Evaluating interhemispheric connectivity during midline object recognition using EEG

Abstract

Functional integration between two hemispheres is crucial for perceptual binding to occur when visual stimuli are presented in the midline of the visual field. Mima and colleagues (2001) showed using EEG that midline object recognition was associated with task-related decrease in alpha band power (alpha desynchronisation) and a transient increase in interhemispheric coherence. Our objective in the current study was to replicate the results of Mima et al. and to further evaluate interhemispheric effective connectivity during midline object recognition in source space. We recruited 11 healthy adult volunteers and recorded EEG from 64 channels while they performed a midline object recognition task. Task-related power and coherence were estimated in sensor and source spaces. Further, effective connectivity was evaluated using Granger causality. While we were able to replicate the alpha desynchronisation associated with midline object recognition, we could not replicate the coherence results of Mima et al. The data-driven approach that we employed in our study localised the source of alpha desynchronisation over the left occipito-temporal region. In the alpha band, we further observed significant increase in imaginary part of coherency between bilateral occipito-temporal regions during object recognition. Finally, Granger causality analysis between the left and right occipito-temporal regions provided an insight that even though there is bidirectional interaction, the left occipito-temporal region may be crucial for integrating the information necessary for object recognition. The significance of the current study lies in using high-density EEG and applying more appropriate and robust measures of connectivity as well as statistical analysis to validate and enhance our current knowledge on the neural basis of midline object recognition.

Fig 1

(A) Electrode layout used for the experiment. Red ovals indicate the electrodes of interest: TP7, TP8, P7, P8 used in Mima et al. 2001. (B) Illustration of experimental design. (C) Example images of object (top panel) and meaningless object (bottom panel) stimuli.

Fig 2. Frequency power spectrum averaged over four electrodes of interests (TP7, TP8, P7, P8) for object (2A) and meaningless object (2B) stimuli plotted across time.

Fig 3. Task-related change in alpha band power averaged over four electrodes of interest (TP7, TP8, P7, P8) and plotted for different time windows.

Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. t1 = 0–500 ms; t2 = 250–750 ms; t3 = 500–1000 ms; t4 = 750–1250 ms. Error bars are SEM. Asterisks indicate p<0.05.

Fig 4

(A) Task-related change in alpha band coherence averaged for four electrode pairs of interest (TP7-TP8, TP7-P8, P7-TP8 and P7-P8) pairs and plotted for four different time windows. Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. t1 = 0–500 ms; t2 = 250–750 ms; t3 = 500–1000 ms; t4 = 750–1250 ms. Error bars are SEM. (B) Task-related change in imaginary part of coherency for alpha band averaged for four electrode pairs of interest (TP7-TP8, TP7-P8, P7-TP8 and P7-P8) and plotted for four different time windows. Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. t1 = 0–500 ms; t2 = 250–750 ms; t3 = 500–1000 ms; t4 = 750–1250 ms. Error bars are SEM.

Fig 5

(A) Results of non-parametric cluster-based permutation analysis of power spectrum data for alpha band. Asterisks indicate electrodes that showed significant difference (p<0.05) in alpha power between the two stimulus conditions (Objects minus meaningless objects) that occurred at 0.6s (left) and 0.7s (right). The prominent cluster visible over the left occipito-temporal region included C3, P3, Pz and O1 electrodes. (B) Task-related change in alpha power averaged for the significant electrode cluster (C3, P3, Pz and O1) and plotted for four time windows. Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. t1 = 0–500 ms; t2 = 250–750 ms; t3 = C_xy (f) = 500–1000 ms; t4 = 750–1250 ms. Error bars are SEM. Asterisks indicate p<0.05.

Fig 6. Results of paired permutation test (objects vs meaningless objects) performed on alpha power spectrum of sources and projected on a template anatomy cortex.

Fig 7. Regions of interest (ROIs) demarcated based on the results of time-frequency analysis for source space.

The left ROI encompassed the occipito-temporal region in the left hemisphere that showed significant alpha desynchronisation associated with object recognition and the right ROI included the homologous region in the right hemisphere.

Fig 8

(A) Task-related change in alpha band coherence between the left ROI and right ROI in source space and plotted for four different time windows. Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. t1 = 0–250 ms; t2 = 250–500 ms; t3 = 500–750 ms; t4 = 750–1000 ms. Error bars are SEM. Asterisks indicate p<0.05. (B) Task-related change in imaginary part of coherency for alpha band between the left ROI and right ROI in source space and plotted for four different time windows. Solid black bars represent object stimuli; checked bars represent meaningless object stimuli. T1 = 0–250 ms; t2 = 250–500 ms; t3 = 500–750 ms; t4 = 750–1000 ms. Error bars are SEM. Asterisks indicate p<0.05.

Fig 9. Results of Granger causality (GC) analysis between left and right ROI in source space for the time window of 250–500 ms.

Solid black bar represents Left to Right GC for object stimuli; solid white bar represents Right to Left GC for object stimuli; solid grey bar represents Left to Right GC for meaningless object stimuli; checked bar represents Right to Left GC for meaningless object stimuli. Error bars are SEM. Asterisks indicate p<0.05.