Word frequency and reading demands modulate brain activation in the inferior frontal gyrus

Abstract

Processing efficiency differs between high- and low-frequency words, with less frequent words resulting in longer response latencies in several linguistic behavioral tasks. Nevertheless, studies using functional MRI to investigate the word frequency effect have employed diverse methodologies and produced heterogeneous results. In this study, we examine the effect of word frequency through complementary analytical approaches and functional connectivity analyses. Furthermore, we examine whether reading demands, which have been shown to influence reading-related activation, modulate the effects of word frequency. We conducted MRI scanning on 54 healthy participants who performed two versions of a single-word reading task involving high- and low-frequency words: a low-level perceptual reading task and a high-level semantic reading task. The results indicate that word frequency influenced the activation of the pars orbitalis and pars triangularis of the inferior frontal gyrus, but only in the semantic reading task. Additionally, the ventral occipitotemporal cortex exhibited stronger regional activation during the semantic reading task compared to the perceptual reading task, with no effects of word frequency. Functional connectivity analyses demonstrated significant coupling among regions within both the dorsal and ventral reading networks, without any observable effects of word frequency or task. These findings were consistent across group- and individual-level analytical approaches. Overall, our results provide further support for the involvement of the inferior frontal gyrus in semantic processing during reading, as indicated by the effect of word frequency and the influence of reading demands, highlighting the role of the ventral reading network. These findings are discussed in line with their implications for lexical and pre-lexical reading processing.

Figure 1

Whole-brain contrasts and low–high frequency simple contrasts. (A) Results of the Words versus Null contrast across all subjects; (B) Low > High frequency contrast in the semantic (high reading demand) task (in green) and in the perceptual (low reading demand) task (in red), p < 0.05 FWE corrected clusterwise (p < 0.001 uncorrected voxel-extent threshold). No clusters survived this threshold in the perceptual task.

Figure 2

ROI analyses. (A) Group ROIs employed, obtained from the Words-Null contrast. (B) Results from the group ROI analyses. ROIs are represented in the X axis, whereas the difference of the parameter estimates between regional activation of low versus high frequency words is depicted in the Y axis. Boxes with straight lines represent the perceptual task, whereas boxes with dotted lines depict the semantic task. Red asterisks at the bottom part indicate that the region showed a significant main effect of Frequency (*p < .05, BF > 1; **p < .01, BF > 5; ***p < .001, BF > 10). Blue asterisks at the bottom part indicate that the region showed a significant main effect of Task (*p < .05, BF > 1; **p < .01, BF > 5; ***p < .001, BF > 10). Black asterisks over the boxes indicate that the region showed a significant Frequency x Task interaction, due to a significant difference in the WFE between the perceptual and the semantic tasks (*p < .05). Supplementary Fig. S2 shows the same group ROI results with separated boxes for low and high frequency % signal change.

Figure 3

Pairwise connectivity between all the 6 group ROIs. Edges were drawn if they exceeded the threshold of r = 0.355, which was the estimated threshold for a pair of nodes to be significantly co-activated, after Bonferroni correction. Thicker edges represent stronger beta-correlation values.