Inferior Longitudinal Fasciculus' Role in Visual Processing and Language Comprehension: A Combined MEG-DTI Study

Abstract

The inferior longitudinal fasciculus (ILF) is a white matter tract that connects the occipital and the temporal lobes. ILF abnormalities have been associated with deficits in visual processing and language comprehension in dementia patients, thus suggesting that its integrity is important for semantic processing. However, it remains elusive whether ILF microstructural organization per se impacts the visual semantic processing efficiency in the healthy brain. The present study aims to investigate whether there is an association between ILF's microstructural organization and visual semantic processing at the individual level. We hypothesized that the efficiency of visual semantic processing positively correlates with the degree of anisotropy of the ILF. We studied 10 healthy right-handed subjects. We determined fractional anisotropy (FA) of the ILF using diffusion tensor imaging (DTI). We extracted N400m latency and amplitude from magnetoencephalography (MEG) signals during a visual semantic decision task. N400m and mean FA of the ILF were left lateralized with the higher FA value in the left hemisphere. Inter-individual analysis showed that FA of the ILF negatively correlated with the N400m latency and amplitude, which suggests that high ILF anisotropy is associated with more efficient semantic processing. In summary, our findings provide supporting evidence for a role of the ILF in language comprehension.

Keywords: diffusion tensor imaging; language comprehension; magnetoencephalography; ventral language pathway; white matter.

FIGURE 1

Evoked response field from MEG signals related to silent reading of word pairs in a single subject. The signals in green are derived from the left hemisphere sensors and the signals in blue are from the right hemisphere. In (A) the “unrelated” condition and in (B) the “related” condition. Early visual components (N100m and M170) are not significantly different between “unrelated” and “related” conditions. The signal in (C) is a subtraction of the “unrelated” condition from the “related” condition.

FIGURE 2

Individual N400m signals of all 10 subjects. These signals are subtractions of the “unrelated” condition average from the “related” condition average to reflect the difference in N400m response only. The green signals represent activities recorded from the left hemisphere and the blue signals represent activities recorded from the right hemisphere. Light blue signals derive from midline sensors. The red bar locates the time point used to measure N400m latency and amplitude, which is the first peak within the N400m time window (300–500 ms) with a signal-to-noise ratio higher than 1.5 for each subject.

FIGURE 3

Registered atlas-based ILF mask of a single subject. The probabilistic left ILF mask obtained from the JHU atlas (color) is co-registered with a subject’s FA map in standard space in radiological orientation. The masked brain areas were included in the weighted mean FA calculation.

FIGURE 4

Behavioral data and mean FA of the left ILF. No correlation was found between mean FA of the left ILF and reaction time (A) or accuracy (B).

FIGURE 5

Lateralization of the ILF determined by the mean FA. Each dot and square represents one individual mean FA value. Error bars represent standard deviation. The mean FA of the left ILF is significantly higher than that from the right ILF (p = 0.012).

FIGURE 6

N400m and MMF latencies and mean FA of the left ILF. A negative correlation between N400m peak latency and mean FA of the left ILF has been identified [r(10) = –0.64, p = 0.023, R² = 0.41]. No correlation was found between MMF and mean FA of the left ILF or between N400m latency and mean FA of the right ILF.

FIGURE 7

N400m amplitudes and mean FA of the ILF. A negative correlation between N400m peak amplitude and mean FA of the left ILF has been identified [r(10) = –0.60, p = 0.033, R² = 0.36]. The right ILF did not show a significant correlation with N400m peak amplitude (p = 0.080).