puzzle**A-S Tom heard the gossip wasn’t true. A-L Tom heard the gossip about the neighbors wasn’t true. U-S Tom heard that the gossip wasn’t true. U-L Tom heard that the gossip about the neighbors wasn’t true. • Previous empirical finding: ambiguity induces difficulty… • …but so does the length of the ambiguous region • Our linking hypothesis: Proportion of parse failures at the disambiguating region should increase with sentence difficulty Frazier & Rayner,1982; Tabor & Hutchins, 2004**Another example (Tabor & Hutchins 2004)**As the author wrote the essay the book grew. As the author wrote the book grew. As the author wrote the essay the book describing Babylon grew. As the author wrote the book describing Babylon grew.**Resampling-induced drift**• In ambiguous region, observed words aren’t strongly informative (P(xi|zi) similar across different zi) • But due to resampling, P(zi|xi) will drift • One of the interpretations may be lost • The longer the ambiguous region, the more likely this is**Model Results**Ambiguity matters… But the length of the ambiguous region also matters!