Specifying the role of the left prefrontal cortex in word selection

Abstract

Word selection allows us to choose words during language production. This is often viewed as a competitive process wherein a lexical representation is retrieved among semantically-related alternatives. The left prefrontal cortex (LPFC) is thought to help overcome competition for word selection through top-down control. However, whether the LPFC is always necessary for word selection remains unclear. We tested 6 LPFC-injured patients and controls in two picture naming paradigms varying in terms of item repetition. Both paradigms elicited the expected semantic interference effects (SIE), reflecting interference caused by semantically-related representations in word selection. However, LPFC patients as a group showed a larger SIE than controls only in the paradigm involving item repetition. We argue that item repetition increases interference caused by semantically-related alternatives, resulting in increased LPFC-dependent cognitive control demands. The remaining network of brain regions associated with word selection appears to be sufficient when items are not repeated.

Keywords: Chronic stroke patients; Language production; Left prefrontal cortex; Proactive control; Semantic interference; Word selection.

Figure 1

A. Lesion overlay of the 6 patients. The color coding indicates the amount of overlap between the different patients’ lesions (red corresponds to 100% overlap and purple to 0% overlap). B. Individual patients’ lesions in red. All were reconstructed on the individual patient’s scans and are presented over each patient’s T1 MRI image except for P3 for whom only a CT scan was available. This patient’s lesion is presented on the MNI template.

Figure 2

Cumulative semantic interference effect per group (left PFC patients, aged-matched controls, young controls) on mean reaction times (A) and median error rates (B). Mean reaction times increase with position-within-category. Left PFC are overall slower and less accurate than controls but do not show a larger semantic interference effect. Medians are indicated by the black horizontal lines in the box-and-whisker plots. Interquartile ranges are represented by the boxes and the total range is depicted by the dotted lines.

Figure 3

Slope of the cumulative semantic interference effect on mean reaction times per participant in the young controls, aged-matched controls, and patient groups. Two patients (P2 and P10) had a slope which was steeper than the average slope of the aged-matched control group plus 2 standard deviations from this mean.

Figure 4

Semantic context effect in the Naming task on mean reaction times (A) and median error rates (B) for aged-matched controls (left), and left PFC patients (right). Values for homogeneous blocks (HOM) are depicted by the solid lines and values for heterogeneous blocks (HET) are depicted by the dotted lines for the reaction times (A) and by the dark grey and light grey boxes respectively for the error rates (B). Values for repetitions 2 to 6 are presented. For reaction times (A), standard deviations are represented by the horizontal lines (only positive values are presented for the homogeneous condition and only negative values are presented for the heterogeneous condition for visual clarity). For error rates (B), medians are indicated by the black horizontal lines in the box-and-whisker plots. Interquartile ranges are represented by the boxes and the total range is depicted by the dotted lines.