Lexical Ambiguity in Sentence Comprehension

1. Lexical Ambiguity in Sentence Comprehension By R. A. Mason & M. A. Just Brain Research 1146 (2007) 115-127 Presented by Tatiana Luchkina

2. Background • Lexical ambiguity demands the reader to select one Mg & retain the possibility of using another Mg at the same time; • This ability is correlated with the memory capacity of the reader + the frequency of the ambiguous W Mgs; • Correct analysis requires inhibition of the alternative Mg, erroneous analysis – creates a garden path effect;

3. Lexical Ambiguity • Biased: a word’s Mgs are asymmetric in frequency, e.g., • This time the ball was moved …. ….because it was always so well attended • Balanced: two equally likely Mgs, e.g., …the cell looked small…. because it was piled high with supplies

4. BallCell Garden path effectMultiple Mgs maintained Q: Which ambiguity type takes longer to process??

5. Previous studies of Lexical Ambiguity • Majority were grounded in behaviorist tradition, measuring: • Reading times/response times • Eye movements • Cross-modal priming effects How about brain activity during the processing of lexical ambiguity?

6. Ambiguity & the brain • Left Hemisphere – rapid fine semantic coding (only relevant Mgs); • Right Hemisphere – slow coarse semantic coding (activates a broad spectrum of meanings);

7. Left inferior (red) & superior (green) frontal regions become active while processing ambiguity

8. Study • Brain imaging (fMRI) used to measure brain activity during the reading of Ss with lexically-ambiguous words vs. matched control words; • Rationale: to measure brain activity when ambiguity occurs in early (biased) or late (unbiased) selection of meaning; • How: brain responses to the processes of ambiguity resolution relate to individual differences in working memory capacity

9. Experiment: • 12 right-handed volunteer college students; • Stimuli: 36 sentences (ambiguous vs. control) with the target appearing before any disambiguating context; • Sentences presented on the screen 1 W at a time, at a normal reading rate; A yes/no comprehension Q followed; • Cerebral activation measured using blood oxygenation level contrast; Dep & Indep Variables: degrees & areas of cortical activation; individual’s working memory capacity; ambiguity types;

10. Findings • Lex. Ambiguity evokes extra processing due to generation, retention, selection of multiple meanings and coherence monitoring; • Reading of ambiguous Ss activated left inferior frontal gyros more than reading of control Ss; • Biased condition only produced additional activation clusters in inferior/superior frontal regions of both hemispheres; • Activation in the right hemisphere - spillover of processing to help resolve secondary Mgs;

11. Biased Ambiguity activates Right Inferior Frontal Region (circled in red)

12. Balanced Ambiguity – left inferior frontal region active only

13. Findings, cont’d. • Activation patterns were correlated with the reading spans of the subjects, which reflected their working memory capacity; • Readers with lower reading spans use right hemisphere, esp. right inferior frontal area, to resolve ambiguities & maintain multiple meanings during disambiguation; • Bilateral extra activation occurs selectively & is minimal in the high-span subjects;