A comparison of brain activity evoked by single content and function words: an fMRI investigation of implicit word processing

Abstract

Content and function words have different roles in language and differ greatly in their semantic content. Although previous research has suggested that these different roles may be mediated by different neural substrates, the neuroimaging literature on this topic is particularly scant. Moreover, fMRI studies that have investigated differences between content and function words have utilized tasks that focus the subjects' attention on the differences between these word types. It is possible, then, that task-related differences in attention, working memory, and decision-making contribute to the differential patterns of activation observed. Here, subjects were engaged in a continuous working memory cover task while single, task-irrelevant content and function words were infrequently and irregularly presented. Nonword letter strings were displayed in black font at a fast rate (2/s). Subjects were required to either remember or retrieve occasional nonwords that were presented in colored fonts. Incidental and irrelevant to the memory task, content and function words were interspersed among nonwords at intervals of 12 to 15 s. Both word types strongly activated temporal-parietal cortex, middle and anterior temporal cortex, inferior frontal gyrus, parahippocampal gyrus, and orbital frontal cortex. Activations were more extensive in the left hemisphere. Content words elicited greater activation than function words in middle and anterior temporal cortex, a sub-region of orbital frontal cortex, and the parahippocampal region. Words also evoked extensive deactivation, most notably in brain regions previously associated with working memory and attention.

Figure 1

Experimental Design. Participants viewed a rapidly changing visual display of words and nonwords (duration=500ms). Most trials were nonwords and word trials occurred every 12 to 15 seconds. Participants performed a matching task in which they decided whether two colored nonword trials, separated by a varying time interval, were identical. The presentation of words was incidental to the task that participants performed.

Figure 2

Activation elicited by content and function words. Regions in which either content or function words elicited a significant activation (green) and regions in which content words elicited significantly greater activation than function words (red) are shown on 3D volume renderings. Throughout the paper and figures, areas of activation are consistently labeled with letters (e.g., the area labeled b in Figure 2 is the same as that labeled b in Figure 3). Activations in left inferior frontal gyrus (a), left middle temporal gyrus (b, d), left angular gyrus (c), right orbital frontal gyrus (e), and right middle temporal gyrus (g) are shown. Hemodynamic time courses to content (red) and function words (blue) from left inferior frontal gyrus (a), left middle temporal gyrus (b) and left angular gyrus (c) are shown. Hemodynamic time courses represent the average time course (-3s - 13.5s) for the region. Time points with significant differences between content and function words are indicated with an asterisk.

Figure 3

Left Middle Temporal Gyrus. Regions in which content words elicited significantly greater activation than function words (red) are displayed on sagittal slices through the left hemisphere. Two regions of activation in left middle temporal gyrus were found: a posterior region (b) and a more anterior, inferior region (d). Significant differences between content and function words in the parahippocampal gyrus can be seen in the coronal view. Hemodynamic time courses to content (red) and function (blue) words from the posterior region (left, b) and from the more anterior region (right, d) are shown. Time points with significant differences between content and function words are indicated with an asterisk. Coordinates are in MNI space.

Figure 4

Orbital Frontal Gyri. Content words elicited significantly greater activation than function words (red) in right (e) and left (f) orbital frontal gyri. Hemodynamic time courses to content (red) and function (blue) words from right (e) and left (f) orbital frontal gyri are shown. Time points with significant differences between content and function words are indicated with an asterisk. Coordinates are in MNI space.

Figure 5

Temporal, Frontal, and Parahippocampal Regions. Regions in which content words elicited significantly greater activation than function words (red) are shown. Hemodynamic time courses to content (red) and function (blue) words in the right parahippocampal gyrus (h) are shown. Time points with significant differences between content and function words are indicated with an asterisk. Coordinates are in MNI space.

Figure 6

Areas of Deactivation. Regions in which either content or function words elicited a significant deactivation (blue) and regions in which either content or function words elicited a significant activation (green) are shown. A coronal slice through frontal gyri further illustrates the pattern of deactivations. Coordinates are in MNI space.