Revisiting the Functional Specialization of Left Inferior Frontal Gyrus in Phonological and Semantic Fluency: The Crucial Role of Task Demands and Individual Ability

Abstract

Despite a large body of research, extant findings on the functional role of left inferior frontal gyrus (LIFG) in phonological and semantic fluency are still controversial. Based on cross-study comparisons, a recent meta-analysis of neuroimaging results suggests that posteriordorsal (Brodmann area, BA, 44) and anterior-ventral parts (BA 45) of LIFG contribute differentially to processes of phonologically and semantically cued word retrieval, respectively. In contrast, a subsequent functional magnetic resonance imaging experiment failed to validate the proposed dissociation using a within-subjects design. In particular, no evidence for a specific role of BA 45 in semantic fluency was found. Here, we resolve this apparent controversy by showing that the conflicting findings can be accounted for when considering the influence of task demands and individual ability on resulting functional magnetic resonance imaging activation patterns. By comparing phonological versus semantic fluency, higher activation was robustly observed in BA 44. For the opposite comparison, higher activation was found in dorsal BA 45; however, this was more pronounced in posterior-dorsal parts of BA 45 for low-performing subjects and was only apparent in anterior-dorsal parts of BA 45 under high demands on controlled semantic retrieval. Our results thus disclose important determinants for detecting a functional segregation of LIFG in verbal fluency that also have implications for the controversial findings in previous lesion studies. Moreover, the present parcellation of dorsal BA 45 corresponds well with anatomical evidence suggesting a subdivision into an anterior (45A) and posterior part (45B) and may therefore represent evidence for its functional significance in humans.

Fahr's Disease Linked to a Novel <b><i>SLC20A2 </i></b>Gene Mutation Manifesting with Dynamic Aphasia

“…Brain [18F]FDG-PET imaging identified a significant hypometabolism in the left middle and inferior frontal gyri, in agreement with previous studies showing that this area is critically involved in the causation of dynamic aphasia [7]. In particular, functional imaging studies have demonstrated that phonological fluency, which was precociously and prominently impaired in our patient, is underpinned by the activation of the posterior left inferior frontal gyrus (Brodmann area 44) close to adjacent premotor areas [8]. …”

Partly segregated cortico-subcortical pathways support phonologic and semantic verbal fluency: A lesion study

“…By contrast, the search strategy in phonologic fluency requires utilizing unusual and thus slower, more controlled associations based on the first letter of the words (Mummery et al, 1996). Variations in task demands between the two types of fluency have also been demonstrated to account for differences in the brain areas involved in the two tasks, most notably at the level of inferior frontal gyri (Katzev et al, 2013).…”

“…In contrast, phonologic fluency requires inhibiting the default semantic associations to search words according to the unusual 'first letter' association between them. Phonologic fluency has thus been advanced to load more strongly on the executive component than semantic fluency (Perret, 1974;Thompson-Schill et al, 1997;Thompson-Schill et al, 1998;Katzev et al, 2013), although control processes are also likely necessary in semantic fluency to shift between subcategories of items and resist the interferences from competing alternatives when a given semantic network is activated (Mummery et al, 1996;Reverberi et al, 2006).…”

“…Moreover, we included a very large sample of 191 patients to optimize the statistical sensitivity and brain coverage of our analyses. Because age and education level have been shown to influence fluency performance (Tombaugh et al, 1999;Katzev et al, 2013;Marsolais et al, 2015), we used the continuous fluency scores corrected for these factors as behavioral inputs in the analyses. We further analyzed the correlation between the score at each of the fluency task.…”

Regional cerebral blood flow estimated by early PiB uptake is reduced in mild cognitive impairment and associated with age in an amyloid-dependent manner

“…Our results provide additional evidence for impaired cerebral metabolism or respectively blood flow in the left IFG in MCI. We believe that such an effect is plausible because of this region's involvement in semantic and phonological processing (Costafreda et al, 2006;Katzev et al, 2013), in processes of empathy and working memory (Liakakis et al, 2011), and in tasks of self-awareness (Morin and Michaud, 2007).…”