Ventral and dorsal aspects of the inferior frontal-occipital fasciculus support verbal semantic access and visually-guided behavioural control

Abstract

The Inferior Frontal Occipital Fasciculus (IFOF) is a major anterior-to-posterior white matter pathway in the ventral human brain that connects parietal, temporal and occipital regions to frontal cortex. It has been implicated in a range of functions, including language, semantics, inhibition and the control of action. The recent research shows that the IFOF can be sub-divided into a ventral and dorsal branch, but the functional relevance of this distinction, as well as any potential hemispheric differences, are poorly understood. Using DTI tractography, we investigated the involvement of dorsal and ventral subdivisions of the IFOF in the left and right hemisphere in a response inhibition task (Go/No-Go), where the decision to respond or to withhold a prepotent response was made on the basis of semantic or non-semantic aspects of visual inputs. The task also varied the presentation modality (whether concepts were presented as written words or images). The results showed that the integrity of both dorsal and ventral IFOF in the left hemisphere were associated with participants' inhibition performance when the signal to stop was meaningful and presented in the verbal modality. This effect was absent in the right hemisphere. The integrity of dorsal IFOF was also associated with participants' inhibition efficiency in difficult perceptually guided decisions. This pattern of results indicates that left dorsal IFOF is implicated in the domain-general control of visually-guided behaviour, while the left ventral branch might interface with the semantic system to support the control of action when the inhibitory signal is based on meaning.

Keywords: DTI; Inferior frontal-occipital fasciculus; Inhibition; Semantic access; Tractography.

Fig. 1

The left-hand panel depicts example stimuli per block. In WORD blocks, participants pressed for words denoting manmade objects and withheld this response for words denoting animals. In PICTURE blocks, participants pressed for pictures of manmade objects and withheld this response for pictures of animals. In Perceptual blocks, participants pressed for slightly slanted boxes and withheld this response for more strongly slanting boxes. Difficulty in the Perceptual trials was manipulated by adjusting the size of the slant. The right-hand panel shows behavioural results for the Go/No-Go paradigm expressed as inverse efficiency scores (a proportion of reaction time divided by accuracy); higher scores indicate greater difficulty. The error bars depict the standard error of the mean. In Experiment 1 (previously reported in Gonzalez Alam et al. 2018), the task was performed inside the scanner, while in Experiment 2 (reported here together with DTI analysis), it was performed outside

Fig. 2

The top panel illustrates the dorsal and ventral tracts of the inferior fronto-occipital fasciculus resulting from the tractography for one of our cases, overlaid on the structural image of the case. The bottom panel depicts four more examples of the dorsal and ventral IFOF from our cases, overlaid on their non-diffusion weighted image to illustrate the inter-individual variability in these tracts. The dorsal IFOF is depicted in red, and the ventral IFOF in green

Fig. 3

Scatterplots depicting the correlations between Dorsal and Ventral IFOF in LH (on the Y axis) and z-scored inverse efficiency scores for the Word, Picture, Perceptual Easy and Perceptual Hard conditions (on the X axis). The regression line and confidence interval are shown as illustration of the trend only

Fig. 4

Scatterplots depicting the correlations between the LH—RH hemispheric difference in tract integrity for Dorsal IFOF (on the Y axis) and the z-scored inverse efficiency scores of the Word, Picture, Easy and Hard conditions (on the X axis)