Interhemispheric connectivity during lateralized lexical decision

Abstract

The well-established right visual field (RVF-lh) advantage in word recognition is commonly attributed to the typical left hemisphere dominance in language; words presented to the LVF-rh are processed less efficiently due to the need for transcallosal transfer from the right to left hemisphere. The exact stage for this hemispheric transfer is currently unsettled. Some studies suggest that transfer occurs at very early stages between primary visual regions, whereas other studies suggest that transfer occurs between the left visual word form area and its right hemisphere homolog. This study explores these conflicting accounts and finds evidence for both. Participants conducted a lateralized lexical decision task with both unilateral and bilateral display conditions. Connectivity analyses were conducted from magnetoencephalography signals that were localized to the left middle occipital gyrus (LMOG), right middle occipital gyrus (RMOG), left visual word form area (LVWFA), and right visual word form area (RVWA). Results from unilateral trials showed asymmetrical interhemispheric connectivity from the RMOG to LMOG and symmetrical interhemispheric connectivity between the LVWFA and RVWFA. Furthermore, bilateral presentations led to reduced interhemispheric connectivity between both homologous region of interest pairs. Together, these results suggest that lateralized word recognition involves multiple stages of interhemispheric interactions and that these interactions are reduced with bilateral displays.

Keywords: functional connectivity; hemispheric interactions; visual word recognition.

Figure 1

Task details. (a) Presents the task sequence. A fixation cross is presented for 3,000 ms. This is followed the stimulus screen for 200 ms and a 200 ms mask. The central arrow from the stimulus screen points to the visual field of the target (CAT). Participants are then given up to 3,000 ms to respond and provided with feedback. (b) Example stimulus presentations for a RVF‐lh word target (CAT) in the unilateral (top) bilateral congruent (middle), and bilateral incongruent (bottom) conditions. (c) Example stimulus presentations for a RVF‐lh pseudoword target (NER) in the unilateral (top) bilateral congruent (middle), and bilateral incongruent (bottom) conditions. Stimuli are repeated here to facilitate explanation; words and psuedowords are never repeated within the experiment

Figure 2

Size and locations for ROI. The VWFA ROIs are 10 mm spheres based on coordinates from (McCandliss et al., 2003) for the LVWFA and the same coordinates with the x‐axis flipped for the RVWFA. The bilateral MOG ROIs are based on the macrolabels automated anatomical labeling atlas (Tzourio‐Mazoyer et al., 2002) [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 3

Behavioral results. Plots present accuracy (top) and inverse reaction time (bottom) separately for words (left) and pseudowords (right). Significant differences between left and right visual field are thresholded at *p < .05 (uncorrected), **p < .05 (corrected), and ***p < .01(corrected)

Figure 4

Time–frequency responses. Plots presents the percentage change in oscillatory power (left) and the standardized change in wPLI (right) compared to a baseline period of −300 to −100 ms before stimulus onset. Both plots are averaged across all 12 conditions of the experiment [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 5

Source‐localized activation maps. Maps present 1–5 Hz oscillatory activity for unilateral and bilateral word trials from 100 to 300 ms poststimulus onset against a baseline period of −300 ms to −100 ms before stimulus onset. All maps thresholded at p < .001, uncorrected [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 6

ERF plots for unilateral trials. Plots present the normalized ERF responses that were time‐locked to stimulus onset, and low‐pass filtered at 5 Hz. Responses are localized to the LMOG (top left), RMOG (top right), LVWFA (bottom left), and RVWFA (bottom right). Horizontal ticks indicate significant visual field differences from the cluster‐based permutation analysis. Cluster‐wise significance is indicated as: •p < .1, *p < .05. The same plot with shaded error bars for the standard error of the mean (SEM) can be found in the Supporting Information [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 7

wPLI for unilateral trials. Plots present raw wPLI values between LMOG–RMOG (top left), LVWFA–RVWFA (top right), LMOG–LVWFA (bottom left), and RMOG–RVWFA (bottom right) channel pairs. Horizontal ticks indicate significant visual field differences from the cluster‐based permutation analysis. Cluster‐wise significance is indicated as *p < .05. The same plot with shaded error bars for the SEM can be found in the Supporting Information [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 8

ERF plots for unilateral and bilateral trials. Plots present the normalized ERF responses that were time‐locked to stimulus onset, and low‐pass filtered at 5 Hz. Responses are localized to the LMOG (top left), RMOG (top right), LVWFA (bottom left), and RVWFA (bottom right). Horizontal ticks indicate significant main effects of visual field (in black), condition (unilateral vs. bilateral congruent, in red), and significant interactions (in blue) from the cluster‐based permutation analysis. Cluster‐wise significance is indicated as *p < .05, **p < .01, and ***p < .005. The same plot with shaded error bars for the SEM can be found in the Supporting Information [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 9

wPLI for unilateral and bilateral trials. Plots present raw wPLI values between LMOG–RMOG (top left), LVWFA–RVWFA (top right), LMOG–LVWFA (bottom left), and RMOG–RVWFA (bottom right) channel pairs. Horizontal ticks indicate significant main effects of visual field (black), condition (red), and significant interactions (blue) from the cluster‐based permutation analysis. Cluster‐wise significance is indicated as *p < .05, **p < .01, and ***p < .005. The same plot with shaded error bars for the SEM can be found in the Supporting Information [Color figure can be viewed at https://wileyonlinelibrary.com]

Figure 10

PSI for unilateral and bilateral trials. Plots present raw PSI values between LMOG–RMOG (top left), LVWFA–RVWFA (top right), LMOG–LVWFA (bottom left), and RMOG–RVWFA (bottom right) ROI pairs. The same plot with shaded error bars for the SEM can be found in the Supporting Information [Color figure can be viewed at https://wileyonlinelibrary.com]