Lexical Ambiguity in Sentence Comprehension

By R. A. Mason & M. A. Just Brain Research 1146 (2007) 115-127

Presented by Tatiana Luchkina

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;

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

Ball Cell

Garden path effect Multiple Mgs maintained

Q: Which ambiguity type takes longer to process??

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?

Ambiguity & the brain

• Left Hemisphere – rapid fine semantic coding (only relevant Mgs);

• Right Hemisphere – slow coarse semantic coding (activates a broad spectrum of meanings);

Left inferior frontal gyrus

Lefttemporal lobe

Left inferior temporal cortex & right inferior frontal cortex

de-contextualized semantic analysis;

categorization tasks

contextualized semantic processing;

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

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

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;

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;

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

Balanced Ambiguity – left inferior frontal region active only

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;