Shifts of effective connectivity within a language network during rhyming and spelling

Abstract

We used functional magnetic resonance imaging to examine task-specific modulations of effective connectivity within a left-hemisphere language network during spelling and rhyming judgments on visually presented words. We identified sites showing task-specific activations for rhyming in the lateral temporal cortex (LTC) and for spelling in the intraparietal sulcus (IPS). The inferior frontal gyrus (IFG) and fusiform gyrus were engaged by both tasks. Dynamic causal modeling showed that each task preferentially strengthened modulatory influences converging on its task-specific site (LTC for rhyming, IPS for spelling). These remarkably selective and symmetrical findings demonstrate that the nature of the behavioral task dynamically shifts the locus of integration (or convergence) to the network component specialized for that task. Furthermore, they suggest that the role of the task-selective areas is to provide a differential synthesis of incoming information rather than providing differential control signals influencing the activity of other network components. Our findings also showed that switching tasks led to changes in the target area influenced by the IFG, suggesting that the IFG may play a pivotal role in setting the cognitive context for each task. We propose that task-dependent shifts in effective connectivity are likely to be mediated through top-down modulations from the IFG to the task-selective regions in a way that differentially enhances their sensitivity to incoming word-form information.

Figure 1.

Group activation for the spelling and rhyming tasks compared with baseline. Extent threshold, >50 voxels.

Figure 2.

Center of ROIs in individual subjects. Pink, IFG; orange, FG; yellow, LTC; blue, IPS.

Figure 3.

Intrinsic connections. The average strengths of influences across individuals are presented. Significant influences (posterior probability, >0.95) are presented.

Figure 4.

Modulatory effects of the spelling (A) and rhyming (B) tasks. The average strengths of effects across individuals are presented. Significant effects (posterior probability, >0.95) are presented in black and nonsignificant effect in gray. Pairs of effects that are significantly different are indicated by ^* (and connected by a dotted arc in distant pairs) with the stronger effect indicated in bold. For example, in A, the dotted arc on the right indicates that the fusiform area had a significantly greater influence on the IPS than on the LTC in the spelling task.

Figure 5.

Differential modulatory effects between tasks. The dotted arrow indicates influences that are modulated by the spelling task significantly more than by the rhyming task. The solid arrow indicates influences that are modulated by the rhyming task significantly more than by the spelling task.