Semantic adaptation and competition during word comprehension

Abstract

Word comprehension engages the left ventrolateral prefrontal (lVLPFC) and posterior lateral-temporal cortices (PLTC). The contributions of these brain regions to comprehension remain controversial. We hypothesized that the PLTC activates meanings, whereas the lVLPFC resolves competition between representations. To test this hypothesis, we used functional magnetic resonance imaging (fMRI) to assess the independent effects of adaptation and competition on neural activity. Participants judged the relatedness of word pairs. Some consecutive pairs contained a common ambiguous word. The same or different meanings of this word were primed (e.g., SUMMER-FAN, CEILING-FAN; ADMIRER-FAN, CEILING-FAN). Based on the logic of fMRI adaptation, trials with more semantic overlap should produce more adaptation (less activation) in regions that activate meaning. In contrast, trials with more semantic ambiguity should produce more activation in regions that resolve competition. We observed a double dissociation between activity in the PLTC and lVLPFC. LPLTC activity depended on the amount of semantic overlap, irrespective of the amount of semantic ambiguity. In contrast, lVLPFC activity depended on the amount of semantic ambiguity. Moreover, across participants the size of the competition effect as measured by errors was correlated with the size of the competition effect in the lVLPFC. We conclude that the lVLPFC is an executive mechanism within language processing.

Figure 1.

Relatedness judgment task.

Figure 2.

Amount of semantic competition and adaptation in the inconsistent, consistent, and control conditions.

Figure 3.

Competition effect in left opercularis. (a) Left opercularis anatomical ROI displayed on a template. (b) Average time series data for each of the 3 conditions: inconsistent, consistent, and control. Percent signal change from zero is on the y-axis, image number (TR) is on the x-axis. (c) Correlation between inconsistent versus consistent difference in the opercularis and the behavioral competition effect across participants. The inconsistent − consistent beta value is on the y-axis. Inconsistent − consistent percent errors is on the x-axis. Each point represents a single subject.

Figure 4.

Semantic repetition effects in posterior temporal ROIs. (a) Group activation maps of the [(inconsistent + consistent) − control] contrast are displayed on a template. Regions that responded to competition [(inconsistent + consistent) > control] are displayed in warm colors and regions responded to semantic adaptation [control > (inconsistent + consistent)] are depicted in cool colors. The figure is thresholded at P < 0.05 (corrected). PLTC functionally defined ROIs are outlined in black. (b) Average time series data in PLTC function ROIs. Percent signal change from zero is on the y-axis, image number (TR) is on the x-axis.

Figure 5.

Semantic competition and adaptation conjunctions. Whole-brain results of the competition and adaptation conjunctions are displayed on a normalized anatomical scan of one subject. The figures are thresholded at P < 0.05 (corrected). (a) The competition conjunction [(inconsistent > consistent) and (consistent > control)], LIFG/MFG region is outlined in red. (b) The semantic adaptation conjunction [(control > inconsistent) and inconsistent > consistent)], LSTG voxels are outlined in blue. (c) LIFG and LSTG activity for the inconsistent + consistent conditions, relative to the control. (d) LIFG and LSTG activity in the inconsistent and consistent conditions. Error bars represent the standard error of the mean.