Wave-ering: An ERP study of syntactic and semantic context effects on ambiguity resolution for noun/verb homographs

Abstract

Two event-related potential experiments investigated the effects of syntactic and semantic context information on the processing of noun/verb (NV) homographs (e.g., park). Experiment 1 embedded NV-homographs and matched unambiguous words in contexts that provided only syntactic cues or both syntactic and semantic constraints. Replicating prior work, when only syntactic information was available NV-homographs elicited sustained frontal negativity relative to unambiguous words. Semantic constraints eliminated this frontal ambiguity effect. Semantic constraints also reduced N400 amplitudes, but less so for homographs than unambiguous words. Experiment 2 showed that this reduced N400 facilitation was limited to cases in which the semantic context picks out a nondominant meaning, likely reflecting the semantic mismatch between the context and residual, automatic activation of the contextually-inappropriate dominant sense. Overall, the findings suggest that ambiguity resolution in context involves the interplay between multiple neural networks, some involving more automatic semantic processing mechanisms and others involving top-down control mechanisms.

Figure 1

Shown are the locations of the 26 scalp electrodes, as seen from the top of the head (with the front of the head at the top of the figure). Frontal electrodes are represented as triangles and central/posterior electrodes are represented as circles. The electrodes used for statistical analysis are shown using filled in shapes.

Figure 2

Grand average ERPs to ambiguous words (dashed line) and unambiguous words (solid line) are plotted separately for syntactic prose sentences (left panel) and congruent sentences (right panel) at 8 representative electrode sites. Positions of the plotted sites are indicated by filled circles on the center head diagram (nose at top). Negative is plotted up for this figure and figure 4. In the syntactic prose sentences, the response to ambiguous words (e.g. ‘the season/to season’) is more negative than the response to unambiguous words (e.g. ‘the logic/to eat’) over the frontal channels, between about 200 and 700 ms post-stimulus-onset. In the congruent sentences, there is no enhanced frontal negativity. Instead, the ambiguous words are more negative over central/posterior sites in the N400 time window (250-500ms).

Figure 3

Grand average ERPs to ambiguous words (dashed lines) and unambiguous words (solid lines) in syntactic prose sentences (thinner lines) and congruent sentences (thicker lines) are overlaid at 4 representative central/posterior electrode sites to highlight the influence of semantic constraints on the N400. N400 amplitudes to both ambiguous and unambiguous words are highly facilitated (made more positive) in the presence of semantic constraints, although this facilitation is greater for unambiguous than ambiguous targets in cloze-probability matched sentence contexts.

Figure 4

Mean amplitude differences are summarized as isopotential voltage maps (on the left) and bar graphs (on the right) for the syntactic prose sentences (upper panel) and congruent sentences (lower panel) respectively. The two isopotential voltage maps show distributions viewed from the top of the head for (upper panel) the frontal negativity effect in the syntactic prose sentences in the 200-700 ms time window and (lower panel) the central/posterior N400 effect in the congruent sentences in the 250-500 ms time window. The right side of the figure plots the amplitude differences averaged over the 11 frontal sites and 11 central/posterior sites for the two sentence types. Longer bars indicate more negative brain responses to the ambiguous words than to the unambiguous words. The statistical significance of each difference is noted next to each bar (with N.S. = not significant).

Figure 5

Grand average ERPs at 3 midline electrode sites for ambiguous words (dashed line) and unambiguous words (solid line) in Experiment 2. The leftmost column overlays the overall brain responses to ambiguous and unambiguous words, irrespective of meaning dominance. Replicating Experiment 1, in syntactically and semantically congruent sentences, N400 responses to ambiguous words are more negative than those to unambiguous words. The two columns on the right side of the figure contrast the brain responses to ambiguous versus unambiguous words when the context favors the dominant interpretation (middle column) or the nondominant interpretation (right column) of the homographs. The data clearly show that when the context picks out the dominant meaning of the homograph, the waveforms from the ambiguous and unambiguous words are indistinguishable. However, when the context picks out a nondominant meaning of the homograph, larger N400s are observed to the ambiguous words.